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University of Groningen

Synthesis and evaluation of novel linear and branched polyacrylamides for enhanced oilrecoveryWever, Diego-Armando Zacarias

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Citation for published version (APA):Wever, D-A. Z. (2013). Synthesis and evaluation of novel linear and branched polyacrylamides forenhanced oil recovery. Groningen: s.n.

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Chapter 3

63

Chapter 3

Branched polyacrylamides:

Synthesis and effect of molecular

architecture on solution rheology

Abstract

Linear, star and comb-shaped polyacrylamides (PAM) have been

prepared by atomic transfer radical polymerization (ATRP) in aqueous media

at room temperature. The influence of the molecular architecture of PAM on

the rheological properties in aqueous solution has been investigated. The

well-known theory of increased entanglement density by branching for

polymers in the melt can also be applied to polymers in semi-dilute water

solutions. We have demonstrated this by investigating the rheological

properties of PAM of similar molecular weights with different molecular

architectures. Interestingly, the solution viscosity of a comb-like PAM is

higher than its linear and star-shaped analogues (both at equal span

molecular weight, Mn,SPAN, and total molecular weight, Mn,tot). In addition to

the pure viscosity, we also demonstrate that the visco-elastic properties of

the polymeric solutions vary as a function of the molecular architecture of the

employed PAM. The elastic response of water solutions containing comb PAM

is more pronounced than for solutions containing either linear or star PAM at

similar Mn,SPAN and Mn,tot. The obtained results pave the way towards

application of these polymeric materials in Enhanced Oil Recovery (EOR).

Based on: D.A.Z. Wever, F. Picchioni, A.A. Broekhuis. Branched

polyacrylamides: Synthesis and effect of molecular architecture on solution

rheology. European Polymer Journal, 2013, 49, 3298-3301.

Synthesis of branched polyacrylamide

64

3.1. Introduction

Polyacrylamide (PAM) is a versatile water soluble polymer which is used

in a number of areas such as oil recovery, wastewater treatment, cosmetics

and biomedical applications.1, 2 For most of these applications the function of

the polymer is to increase the solution viscosity or to behave as a flocculating

agent. Looking more closely at the polyacrylamides currently used, one can

observe that in all the applications linear PAM is employed. This is probably

due to the fact that PAMs with different architectures (i.e. other than linear)

are difficult to prepare. The relatively high propagation rate3 during

polymerization prevents achieving control over the molecular architecture. It

was demonstrated that uncontrolled grafted PAM can be prepared using free

radical polymerization at higher temperatures.4, 5 Alternatively, branched PAM

has been synthesized through the usage of transfer agents.6, 7 Although a

high degree of branching could be obtained8 there is little to no control in the

reaction and thus no control over the molecular architecture of the resulting

polymer.

The difficulties become even more relevant when attempting a controlled

radical polymerization, i.e. when trying to prepare PAM homo- and co-

polymers with a well-controlled macromolecular architecture. Historically,

controlled polymerization has been achieved by living anionic polymerization,

reversible addition-fragmentation chain transfer (RAFT) or atomic transfer

radical polymerization (ATRP). Unsuccessful controlled radical polymerization

of acrylamide has been reported.9-12 Similar to N-isopropylacrylamide13, living

anionic polymerization cannot be considered given the acidity (pKa ~ 25-26)

of the amide protons of acrylamide. Recently, the controlled preparation of

hyperbranched PAM has been demonstrated by copolymerizing acrylamide

and N,N-methylenebis(acrylamide) using a semi-batch RAFT

polymerization.14 However, in order to prepare comb-shaped polymers with

long arms, more specific methodologies15, i.e. “grafting from” (backbone

functionalized with a RAFT agent or radical initiator) or “grafting through”

(through the use of macromonomers), have to be used leading to more

cumbersome and lengthy preparation routes.

