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SIZE EXCLUSION CHROMATOGRAPW OF POLYOLEFINS AND
EVALUATING LOCAL POLYDISPERSITY
A thesis submitted in conformity with the requirements for the degree of Master of Applied Science
Graduate Department of Chemical Engineering and Applied Chemistry University of Toronto
0 Copyright by Baihua Rao, 1998
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Size Exclusion Chromatography of Folyolefins and Evaluating Local Polydispersity
Baihua Rao, Mmster of Apptied Science, 1998
Department of Chernial Engineering and A p p M Chemistry, University of Toronto
This thesis focuses upon two current problem areas in size exclusion chromatography
(SEC): (1) the high temperature (145°C) required to analyze polyolefhs and, (2) the fdure of
the assumption that size separation is molecular weight separation (the presence of local
polydispersity). A new mobile phase, methyl cyclohexane, pennitted analysis at 90°C with
improved refractometer (DRI) sensitivity, lower toxicity than traditional solvents and
simultaneous W-DR1 analysis of grafted polyethylenes. Solutions to the increased
flammability and the tendency for some materials to adsorb to the column packing were
proposed. Of three new methods for detecting the presence of local polydispersity, only one
based upon the cornparison of "reconstnicted" and the experimental DEU chromatograms was
shown to be both practical and general. The method was successfully applied to the analysis of
two new polymer blends and local polydispersity became undetectable when one of the
components in the blends examined was less than 5 to 10 wt%. Finally, inter-detector volume
was shown to have a major influence on the "reconstructed" chromatogram.
To Bing Wm and my Parents,
bofh whom I love and owe more gratitude than 1 can mer express.
The beauty of nuture lies in detail
The ptinciples exist in generality
Tu understand the world
Demands bo th
I am very grateful to Rofessor Stephen Thomas Balke for al1 his guidance and assistance
throughout this research. 1 also wish to thank Dr. Thomas H. Mourey and Dr. Timothy C.
Schunk (Eastman Kodak Company, Rochester, NY) for giving me many helpful comments and
suggestions.
I wish to acknowledge the Eastman Kodak Company (Rochester, N.Y) for providing
hancial support to this work. 1 wouid also like to thank the following companies for kindy
contributhg materials required for the completion of this work: Eastman Kodak Company
(Rochester, N-Y) for dimethyl siloxane-b-ether imide) and dimethyl siloxane-b-ester copolymers,
Imperid Oil, Dow Chernical Canada (Sarnia, ON) and Mitsubishi Petrochemicals (Japan) for
Iinear low density and high density polyethylene, Dupont for maleated linear low density
polyethylene, Himont Canada (Montreal, PQ) and Phillips (OK, USA) for polypropylene.
1 wish to thank many fiends and colleagues who have generously given assistance and
shared ideas throughout this work, in particda., Askar Karami, Carrie Hu, Joseph Sibichen,
KeivanTorabi, Ramin Res hadat, Rueng sak Thitiratsakul, S ina Say ed and Teny Borer .
Findy, 1 wish to thank my wife, parents, brother and sisten for their love and
encouragement throughout the course of this work.
TABLE OF CONTENTS
ABSTRACT
ACKNOWLEDGEMENTS
TABLE OF CONTENTS
LIST OF FIGURES
LIST OF TABLES
1.0 INTRODUC'MON
2.0 TFIEORY
2.1 Size ExcIusion Chromatography
2.1.1 The Differential Refractorneter @Ri)
2.1.2 The Ultra-violet Spectrophotometer 0
2.1.3. The Differential Viscometer @V)
2.1.4 The Low Angle Laser Light Scattering Detector (LALLS)
2.1.5 Molecular Weight Calibration Methods in SEC
2.2 High Temperature SEC of Polypropylene and Polyethylene
2.2.1 Problems of High Temperature SEC of Polyethylene and Polypropylene
2.2.2 Previous Attempts to Reduce SEC Analysis Temperature for Polyolefins
2.2.3 Anaiysis of Dye-grafted Polyolefins
2 3 Polymer Local Polydispersity
2.3.1 Oiigin of Polyrner Local Polydispersity
2.3.2 The Systematic Approach
2.3.3 Method 1
2.3 -4 Method II
2.3-5 Method III
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2.4 Summary of Theoretical Development
3.0 EXPERIMENTAL
3.1 SEC of PE and PP at 90 "C
3.1.1 SEC Materials
3.1 2 Solubility Evaluaîion
3.1 -3 SEC Analysis
3.1 -4 Calibration of PP Using Numerical Optimization Methods
3.1.5 Calibration of LLûPE Using Broad MWD Standard
3.2 Elucida tion of Polymer Local Potydispenity
3 -2.1 Materiais
3.2.2 Dual Detectors and Selective Precipitation
3.2.3 Triple Detector System
4.0 RESULTS AND DISCUSSION
4.1 High Temperature SEC of Polyethylene and Polypropylene
4.1.1 Mobile Phase Selection
4.1.2 Sarnple Preparation
4.1.3 Quantitative Assessment
4.1.3.1 Evaluation of PE and PP Chrornatograms
4.1 -3.2 Calibration
4.1.3.2.1 Polypropylene
4.1.3.2.2 Linear Low Density Polyethylene
4.1.3.3 Molecular Weight Averages and MWD
4.1 -3.3.1 Polypropylene
4.1 -3.3 2 Linear Low Density PoIyethy lene
4.1.4 Analysis of Dye-grafted PE and PP
4.1.5 Assessment of Methyl Cyclohexane as the Mobile Phase
4.1.5.1 Reproducibility 66
4.1-52 Dissolution 66
4.1.5.3 Flammability 67
4-1 5.4 Adsorption 68
4.2 Elucidation of Polymer Local Polydispersity 70
4 . Assesment of Methods involving Sample Treatment 70
4 2 3 Local Polydispersity Detection Using Method 1 70
42.3 Local Polydispersity Detection Using Method II 71
4.2.3.1 Utilizing the Systernatic Approach for Method II 72
42.32 Local Polydispersity of CopoIymers in THF Using Method II 76
4.2.4 The "Reconstructed" DR1 Chromatogram Method (Method III) 79
4.2.4.1 New Polymer Blend Examples for Method III 80
4.2.4.2 Evaluating the Detection Limit 86
42.4.3 Effect o f Interdetector Volume on Calibration Curves 87
4.2.4.4 Effect of Interdetector Volume on the "Reconstructed
DR1 Chromatogram
5.0 CONCLUSIONS
6.0 RECOMMENDATIONS
7.0 REFERENCES
APPENDIX 1 Equations for Analysis of Dye-Grafted Polyethylene
APPENDIX 11 Evaluating the Detection Limit of PMMA / PVA
APPENDIX m Evaluating the Detection Limit of PVA / PVC
vii
LIST OF FIGURES
Figure 2.1: Schematic diagram of the SEC apparatus
Figure 22: Procedure to obtain a calibration cuve using a broad standard with known MWD
Figure 2.3: Converting a chromatogram to a molecular weight distribution
Figure 2.4: Detecting local polydispersity using DR1 and DV detectors
Figure 2.5: Local polydispersity flow chart
Figure 4.1 : SEC procedure using methyl cyclohexane as the mobile phase
Figure 4.2: Area under the chromatogram of PP as a firnction of mass injected
Figure 4.3: Chromatograms of PP and HDPE in methyl cyclohexane mobile phase at 90 OC
Figure 4.4: Search for grouped Mark-Houwink constants
Figure 4.5: Calibration cuves of polyiso butylene and polypropy lene
Figure 4.6: Calibration curve of linear low density polyethylene
Figure 4.7: Normalized molecular weight distribution of polypropylene
Figure 4.8: Overlay of UV chromatogram of anthracene-grafled LLDPE on
its DM chromatogram
Figure 4.9: Molecular structure of dye-grafied LLDPE and mode1 molecules
Figure 4.10: Area under the W chromatogram versus content of 9-methy! anthracene
Figure 4.1 1 : Area under the UV chromatogram versus content of 1-methyl naphthalene
Figure 4.12 Flow diagram shows transformation fiom UV chromatogram to dye content
Figure 4.13: Dye content versus molecular weight and moIecu1a.r weight distriiution curve
Figure 4.14: D(1ogM) and N(1ogM) as a function of molecular weight
Figure 4.15: The ratio of moles of dye molecules to moles of polymer versus molecular weight
Figure 4.16: Normalized chromatograrns of polyester "before" and "after" sample preparation
Figure 4.17: Polystyrene moleculat weight calibration c m e of the DRI-DV detection systern
Figure 4.18: Apparent intrinsic viscosity versus retention volume of four block copolymers
PAGE
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45
46
48
5 1
52
56
5 8
59
60
60
62
63
65
65
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73
77
Figure 4-19: Chromatograms and residual plots of PMMA / PVA (Case IV)
Figure 420: Chromatograms and residual plots of PVA 1 PVC (Case IV)
Figure 4 2 1 : Chromatograms and residual plots of PMMA / PVA (Case V)
Figure 4.22 Chromatograms and residual plots of PVA / PVC (Case V)
Figure 423: Effect of interdetector volume on local number average molecular weight
Figure 424: Effect of interdetector volume on local weight average molecuIar weight
Figure 4.25: Effect of DRI-DV interdetector volume on the "reconstmcted" chrornatogram
Figure 426: Effect of DRI-LS interdetector volume on the "reconstructed" chromatogram
Figure 4.27: Effect of both DM-DV and DN-LS interdetector volume on the "reconstmcted"
Chrornatogram (positive or negative error in both)
Figure 4.28: Effect of both Dm-DV and DRI-LS interdetector volume on the "reconsûucted"
Chrornatogram (positive error in one, negative error in the other)
LIST OF TABLES
Table 1: Selected properties of PE and PP
Table 2: Selected properties of some possible eluting solvents
Table 3: The solubility of polyolefins in decalin at different temperatures
Table 4: Co-solvent selection
Table 5: Polyisobutylene standards used for SEC calibration
Table 6: Numerical opthkation search results for polypropylene calibration
Table 7: Molecular weight and retention volume data of LLDPE
Table 8: Molecular weight results of PP standards compared with vendor values
Table 9: MoIecular weight averages of LLDPE samples
Table 10: Average number of dye groups per polymer chah
Table 1 1 : Molecular weight averages of PP re-injected four tirnes
TabIe 12: Flash points and autoignition temperatures of SEC solvents
Table 13: Effect of polar solvent on the SEC of polymers
TabIe 14: Polystyrene standards used for calibration
Table 15: Molecular weight averages of the NBS SRM706 polystyrene standard 74
Table 16: Intrinsic viscosity of the NBS SRM706 polystyrene standard 75
PAGE
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1.0 INTRODUCTION
Polymers are generally mixtures of molecules of different molecular weights. The
molecules of indusfridly usefbi polymers often also M e r in other properties: the number of
fiinctional end groups, composition, branch length, branch fiequency, etc. Thus, the chemical
and structural details of the long c h a h (i.e. the molecular properties of polymers), are of great
importance in polymer processing and product development
Polydispersity is a measure of the variability of a particular molecula. property in a
polymer. For example, the ratio of two differently calculated molecular weight averages, the
weight average molecular weight and the number average molecular weight, provides a measure
of the molecular weight diversity present (the larger the value the greater the diversity). M e r
polydispersity measutes can be devised to express variability in other molecular properties.
Size exclusion chromatography (SEC) is now the most commonly used method for
measuring molecular weight and size of polymers and for detennining polydispersity. SEC is a
liquid chromatography method in which the molecules are separated according to their size in
solution. One or more detectoa are used to examine the molecules of each size as they exit the
instrument It is normally assumed that separation by SEC is perfect: each different molecular
size requires a different time to elute and each molecular size is uniquely related to a different
molecular weight.
Two curent problem areas in SEC analysis are the relatively high temperature (145 OC)
1
2 required to d y z e polyolefins and the failtue of the perfect molecular weight separation
assumption (the presence of Yocal polydispersity" in SEC). High temperatme analysis
necessitates the use of toxic solvents and is associated with a variety of operational problems.
However, polyethylene (PE) and polypropylene (PP), two of the world's most commonly used
polymers, require such analysis. Local polydispersity, molecular variety at each retention
volume, is potentidy a major source of error in quantitative SEC. However, until very recently,
it could exist undetected in the analysis of more complex polymers (e.g. copolymers and
branched polymers).
