quantitative determination of short-chain branching content and distribution in commercial...

$: Quantitative determination of short-chain branching content and distribution in commercial polyethylenes by thermally fractionated differential scanning calorimetry$
Quantitative Determination of Short-Chain Branching Content and Distribution in Commercial

Pol yet hy I enes by The rma I I y Fractionated Differential Scanning Calorimetry

MINGQIAN ZHANG and SIEGHARD E. WANKE*

Department of Chemical and Materials Engineering, University of Alberta Edmonton, Alberta, Canada T6G 2G6

A method for rapid quantitative analysis of the content and distribution of short chain branching (SCB) for a-olefln/ethylene copolymers based on thermally fractionated DSC is presented. Eight commercial polyethylenes, four made with conventional Ziegler-Natta catalysts and four made with metallocene catalysts, were analyzed by dmerential scanning calorimetry (DSC), after having been thermally segregated by successive nucleation annealing (SNA). The polyethylenes were also analyzed by temperature rising elution fractionation W F ) and carbon-13 nuclear magnetic resonance ( 13C-NMR). The SNA-DSC procedure segregates polyethylenes according to methylene sequence lengths (MSL). The relationship between DSC melting temperature and SCB content was obtained by calibration with linear hydrocarbons: TREF results were not used in the SNA-DSC calibration. Deconvolution of the SNA-DSC endotherms yielded estimates of the average SCB contents and SCB distributions. The SCB contents obtained from the SNA-DSC for linear low density polyethylenes agreed very well with the SCB contents obtained by 13C-NMR and TREF, and the SCB distributions measured by SNA-DSC were very similar to those obtained by TREF. The SCB contents obtained by SNA-DSC for ultra-low density polyethylenes, made with metallocene catalysts, were about 20% lower than the values obtained by 13C-NMR; the values obtained by TREF were even lower.

INTRODUCTION most commonly used technique (3 - 6). Although TREF

olyethylenes synthesized by the copolymerization P of ethylene and a-olefins with either Ziegler-Natta or metallocene catalysts are major commodity polymers worldwide. The properties, and hence the end uses, of commercial ethylene/a-olefin copolymers are largely governed by the degree and distribution of the short chain branching (SCB) (1, 2). The average content and distribution of short chain branchmg (SCB) are varied and controlled by the use of Merent catalysts and variation in polymerization conditions. The development of a fast and reliable technique for the quantitative analysis of SCB content and distribution in polyethylenes is of practical importance for correlating molecular structure and physical properties.

The SCB content and distribution in LLDPEs are usually determined by a fractionation procedure: temperature rising elution fractionation (TREF) being the

*To whom correspondence should be addressed. E-mail: [email protected]

is generally established as the most reliable technique for the quantitative analysis of LLDPE in terms of average SCB content and SCB distribution, it suffers from being time-consuming and involving the use of toxic solvents (6, 7). Differential scanning calorimetry (DSC) has been considered as an alternative to TREF because DSC is a much faster technique than "REF, and is solvent-free as well. The earlier studies done by Wild et aL (8) and Karbashewski et aL (9) showed that DSC analyses of LLDPE samples that had been crystallized slowly provided the same qualitative information as analytical TREF in terms of estimating the breadth of the SCB distribution, but the resolution of DSC was not as good as that of TREF. These studies also indi- cated that the quantitative analysis of DSC is difficult because the intensity of the DSC response is a prod- uct of the amount of material melted at a particular temperature and the enthalpy of fusion. For the past several years, the use of various thermal treatment procedures has considerably improved the resolution and reproducibility of DSC (6, 7,lO-14), and the thermally

1878 POLYMER ENGINEERING AND SCIENCE, DECEMBER 2003, Vol. 43, No. 12

Short-Chain Branching Content and Distribution

fractionated DSC method has shown promise as a quantitative technique for the analysis of LLDPE with respect to SCB.

In the present paper, the determination of SCB by DSC of thermally fi-actionated polyethylenes is reviewed, and the results of this method are compared to SCB concentrations determined by TREF and 13C-NMR. Techniques used for deconvoluting DSC endotherms, converting melting temperatures to SCB content, and correcting weght fraction from enthalpy of fusion are presented. The SCB concentrations obtained by thermal fractionation-DSC of eight commercial polyethylene samples, four made with conventional Ziegler-Natta catalysts and the other four made with metallocene catalysts, are compared with TREF and 13C-NMR results.