ATRP has enabled the synthesis of a variety of molecular architectures of

an even wider variety of different monomers.16 Nevertheless, given the

difficulty for the ATRP of acrylamide, the synthesis of branched PAM in a

controlled fashion has not been reported so far. However, with the recent

accomplishment of ATRP of acrylamide, either in water 17 or a water-alcohol

mixture 18, controlled polymerization of acrylamide yielding grafted, comb

and star-shaped PAM can be envisaged. Star-shaped PAM can be easily

prepared using the well-known multifunctional initiators widely used for the

Chapter 3

65

preparation of star polystyrenes and polyacrylates19. Other methods aimed at

the synthesis of comb-like structures of different monomers have been

published 20-22, but are based on multiple and cumbersome synthetic steps to

prepare the appropriate macroinitiators. This paper describes the preparation

of a multifunctional macro-initiator based on aliphatic alternating polyketone

(PK) oligomer. The latter was functionalized through the classic Paal-Knorr

reaction leading to the desired macro-initiator, which was subsequently used

in the ATRP of acrylamide yielding the envisaged comb-like PAM. Linear and

star-shaped polymers were also prepared using the published method.17 The

rheological properties for these polymers were compared in aqueous

solutions.

In this work, the aim is to (1) synthesize branched (comb) PAM using

novel macro-initiators based on aliphatic perfectly alternating polyketones

and (2) to investigate the effect of the architecture of the polymer on the

aqueous solution rheology. The choice of chemically modified PK (a polymer

of industrial origin with relatively broad molecular weight distribution) as

initiator stems for the future applicability of the proposed method at

industrial level.

3.2. Experimental section

Chemicals. Acrylamide (AM) (electrophoresis grade, ≥99%), PAM (Mw =

5-6·106 g/mol), tris[2-(dimethylamino)ethyl]amine (Me6TREN), 2,2-

bipyridine (bpy), copper(I) chloride (CuCl, 98%), copper(I) bromide (CuBr,

98%), methyl 2-chloropropionate (MeClPr, 97%), methyl chloroacetate

(MClAc, 99%) pentaerythritol tetrakis(2-bromoisobutyrate) (97%), 3-

chloropropylamine hydrochloride (98%), and sodium hydroxide (pellets) were

purchased from Sigma Aldrich. CuCl and CuBr were purified by stirring in

glacial acetic acid (Aldrich), washing with glacial acetic acid, ethanol and

diethyl ether (in that order) and then dried under vacuum. All solvents were

reagent grade and used without further purification. The alternating

polyketone with 30 mol% ethylene content (PK30, Mn = 2797 g/mol, PDI =

1.74) was synthesized according to the published procedure.23, 24

ATRP of AM in aqueous media using a primary halogen. The

polymerization was performed in analogy with literature17. A 250 mL three-

necked flask was charged with AM (5 g, 70 mmol). A magnetic stirrer and

distilled water were added and subsequently degassed by three freeze-pump-

thaw cycles and left under nitrogen. The flask was then placed in a water

bath at 25 °C. Afterwards CuCl (21 mg, 0.21 mmol) and Me6TREN (48 mg,

0.21 mmol) were added, and the mixture was stirred for 10 min. The


66

reaction was started by adding MClAc (15 mg, 0.14 mmol) with a syringe. All

the operations were performed under nitrogen. The polymer was isolated by

precipitation in a ten-fold amount of methanol and subsequently dried in an

oven at 65 °C. Aliquots of the reaction mixture were removed at different

time intervals using a degassed syringe and frozen immediately in liquid

nitrogen. AM conversion was determined using a GC and the molecular

weight and distribution were determined by GPC (after precipitation in

methanol).

Synthesis of the macro-initiator. The chemical modification of the

original PK was performed according to the published method25 (Scheme

3.1). The reactions were performed in a sealed 250 ml round bottom glass

reactor with a reflux condenser, a U-type anchor impeller using an oil bath

for heating.

Scheme 3.1: Synthesis of the macro-initiators

The chloropropylamine hydrochloride (9.89 g) was dissolved in methanol (90

ml) to which an equimolar amount of sodium hydroxide (2.16 g) was added.

After the polyketone (10 g) was preheated to the liquid state at the

employed reaction temperature (100 °C), the amine solution was added drop

wise (with a drop funnel) into the reactor in the first 20 min. The stirring

speed was set at a constant value of 500 RPM. During the reaction, the

mixture of the reactants changed from a slightly yellowish, low viscosity

state, into a highly viscous brown homogeneous paste. The product was

dissolved in chloroform and afterwards washed with demineralized water in a

separation funnel. The polymer was isolated by evaporating the chloroform at

low pressure (100 mbars). The product, a brown powder, was finally freeze

dried and stored at -18 °C until further use. The macro-initiator was

characterized using elemental analysis, 1H-NMR spectroscopy (in chloroform),

Chapter 3

67

and Gel Permeation Chromatography (GPC). The conversion of carbonyl

groups of the polyketone was determined using the following formula:

(3.1)