The objectives of this thesis are:
i. to determine a method for analyzing plyolefins by SEC at a temperature which is
suniciently low to aileviate currrnt operational problems;
ii. to assess new methods for detecting local polydispersity using SEC.
In the next section, quantitative SEC analysis is bnefly summarized and the idea of
selecting a new solvent for polyolefins to permit lower temperature operation is examined.
Following that, methods of detecting local polydispersity are described with emphasis on the
"reconstmcted chromatogram" method. The thesis then centers upon accomplishing the above
two objectives.
2.0 THEORY
2.1 SIZE EXCLUSION CHROMATOGRAPEIY
Molecular size sepration in SEC occurs as a resdt of stenc exclusion fiom the pores of
the packing material. Larger polymer molecules penetrate fewer pores within the cross-linked gel
beads and thus elute before smaiier molecules. As shown in Figure 2.1, a size exclusion
chromatograph consists of a solvent reservoir, a hi& pressure pump, a sample injection device, a
series of packed columns, one or more detectors, and a solvent waste reservoir. Column packing
usually consists of cross-linked polystyrene beads, 3-20 microns in diameter. Microcornputers
are used to both control the instrument and to interpret data. The type of data interpretation
software employed depends upon the detectors used. In addition to the traditional ciifferentai
rehctive index detector and ultraviolet and infrared detectors, there are now five different types
of light scattering detectors and two different types of i n d i c viscosity detectors [1,2].
Thennostated oven
Data pmcessor
Figure 2.1 : Schematic diagnim of the SEC apparatus
4 For the f k t objective of this thesis, analysis of polyoleh at a Low operating
temperature, the Merentiai r e k t i v e index @RI) detector was used, sumetimes in combination
with an ultraviolet spectrometer. For the second objective of the thesis, assessrnent of local
polydispersity, a DR1 detector and a differential viscometer were used to collect data Data fiom
Eastman Kodak Company obtahed using three detectors: a DR1 detector, a low angle laser light
scattering detector and a Merential viscometer were also supplied to us for the evaluation. The
following sections briefly describe these pdcular detectors.
2.1.1 The Differential Rehctometer
The differential rehctometer (DRI) is the most commody used detector in SEC [3]. In
the conventional DRI, monochromatic light is split into two beams, one diverted into a
transparent sample ce11 containing the flow of the polymer solution exiting the columns, and the
other into the reference cell containing the pure eluent. If the contents in the sample and
reference cells are the same, (Le. no polymer is present), the r e k t i v e indices are the same. The
light beams faIl on the surface of a position sensitive to photodetector that produces an electronic
signal and establishes the baseline of the chromatogram. In the presence of polymer, the sample
ce11 contents are different and the rehctive indices change, causing deflection in the light beam
directly proportional to the mass concentration of the polymer, thus producing the
chromatographie peak. The extent of the deflection is a function of how much the rehctive
index changes with a change in polymer concentration (the specific rehctive index increment,
dddc) as well as the instnunent gain (B).
When polymers of the same Wdc are present in the detector cell, their concentration at
retention volume (vi ), ci , is given by:
where Wi is the detector response.
The advmtage of the DR1 detector is that it offers a linear response over a wide range of
concentrations. As well, it cm be considered a universal concentration detector because most
polymen in a solvent have a measurable rehctive-index difference and response can be afTected
by the proper choice of solvent. A disadvantage, however, is that since the response depends
upon Wdc, if two polymer types are present and CO-elute at one particular retention time, each
with Merent dddc values, the detector response will reflect the composition of the molecules as
well as their total concentration. M e r disadvantages are that the DR1 has a relatively low
sensitivity (1 O-' - 1 O" g/mL) [4] and is more sensitive to changes in ambient temperatures than
most detectors.
2.1.2 The W Spectrophotometer
The ultraviolet 0 spectrophotorneter detector is limited to samples containing
chromophores that will absorb in the UV region, and that will dissolve in solvents which do not
absorb W. These detectors c m only be operated at moderate temperatures (below 100 O C ) but
they c m be very sensitive (1 od - 1 0 ' ~ ~ g/mL) [4]. The W detector is much less sensitive to
6 temperature variations than the DR1 detector. Analysis of dye-grafted polyolefins would benefit
if an W detector could be used in combination with a DR1 detector in SEC analysis. As shown
in Appendix 1, the moles of dye per mole of polymer at each retention time could then be
determined across the chromatogram. However, the present solvent used for high temperature
SEC (trichlorobenzene, TCB) strongly absorbs UV light.
2.13 The Differential Viscorneter
The viscometer penorms on-line uitrinsic viscosity measurernents by measuring the
pressure drop across a section of capillary tubing. The Hagen-Poiseuille Equation is used to
convert the pressure drop to a solution viscosity [5,6]. In the differential version, a special
"Wheatstone Bridge" configuration of tubhg is used to cancel flow rate effects on the measured
pressure drop so the equation actually used is as follows:
where q , i , , and Pi are specific viscosity, pressure drop in hvo branches of the
"Wheatstone Bridge", and inlet pressure, respectively, at each retention volume (the "local"
value).
Intrinsic viscosity at each retention volume (the "local" intrinsic viscosity) at the low
concentrations present in the chromatopph is estimated as:
Thus, by combining the local concentration, ci, fiom the DR1 detector (Equation 2.1) with
the local specifïc viscosity, q , , fiom the DV detector (Equation 2-21, the local intrinsic
viscosity, [q], can be obtained fiom Equation 2.3.
2.1.4 The Low Angle Laser Light S c a t t e ~ g Detector
In the low angle laser Iight scattering detector (LALLS) a laser beam of wavelength 632.8
nrn (heliurn-neon laser) or 670 nm (diode laser) is directed at the eluting solution in a 10-75 pL,
flow celi [3]. Scattered light is collected at a very low angle (2-15'). At this low angle the shape
of the rnolecuie in solution does not affect the response so the local weight average molecular
weight at each retention volume, Kj, can be estimated fkom (at low concentrations, the product
of the second Wial coefficient and concentration is negligible):
4.i M , , = - Kc,
where Re. and ci are the Rayleigh scattering ratio and polymer concentration at each retention
volume, respectively. K is the proportionality constant and is a function of dddc. Thus,
interpretation of the light scattering data can be confounded by both molecular weight and
composition if different types of molecules are present at the same retention volume.
8 2-15 Molecular Weight Calibration Methods in SEC
SEC is a secondary technique and the correlation between molecular weight and retention
time (or retention volume) is established by caiibration with standards whose molecular weight
has been detemiined by absolute methods (e.g. osmorneter, laser light sca t te~g) . Very narrow
molecular weight distribution polymer standards (e.g. polystyrene narrow standards) can be used
to define this correlation. A series of these standards of different molecular weight are injected
into the instrument and the loga&hm of th& peak molecular weight versus peak retention
volume data is fitted by a polynomial[7,8]:
When the polymer standards are a different type of polymer than the polymer to be
analyzed, then, "Universal Calibration" can be used [9]. A Universal Calibration c w e is a plot
of hydrodynamic volume vernis retention volume. Hydrodynamic volume, Ji, is defined by
where [qli is the intrinsic viscosity, M, is the molecular weight, at each retention volume.
If narrow standards are available and they obey the Mark-Houwink equation:
9 where K and a are Mark-Houwink constants and are dependent on the polymer, the solvent and
temperature. K and a are usuaily obtained either fkom the viscosity measurement on narrow
molecular weight distribution standards or from the literature[lO].
Then, molecular size at each retention volume is given by:
J, = K M,~+'
where M, is obtained from Equation (2.5)
Now, if the sample to be analyzed also obeys the Mark Houwink Equation, then the
calibration curve for the sample can be obtained fiom:
1 Kstd astd+l log M, =-log-+-
a+l K a+l log Mstd. i
where the subscript "std " indicates the standard polymer. Log M s t ~ is obtained fiom Equation
However, accurate K and a values are difficult to obtain experimentally since there will
not be a suficient number of polymer standards of known molecular weight to allow accurate
determination of the molecular weight dependence of intrinsic viscosity ([q 1). Imprecision in the
viscosity measurements needed to d e t e d e K and a is also a problem. Another problem is non-
linearity of log [rl] versus log M at low molecular weight (40,000).
If the Mark-Houwink constants are not known, as is the case for polypropylene and
polyethylene in this study, then there are two rnethods to obtain the calibration.
The first method, termed "the n u d c a l opthkation search method", consists of
injecting broad molecular weight distribution standards and searching for the value of the
grouped constants necessary to obtain a molecular weight calibration curve which, when applied
to the chromatograrn of the standards, provides the known rnolecular weight averages [Il]. The
following paragraph provides more details on this method.
Equation 2.9 can be rewritten as:
The grouped constants "S" and 'T' are successively guessed using a modified Nelder-
Mead Simplex search, and substitute into Equation 2.10 to obtain a trial calibration for the broad
standards. The trial calibration curve resulting fiom each guess is then applied to the
chromatograms of broad standards and the search for 'Y' and "P' continues until the Y, and M,
calculated fiom the chromatognun match those known for the standards. The molecular weight
averages are compared in an "objective function" fomulated by assuming that the error variance
of the averages is proportional to the square of the experimental value of the averages [12].
Molecdar weight averages are obtained h m the whole chromatogram and the desired
caiibration curve with the searched grouped Mark-Houwink constants ushg Equation 2.1 1 and
Number average molecular weight:
Weight average molecular weight:
where, WW, the normalized chromatogram height is given by:
Wi is the chromatogram height, Av, is the retention volume increment between two adjacent data
points, and molecular weight, M, , is defined by Equation 2.10.
The second method used to obtain the caiibration curve for polymers with unknown
Mark-Houwink constants employs a broad molecular weight distribution standard with known
molecular weight distribution [3]. This method is based upon the molecular weight distribution
for a polymer sample being unchanged in different SEC systems.
For a known polymer MWD curve (Figure 2.2a), there is a unique correlation between
molecular weight and the weight hct ion of the polymer below a given molecular weight (Figure
2.2b). Sirnilarly, fkom the SEC chromatognun (Figure 2 . 2 ~ ) ~ there is a unique correlation
between the retention volume and the weight fraction above a given retention volume (Figure
2.2d). Therefore, a calibration curve is obtained by matching those molecular weight and
retention volume values that correspond to the same value of cumulative weight fraction on the
MWD c w e and SEC chromatograrn.
Once the calibration curve is obtained from this second method, it can be used with the
SEC chromatograms of unknowns of the same po!ymer type to provide the molecular weight
distribution (Figure 2.3).
(a) Normalized molecular weight distribution (c) Normalized chromatogram
(b) Cumulative molecular weight distribution (d) Cumulative chrornatogram
Figure 2.2: Procedure to obtain effective calibration curve using a broaa standard with known
molecular weight distribution.
14 Figure 2.2: Procedure to obtain an effective calibration curve using a broad standard with
known molecular weight distribution.
Molecular Weight Distribution
Cal ibration Curve
Log M
Figure 2.3: Converting a chromatognun to a molecular weight distribution
2.2 HIGH TEMPERATURE SEC OF POLYETHYltENE AND
POLYPROPYLENE
22.1 Problems of High Temperatore SEC of Poiyethylene and Polypropylene
Size exclusion chromatography (SEC) analysis requkes polymer molecules to be in tme
solution, that is, dispersed on a molecular level. Many polymers, polystyrene and poly(methy1
methacrylate) for example, can easily be dissolved at room temperature. However, the
dissolution of semi-crystalline polymers, such as polyethylene (PE) and polypropylene (PP), is
particularly difficult and generally requires temperatures above their melting points. Thus, SEC
anaiysis of PE and PP is normally accomplished at 135 to 145 O C 113, 141.
Over the past three decades, a number of review papers on hi& temperature SEC of PE
and PP have been published [15- 171. Frequently used solvents in the SEC characterization of
these polymers are O-dichlorobenzene (ODCB), 1,2,4-trichlorobemene (TCB),
chloronaphthalene (CN), decalin, tetralin, and xylene [18-201 with the first two mentioned being
the most common 113- 17 .
A variety of problems have been reported [21,22]:
Molecular weight results from SEC increase with increasing temperature. This trend is
more pronounced at higher molecular weight, and is particularly noticeable at solution
temperatures close to that of the normal crystallhe melting temperature of the polymer
[23 1. It can be attributed to the existence of some aggregates in the polymer solution
under experimental conditions.