EXPERIMENTAL Materials

The eght commercial low-density polyethylenes used in this study are described in Table 1. All the polyethylenes were produced by the copolymerization of ethylene with an a-olefin on either conventional Ti-based Ziegler-Natta or metallocene (single-site1 catalysts; these polyethylenes will subsequently be referred to as Ziegler-Natta and metallocene polyethylenes. The mass average molar masses of all eight samples were similar, ranging from about 80 to 110 thousand: the polydispersities of the Ziegler-Natta polyethylenes were all about 3.3 ( 2 0.1) and the metallocene polyethylenes had polydispemities of about 2.1, indicating single-site type behavior. The densities of the Ziegler-Natta polyethylenes varied from 0.918 to 0.930. Three of the metallmene polyethylenes were ultra-low density polyethylenes (0.87 to 0.88 g/cm3), while the fourth metallocene polyethylene (Attane420 1) had a density of 0.912 g/cm3, which is comparable to the densities of the Ziegler-Natta polyethylenes. Ziegler-Natta and metallocene polyethylenes made with 1-butene, 1-hexene and 1-octene cornonomem were included in the commercial samples examined.

Temperature Rising Elution Fhctiodon (TREF)

A sample of each polyethylene (ca. 5 ma, along with about 1.5 g support (glass beads, 80-100 mesh) was

added to 5 mL o-xyl.ene. The polymer, support, and solvent were heated at 125°C for 4 h, and then cooled from 125 to -8°C at a constant coollng rate of 1.5"C/h. During this controlled cooling process, separation on the basis of crystallizability occurred, in which the less branched components crystallized fiist at high temperature, followed by crystallization of molecules with increasing level:: of branching at lower temperatures. The crystallized sample was then eluted at a heating rate of l.O"C/min in a TREF system. The de- tailed description of the TREF experimental process has been given elsewhere (12).

Thermally Frpctionated Differential Scauning Calorimetry (DSC)

The thermally li-actionated DSC analysis involved the thermal treatment of the polyethylene samples by a successive nucleation/annealing (SNA) procedure prior to DSC analysis. The SNA procedure used for the thermal treatment of the samples was similar to that used in an earlier study (121. The polymer samples were heated at 5"C/min to a selected temperature and were annealed at that temperature for 10 min. The crystallization was achieved by subsequently cooling the samples to 25°C at a coo- rate of 5"C/min. The heating- annealing-cooling cycle was repeated at a temperature interval of 5°C from 135 to 30°C for the Ziegler-Natta samples and 105 to 310°C for the metallocene samples. Zhang (15) confirmed the observations of Miiller et aL (6) that the SNA procedure resulted in better fractionation of polymers than the stepwise crystallization procedure used by some investigators (e.g. 14,16,17).

DSC analysis of the polyethylene samples was per- formed on a TA Instrument Model DSC2910. An in- dium standard was used to calibrate the instrument. The SNA-treated samples (ca. 10 mg) were heated from 0°C at a heating rate of 10°C/min to 160°C. held at that temperature for 2! min, and subsequently cooled to 0°C at the same rate. Heating rates of 10°C/min were used because this i:; the most commonly employed heating rate for DSC analysis of polyethylene (e.g. 6, 8-10, 12, 13, 15, 16, 18, 191. Other heating rates, such 5"C/min, are used less frequently, (e.g. 7, 14, 17). A heating rate of 10°C/min was also used in all the calibration experiments with standards of known

Table 1. Description of Polyethylenes Used.

Designation Manufacturer Type Type (s/cm3) M, x 10-4 Pd Polymer Comonomer Catalyst Density

PFOl18F NOVA 1 -butene Ziegler-Natta 0.918 PFO218F NOVA 1 -butene Ziegler-Natta 0.918 TFOl19F NOVA 1 -hexene Ziegler-Natta 0.918 Sclairl3J7 NOVA 1 -octene Ziegler-Natta 0.930 Exact4033 Exxon 1 -butene metallocene 0.880 SLP9095 Exxon 1 -hexene metallocene 0.883 Engage81 00 Dow 1 -octene metallocene 0.870 Attane4201 Dow 1 -octene metallocene 0.91 2

M, = m a s average molar mass. Pd = polydispersity = KIM, .

10.6 8.5

10.6 9.7

11.0 8.6

10.8 8.2

3.28 3.29 3.28 3.43 2.14 2.14 2.00 2.09

POLYMER ENGINEERING AND SCIENCE, DECEMBER 2003, Vol. 43, No. 12 1879

Mingqian Zhang and Sieghard E. Wanke

methylene sequence lengths (13). The results of the calibration experiments were used to convert the DSC heating curves (endotherms) into SCB concentrations and distributions. The DSC analyses on samples with unknown methylene sequence lengths have to be done at the same heating rate as the calibration experiments in order to convert melting temperatures to methylene sequence lengths because melting temperatures determined by DSC are a function of heating rates (20).