, being the average number of carbons in n-m (see Scheme

3.1)

, being the average number of carbons in m (see Scheme 3.1)

molecular weight of nitrogen

molecular weight of carbon

The average number of pyrrole units was determined using the conversion of

the carbonyl groups of the polyketone and formula 3.2:

(3.2)

= the average molecular weight of the parent (unmodified)

polyketone

= the average molecular weight of the polyketone

repeating unit

Comb polymerization. A 250-ml three-necked flask was charged with the

macro-initiator (e.g. entry 11: 0.3293 g, 0.117 mmol). Sufficient acetone

(typically 5-10 ml) was added to dissolve the macro-initiator. Demineralized

water (60 ml) and acrylamide (10 g, 140 mmol) were then added to the

solution. Subsequently, the mixture was degassed by three freeze-pump-

thaw cycles. A nitrogen atmosphere was maintained throughout the

remainder of the reaction steps. CuBr (27 mg) was then added to the flask

and the mixture stirred for 10 minutes. The flask was then placed in an oil

bath at 25 °C. The reaction was started by the addition of the ligand

(Me6TREN, 34 mg) using a syringe. After the pre-set reaction time, the

mixture was exposed to air and the polymer was precipitated in a tenfold

amount of methanol. For the higher molecular weight polymers the solution

was first diluted with demineralized water before being precipitated. The

polymer was isolated by filtration and subsequently dried in an oven at 65

°C.


68

To investigate whether all the initiation sites on polyketone are reactive (for

acrylamide) a lower monomer to initiator ratio was chosen. The

polymerization using PK30-Cl12 as the macro-initiator was analogous to the

comb polymerization described earlier. The chosen monomer to macro-

initiator ratio was relatively low (150:1) so that even at a high conversion

only a few acrylamide units are inserted. A sample was taken after 30

minutes and a 1H-NMR spectrum was recorded. ChemBioDraw Ultra 12.0

(CambridgeSoft) was used to simulate the 1H-NMR spectrum of the macro-

initiator with only few acrylamide units attached, and interpretation was

performed according to literature.26

Block copolymerization. The macroinitiator was prepared according to

the aforementioned procedure. A round bottomed three necked flask was

charged with the macroinitiator (3.6 g, 0.006 mmol) and NIPAM (36 g, 318

mmol). Double distilled water was added, and the mixture was degassed by

three freeze-pump-thaw cycles. Afterwards CuBr (4 mg, 0.028 mmol) was

added and the solution was stirred for 10 min. The flask was placed in a

water bath at 25 °C and the reaction was started by adding Me6TREN (6.5

mg, 0.028 mmol). All the operations were performed under nitrogen. At set

time intervals aliquots were taken and analyzed by 1H-NMR.

Star polymerization. A 250-ml three-necked flask was charged with

AM (e.g. entry 8, Table 3.2: 5.0 g) and the initiator (pentaerythritol

tetrakis(2-bromoisobutyrate), 26 mg). A magnetic stirrer and distilled water

(30 ml) were added and subsequently degassed by three freeze-pump-thaw

cycles. The flask was then placed in an oil bath at 25 °C, CuCl (31 mg) was

added and the mixture was stirred for 10 minutes. The reaction was started

by adding the ligand (Me6TREN, 44 mg) using a syringe. After the reaction

the mixture was exposed to air and the polymer was precipitated in a tenfold

amount of methanol. The polymer was dried in an oven at 65 °C up to

constant weight.

Characterization. The acrylamide conversion was measured by using

Gas Chromatography (GC). The samples (taken from the reaction mixtures)

were dissolved in acetone (polymer precipitates) and injected on a Hewlett

Packard 5890 GC with an Elite-Wax ETR column. The total molecular weight

(Mn,tot) is calculated by using the acrylamide conversion (monomer-initiator

ratio multiplied by the conversion). The span molecular weight (Mn,SPAN) is

calculated using the Mn,tot and is defined as two times the molecular weight of

one arm (star PAM) or two times the molecular weight of one arm plus the

molecular weight of the macro-initiator (comb PAM).