The MW values obtained h m SEC are Iower than those fiom "off-line" light scattering
analysis [24]. This suggests that some high molecula. weight species cannot be detected
either because they re-aggregated during the sample preparation and were fiitered out
before entering the column or because they were too diluted on passage through the SEC
columns to be observed by the detector.
Experimental data are less reproducible than with room temperature instruments used for
other polymers and column clogging sometimes occurs [2 1,22,25].
ODCB and TCB are toxic and have an unpleasant odor. Because of sample handling
requirements, this is an important consideration despite the use of well-ventilatecl,
automated instruments.
The s m d difference in rehctive index (nd29 between the halogenated aromatic eluting
solvents (nd2': ODCB 1.549 1, TCB 1.5524) and the polymers (nd20: PP 1.5030, HDPE
1 S450) [IO] results in a low sensitivity for the differential rehctive index @RI) detector.
Degradation of polyolefins may occur at the high (> 135OC) SEC operating temperature.
The lack of spectrophotometric "windows" in the unial solvents prohibits the use of W
detection and severely limits the use of i . d detection.
2.2.2 Previoos Attempts to Reduce SEC Analysis Temperatures for Poïyolefins
Ying et al. [26-281 reported the use of cyclohexane as eluent for the SEC of PP at 70°C.
Later, Ibhadon [29] descrïbed a similar study using cyclohexatle-decalin mixtures at 60°C and
extended to a study of the fÏactionation of PP and ethylene I propylene copolymer sarnples. Both
authors used a simila. sample preparation procedure: PP was dissolved in decalin at 140°C for 1 -
2 hours; the solution was diluted with hot cyclohexane and then maintained at about 70°C.
Nevertheless, application of this technique directly to the SEC of HDPE and PP samples with
hi& molecuiar weight (say MW >250,000) encountered diffîcult problems because maintainhg
these polymers in solution requires temperatures above the boiling point of cyclohexane.
Recently, Grinshpun et al. [30] reported a dissolution procedure to obtain aggregate-fiee
solutions. PE solutions were prepared in TCB by a thermal treatment at 160°C with the addition
of antioxidant before operating SEC at 135-145°C. This procedure is not applicable for
dissolving PP because PP has a wider crystal melting range (1 40- 1 70°C) [l O] and a poorer
thennal stability than PE. Although PP dissolution can be obtained in TCB by controllhg the
storage time at 14S°C with added stabilizers [3 11, in practice it is inconvenient to determine the
optimum time.
2.2.3 Analysis of Functionaiized Polyethylene and Polypropyïene
Chromophore-labeled polymers are widely used in the study of morphology of polymer
blends. If such polymers are used in an extruder, using fluorescence analysis, one has the
possibility to monitor the interface structure which is believed to control the morphology of
polymer blends in the melt and adhesion in the solid state. If these chromophore-labeled
18 polymers are used in a melt rheometer, one can follow the effect of shear on coalescence rates.
Polyethylene and polypropylene are important industrial polymers. It is highly desirable to
employ such interface monitoring technology in the study of PE 1 PP blend miscibility. The
difficulty is to ver@ that the chromophore hctional groups are indeed covalently grafted to the
rnacromolecular backbone rather than being merely trapped in the polymer rnatrk 132-341.
Fractionation by size exclusion chromatography (SEC) followed by the use of
spectroscopic detection is one method of obtaining this type of information. This method has
been successfb.lly applied in the analysis of dye-grafted polystyrene [35] and poly(methy1
methacrylate [36]. However, it has not been applied to dye-grafted FE and PP. The reason is
that SEC analysis of PE or PP has to be conducted at temperatures above 135 OC using
halogenated aromatic compounds as solvents (Section 2.2.1). The hi& operating temperature
and the lack of spectrophotornetric "windows" in these solvents prohibit the use of an ultraviolet
0 detector or a fluorescence (FL) detector and severely limit the use of infiared (IR)
detection.
2.3 POLYMER LOCAL POLYDISPERSIW
23.1 Origh of Polymer Local Polydispersity
In SEC, the variety in molecular weights at a particular retention volume is termed local
polydispersity in molecular weight. Axial dispersion as a source of such local polydispersity has
been recognized for many years and many methods have been developed to detect and correct
19 for its effect [3]. Today, with high resolution columns, this source of error is generally assumed
negligible. Conventional SEC interpretation assumes a unique relationship between each
molecular size and the corresponding molecular weight. That is, for high resolution it is
assumed that there is only one molecular weight present at each retention volume. However, for
complex polymers, this assumption may not be valid because, for copolymers for example,
different combinations of composition and molecular weight can result in the same molecular
size. They will thus elute together at the same retention volume. This is a second source of local
polydispersity but one that is not easily detected even let alone corrected for. There has been a
considerable amount of work directed at chromatographie cross-hctionation of copolymers [37-
441. However, cross-hctionation is not easily accomplished. It tends to be very expensive and
tirne-consuming .
In the development of methods for detecting local polydispersity using SEC, blends of
different types of polymers are usually analyzed. The chromatogram of a blend of two polymers
is sirnply the addition of the chromatograms of the individual polymers. The region of local
polydispersity for the blend is the range of retention volumes over which the chromatognuns of
the two individual polymers overlap. That is, in that overlap region there will be two different
types of polymer molecules at each retention volume. Thus, polymer blends provide readily
available standards of known local polydispersity.
Very recently, by using polymer blends, three practical methods have been developed for
detecting local polydispersity using SEC f45-471. These methods utilize different detection
systems to elucidate the existence of different molecules at each particular retention volume. A
20 necessary prerequisite to using these methods is that the detection system be carefdly set up.
The following section WU briefly describe the 'Systematic Approach' for setting up a multi-
detector system in SEC. Subsequent sections wiil focus on descnbing the methods for local
polydispersity detection.
232 The Systematic Approach
In mdti-detector SEC, resuits become very sensitive to many interacting experimental
variables. A systematic method has been developed [48] to set up rnulti-detector SEC systems.
There are four steps involved. Following each step, sources of error are diagnosed and removed
before proceeding to the next step. The steps are as follows for a system containhg DRI, DV
and LALLS detectors:
Step 1. Inject a broad molecular weight distribution polystyrene standard (NBS SRM706)
of known molecular weight averages. Apply the conventional molecular weight calibration
curve to the DR1 chromatogram and calculate molecular weight averages. If these averages are
of satisfactory accuracy and precision, go on to Step 2.
Step 2. Calculate the whole polymer intrinsic viscosity , [iI], for NBS SRM706
polystyrene standard nom the DV output, and the whole polymer weight average molecular
weight, MW, fkom the LALLS output. [rl] is given by:
where qW,i is the specinc viscosity at each retention volume v , and m is the total mass injected.
Mu is obtained fiom:
where K is the LALLS optical constaut and is the Rayleigh scattering at each retention
volume v. Quantities calculated using the above two equations are independent of
chromatographie resolution and require only that al1 the sample molecules elute fiom the
column.
I f [fi] and MW agree with known values for the standard, then proceed to Step 3.
Step 3. For the DV-DEU detectors, search for the interdetector volume, which when used
with the universal calibration curve, will superimpose the calculated intrinsic viscosity
calibration cuve of the standard sample used on the tnie intrinsic viscosity calibration curve.
The true curve is obtained from the determined intrinsic viscosities of narrow molecular weight
distribution standards (or from a well-established Mark-Houwink relationship). The
interdetector volume between the LALLS and DR1 detectors is determined by superimposing
Log M versus retention volume curves.
Step 4. Assess and, if necessary, correct for axial dispersion effects.
This approach has been found to work weil in the interpretation of SEC chromatograms
fiom broad molecular weight distribution polymers, but the method does not work well for
chromatognims of nmow molecdar weight distribution po lpe r s [49,50].
Once the muiti-detector systern has been set up using the Systematic Approach, then local
polydispersity detection can be attempted.
233 Method 1
This method utilizes only a single concentration detector for local polydispersity detection
but requires detailed sample preparation knowledge to be useful. If the nonnalized
chromatogram appears different before and d e r special sample preparation (eg. adsorption to a
selective caiaidge), local polydispersity may be present [45]. For this method to work,
knowledge of exactly what molecules are being removed by the sample preparation is necessary.
Thus, this method is expected to be a very Limited technique, applicable only to a few polymer
blends and not to copolymers.
23.4 Method II
In Method II, dual detectors are used in combination to investigate local polydispersity.
The sample is prepared using selective precipitation. If two W wavelengths are used for
detection, each molecule type absorbs W differently and the local composition values cm be
readily calculated. If a DV-DR[ combination is used, then the apparent local intrinsic viscosity
before selective precipitation is compared with that after the precipitation. An obvious change
23 caused by sample selective precipitation indicates that those molecules removed are different
fkom those remaining and, thus, elucidates the presence of local polydispersity. No change of
local intrinsic viscosity may indicate that either there is no local polydispersity or the sample
selective precipitation fails to preferentially remove one polymer type [45]. The following
paragraphs will elaborate on this description.
As previouly mentioned in Section 2.1.3, in a multi-detector SEC using DRI and DV
detectors, the local htrinsic viscosity, [Illi c m be obtained fkom Equation 2.3.
The output from a differential refiactometer measured at each retention volume, Wi, is
proportional to the concentration at the corresponding slice, ci:
where K is the DRI detector response constant.
I f w i ~ and W~+B are the weight fiactions of A units and B units of polymer blends (A+B)
or copolymer (A+B) at retention volume v, then:
In actual application of a DR1 and a DV detector, a practical problem is encountered: the
local concentration value cannot be caiculated fiom the DR1 response because the DR1 is
24 assumed to respond differently to each type of molecule. This problem is circumvented by
denning an apparent intrinsic viscosity.
The apparent ineinsic viscosity cm be Wntten as:
If precipitation contains identical molecules to those lefi in the solution at a retention
volume then [qliY wny and wie (and hence [qJi,np) will be unchanged at that retention volume
(Figure 2.4).
Although this method simplifies the sample preparation procedures to some extent, it
employs selective precipitation that may be experimentally difficult.
1 \ If molecules removed are ali identical at v (i.e. no local poiydispersrty), then, there is no change in local apparent intrinsic
SEFORE vie1
AFTER
Figure 2.4: Detecring local polydispersity using DRI-DV detectors
2.3.5 Method III
Method III uses three detectors @RI, DV, LS) 146,471. This method is based upon the
"reconstructed DR1 chrornatogram generated fiom the viscosity and light scattering signal,
combined with a universal calibration cuve by assuming that no local polydispersity is present.
Local concentration (the total polymer concentration at each retention volume) is used to
calculate local intrinsic viscosity fiom DV detecton and local weight-average molecular weight
nom light scattering detectors. The mie value of local concentration can be obtained from the
sum of the DRI chrornatogram heights of j different polymer molecules, W,, at each retention
volume, vi, and the specific refiactive index increment of the different moiecules, ( d d d ~ ) ~ ~ .
Y.,
M,,, where p is the DRI detector response factor. If no local polydispersity is present that affects
dddc, then :
The work of Hamielec and h o [5 11 showed that the DV detector gives number
average molecular weight :
where Ji is the universal value in Mark-Houwink equation. If no local polydispersity is presenf
then, fiom Equation 2.20 and 2.21 :
From the LALLS detector, the local weight-average molecular weight is given by:
where a is the light scattering optical coIlStant and P(0)i is the particle scattering constant.
When LALLS is used, P(0)i - 1 . 1 f no local polydispersity is present, we have:
Thus, an equation that pertnits the reconstruction of DR1 chromatograrn can be derived by
equating Equation 2.22 and 2.24, assuming that no local polydispersity is present:
Equation (2.25) is valid only if no local polydispersity is present = MGi), or the
eEect of local polydispersity on the local (Wdc) is negligible. Thus, any difference between this
reconstnicted DRI chxomatognun and the experimental one obtained from the DIU detector
indicates the presence of local polydispenity.
For the analysis of unknown polymer sample, five cases were summarized in the
literature and shown with some specific examples [47]. These cases were as follows:
Case 1. Constant dn/dc across the chromatogram and at each retention volume,
no local molecular weight polydispersity is present.
Case II. Constant dddc across the chromatogram and at each retention volume,
local molecular weight polydispersity is present.