Nuclear Magnetic Resonance (NMR) Spectroscopy l3C nuclear magnetic resonance spectroscopy

was used to determine the average SCB contents of the polyethylene samples. A 1.1 g portion of the pre- heated polymer solution, prepared by adding -1.4 g PE copolymer to 12.2 g 1,2,4-trichlorobemene, was transferred into a 4 mm NMR tube. About 10 VOWO de- uterated benzene was added as a lock solvent. The sample solution tube was further heated at 140°C for 4 h prior to accumulation of the spectrum. NMR spectrum was recorded with 90" pulse angle, 10 s pulse delay, and 3 s acquisition time on a Varian Unity 50 instrument. The average SCB content was determined according to the method presented by Pooter et al. (21) and described in- D5017-96.

RESULTS AND DISCUSSION

TREF Analysis

"REF has been shown to be effective in determining differences in SCB content and distribution among different polyethylenes (3, 4). The TREF profiles of the commercial polyethylenes used in this study were used as the basis of comparison with the DSC analysis. TREF profiles of the four Ziegler-Natta polyethylenes are shown in Rg. 1. It can be seen that the Ziegler- Nat ta polyethylenes, regardless of their branch type, had a broad and bimodal elution temperature distribution; this corresponds to broad and bimodal SCB distributions. The ethylene/l-butene and ethylene/l- hexene copolymers showed a distribution in the elution temperatures ranging from 20 to lOO"C, while the ethylene/ 1 -octene copolymer showed a narrower distribution between 50 to 100°C. The bimodal TREF pro- file is characteristic of Ziegler-Natta LLDPE (3, 4). The peak at low temperature with the long tail toward lower temperatures is due to ethylene/u-olefm copolymer produced on the catalytic sites responsible for copolymerization, while the sharp peak centered at about 95°C corresponds to the "homopolymer" fraction.

The TREF profiles of the four metallocene polyethylenes are shown in Rg. 2. I t can be seen that the metallocene polyethylenes exhibited narrower distributions of elution temperatures, i.e. crystallinity, than the Ziegler-Natta polyethylenes shown in Rg. 1. The broadness of the distribution, i.e. the range of elution temperatures, vaned considerably from sample to sample. Engage8 100, an ethylene/ 1 -octene copolymer, exhibited a very narrow range in elution temperatures fi-om 15 to 40°C. Exact4033 and SLP9095

L

c

Elution Temperature, "C Hg. 1. TREF proJZes of commercial Ziegler-Natta polyethylenes (patterns are oflset for clarity).

both had elution temperature ranges from 20 to 60"C, although the shapes of their TREF profiles were different. Attane420 1 had the broadest distribution among the four metallocene polyethylenes with an elution temperature range of 20 to 80°C.

DSC Endotherms

Successive nucleation/anneahg (SNA) is a thermal fractionation procedure which segregates polyethylene molecules based on recrystallization and reorganiza- tion from the melt according to methylene sequence lengths (MSL) (6, 10-14). Methylene sequences of different lengths in a polymer molecule can become part of crystals with different sizes and these crystals in subsequent DSC analysis melt at temperature corre- sponding to their sizes (18). As a result, each peak of SNA-DSC endotherms represents a group of chain segments having similar methylene sequence lengths.

mure 3 shows DSC endotherms of four Ziegler-Natta polyethylenes treated by SNA. These results show that SNA-DSC is capable of differentiating between different samples. All four samples show a broad distribution of melting temperatures, fi-om about 60 to 130"C, but the shapes of the endotherms differed sigmficantly from sample to sample. The SNA-DSC endotherms in Ffg. 3 are similar to the TREF profiles in 8Tg. 1. A quantitative comparison of the SNA-DSC and TREF results is presented later in this section.



0 20 40 60 80 100 120

Elution Temperature, "C

Melting Temperature, "C

Ftg. 2. TREF profles of commercial metallocene polyethylenes @atterns are offset for clarity).

Fig. 3. PSC endotherms of commercial Ziegler-Natta polyethylenes a@ SNA treatment lpat tm are oflset for clwity).