Gas Chromatography-Mass Spectrometry (GC-MS) was used to

investigate the presence of initiator after the ATRP of AM (using 3-chloro-1-

Chapter 3

69

propanol as the initiator). A sample of the reaction mixture was taken and

precipitated in acetone. An acetone sample, containing 1000 ppm of 3-

chloro-1-propanol, was used as the blank. GC-MS measurements were

performed on a Hewlett Packard (HP) 6890 Series GC system coupled to a HP

6890 Series Mass Selective Detector. The GC was operated splitless and in

order to blow off the solvent a flow of 80 mL/min of Helium was applied 1

minute after injection, the injector temperature was 250 °C, and an injection

volume of 1 l was used. The temperature program for the oven was as

follows: 40 °C for 5 min followed by heating with 10 °C/min to 280 °C.

Helium was used as the carrier gas with a constant flow rate of 0.8 ml/min.

Nuclear Magnetic Resonance (NMR) spectra were recorded on a Varian

Mercury Plus 400 MHz spectrometer. For analysis D2O was used as the

solvent.

GPC analysis of all the water-soluble samples was performed on a Agilent

1200 system with Polymer Standard Service (PSS) columns (guard, 104 and

103 Å) with a 50 mM NaNO3 aqueous solution as the eluent. The columns

were operated at 40 °C with a flow-rate of 1 ml/min, and a refractive index

(RI) detector (Agilent 1200) was used at 40 °C. The apparent molecular

weights and dispersities were determined using a PAM based calibration with

WinGPC software (PSS). The macroinitiators were analyzed by GPC using THF

(used as received) as the eluent with toluene as a flow marker. The analysis

was performed on a Hewlett Packard 1100 system equipped with three PL-gel

3 m MIXED-E columns in series. The columns were operated at 42 °C with a

flow-rate of 1 ml/min, and a GBC LC 1240 RI detector was used at 35 °C.

The apparent molecular weights and dispersities were determined using

polystyrene standards and WinGPC software (PSS).

The particle sizes of the different polymers were measured using a

Brookhaven ZetaPALS zeta potential and particle size analyzer. Dilute

(polymer concentration < 0.1 wt. %) aqueous solutions were prepared and

filtered prior to the measurement. The laser angle for the measurements was

set at 90 ° and a total of 10 runs were performed for each sample (the

reported value is the average).

Elemental analysis of the macroinitiators was performed on the

EuroEA3000-CHNOS analyzer (EUROVECTOR Instruments & Software).

Approximately 2 mg of each sample is weighed and placed in tin sample-

cups. The reported values are the average of 2 runs.

Rheological characterization. The aqueous polymeric solutions were

prepared by swelling the polymers in water for one day and afterwards gently

stirring the solution for another day.


70

Viscometric measurements were performed on a HAAKE Mars III

(ThermoScientific) rheometer, equipped with a cone-and-plate geometry

(diameter 60 mm, angle 2°). Flow curves were measured by increasing the

shear stress by regular steps and waiting for equilibrium at each step. The

shear rate ( ) was varied between 0.1 – 1750 s-1. Dynamic measurements

were performed with frequencies ranging between 0.04 – 100 rad/s (i.e.,

6.37·10-3 – 15.92 Hz). It must be noted that all the dynamic measurements

were preceded by an oscillation stress sweep to identify the linear

viscoelastic response of each sample. With this, it was ensured that the

dynamic measurements were conducted in the linear response region of the

samples.

Fluorescence spectroscopy. Fluorescence spectra of the aqueous

polymer solutions were recorded on a Fluorolog 3-22 spectrofluorimeter. The

excitation wavelength was set at 350 nm and the spectra were recorded

between 365 and 600 nm. The slit width of the excitation was 3 nm while

that of the emission was maintained at 2 nm. All the measurements were

performed in demineralized water at 10 °C.

3.3. Results and discussion

Macroinitiator. The synthesis of the macroinitiator was performed

according to the Paal-Knorr reaction of a halogenated primary amine with

aliphatic perfectly alternating polyketones (Scheme 3.1). The conversion of

the reaction was determined using elemental analysis (Table 3.1). Resonance

peaks corresponding to the pyrrole units were observed with 1H-NMR at

=5.68 ppm and validated by using model compounds.25 The average

number of pyrrole units equals the number of side chains which is obtained

after the polymerization of acrylamide by ATRP.