Case III. Constant dddc across the chromatogram but varying at each retention volume.
Case N. W d c different across the chromatogram but identical at each retention volume.
Case V. dddc different across the chromatogram and at each retention volume.
The flow chart shown in Figure 2.5 summarizes the molecular weight calibration curves
and the reconstructed chromatograms compared to the experimentai DRI chrornatograms
concentration
chmmatogfam dddc constant dnldc different
acmss chromatagram across chromatogram I \
I A. dnldc identical
\ B. dddc different
/ C. dn/dc identical
within slice within slice D. dnldc different
within slice within ske
DV, LS MWD correct
DV, LS MWD correct
both MWD incorrect
both MWD incorrect correct
LS incorrect
Case I Case I I Case I I I Case IV Case V
Figure 2.5: Local polydispersity flow chart [473
30 obtained for each case. In this work the two most common cases encountered in practice, Cases
N and V are examined using new data obtained from Eastman Kodak Company. To compare
the reconstructed and experirnentd chromatograms, two types of residuals were used:
Chromatogram residuais, Ri , are d e h e d by:
where Wap, i is the experirnentai chromatogram height and Wmi is the height of the
reconstructed chromatogram at retention volume q. This residual has been shown in the
literature to be an effective way of seeing the difference between the two chromatograms.
However, a measure of random error is needed so that the significance of the residual can be
judged. To provide this measure, sequential residuals are used in this work [52].
Sequential residuals, Si , are defined by:
Sequential residuals will show the random error in the DR1 chromatogram if the
systematic variation of the chromatogram heights is sufficiently mal1 over the t h e increment
separating Wi+, from Wi . When that is not the case the sequential residual will show a
combination of systernatic variation and random error. For the very small time increments used
here (2 seconds) random emr is expected to dominate in the tails of the chrornatograms and can
3 1 be distinguished by its appearance compared to the smooth appearance of systematic variation.
Thus, by comparing the chromatogram residuals, Ri, with the sequential residuals, Si, we c m
attempt to determine over what range of retention volumes the recoastnicted chromatogram is
significantly Merent fÎom the experirnental chromatogram. Accordingiy, this range of retention
volumes will correspond to the range of detectable local polydispersity in the sample.
2.4 SUMMARY OF THEORETICAL DEVELOPMENT
In the development of hi& temperature SEC of PE and PP in the published literature,
cyclohexane has been used to replace the conventional halogenated aromatic (e-g.
trichlorobenzene) mobile phase. Previous results are encouraging. However, previous systems
failed to analyze industrial grade (high molecular weight) PE and PP. New solvent systems in
SEC analysis must permit quantitative results to be obtained for industrial grade PE and PP. To
obtain such results, calibration is necessary. Since narrow molecular weight PE and PP
standards are rarely found, numerical search methods using different objective h c t i o n s can be
applied to obtain the necessary calibration c w e s . Another method using a broad standard with
known molecular weight distribution can also be considered.
Three SEC analytical methods have k e n developed to investigate polymer local
polydispersity :
Method 1 is a cornparison of DRI chromatograms before and after special sample
preparation. Method II is an examination of the change in a local apparent property value (e.g.
local apparent i n d i c viscosity) obtained fiom a dud detection system (e.g. DV-DRI). Method
III is a cornparison of a "reconstnicted" DR1 chromatogram obtained using DV and LALLS
detectors with the experimentally obtained DR1 chromatogram.
From the above information, the objectives of this thesis may be stated in more detail as
follows:
1. To select a new solvent for the SEC analyses of PE and PP that will:
(a) allow high molecular weight polymers to be analyzed at lower operating
temperatures;
(b) be less toxic than trichlorobenzene;
(c) have superior rehctive index sensitivity to trichlorobenzene;
(d) d o w W and other spectrophotometric detection for dye grafled PE and PP.
2. To assess and M e r develop three previously published methods for local polydispersity
detection using SEC with emphasis on Method III, the "reconstructed" chromatogram
metbod.
3.0 EXPERIMENTAL
3.1 SEC OF PE AND PP AT 90 OC
3.1 .l SEC Materiais
Solvenfs: Decalin, tetmh, trichlorobenzene, toluene, heptane, cyclohexane and methyl
cyclohexane were purchased nom the Aldrich Co. (Milwaukee, WI, USA) and were used as
received (chromatographic grade).
Homopolymers: Linear low density polyethylene (LLDPE) from Imperia1 Oil Co., hi&
density polyethylene (HDPE) PL 1840 from Dow Chernicals (Sarnia, ON, Canada), HDPE
HIZEX fkom Mitsui Petrochemicals Inc. Ltd. (Japan), PP HGX-010 (Ml 1.2) fiom Phillips (OK,
USA) and PP 6823 fiom Himont (Montreal, PQ, Canada) were the polyolefins analyzed.
Fluorescent dye la6eledpoZyolef»ls were prepared by reacting maleic anhydride grafted
polymers with alcohol derivatives containhg fluorescent dyes. The macromolecular
esterification reaction was performed in xylene at 145 O C with reflux under nitrogen atmosphere
[33]. The polyolefins used were Dupont Fusabond maleated LLDPE (MB-226D and MB- 1 10D)
and PP (MZ- 1 O9D). The alcohol derivatives used were 9-anthracene methanol and 1 -
naphthalene ethanol (both fkom Aldrich).
Polymer Standmds: Poly(isobuty1ene) (PB) narrow molecular weight distribution
fiactions fiom Polymer Standard Service (Houston, TX, USA), polypropylene broad molecular
weight distribution standards (PP95K, PP135K, PP180K, PP230K and PP350K) fiom American
Pol ymer Standards Corp. (Mentor, OH, USA) and polystyrene narrow molecular weight
distribution standards fiom Polymer Laboratones (üK) were used.
3.1.4 Calibration of PP Using Numericd Optimization Methods
Poly(isobutylene) narrow firactions were prepared ovemight at arnbient temperature
without agitation. To ensure data repmducibility, 0.1 &/O butylated h y h x y toluene was added
as a stabilizer and flow marker to all samples although no degradation was evident under the
experimental conditions. The prepared samples were then transferred to the SEC injection
chamber and injected d e r 3 hours. The resulting logarithm of peak molecular weight versus
peak retention volume data was fitted by a cubic polynomial to provide a calibration curve for
PIB. Since narrow standards of PP were wvailable, a numerical opthkation search method
was used to derive the PP calibration cuve fiom the PIB calibration curve (Section 2.1 S).
3.1.5 Calibrntion of LLDPE Using a Broad MolecuIsr Weight Distribution Standard
A broad molecular weight distribution sample (LLDPE 705) with molecular weight
distribution (MWD) known fiom SEC analysis iii trichlorobenzene was used as a standard. The
normalized integral MWD curve was plotted from the known distribution. The SEC of the broad
standard was nui in methyl cyclohexane at 90 O C. A cumulative chromatogram was obtained.
Ordinate values in the upper and lower 10 % were excluded in establishing the calibration curve.
By converting these two cumulative curves, an effective calibration curve for the new eluting
system was obtained (see Calibration in the Theory Section).
3.2 ELUCIDATION OF POLYMER LOCAL POLYDISPERSITY
3.2.1 Materiais
Narrow moiecular weight distribution polystyrene standards nom Polymer Labs were
used for calibration. Concentrations of the standards which were between 3,150,000 MW and
1,320 MW, ranged fiom 0.5 - 2.0 mg/mL, for hi&-to-low molecular weights, respectively. NBS
SRM 706 broad molecular weight distribution polystyrene standard (National Institute of
Standards and Technology, Gaithersburg, MD) was dissolved at a concentration of 1 .O mg/mL.
Four dimethylsiloxane multi-block copolymers were w d to study polymer local
polydispersity. They are two types of dunethylsiloxane-b-ester (50A and 65A, with 14,000 M,
PDMS starting block, fiom the Eastman Kodak Company, Rochester, NY.) and two types of
dimethylsiloxane-b-etheriniide copolymers (GE 1300 and GE1 500, nom General Electric
Company, USA).
3.2.2 Dual Deteetors and Selective Precipitation
The SEC system consisted of a model 5 10 purnp, a model 4 10 differentiai refractometer
(Waters Corporation, Midford, MA), an auto-sampler (Hewlett Packard Ltd.), and a model 1 1 0
Viscotek differential viscorneter (Viscotek Corporation, Houston, TX) in a series configuration.
The colurnn set consisted of three Plgel 10 p-7 .5 mm i.d. x 300 mm mixed-B bed columns
(Polymer Laboratones, Amherst, MA). The mobile phase was tetrahydrofunin (W, BDH, Inc.)
at 30 O C. Acetone (0.03 vol %) was added to the THF mobile phase as flow marker (- 28.5 mL,
negative peak). The flow rate was maintained at 1 .O ml/min.
The selective precipitation procedure was as foilows: copolymers were dissolved in THF
(The concentration for dimethylsiloxane-b-ester solution was 4.0 mg/mL, and for
dimethylsiloxane-b-etherimide solution was 7.0 mg/mL) as the stock solutions, methanol was
added to the stock solution as the precipitation agent until the ratio of methanol to THF in the
final solution was 2 : 1.
3 2 3 Triple Detector System
Data nom triple detector system were provided by Eastman Kodak Company. A Waters
differential refiactometer mode1 4 10, a PD2000 two-angle Light scattering photometer (Precision
Detectors, Franklin, MA, USA), and a Viscotek Merentid viscorneter mode1 H502A (Viscotek
Corporation, Houston, TX) were used in series to detect the fiactions eluthg fiom columns.
Three Polymer Lab IO-micrometer mixed-B bed columns were used. The mobile phase was
THF. The Systematic Approach was used at Eastman Kodak Company to determine the required
interdetector volumes.
The polymer blends used in this study were: Iinear / branched polyester (Kodak),
poly(methy1 methacrylate) (Sp) / poly(viny1 acetate) (American Polymer Standards), and
poiy(vuiy1 acetate) (American Polymer Standards) / poIy(vinyl chloride).(Eastman).
RESULTS AND DISCUSSION
HIGH TEMPERATURE SEC OF PE AND PP
Mobile Phase Selection
A prime requirement for a candidate mobile phase is that it is sirnilar in solubility
parameter and polarity to polyolefins. The toxicity and boiling point @.p.) of the solvent are
also of concern. Table 1 lists solubility parameter (6), melting point (M.P.) and rehctive index
(%2") at 20 O C of polyethylene and polypropylene. The solvent candidates are shown in Table 2.
Cyclohexane is a less toxic solvent than halogenated aromatic solvents and has a similar polarity
to polyolefins. Also, its solubility parameter (8.2 [cal/cm3 ]IR) is near that of PP (9.2 [cal l~rn~] '~)
[10,53]. However, PP cannot be dissolved directly in cyclohexane because of its high
crystallinity and melting point. Dissolving the polymer in decalin at elevated temperature
overcomes this problem, and once the crystalline polymer dissolves it does not appear to re-
aggregate fiom either decalin or cyclohexane or deposit on the PS gel in the column. Also, the
SEC c m be operated at a lower temperature (70°C). There is no trouble with filter blocking as
reported in halogenated aromatic eluents at 145OC, and since the rehctive index difference with
cyclohexane is greater than that with the usuai eluents, sensitivity is improved [26-291.
However, as mentioned eariier, high molecular weight polyolefins (M,,, > 250,000) require
temperatures above the boiling point of cyclohexane to remain in solution.
Table 1. Selected Properties of PE and PP*
Polyprop y lene 1 9.2 - 9.4 1 140 - 170 1 1.47 - 1 .52
Polyethylene
+ ~ e f ['O1. Data varies with the content of crystalline in PE, and isotactic in PP.
Table 2. Selected Parameters of Some Possible Eluting Soivents*
6 [cal/cm3]"
7.7 - 8.4
Tetralin 1 9.5
Xylene 1 8.8
m.p. [OC]
130 - 140
Toluene 1 8.9
N,"
1-48 - 1 -56
Cyclohexane 1 8.2
Methyl Cyclo hexane 1
Relative polarity**
- - -
relative toxiciîy* *
* ~ e f ['O1; *+ Ref. ; H-highly, L-lower, M-moderate
40
The focus of the work was finding a solvent with simïiar advantages to cyclohexane but
that can ais0 permit the analysis of higher molecula. weight polyolefjns. Methyl cyclohexane
appeared to be the best candidate: it has a higher boiling point (lOl°C) than does cyclohexane
(8 1°C). Thus, it can be used at higher temperatures. Another advautage is that the reffactive
index ciifference with methyl cyclohexane is even slightly greater than that with cyclohexane.