POLYMER ENGINEERING AND SCIENCE, DECEMBER 2003, Vol. 43, No. 12 I881

Mingqian Zhang and Sieghard E. W&

The DSC endothems of the metallocene LLDPEs treated with SNA are plotted in Fig. 4. Again, multiple- peaked DSC endotherms were observed, which indi- cate different MSL distributions among these metallocene polyethylene samples. The Attane420 1 showed the broadest distribution, while the Engage8100 had the narrowest among these four metallocene polyethylenes. The melting temperature distribution curves in Rg. 4 showed the same trends as the TREF profiles for these samples shown in Rg. 2, with the exception of a pronounced peak at about 40°C in Flg. 4 for three of the metallocene polyethylenes. It is likely that this peak at 40°C represents a group of short methylene sequences which form a fringed micelle-like nucleation structure (1, 22). These short MSL can crystallize and melt at low temperatures under favorable conditions, but they were not detected by TREF. Similar pronounced peaks at 40°C were observed for all prepara- tive TREF fractions prepared from Exact4033 (1 2).

Quantitative DSC Analysis

The results shown in Flgs. 3 and 4 show that the SNA-DSC technique results in excellent segregation of polyethylene samples with different SCB structures: this segregation is on the basis of methylene sequence length (see above). For ethylene/a-olefin copolymers, the branches, except methyl (propylene as comonomer), are usually excluded from the crystalline domains for energetic reasons (23, 24). I t follows that the MSL controls the attainable crystal thickness and thereby the thermal segregation when the maximum MSL is

less than the critical value for the onset of chain folding. The critical value for the onset of chain folding is about 250 carbons (24-26). The MSL values calculated from the peak temperatures of the SNA-DSC endotherms for typical polyolefms were less than or close to the critical value for almost all the peak melting temperatures (13). indicating that the SNA-DSC segregation is controlled by MSL for the commercial u-olefW ethylene copolymers used in the current study.

The calibration curve, in our previous study (13), re- lating MSL to melting temperatures was based on DSC measurements of linear hydrocarbons after SNA treat- ments. The calibration curve we obtained was

142 2 T

In ( X ) = 0.3451 - - ; r2 = 0.9997 (1)

where Xis the mole fraction of carbon in CH, groups in the linear hydrocarbon, e.g. X = 0.900 for n-C,&,,, and T is the melting temperature in K (location of the maximum in the endotherm). Previously, Keating et aL (1 1) had used a similar calibration method, but they used stepwise crystallization rather than SNA, prior to the determination of melting points of linear hydrocarbons by DSC. The calibration curve obtained by Keat- ing et aL (1 11 was

In ( X ) = 0.331 - - - 135*5 , r2 = 0.9992 T

Values of X can be converted to methylene sequence lengths, MSL, by

Melting Temperature, "C Flg. 4. Dsc endotherms of commercial metdocme polyethylenes afier SNA treatment (patterns are offset for clarity).



2 x MSL = -

1 -x (3)

MSL as a function of the melting temperature T, in K, is obtained by substitution for X in terms of T from a calibration equation; using Eq 1 yields

2 MSL = (4) -.[+ o.wji] - 1

Plots of MSL as a function of melting temperature according to Eq 4, as well as for a similar equation using calibration Eq 2, are shown in Ftg. 5.

For a-olefm/ethylene copolymers, the short chain branching concentration, C in SCB per 1000 carbons, as a function of MSL, assuming constant values of MSL, is given by

1000 M S L + i + l

C = (51

where i is the number of carbon atoms in the SCB, i.e. i = 4 for a 1-hexene/ethylene copolymer. The concentration of SCB as a function of melting temperature can then be obtained by substitution of MSL from Eq 4 into Eq 5. Plots of C as a fimction of melting temperature for values of i = 2 , 4 and 6 are shown in Fig. 6. Hosoda (27) reported the effects of short chain branch concentration and length on melting temperature for melting temperatures > 80°C. Hosada's results for the 1-octene copolymers were very similar to the results shown in FYg. 6; his results for 1-butene copolymers yielded slightly higher SCB concentrations compared to the results shown in FYg. 6. Addison et al. (10) reported curves of SCB concentration as a function of melting temperature for ethylene/ 1-butene

;2

3 X*

1 0

u tw 0

E iz ?a E

and ethylene/ 1-hexene copolymers. Their calibration gives values of C at low melting temperatures (40 to 50°C) which are 1596 to 20% higher than our values; the differences were less at higher melting temperatures. The results b:y Addison et aL (10) and Hosada (27) are based on T'REF fractions and not on linear hydrocarbons as was done in the current study and the study by Keating et al. (1 1).