This significantly improves the DRI sensitivity. Furthemore, this new eluent has some potential
applications in the analyses of functionalized polyolefins using W, fluorescence, and i d k e d
detection because it has no significant W absorbance and wider infhred windows as weil as
lower operating temperatures than the usual solvents. A concem with methyl cyclohexane is that
its flash point (-3°C) and autoignition temperature (284°C) are much lower than those of
trichlorobenzene (flash point: 1 1 O°C, autoignition temperature: 57 1 O C ) . Thus, additional
operating precautions are necessary. In the Waters 150C, vapor detectors should be operationai,
constant ventiiation must be assured and considerable vigilance exercised for serious solvent
leaks. Use of a nitrogen generator to displace air in the instrument was not used here but can be
considered as an additional precaution, especiaily in heavily used instruments.
4.1.2. Sample Preparation
In order to h d an acceptable sample preparation procedure, the solubility of PE and PP
in different solvents was qualitatively investigated. As mentioned in the Experimental Section,
this involved observing the light scattering pattern while a He-Ne laser beam passed through the
polyoleh solution. The foilowing qualitative solubility results were found:
For identical polyolefins at 120°C, the order of decreasing ability to dissolve po lyo l eh
was: decalin, tetralin, TCB.
41
For identical solvents at above 120°C, in decalin, PP is more soluble than PE while, in
tetralin, PE is more soluble than PP. In TCB, the solubility of PE was about the same a s
PP.
a For dissolution, the folIowing temperatures were required for both PE and PP: in decalin
greater than 120°C; in TCB greater than 1 40°C.
Obviously, decalin is a better solvent with an advantage of less toxicity for PE and PP.
Table 3 summarizes solubility test results of polyolefins in decalin at different temperatmes.
Below 70°C, d polyolefin samples are precipitated except PP HGX-0 10. Above 90°C, al1
samples rernained in solution. It thus appeared that SEC of polyolefh can be nin at temperature
above 90°C using decalin as the eluent However, the refhctive index of decalin is very close to
that of both PE and PP [IO]. To overcome this weakness, Ying et al. [26,27] have described a
special procedure that involves dissolving PP of moderate molecular weight in decaiin at 140°C
for 2 hours and diluting with hot cyclohexane (70°C) so that the content of decalin is less than
8% by weight. Once the crystalline molecular aggregation is broken, the macromolecules
apparently do not reaggregate in hot cyclohexane because they are solvated by cyclohexane
molecules. Another way of dissolving PP is by placing the polyrner in cyclohexane under a
pressure of 3-4 atm. at 140°C for 2-3 hours and then cooling to 70°C under normal pressure [28].
SEC can be performed at 70°C using cyclohexane as eluent. However, as already rnentioned,
higher M, HDPE and PP (e.g. those with M,,, greater than 250,000) do not remai. in solution at
70°C.
42
Table 3. The Solubility of Poiyolehs in De& at Dinerent Temperatures*
HDPE
HIZEX
LLDPE
LL6 1 O 1.00
* Polymer concentration 0.5 wt%. S - soluble, 1 - insoluble.
HDPE
PL 1 840
Methy 1 c y clo hexane and heptane, which have higher boiling points, were alternatives.
Charlet and Delmas [54] reported the lower critical solubility temperature (LCST) of PE and PP
in these two solvents, but no mention on either the upper critical solubility temperature (LJCST)
or the application of these solvents as a mobile phase in SEC was found. Cyclohexane was
replaced with methyl cyclohexane or heptane to dilute decalin solutions with the final
concentration of decalin being approximately 2 vol%. The final solutions were maintained at the
dilution temperature for three days and then observed with the aid of the He-Ne laser beam to
determine possible aggregation. Results are show in Table 4.
Table 4. Co-soivent Seledion*
Co-solvent with c 2 vol % Decaiin
Cyclohexane, 73 OC
-
LLDPE / HDPE 1 HDPE
Heptane, 90 OC
Methyl cyclohexane, 90 OC
* S-soluble, 1-insoluble.
** Polymer concentration in decalin: 4.0 wt %.
S
From the above information and Table 4, methyl cyclohexane readily appeared as the bea
choice. PE and PP samples were prepared by dissolving the polyrners in decalin at above 140°C,
then diluted to the required concentration with methyl cyclohexane at 90°C previous to injection
Uito the SEC. The SEC was operated at 90°C using methyl cyclohexane as the mobile phase.
The procedure is sumrnarized in Figure 4.1
1
S
S
I
S
I Poiyoleîh dissohied in decaiin at temperature above 140°C for 2hr I
Decalin solution was diluted with methyl cyclohexane at 90°C
I Sample was injected into SEC run at 90°C with methyl cyclohexane as the mobile phase
Figure 4.1 : SEC procedure using methyl cyclohexane as the mobile phase
4.13 Quantitative Assessrnent
4.13.1 Evaluation of PE and PP Chromatograms
Figure 4.2 is a plot of chromatogram areas for PP (M,,,: 230,000) versus the mass of PP
injected. The linear relationship indicates no loss of sample injected by adsorption on the
column. Also, the number of theoreticai plates remained at 24,000 I 900 during 60 days of
45
consecutive operation (indicating no change in the column characteristics). Figure 4.3 shows
SEC chromatograms of PP350K and HDPE PL 1840. The former has a significantly broder
range of molecular mass than the latter. The large deviation at approximately 30 mL results h m
the presence of decalin in the injected sample. The peak that originated from stabilizer
(butylated hydroxy toluene) at approxhnately 35 mL was used as the flow marker. Identical
chromatograms were obtained two months later when the sample was re-injected.
O 0.5 1 1.5 2 2.5 3 3.5
Relative Concentration
Figure 4.2: Area under the chromatognuns of polypropylene @&: 230,000) as a
function of mass using methyl cyclohexane as the mobile phase at 90°C.
Relative concentration refers to the injection of successive dilutions.
Figure 4.3 : Chromatograms of polypropylene (MW : 350,000, MW/ MN : 8.0, bold line) and
high density polyethylene (HDPE PL 1840, dash line) using rnethyl cyclohexane
as the mobile phase operating at 90°C. The sharp peaks at larger retention volume
(- 30 mL) result from decaiin in the injected sarnple, while the peaks (- 35 mL)
that originated fiom stabilizer (butylated hydroxy toluene) were used as flow
marker.
4.1.3.2 Calibration
4.1 A2.1 Polypropy lene (PP)
Ying et al. [26] compared the chromatogram of a PS sample (M, = 2,300,000) with that
of a PP hction (M, = 103,000), which appeared in the same retention volume region in
cyclohexane eluent and found that the peak of the PS appeared to be sharpened on the high-
molecular-weight side, while that of the PP remained normal. This distortion of the PS
chromatognuns became more pronounced in our experiments with methyl cyclohexane as the
mobile phase and can be attributed to adsorption of PS on the column packhg [55-57. This is
understandable since methyl cyclohexane is a poorer solvent for PS (9 temperature 65°C [26])
than cyclohexane (8 temperature 3 SOC [26]). Addition of a s m d amount of a second solvent
(such as toluene or trichlorobenzene) to the methyl cyclohexane mobile phase was found to
partially suppress this distortion of the PS chromatogram. However, column clogging occurred
when injecting PP or PE samples into this mixed mobile phase. In general, polyçtyrene standards
are therefore not suited to the methyl cyclohexane system.
Ying et ai. [26-281 used PP fiactions to obtain a PP calibration curve. Ibhadon's results
[29] were based on the calibration data of Ying et al. In our case narrow hctions were not
available so the calibration c u v e was obtained by a nurnencai search program as described in the
Experimental Section and shown in Figure 4.4. Commercially available poly(isobutylene)
narrow fiactions with peak molecular weight (MJ fiom 1,150 to 1,020,000 provided the basis for
calibration (Table 5) fiom which a universal plot (Log J versus retention volume) is obtained.
There have been some controversial reports on the universai calibration of PIB in t e t r ahyd rob
at very low molecular weights (< 5000) [58,59]. However, the situation in methyl cyclohexane is
not known and much higher rnolecular weights are the main interest here.
Log J
Log M
Search for "S" and "1"
Calculatuf Truc
Figure 4.4: Search for grouped Mark-Houwink constants
Table 5. Polyisobutylene Standards Used for SEC Calibration
1 Standard 1 Peak Molecular Weight 1 Polydispersity 1 Retention volume*
* After flow rate correction (ASTM D 5296-92).
Table 6. Numerieal Optimization Search Resuits for Polypropylene Caiibration
*Based on the assumption that $, = 0.69 and I<pb = 0.000265 U g .
Table 6 shows the values of S and I in Equation 2.10 obtained fkom the search program
for different combinations of PP standards. Fox and Flory [60] reported the Mark-Houwink
parameters of P B in cyclohexane at 30°C: 4, = 0.69 + 0.02, I$, = 0.000265 W g . Combining
these data with the S and 1 values obtained fiom the numerical search, Mark-Houwink
parameten of PP were calculated and are ais0 shown in Table 6. Mthough the data for K, and
s, in Table 6 are only approximate due to the use of values of Qi,, and %ib in cyclohexane at
30°C rather than in methyl cyclohexane at 90°C in the calculation, the value of a, above 0.7
provides some evidence that methyl cyclohexane is a good solvent for PP at 90°C.
Figirre 4.5 shows the calibration curves using d l five broad PP standards compared to
that when only two lower molecular weight (narrower molecular weight distribution) standards
51
were used. Calibration curves obtained using other combinations of PP standards in the
numerical optimi7ation search provided calibration cwes close to the fornier c w e .
2.8 18 20 22 24 26 28
Retention Volume [ml]
Figure 4.5: Calibration Curves: Curve 1 : polyisobutylene (PIB) calibration curve obtained by
fitting logarithm of peak molecular mass vs. peak retention volume for the narrow
MWD fiactions @eak data points shown). Curve 2: PP calibration curve obtained
from a numerical search for S and 1 using Equation (2.10) while employing five
PP broad MWD standards (PP95K, PP135K, PP 180K, PP230K and PP350K).
Curve 3: PP calibration curve obtained as in Curve 2 but employing only two
broad standards (PP95K and PP MK).
4.1.3.2.2 Linear Low Density Polyethylene (UDPE)
A broad molecular weight distribution linear low density polyethylene sample
(LLDPE705) with rnolecular weight distribution known fiom SEC analysis in trichiorobenzene
mobile phase was used as a broad molecular weight distribution standard [6 11. The calibration
procedure shown in Figure 2.2 was used. Data are summarized in Table 7. The calibration curve
is shown in Figure 4.6.
Figure 4.6: Cdibration c w e of linear low density polyethylene in rnethyl cyclohexane
mobile phase at 90°C obtained by using the procedure described in Figure 2.2.
Table 7. Moleculnr Weight and Retention Volume Data of LLDPE
* Mer flow rate correction (ASTM D 5296-92).
I (%) v [mu* I Log M
4.133 Molecular Weight Averages and Molecaler Weight Distribution
4.1.3.3.1 Poiypropy lene (PP)
The molecular weight averages and polydispersity for each of the five PP standards
calculated using the calibration curve derived by using all of these standards in the numerical
optimkation search are Listed in Table 8. Deviations fiom the vendor values ranged fkom 4.3%
to 4.2% for M, and - 14.4 to 16.5% for M, . With regards to utilizing a calibration cuve derived
using fewer standards, it was found that a better weight-average molecular weight, M, , could be
obtained using higher molecular weight standards only and, a better number-average data, &,
could be obtained by using lower molecular weight standards. Also, the data in Table 8 and the
caiibration curves in Figure 4.5 show that, at least for our standards, the number of broad PP
standards used was of secondary importance to the polydispersity of those standards for
infiuencing calibration and rnolecular weight averages. The broader the molecular weight
distributions of PP standards, the more satisfactory the resulting calibration curve.
Using the calibration curve obtained from five PP standards, the molecular weight
distributions of each PP standard was calculated. Two typical molecular weight distribution
c w e s are shown in Figure 4.7. Al1 of these distributions show a long low-molecular-weight tail
extending to a molecular weight of 103. The high-molecular-weight tail of the highest molecular
weight sample, PP350K, extends to a molecular weight of approximately 107, thus
encompassing the whole range of industrial grade PP polymers.
Table 8. Molecular Weight Resuits of PP Standards Compared with Vendor Values
WJW (vendor)
Figure 4.7: Normaiized molecular weight distributions of PP135K (bold line) and PP350K
(dash line), their rnolecdar weight averages are listed in Table 8).
4.1.3.3.2 Linear Low Density Polyethylene (LLDPE)
Table 9 sumarizes the results of applying the LLDPE calibration curve to three samples
previously anaiyzed in trichiorobenzene mobile phase and not used in formulating the calibration
c w e . Deviations fkom the previously determined values were fiom 3.4 to -9.7 % for M, , and 5
7 % for K.
Table 9. Moleealar Weight Averages of LLDPE Samples
l Error (%)
M, (in methyl cyclohexane)
(in trichlorobenzene)
Error (%)
LLDPE LLDPE
710
LLDPE
4.1A. AnaIyses of Dye Labeleà PE and PP
In o u research we wish to use SEC to measure the quantity of an ultraviolet (UV)
absorbing dye which has been @ed onto polyethylene and poiypropylene macromolecular
main ch&. As already mentioned, methyl cyclohexane as a new mobile phase for the SEC of
polyolefïns provides an opportunity for UV detection because it has no significant W
absorbance and lower operathg temperature than the usual mobile phase. Figure 4.8 shows the
overlay of the UV chromatogram for anthracene-grafted h e a r low density polyethylene on its
corresponding DR1 chromatogram.
15 20 25
Retention Time [min]
Figure 4.8: Overlay of W chromatogram @old line) of anthracene labeled linear low density
polyethylene on its corresponding DM chromatogram (dash line), both
chromatograms were nonnalized.
In order to obtain the W detector response factor, 9-methyl anthracene and 1 methyl
naphthalene were used as mode1 molecules for anthracene labeled polyolefins and naphthalene
59
labeled linear low density polyethylene, respectively. Their molecular structures are show in
Figure 4.9. This approach assumes that the molar extinction coefficients do not change when the
dye is grafted to the polymer.
(A) anthracene gmfted LLDPE (B) naphthalene grafted LLDPE
(C) 9-methyl anthracene (D) 1 -methyl naphthalene
Figure 4.9: Molecular structures of dye grafted linear low density polyethylene and their
correspondhg mode1 molecules.
Dye Content x 10' [mol]
Figure 4.10: Area under the UV chromatognun as a function of the content of 9-rnethyl
anthrac ene .
O 5 10 15 20 25 30 35
Dye Content x 10' [mol]
Figure 4.1 1 : Area under the W chromatogram as a function of the content of 1-methyl
naphthalene.
Figures 4.10 and 4.1 1 show the area under the W chromatogram as a h c t i o n of dye
content for Cmethyl anthracene and 1-methyl naphthalene. The plot of anthracene shows
notable deviations fiom a straight he. Adsorption of the dye on the column packing appeared to
be taking place since dye retention time exceeded the total permeation time and the
chromatograms were skewed. Thus, loss of dye in the columns was possible. Once the UV
detector response is known, the average number of dye fimctiond groups per polymer chah can
be obtained. The results are summarized in Table 10. Based upon the W chromatogram and
the calibration curve, dye content as a function of Log M can be plotted, as is demonstrated in
the following fiow chart (Figure 4.12). This plot shodd have a similar shape to the molecular
weight distribution curve of the dye labeled polymer sample if every unit of polymer chah has
equal chance to be grafted. Obviously, afler comparing the cuve shapes in Figure 4.13, one
readily h d s that polymer chains with lower molecular weight have much more chance to be
grafted.
W chromatogram
[Dye] = f (ara of slice)
Figure 4.12: Flow diagram shows the transformation fiom W chromatogram to dye content as
a function of molecular weight.
1 2 3 4 5 6
Log M
1 2 3 4 6
Log M
Figure 4.13: Dye content as a function of logarithm of molecular weight (upper) and the
molecular weight distribution curve (lower) of anthracene grafted linear Iow
density polyethylene (MJ3-226D). A much greater tail of the former at the low
molecular weight end indicates that short polymer chains have more chance to be
grafted given that the cdibration cuve is nearly linear in the molecdar weight
range of interest.
Table 10. Average Number of Dye Groups per Polymer Chah
N = [moles of dye] 1 [moles of polymer injected]
Figure 4.14 shows plots of D(Log M) and N(Log M) as a fiuiction of molecdar weight
(see equations in Appendix 1 ). In the lower molecular weight range (Log M < 3 . 9 , the number
of dye molecules per polymer chain is less than one. This is because the number of monomer
uni& (the grafting point) per polymer chain is s m d although the number of polymer chains is
large. As the molecular weight increases, the number of grafting points increases and D(Log M)
also increase. At Log M -3.5, each polymer chah has nearly one dye molecule grafted After
D(Log M) reaches its maximum (around the maximum peak point of the molecular weight
distribution curve), the number of polymer chains decreases dramatically and D(Log M) also
follows this trend. Figure 4.1 5 is the plot of the ratio of D(Log M) to N(Log M) as a function of
molecdar weight. As the molecular weight increases, the total number of grafting points per
polymer chain increases and the ratio of DGog M) to N(Log M) increases.
Anthracene-PP
MZ- 1 09D
20000
0.2
Mn
N
Anthracene-LLDPE
MB-226D
23000
3 .O
Naphthalene-LLDPE
MB-1 1OD
1 1000
3.6
3 4
Log M
Figure 4.14: D(Log M) and N(Log M) as a fùnnion of molecular weight.
4
Log M
Figure 4.15: The ratio of moles of dye molecules to moles of polymer
as a funnion of molecular weight.
4.1.5 Assessrnent of Methyl Cyclohexane as the Mobile Phase
w 4.1 S. 1 Reproducibility
To assess the reproducibility of SEC in the new mobile phase, a broad molecula. weight
distribution polypropylene standard (M. : 41 800, MW : 23 1300) was re-injected four t h e s at
different dates. As shown in Table 1 1, the standard deviation for Mn was 7.2 %, and 1.2 % for
MW . These results are encouraging. However, more data are needed.
Table 11. Molecdar Weight Averages of PP Re-injected Four Times
4.1.5.2 DissoIution
As already mentioned, both PE and PP are semi-crystalline polymers. Therefore
complete dissolution is of importance in this work. Evidence supporting complete dissolution
includes:
a Off-Line He-Ne laser light scattering resdts;
No pressure increase during the SEC nui;
Molecdar weight average results are in good agreement with vendor values;
Molecular weight averages have good reproducibility.
Standard
Deviation (%)
7.2
1.2
Mean
44100
257400
Sep. 1 1
42550
253160
Sep.4
41640
259960
Date
Mn
MW
Jd.31
48740
256600
Sep.2
43400
259730
4.1 S.3 Flammabitity
Both flash point and autoignition point are often used to assess the flammability of a
soivent. Flash point is the lowest temperature of a iiquid at which it releases enough vapor to
form an ignitable mixture of vapor and air immediately above the liquid surface. The
autoignition point is the minimum temperature required to initiate or cause self-sustained
combustion, in the absence of a spark or flame. Table 12 lists these parameters of three SEC
solvents
Table 12. Flash Points and Autoignition Tempemtiires of SEC Soivents
Operating Temperature ["Cl
*
Solvent
Trichlorobenzene
Methy 1
It can be seen that the autoignition temperatures of these solvents are well above their
operating temperatures. This suggests that there should be no hazard of self-sustained
combustion under normal operatiom. However, the flash points of these solvents are below the
operating temperatures. Thus, f i e hazard is possible if an ignitiog source is present.
Three precautions should be exercised:
vapor detector shouid be operational;
good ventilation of the SEC system should be ensured;
a nitrogen generator can be considered to replace the air in the SEC cabinet.
cyclohexane
Tetrahydro fûran
Flash Point ["Cl
110
-3
Autoignition Temperature K I
-14
571
284
135-145
90
321 20-30
4.1 S.4 Adsorption
Adsorption is a main disadvantage of this new mobile phase. As mentioned earlier, it
restricts the use of narrow polystyrene standards. Also it was observed that the column
efficiency decreased after repeatedy injecting 9-methyl anthracene and 1-methyl naphthalene
mode1 molecules for the determination of UV detector response factors. Thus, the adsorption of
mode1 molecules to the col- is considered a main source of error in the analysis of dye-
grafted PE and PP.
It has been reported that the adsorptive activity of PS I DVB columns depends rather
strongly on the column manufacturer. "Normal" SEC results of poly(methy1 methacryIate) were
obtained when a srnail amount of t e t rahydroh was added into toluene mobile phase 1621. In
this work, srnail arnounts of polar solvents were added into methyl cyclohexane mobile phase to
overcome the adsorption problem. A ~o~~s ty renk sample was analyzed. Preliminary results
were summarized in Table 13. Neither toluene nor trichlorobenzene is a good candidate. High
alcohols may be better alternates. However, the folIowing constraints need to be taken into
accom:
miscibility with methyl cyclohexane;
boiling points;
refkactive index of the solvent mixture;
solubility of PE and PP in the solvent mixture;
temperature effects on adsorption / desorption;
alcohol peak may interfere with the peaks of low molecular weight polymer hctions.
Table 13. Effect of Polar Solvent on the SEC of Poiymers*
Toluene
Trichlorobenzene
Pressure increase when injecting
Effect on the SEC of PS Polar Soivent
sample, slightly baseline drift
Effect on the SEC of PE & PP
Si@cant pressure increase
when injecting sample (fiom 9
bars to 20 bars) --
Significant pressure increase
when injecting sample (eom 1 1
bars to 25 bars)
Baseline noisy and drifting
Very weak peak at 8-8.5 mL
Weak and broad peak at 7.7-
8.0 mL
Broad peak taihg at low
molecular weight side, V =
Peak tailing at low
molecular weight side, V =
6.9-7.1 mL
Baseline noisy and drifthg
* Detailed SEC conditions as descnbed in Experimental Section but only one Plgel (Polymer
Lab) cross-linked polystyrene column was used.
4.2 ELUCIDATION OF POLYMER LOCAL POLYDISPERSITY
42.1 Assessrnent of Methods Invohring Sample Treatment
As mentioned in the Theory Section, these methods rely on a comparison of SEC data
before and af'ter special sample preparation. There are two nich methods: in Method 1 only a
single DRI detector is required and nonnalized DR1 chromatograms are compared; in Method II,
when a DV and a DR1 detector are use& plots of local apparent intrinsic viscosity versus
retention volume are compared.
42.2 Local Poiydispersity Detection Using Method 1
The sample preparation method removes molecules based upon both molecular weight
and composition but, as mentioned above, Method I employs only a single DRI detector. A
typical result for polyester is shown in Figure 4.16. Without additional information regarding the
relative removal of molecular weight and composition by the sample preparation method, the
local polydispersity situation cannot be elucidated. The method was therefore considered
impractical for most polymen.
Figure 4.16: The "before" and the "after" normalized chromatograrns of a polyester are
different because the sample treatment removed higher molecular weight
fiactions.
43.3 Local Poiydispersity Detection Using Method II with a DV-DR1 Deteetor System
As shown in Figure 2.3, a change in the apparent Iocd viscosity after sample treatment
indicates the presence of local polydispersity. In this work the "Systematic Approach" was used
to ensure that the DR1 and DV detectos were operating sufficientiy well. For Method II, only
the first two steps of the approach had to be used; detexmination of interdetector volume was
omitted because interdetector voIume did not need to be included in the calculation of the
apparent intrinsic viscosity for local polydispersity to be detected.
72
4.2.3.1 Utilizing the Systematic Approach for Method II
In the f k t step of the approach, emphasis was on matching the "tnie" molecular weight
average r ed t s (obtained fiom averages of hundreds of duplicate measurements by Eastman
Kodak Company) for the broad molecular weight distribution polystyrene standard O\IBS
SRM706) by using only the DR1 detector. The polystyrene molecular weight calibration curve
was fkt d e t e d e d using the polystyrene standards shown in Table 14. Figure 4- 1 7 shows the
fit to the calibration curve data.