The DSC endothe,ms of SNA treated samples can be converted to SCB distributions by measuring the areas under each of the peaks in the endotherm and using Eqs 4 and 5 to convert the temperatures of the various minima in the endotherms to SCB concentrations. Commercial software PeakFit, Version 4.06 from SPSS Science Inc) was used to deconvolute the SNA- DSC endotherms of Merent polyethylene samples. The functions used in the deconvolution were the Gauss- ian area function:

or the Gaussian-Lorentzian area sum function: r - a3 vln 2 -."h12(=3] + 1 a 2 G

Y = U o

1-a, 1 (7)

where y is the heat flow, Tis the DSC temperature: A, is the area under a 1)SC peak centered at a temperature equal to a,; a2 is the peak width at half height, and a3 is a shape factor.

60

50

40

30

20

10

0

from Reference 11 - from Reference 13 I

-----I

25 100 125 150

Melting Temperature, "C Rg. 5. Calibration w e s of MSL as a m t i o n of DSC melting temperature (based on linear hydrocarbons).

POLYMER ENGINEERING AND SCIENCE, DECEMBER2003, Vol. 43, No. 12 1883

Mingqian Z h a q and Sieghard E. Wanke

0 0 0 - 50 L Q) a

40 &

E g 20 c) E

8 10 cp u ' / 3 0

c Length ofShortBranches 1 \ \ \ - - - - 2 Carbon Atoms -

4 Carbon Atoms - -.-. 6 Carbon Atoms : -

- -

-

-

25 50 75 100 125 150 Melting Temperature, "C

Rg. 6. Dependence of SCB concentration on melting temperature as afimctbn of length of short chain branches.

prior to the fitting, the sign of the DSC signal was changed from negative to positive so that all the sig- nals were positive: positive values of the dependent variable are required by the software. Typical fits of DSC endotherms of SNA-treated polyethylenes are shown in Figs. 7 and 8. It can be seen that excellent fits of the multi-maxima endothexm were obtained. The individual peaks resulting from the deconvolution of the endothexm are shown in the bottom portion of Figs. 7 and 8. The areas of each peak and the temperatures at the m u m of each peak are part of the PeakFit output.

The cumulative area fraction distributions as a function of melting temperature from the deconvoluted SNA- DSC endotherms of the eight polymer samples are shown in Fig. 9. These distribution curves illustrate the difference in methylene sequence length distribution between the metallocene and Ziegler-Natta polyethylenes. The slopes of the cumulative distribution curves for the Ziegler-Natta polyethylenes are lower than the slopes for the metallocene polyethylenes indicating broader MSL distribution for the Ziegler-Natta polyethylenes.

The average SCB content of each polyethylene sample can be calculated if the SCB content and the mass fnction of each peak in the SNA-DSC endotherms are known. Average short chain branch contents, C, and C,, based on the first and second moments of the SCB distributions, similar to the number and mass average molar mass distributions, can be defined and calculated according to Eqs 8 and 9:

WlC1 + w,c2 + ... + w,c, - w, + w2 + ... + w,

- c, =

Wl w2 Wn - c1 + - c2 + ... + - c, = W W W i= 1

n W,C, (8)

wc2 + wc2 + ... + 11rf-2

WlC1 + w2c2 + ... + w,c, 1 1 2 2 c, =

i = 1

where W is total mass of the sample; n is the number of peaks; wi is the mass fraction of peak i, and Ci is the SCB of the mass fraction w,. The value of C,/C, is an indicator of the broadness of short chain branch distribution.

Values of C, as a function of melting temperature can be obtained from Eq 5, by substitution for MSL from calibration Eq 4. Assuming that the mass fractions are equal to the area fractions for each of the peaks pro- vides initial approximations of wi (19.28). Table 2 gives the average SCB contents of the Ziegler-Natta and metallocene polyethylenes based on the assumption that the mass fraction is equal to the area fraction: this assumption is the same as the assumption that the enthalpy of fusion is independent of SCB content. The results in Table 2 show that C , values calculated by the above procedure are generally lower than the SCB concentrations obtained from 13C-NMR analyses (SCB concentrations obtained by 13C-NMR should be compared to the first moment average C,). Values of C,



121 - - - - - - - - - - - - - . - - - - . - - - -

"1 b.

I . - - - . . - - - * - . . . . . . . . 50 70 90 110 130 150 20 40 60 80 100

Melting Temperature, "C Melting Temperature, "C

Flg. 7. Results of _fittins of SNA-DSC endotherm of Zegler- Natta polyethyhe {PFOI 18F): a Comparison of measured (dotted line) a n d p d (solid line) endot- b. Fitted &on- voluted peaks (solid line in a. is sum of the deconvoluted

Flg. 8. Results of fsmng of SNA-DSC endotherm of metal- bcene polyethylene (Exmt4033): a Comparison of measured (dotted line) a n d e d (solid line) endotherm: b. Fitted &on- voluted peaks (solid line in a. is sum of the deconvoluted

Peaks). peaks).

1 .o

0.8

0.6

0.4

0.2

0.0

0 Engage8100 . ASLP9095 - oExact4033

oAttane4201 -

APF0218F - 0 TF0119F -

. -

-

Melting Temperature, "C Q. 9. CwnulatSe area distributions (approximate SCB distributions) of polyethylenes determinedfrom the SNA-DSC endotherms.


Mirgq ian Zhang and Sieghard E. Wanke

Table 2. Comparison of Average SCB Contents Obtained From 13C-NMR, SNA-DSC Endotherms and TREF for Various Ziegler-Natta and Metallocene Polyethylenes.

c n cw CJCn Polyethylene SCB from

W-NMR DSC TREF DSC TREF DSC TREF

PFOl18F 16.8 12.8 17.7 23.0 26.5 1.48 1.50 PF0218F 15.1 12.4 15.7 18.8 23.6 1.51 1.50

11.6 9.8 13.4 15.9 21.7 1.62 1.62 8.9 8.0 12.9 11.2 1.45 1.40

TFOll9F Sclairl3J7

SLP9095 40.5 36.7 28.7 37.6 29.7 1.03 1.04

Attane4201 21.5 21.6 19.6 24.2 22.5 1.12 1.15

Type

Exact4033 55.0 38.8 32.4 40.5 33.3 1.04 1.02

Engage8100 43.6 36.2 40.2 36.8 40.6 1.02 1.01

'Difficunies were encountered in preparing dissolved sample lor NMR analysis.

obtained by TREF were slightly lower than the values obtained by DSC for the Ziegler-Natta polyethylenes: for metallocene polyethylene the DSC values were usually higher than the TRl3F values.

To determine whether the assumption of constant enthalpy of fusion was valid, heats of fusion as function of SCB content previously determined for PFOl18F and Exact4033 were used to convert the area fractions to mass fractions (13). The correlation obtained for PFO 1 18F. given by Eq 10, was used for the Ziegler- Nat ta polyethylenes.

AHL = 0.0443Cf - 4.6489Ci + 164.8 (lo) The correlation obtained for Exact4033, given by Eq 1 1, was used for the metallocene polyethylenes.

(1 1)

AHi is the enthalpy of fusion and Ci is the correspond- ing SCB content of peak i . The corrected weight fraction is then given by:

AHi = -0.4713Ct + 49.745

I1 2)

Substitution of E q 12 into Eqs 8 and 9 yields the SCB contents based on the corrected weight fraction. In Table 3 the 'corrected average SCB contents and the SCB distribution broadness are compared to the 'uncorrected' values. The difference between the corrected and uncorrected SCB contents for the three

ultra-low density metallocene polyethylenes (Exact 4033, SLP9095 and Attane4201) are relatively small, i.e. differences are 53.3%. The correction for the ef- fect of enthalpy changes with SCB content is small for these samples because the SCB distribution is narrow (see Fg. 9 as well as values of CJCW in Table 3) and the variation in the heat of fusion with SCB content is relatively weak (see Eq 11). The corrections for the Ziegler-Natta polyethylenes were more significant, ranging from 12% to 18%. because the heat of fusion varied significantly with SCB content (see Eq 10) and the SCB distributions were broad.

In Rg. 10, the SCB contents determined by 13C-NMR are compared to the results obtained by SNA-DSC and TREF. The corrected SCB contents for the three Ziegler Natta polyethylenes, for which reliable I3C-NMR analyses were obtained, and for the Attane4201 metallocene polyethylene were in excellent agreement with the results of 13C-NMR analyses. The difference between the corrected DSC results and the 13C-NMR results for these four samples varied from -7.1% to + 3.7%. with an average difference of -0.04%. The difference between the SCB contents determined by TREF and 13C-NMR for these four samples varied from -8.8% to + 15.5Y0, with an average value of + 2.2%. It should be kept in mind that the calibrations for the DSC analyses and the TREF analyses were totally independent, i.e. TREF results were not used for the DSC calibration: the DSC calibration (Eq 1) is based on SNA-DSC measurements using linear hydrocarbons (13). TREF was used to obtain DSC calibration curves (i.e. SCB

Table 3. Comparison of Average SCB Contents from SNA-DSC Endotherms With and Without Corrections for the Dependence of Enthalpy of Melting on SCB Content.