Table 14. Poiystyrene Standards Used for Caiibration Curve
Weight Duplicate I Duplicate II Duplicate III
Figure 4.17: Polystyrene molecular weight cdibration cuwe
In accord with the Systematic Approach, whole polymer M. and MW values were then
obtained by applying this calibration curve to the DR1 chromatograrn of the broad molecular
weight distribution standard (NBS SRM706).
Tables 15(a), 15@) and 15(c) show the rneasured values of molecular weight averages at
three consecutive days compared to the "true" results. The results were judged sufncientiy close
to the standard values for Step 1 of the Systematic Approach to be successfully completed. The
polystyrene molecular weight cdibration will be used in subsequent work.
Table 15. MolecuIar Weight Averages of the NBS SRM706 Polystyrene Standard
(a) Measured on Day 1
M z
435,000
456,000
4.83
"True" Data
Run #1
Deviation (%)
Deviation (Yo)
(b) Measured on Day 2
Deviation (%)
Average
Deviation (Yo)
M n
123,000
12 1,000
-1.63
-0.8 1
MW
276,000
268,000
-2.90
-1.63
121,300
-1.38
I
1
"True" Data
Run #1
Deviation (Yo)
-3.26
-4.7 1
266,000
-3 -62
Deviation (%) 0.8 1 -2.54 6.44
M W
276,000
269,000
Mn
123,000
124,000
Deviation (%)
Average
4.83
3.91
454,700
4.52
M z
43 5,000
463,000
0.8 1
-0.8 1
123,300
-3.26 6.67
-4.7 1
266,300
Deviation (%)
3.45
459,000
-3 -5 1 0.27 5.52
With regard to Step II of the Systematic Approach, values of the whole polymer intrinsic
viscosity for NBS SM706 were obtained from the DV detector and are shown in Table 16 in
cornparison with the "true" resdts.
75
(c) Measured on Day 3
Both the molecuiar weight averages and the whole intrinsic viscosity were considered
suniciently close to the "mie" values for Step II of the Systematic Approach to be complete.
Mz 1
43 5,000
450,000
3 -45
457,000
5.06
446,000 I
2.53
45 1,000
3-68
Table 16. Intrinsic Viscosity of the NBS 706 Poiystyrene Standard [dL/g]
M W
276,000
264,000
-4.35
269,000
-2.54
266,000
-3.62
266,300
-3.51
"True" Data
Run #1
Deviation (%)
Run #2
Deviation (%)
Run #3
Deviation (%)
Average
Deviation (%)
M n
123,000
120,000
-2.44
123,000
0.00
122,000
-0.8 1
12 1,700
- 1 .O6
"True" Value
0.94
Run #1
0.90
Average
0.95
Run #2
0.99
Run #3
0.96
4.2.3.2 Local Polydispersity of Copolymers in THF Using Method II
This method has k e n applied to PS / PDMS blend in toluene [45]. There, local
polydispersity was quaiitatively detected. This research attempted to extend its application to
copolymers and employ a more widely used mobile phase, tetrahydrofuran (THF).
As described in the Experimental Section, a selective precipitation procedure was
employed in this method. The supernatant, precipitate, and the original polymer sample were
injected in the SEC system to obtain the DR1 and DV chromatognuns. Apparent intrinsic
viscosity was calculated from the DR1 and DV data using Equation 2.18. Figure 4.18 shows the
plot of apparent intrinsic viscosity versus retention volume for four different types of industrial
grade multi-block copolymea. It c m be seen that, for al1 four samples, the plots of the
supernatant deviate signincantly fiom the other two plots. Thus, local polydispersity is indicated
to be present in al1 four block copolymer samples.
(a) PDMS-Pol yetheramide (GE- 1 5 00)
(b) PDMS-Polyetheramide (GE- 1 300)
(CI PDMS-Polvester (Kodak 50A)
(d) PDMSPolyester (Kodak 65A)
Figure 4.18: Apparent intnnsic viscosity versus retention volume. A difference of the apparent
intrinsic viscosity between the supernatant and the original polymer indicates the
presence of local polydispersity. p-precipitate, s-supernatant, O-original polymer
sample.
79
However, as the study progresse4 the reproducibility of the r ed t s became suspect. It is
known that some block copolymers can be used as emulsifiers. In this work, emulsincation was
observed as methanol was added. The precipitation occurred ody after an excess amount of
methanol was added. The formation of emdsion was a major difEculty in attempting to apply
this method to these copolymers. It was concluded that this rnethod dso was not a general one
for detecting local polydispersity .
42.4 The "Reconstructed" DR1 Chromatogram Method (Method III)
As mentioned in the Theory Section, for Method III, data fiom a viscometer @V) and a
light scattering detector (LS) are used together with a universal calibration curve to generate a
DR1 chromatogram for the sample, while assuming that local polydispersity is not present This
"reconsmicted" DRI chromatognim is compared to the actuai one. Any signifïcant difierence
between the two indicates the presence of local polydispersity. This work was directed at
extending and assessing Method III in three ways:
(i) by applying the method to new data for two polymer blends not previously
examined;
(ii) by investigating the effect of concentration of one of the blend components on the
detection of local polydispersity and;
(iii) by examining the effect of the value of inter-detector volume on the calibration
curves and the "reconstnicted" DR1 chromatograms.
Results for these three topics are presented in him in the following sections.
4.2.4.1 New Polymer Blend Examples for Method III
Of the five different local polydispersity cases defined for Method III (see Figure 2.5),
Case N and Case V are particularly important because they are so commonly encountered. The
two new polymer blend examples were used for each of these cases.
For Case IV, dddc varies across the chrornatogram but, at each retention volume, al1
molecules had identical dnldc values. Data for two polymer blends were used. The first blend
was a 5050 @y weight) blend of poly(methy1 methacrylate) (PMMA) and poly(viny1 acetate)
(PVA). The dnldc of PMMA is 0.086 while that of PVA is 0.055. Thus, normally molecules of
Merent Wdc values would be present wherever the experimental DEU chromatograms of the
polymer components overlapped. Therefore, to examine this case a computer simulation was
used. This simulation added the chromatograms of the pure polymer components together to
form the "kxperimentd" chromatogram of the blend. However, when it did so it also assigned
al1 the molecules at a particular retention volume the weighted average dddc at that point. Thus,
the dddc was identicai for al1 molecules at a particular retention volume but varied across the
chromatogram. The "recomtructed" chromatogram was generated using Equation 2.25 and was
compared to this "experimental" chromatognun of the blend. A cornparison of the
"recoostnicted" and the "experimental" chromatograms is shown in Figure 4.19.
12 t 4 16 18 20 P 24 Retention Volume (ml1
b
[Roidu311 10
-10 1 12 14 18 18 20 22 24
Retention Volume (ml]
12 14 16 18 20 22 24 Retenüon Volume [ml]
Figure 4.19: Chromatograms aud residual plots of PMMA / PVA (Case IV)
12 14 16 18 20 P 24 Rasrrbion Vdume (mL]
12 t 4 t6 18 20 22 24 Retention Volume [mL]
12 14 16 18 20 P 24 Raantion Volume [ml]
Figure 4.20: Chromatograrns and residual plots of PVA I PVC (Case IV)
83
A similar operation was carried out with data for a second pair of polyrners: poly(viny1
acetate) (PVA) and poly(viny1 chloride) (PVC). In this case the ûue dnldc values were 0.055
and O. 109 respectively. However, again, the weighted average value was assigned to aU
molecules at a particular retention volume. R e d t s are shown in Figures 4.20.
Figure 4.19 and 4.20 corroborate the conclusion fiom the published Literature [46] that
the "reconstructed" chromatogram is not signincantly affected by W d c variation across the
chromatogram. The chromatogram residuals (Equation 2.26) are of the same order as the
sequential residuals (Equation 2.27) over the whole retention volume range.
For Case V the dddc is different across the chromatogram and at each retention volume.
The same polymer blends were used as in Case IV. However, the achial difference in dnldc at
each retention volume was allowed to remain (the molecules at a particdar retention volume
were not aiI assigned the same value at each retention volume).
Figure 4.21 shows the results for the PMMMVA blend and Figure 4.22 for the
PVA/PVC blend. In addition to the chromatograrns and plots of residuals shown in these
figures, the range of overlap of the two polymer blend component chromatograms is also show.
This range of overlap is the region over which a local polydispersity a c W y exk-. The plots of
residuals show that only a portion of this range exhibits local polydispersity using Method III.
That is, noise in the "recotlstructed" chromatogram limits the sensitivity of the method to detect
local polydispersity.
12 14 16 18 20 22 24 Retention Volume [ml]
12 14 16 18 20 22 24 Retention Volume [mL]
-10 1 1 12 14 16 18 20 22 24
Retention Volume [mtj
Figure 4.21 : Chromatograms and residual plots of PMMA I PVA (Case V)
12 14 16 18 20 22 24 Retention Volume (ml]
[ . R a i d u i l ( l
I ' O 1 l
-10 1 I 1 1 12 14 16 18 20 22 24
Retention Volume [mC]
12 14 16 18 20 22 24 Retention Volume [ml]
Figure 4.22: Chromatograms and residual plots of PVA / PVC (Case V)
43.4.2 Evaluating the Detection Limit
Considerable literature exists concerning the dehition of the detection limit for a given
analysis [52,63]. U d y , detection Limit cm be defined as the lowest concentration (or the
lowest amount of mass) of the solute in the sample that can be detected d e r the prescribed
experimental conditions. To be detectable, the detector response must sipnincantly exceed the
noise level. The detection limit for Method III is dependent on both the ciifference in the dn/dc
values of the two polymer components and on the composition of the polymer blend (the mass
ratio of the two components).
Two polymer blend samples were examined. PMMA / PVA is an example where the
component chromatograms only partiaily overlap (local polydispersity is present over a limited
range of the eluted sample). PVA / PVC represents an example where the DM chromatograms
of the two components completely overlap (i.e. local polydispersity is present across the entire
elution volume range of the blend sample). To examine detection lunits, the DRI residual was
compared with the sequential residual as the concentration of one blend component was
decreased with a computer program that summed up the cornponent chromatograms. A
composition range fiom 5 wt% to 50 wt% of one polymer component in the blend was
investigated. The experimental and the "reconstructed" DR1 chromatograms, the DR1
chromatograms of two components and two types of residuals were plotted against retention
volume, as shown in Appendk II (PMMA / PVA) and Appendix III (PVA /PVC).
Both Appendix II and Appendix III demonstrate that the magnitude of DR1 residual
approaches that of the sequential residual as the content of one component decreases fiom
87
50 wP! to 5 wtO/o. It can be judged thaî the detection b i t for local polydispersity is about 10
Wto! PVA in a PMMA 1 PVA blend, 10 wt?! PMMA in a PMMA / PVA blend, 10 Wto! PVC in
a PVA / PVC blend, and 5 Wto? PVA in a PVA / PVC blend.
4.2.4.3 Effect of Interdetector Volume on Caiibration Curves
Error fkom the detennination of interdetector volume can propagate to the final resdts
fiom multi-detector SEC. In the DRI-DV-LS triple detector system, two interdetector volumes
are important: the volume between the DR1 and the DV detector (the DM-DV interdetector
volume); the volume between the DR1 and the LS detector (the DRI-LS interdetector volume).
From Equation 2.22 it can be seen that the local specific viscosity (qsp, fiom the DV
response) and the DRI response are included in the calculation of local nurnber average
molecular weight (Mn,i). A change in MRi is expected if the DRI-DV interdetector volume is
varied. However, variation of the DM-LS interdetector volume wiil have no effect on Mni. The
experimentai results in Figure 4.23 demonsirated these expectations. In contrast, as shown in
Figure 4.24, local weight average rnolecular weight (hluvi) will change if the DRI-LS
interdetector volume varies, while variation of DRZ-DV interdetector volume has no effect on
This is expected fkom Equation 2.23, where the response fiom both DR1 and LS are
included in the calculation of MWj. As reported in the literature [64-661, the local molecdar
weight curves rotate about the curve corresponding to the best value of interdetector volume.