Cn c w CJCn Polyethylene Type Corrected Uncorrected Corrected Uncorrected Corrected Uncorrected

PFOl18F PF0218F TFOl19F Sclairl3J7 Exact4033 SLP9095 Engage8100 Attane42Ol

15.6 15.2 11.9 10.1 40.0 37.2 36.5 22.3

12.8 12.4

8.9 38.8 36.7 36.2 21.6

9.8

~

22.0 21 .a 18.3 14.34 41.4 38.1 37.2 25.1

~~

19.0 18.8 15.9 12.9 40.5 37.6 36.8 24.2

____~

1.41 1.43 1.53 1.41 1.04 1.02 1.02 1.12

1.48 1.51 1.62 1.45 1.04 1.03 1.02 1.12



1 - 1 . 1 . 1 . 1 . 4 - 4 .

4 0

SNA-DSC 4 8

TREF t 4

4

x = y line ,4' - - - -

60

50

40

30

20

10

0 0 10 20 30 40 50 60

SCB Content Measured by I3C-NMR, SCB per 1000 Carbon Atoms

Fig. 10. Comparison of SCB concentmtions obtained by 13C-NMR with those obtained by SNA-DSC and TREF.

content as function of melting temperature) in the majority of DSC studies for the determination of SCB content by DSC (8,10,19, 27). In such studies a comparison of TREF and DSC results is questionable be-

SCB Content by (SNA-DSC) =

x + x2 (13) 1 + 0.81~ + 0.0095~~

cause the TREF and DSC results are highly correlated if TREF results were used to obtain DSC calibration curves. Our results show that DSC analysis of samples after appropriate thermal treatment, i.e. SNA, can be used to obtain SCB contents and SCB distributions without the use of TREF for calibration, As can be seen in FXg. 10, the agreement between

DSC and 13C-NMR results is much poorer for the three ultra-low density metallocene polyethylenes (high SCB contents); the differences vary from -27.3% to -8.1% with an average value of - 17.2%. The differences between the TREF and 13C-NMR are even larger, with a range of -41.1 to -7.8% and an average of -26.00/. The probable cause for the underestimation of SCB contents by SNA-DSC and TREF for samples with high SCB contents is the lack of either of these techniques to detect short MSL. For samples with SCB contents of 40 or higher the average MSL is less than 25; such samples will contain significant number of methylene sequences shorter than 10; such short MSL will not crystallize during the SNA or the TREF crystallization procedures. The SNA-DSC method will yield low SCB content values for polyethylenes with SCB contents above 40 branches per 100 carbons if the calibration is based solely on linear hydrocarbons.

A correlation between the measured SCB contents by SNA-DSC and 13C-NhIR is given by the solid line in R g . 10; the equation for this trend line is

where x is the SCB content obtained by 13C-NMR (as- sumed to be the correct SCB concent). Equation 13 is a totally empirical Correlation that describes the data well, and it can be used as a guide to correct SCB contents determined by SNA-DSC for polyethylenes with SCB concentrations higher than 30 SCB per 1000 carbon atoms. Additional SNA-DSC and 13C-NMR measurements with ultra low density polyethylenes with narrow SCB distributions are required to obtain better correlations for high SCB content polyethylenes by the proposed SNA-DAS procedure.

SUMMARY AND CONCLUSIONS

The combination of DSC and successive nucleation annealing procedure effectively segregated a variety of commercial polyethylenes on the basis of methylene sequence length. The quantitative analysis of the SCB contents and distributions for these polyethylenes was achieved by deconvol.uting the SNA-DSC endotherms, and by converting melting temperature to branching information via calibration equations obtained from DSC endotherms of SNA-treated linear hydrocarbons. The SCB contents from SNA-DSC analyses of Ziegler- Nat ta polyethylenes were very close to those measured by 13C-NMR. Corrections for the variation of the heat of fusion with SCB content improved the agreement between the SNA-DSC and 13C-NMR results.


Mingqian Zhang and Sieghard E. Wanke

The agreement between SNA-DSC and 13C-NMR results was also good for a metallocene polyethylene which had a density comparable to those of the Ziegler-Natta polyethylenes. However, SNA-DSC analysis of the three ultra-low density metallocene polyethylenes yielded lower SCB contents than the 13C-NMR analyses. TREF analyses of these ultra-low density polyethylenes yielded, on average, even lower SCB contents than the SNA-DSC analyses. SCB distributions were also obtained from SNA-DSC analyses: the Ziegler-Nab polyethylenes had broad distributions with a CJC, ratio of 1.4 to 1.5: the same ratio for the metallocene polyethylenes ranged from 1.0 to 1.1. TREF analyses yielded the same range of CJC, ratios. The good agreement in the SCB contents determined by the SNA-DSC and 13C-NMR for linear low density polyethylenes and the good agreement in the SCB distributions determined by SNA-SCB and TREF leads to the conclusion that the SNA-DSC method is a reliable method for the determination of SCB content and distribution in linear low density polyethylenes. The SNA-DSC method can also be used for ultra-low density polyethylenes, but measured SCB contents for such samples are lower than the values measured by 13C-NMR.

ACKNOWLEDGMENTS The authors acknowledge the support of this work

by the Natural Sciences and Engineering Research Council of Canada. The authors thank Ms. N. Bu for the molar mass measurements.

REFERENCES 1. M. Peeters, B. Goderis. C. Vonk, H. Reynaers, and V.

Mathot, J. Polym Sci, Part B: Polym Phys., 35, 2689 (1997).

2. M. Y. Gelfer and H. H. Winter, Macromolecules, 52, 8974 (1999).

3. L. Wild, A h . Polym Sci, 98, 1 (1991). 4. J. B. P. Soares and A. E. Hamielec. Polymer, 36, 1639 (1995).

5. M. G. Pigeon and A. Rudin. J. Appl. Polym Sci, Sl, 303

6. A. J. Mtiller. 2. H. Hernandez. M. L. Arnal. and J. J. (1994).

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25. 26.

27. 28.

Sanchez, Polyrn Bulletin, 39,465 (1997). C. Vandermiers, J.-F. Moulin, P. Damman, a n d M. Dosiere, Polymer, 41, 2915 (2000). L. Wild, S. Chang, and M. J. Shankemarayanan, Po@- merPreprints, 31, 270 (1990). E. Karbashewski, L. Kale, A Rudin, E. J. Tchir, D. G. Cook. and J. 0. Pronovost, J. Appl. PoZym Sci, 44. 425 (1992). E. Adisson, M. Ribeiro, A. Deffieux, and M. Fontanille, Polymer. 33, 4337 (1992). M. Y. Keating, I. H. Lee, and C. S. Wong, Thermochimica Acta, 284, 47 (1996). M. Zhang, D. T. Lynch, and S. E. Wanke, J. AppL Polym Sci. 75, 960 (2000). M. Zhang, D. T. Lynch. and S. E. Wanke, Polymer, 42, 3067 (2001). P. Starck, P. Lehmus, and J. V. Seppala, Polym Eng. Sci, SS, 1444 (1999). M. Zhang, M.Sc. Thesis, Dept. of Chemical and Material Engineering, Univ. ofAlberta (1999). M. Y. Keating a n d E. F. McCord, 'lhenochimica Acta, 245, 129 (1994). F. Zhang, Q. Fu, T. La, H. Huang, and T. He, Polymer, 43, 1031 (2001). F. M. Mirabella, J. Polyrn Sci: Part B: Polym Phys., 39, 2800 (2001). S. A. Karoglanian and I. R. Harrison, Themchimica Acta, 288,239 (1996). A. K. Gupta, S. K. Rana, and B. L. Deopura, J. Appl. PoZym Sci, 51,231 (1994). M. De Pooter, P. B. Smith, K. K. Dohrer, K. F. Bennett, M. D. Meadows, C. G. Smith, H. P. Schouwenaars, and R. A. Geeraeds, J. Appl. Polym Sci, 42, 399 (1991). M. Peeters, B. Goderis, H. Reynaers, and V. J. Mathot, Polym Sci, Part B: Polym Phys., 37, 83 (1999). L. Mandelkem and G. Stack, Macromolecules, 17, 871 (1984). J. G. BoMer, C. J. Frye, and G. Capaccio, Polymer, 34, 3532 (1993). G. Ungar and A. Keller, Polymer, 28, 1899 (1987). G. M. Stack and L. Mandelkem, Macromolecules, 21, 510 (1988). S. Hosoda, Polym Jounal, 20,383 (1988). A. Prasad, Polym Eng. Sci, 38, 1716 (1998).


quantitative determination of short-chain branching content and distribution in commercial...

Documents