(a) Mismatch DRI-DV interdetector volume
(b) Mismatch DRI-LS interdetector volume
Figure 4.23: Effect of interdetector volume on local number average molecular weight
(a) Mismatch DM-DV interdetector volume
I
D R C L S : O mL
DRI-LS: 0.2 mt
(b) Mismatch DM-LS interdetector volume
Figure 4.24: E f k t of interdetector volume on local weight average molecular weight
43.4.4 Effect of Interdetector Volume on the "Reconstrncted" DR1 Chromatogram
The ureconstmcted" DR1 Chromatogram is highly dependent on the accuracy of the DM-
DV and DM-LS interdetector volumes. To demonstrate this, a series of different interdetector
volumes were used with data representing a blend of a linear and branched polyester. Both blend
components have the same dn/dc values. Figures 4-25 and 4.26 show that: when either
interdetector volume is over estimated, the "reconstructed" DR1 chromatogram shifts to lower
retention volumes; it shifts to higher retention volumes if they are underestimated. Figure 4.27
illustrates that when both DV-DR1 and LS-DR1 interdetector volumes are over estimated
together, the "reconstructed" chromatognun shifts more to lower retention volumes than when
only one interdetector volume is over estimated. Similady, the "reconstructed" chromatogram
increasingfy shifts to higher retention volume if both interdetector volumes are under estimated.
Figure 4.28 shows that the "reconstructed" chromatogram will superimpose on the experimental
one if one interdetector volume is over estimated while the other interdetector volume is under
estimated by the same amouut. Thus, it can be seen that mismatching interdetector volumes can
cause incorrect results for estimating the local polydispersity.
10 1s 20 25 30 Re-on Volume [mL]
10 15 20 25 30 Retention Volume [ml]
10 15 20 25 30 Retention Volume [mL]
Figure 4.25: EEect of DRI-DV interdetector volume on "cec~nstructed'' DR1 chromatogram
10 15 20 25 Retention Volume [ml]
10 15 20 25 Retention Volume [mu
10 15 20 25 30 Retention Volume [mi.]
Figure 4.26: Effect of DRI-LS interdetector volume on "~constructed" DRI chromatogram
10 15 20 25 Retention Volume [mtJ
10 15 20 25 30 Retention Volume [mL]
10 15 20 25 30 Retention Volume fmL]
Figure 4.27: Effect of both DRI-DV and DM-LS interdetector volume (positive or negative
error in both) on " r e c o ~ DRI chromatogram
10 15 2Q 2s 30 Retention Volume (mLI
10 15 20 25 30 Retention Volume [mL]
10 15 20 25 30 Retention Volume [ml]
Figure 4.28: Effect of both DRI-DV and DRI-LS interdeteaor volume (positive error in one,
negatïve error in the other) on "reconsûucted" DR1 chromatogram
5.0. CONCLUSIONS
With respect to the nrst objective of this thesis, analysis of PE and PP at a
significantly lower operating temperature, the accomplishments of the work have been
pubfished [671 and presented at an intemational conference [68]:
A new mobile phase, methyl cyclohexane, was proposed and evaluated as a solution to
the problem of high operation temperatures for the SEC analysis of polyethylene and
polypropylene. The solvent was shown to enable even high molecular weight
industrial PE and PP to be analyzed at 90 O C instead of the customary 145 OC. This
lower temperature c m greatly alleviate SEC operational problems.
a In addition to permitting 90 O C operation, methyl cyclohexane has other advantages
over traditional solvents: lower toxicity, improved sensitivity for DR1 detectors and
UV t r q a r e n c y .
W transparency of methyl cyclohexane means that UV absorbing, functionalized PE
and PP may now be analyzed by SEC to examine the distribution of functionaiity with
rnolecular weight. That is, both DRX and W detectors can be used sirnultaneously.
Dye coupled PE was andyzed to demonstrate this signincant advantage.
Disadvantages of the new mobile phase are its flammability and increased molecular
adsorption of samples on the column packing. Flammability hazards can be overcome
by nitrogen blanketing. Adsorption issues are troublesome with some samples.
95
96
However, doping of the mobile phase with appropriate additives which can de-activate
adsorption sites appears to be a potential solution.
With respect to the second objective of this thesis, assessing new methods of detecting
local polydispersity, three new methods were examined:
0 Method 1 relied on a single DRI detector and so was almost immediately found
impractical for analysis of most polymers: a very large amount of information on the
relative importance of molecuiar weight and composition in the special sample
preparation procedure was needed.
0 Method iI utilued a DR1 and a DV detector dong with precipitation as a special
sample preparation method. Comparison of apparent local intrinsic viscosity values
before and after the specid sample preparation was done. However, serious sarnple
preparation problems were encountered because the copolyrners tended to fom an
emulsion. Both the precision and accuracy of the results were questionable so this
method too was considered unsuitable.
Method III utilized DRI, DV and LALLS detectors to obtain a "reconstnicted" DM
chromatogram which could then be compared to the experimentally obtained DR1
chromatogram. This method appeared both practical and general. The method was
used with triple detector data nom Eastman Kodak Company on two new polymer
blends and was shown to work as expected. Sensitivity of the method to the relative
97
concentration of the components was examined ushg plots of residuais and it was
found that local polydispersity became undetectable when one of the components was
less than 5 to 10 wt % of the blend. Finaily, inter-detector volume was varied and
shown to have a major influence on the shape of the reconstnicted chromatogram.
6.0. RECOMMENDATIONS
In the hi& temperature SEC work, adsorption should be fiirther investigated.
High alcohols cm be considered as a displacer in methyl cyclohexane mobile phase. The
relationship between the adsorptive activity and the composition of column m e r should
be defined. m e r spectrometers can be used with this SEC system to exploit the
detection features of this new mobile phase. Low molecular weight poly(isobuty1ene)
can be considered a s a flow marker.
With respect to local polydispersity, the "reconstnicted" DR1 chromatogram
method is a promising universal technique for detecting local polydispersity. The
residual of the "reconsmicted" cbromatogram and the one measured directly fiom DR1
detector can be used as a measure to quantitatively determine local polydispersity. The
low sensitivity of light scattering at low molecular weights is a problem. New lower
noise instruments and the M e r application of statistical methods are potential solutions.
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IO5
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International GPC Symposium, San Diego, 1996
APPENDM 1. EQUATIONS FOR ANALYSIS OF
DYE-GRAFTED POLYETHYLENE
W Detector
PE is transparent to W. Thus the üV response is exclusively fiom the dye groups.
Let D(Log M) dLog M = moles of dye in polymer with logarithm of molecular weight fiom
Log M to (Log M + dLog M);
D(v) dv = moles of dye in retention volume range fiom v to (v + dv);
U(v) = UV detector response at retention volume v.
Then, we have:
where K, is the UV detector response factor and is defïned by:
DRI Detector
Since the dye content is very low, we assume that every unit in polymer main chah bas the
same contribution to the DR1 tespoiîtje.
Let N&og M) dLog M = moles of polymer with logarithm of molecular weight from Log M
to (Log M + dLog M);
W(v) = DR1 detector response at retention volume v.
We have:
where KM is the DR1 detector response factor and is defined by:
where m is the total mass of polymer injected into SEC.
where WN(v) is the normalid height of DR1 chromatogram.
Moles of Dye per Mole of Polymer
Having matched retention volume using the interdetector volume between two detectors, we
combine Equation 0-5) and (I-1 O),
From Equation (I-1 1), the local values of moles of dye per mole of poiymer can be
determined.
APPENDM II EVALUATING THE DETECTION
LIMIT OF PMMA 1 PVA
12 14 16 18 20 22 24 Retention Volume (mLj
12 14 16 18 20 22 24 Retention VoIurne [ml]
12 14 16 18 20 P Retention Volume [mL]
I Retention Volume [ml] I
Figrne II-l : Cbromsitngrams and residual plots
(50 wP/o PVA in PMMA 1 PVA)
12 14 16 18 20 22 24 Retention Volume [ml]
-101 - - l 12 14 16 18 20 22 24
Retention Volume [mL]
12 14 16 18 20 22 Retention Volume [ml]
-10 1 12 14 16 18 20 22 24
Retention Volume [ml]
Figure II-2: ChrI,matograms and residual plots
(20 wt% PVA in PMMA / PVA)
12 14 16 18 20 P 24 Retention Volume (mL]
-10 I 12 14 16 18 20 22 24
Retention Volume [mL]
12 14 16 18 20 P Retention Volume [ml]
-10 1 12 14 16 18 20 22 24
Retention Volume [ml)
Figure II-3: Chromatograms and residual plots
(10 Wto? PVA in PMMA I PVA)
12 14 16 18 20 22 24 Retention Volume [mt]
4
12 14 18 18 20 22 Retention Volume fmL) 24 I
12 14 16 18 20 Z 24 Retention Volume [ml)
12 14 16 18 20 22 24 Retention Volume [ml]
Figure I I 4 Chromatngrams and residuai piots
(5 Wto? PVA in PMMA / PVA)
l
12 i 4 16 18 20 22 I Retention Volume [ml]
12 14 t6 18 20 22 Retention Volume [mL)
24 I
12 14 16 18 Zû 22 24 Retention Volume [ml]
-101 - - * . - . I 12 14 16 18 20 22 24
Retention Volume [mL]
Figure II-5: Chromatograms and residual plots
(20 wt% PMMA in PMMA / PVA)
12 14 16 18 2û 22 24 Retention Volume [ml]
12 14 16 18 20 22 Retention Volume [mL] I
12 14 16 8 îû 22 Retention Volume [mL]
12 14 16 18 20 î2 24 Retention Volume [mLl
Figure II-6: Chromatograms and residual plots
(10 &/O PMMA in PMMA / PVA)
12 14 16 18 20 22 24 Raention Volume [mt]
12 14 16 18 20 22 24 Retention Volume [ml]
12 14 16 18 20 22 24 Retention Volume [ml]
Retention Volume [mLj l
Figure 11-7: Chromatograms and residuai plots
(5 wt?A PMMA in PMMA 1 PVA)
APPENDIX m EVALUATING THE DETECTION
LIMIT OF PVA 1 PVC
12 14 16 18 20 22 24 Retention Volume [mL)
12 14 16 18 20 22 24 Retention Volume [mL]
12 14 16 18 20 22 24 Retenüon Volume [ml)
-1 O 0 12 14 16 18 20 22 24
Retentim Volume [ml]
Figure III- 1 : Chromatograms and raiduai plots
(50 Wto? PVC in PVA / PVC)
12 14 16 18 20 22 24 Retention Volume [mL)
12 14 16 18 20 22 24 Retention Volume (ml]
12 14 16 18 20 22 24 Retention Volume [mL]
12 14 16 18 20 22 24 Retention Volume [ml]
Figure III-2: Chromatograms and residd plots
(20 wt?! PVC in PVA / PVC)
12 14 16 18 20 22 24 Retention Volume [ml]
12 14 16 18 20 22 24 Retention Volume [ml]
12 t4 16 18 20 Z 24 Retention Vdume (mLJ
12 14 16 18 20 22 24 Retentian Volume [ml]
Figure III-3: Chromaîograms and residual plots
(10 wt% PVC in PVA / PVC)
12 14 16 18 20 22 24 Retention Volume [mL]
12 14 16 18 20 22 24 Retention Volume [ml]
12 14 16 18 20 22 24 Retention Voiume (mL]
-1 0 0 12 14 16 18 20 22 24
Retention Volume [mL]
Figure III-4: Chromaiograms and residuai piots
(5 wî!?? PVC in PVA / PVC)
12 14 16 18 20 î2 24 Retention Volume [ml]
-10 ' 1 12 14 16 18 20 22 24
Retention Volume [mC]
12 $4 16 18 20 24 Retention Volume (mt]
-1 O 12 14 16 18 20 22 24
Retention Volume [ml]
Figure III-5: Chromatograms and residual plots '
(20 wt?h PVA in PVA / PVC)
12 14 16 18 20 22 24 Retention Volume [ml]
4 -1 0 12 14 16 18 20 22 24
Retention Volume [ml]
12 14 18 18 20 22 24 Retention Volume (ml]
12 14 16 18 20 22 24 Retention Volume [ml)
Figure ma: Chromatopms and residual plots
(10 wt?? PVA in PVA / PVC)
12 14 16 18 20 22 24 Retention Volume [ml]
-10 L I 12 14 16 18 20 22 24
Retention Volume (mL)
12 14 16 18 20 22 24 Retentian Volume (ml]
;
12 14 16 18 20 22 24 Retention Vdlume [ml]
Figure III-7: Chromatngrams and residual plots
(5 wt% PVA in PVA / PVC)
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