Table 3.1: Properties of the macroinitiator and parent polyketone

Sample

(PK00-xa)

Elemental composition

(C : H : N, wt%) XCO (%)b

Pyrrole

unitsc Mn,GPC (g/mol) PDI

PK30 67.0 : 8.4 : 0 - 0 2 797 1.74

PK30-Cl12, R1 = Cl 64.2 : 7.8 : 4.6 55.10 12 2 093 1.96

a. Number indicates the ethylene content (%) and Cl indicates the halogen present

b. The conversion of the carbonyl groups of the polyketone

c. Average number of pyrrole units per chain

The macroinitiator was analyzed by 1H-NMR (Figure 3.1). As can be

observed, the resonances corresponding to the pyrrole units (a) and the

Chapter 3

71

aliphatic protons of the amine moiety (b-d) appear in the spectrum of the

chemically modified polyketone.

7 6 5 4 3 2 1

d

cb

aa

PK30-virgin

ppm

PK30-Cl12

ab

c

d

Figure 3.1: 1H-NMR spectra of the macroinitiator and the virgin polyketone

The obtained, chemically modified polyketone can be used as macroinitiator

in the ATRP of acrylamide for the preparation of comb-shaped polymers.

ATRP of AM using a primary halogen. The macroinitiator contains

primary halogens. This has mainly to do with better commercial availability of

the corresponding reagent (amino compound in Scheme 3.1) with respect to

ones containing a secondary or tertiary halogen. Despite the reported worse

performance in ATRP for primary halogens with respect to secondary or

tertiary ones27, this choice is driven by the possible future application at

industrial level. However, before proceeding to the ATRP of AM using the

macroinitiator, it is of paramount importance to confirm that primary

halogens can also lead to the ATRP of AM. This is particularly true when

making allowance for the reported lack in initiation efficiency27, which would

lead to the preparation of poorly defined structures. We started by

investigating the controlled nature of the polymerization. Similar to the ATRP

of AM using MeClPr as the initiator17, the reaction kinetics for the

disappearance of AM, using either chloro acetate or the macroinitiator, show

a non-linear relationship (Figure 3.2). It fits the model presented by Goto

and Fukuda28 quite well, thus, indicating that the non-linearity of the plot


72

stems from the progressive deactivation of the catalyst by complexation with

the growing polyacrylamide. The conversion index (ln[ / ]) is represented

by equation 3.3.

(3.3)

where is the equilibrium constant in ATRP, is the propagation rate

constant, is the termination rate constant, is the monomer

concentration at time zero, is the monomer concentration at any time, and

is the initial initiator concentration.

0 10 20 30 40 50 60

0,0

0,4

0,8

1,2

1,6

2,0

Entry 1 (Table 2), R2 (model) = 0.99

Entry 14 (Table 2), R2 (model) = 0.82

ln (

M0/M

)

Time (min)

0 2 4 6 8 10 12 14 16

0,0

0,4

0,8

1,2

1,6

2,0

ln (

M0/M

)

Time2/3

(min2/3

)

Figure 3.2: Kinetic plot for the ATRP of AM (entry 1 & 14, Table 3.2), on a linear (A)

time scale, and (B) on a scale of time2/3

Throughout the reaction for the linear PAM, the molecular weight increases

linearly with conversion and the dispersity remains relatively low (PDI < 1.5).

The molecular weight values are close to the theoretical ones (Figure 3.3,

Entry 1). Although the initiation of primary halogen suffers from low

activity27, the combination of a highly active ligand27 (Me6TREN) with water

(known to accelerate ATRP reactions17) provides control over the

polymerization of AM. For the branched PAM, the molecular weights differ

from the theoretical values, possibly as a result of the architectural difference

between the standards used for the GPC (all linear polymers) and the

synthesized PAM. Indeed, as the branches increase in size the differences (in

hydrodynamic volume) with a linear polymer increase.29 Nevertheless, the

increase in apparent molecular weight with conversion and the decrease in

Chapter 3

73

the PDI (and later on the block copolymerization with NIPAM) provide strong

evidence for the controlled nature of the polymerization.

0 2 4 20 30 40 50 60 70 80 90 100

0,05,0x10

1

1,0x104

1,5x104

2,0x104

2,5x104

3,0x104

Mn,GPC

Mn,theoretical

Mo

lec

ula

r w

eig

ht

(g/m

ol)

0 2 4 20 30 40 50 60 70 80 90 100

1,0

1,2

1,4

1,6

1,8

2,0

2,2

2,4

2,6

2,8

3,0

PDI

Po

lyd

isp

ers

ity

in

de

x (

PD

I)

Conversion (%)

Entry 1, Table 2

0 5 10 15 20 25 30 35 40 45 96 98 100

0,0

5,0x104

1,0x105

1,5x105

2,0x105

2,5x105

3,0x105

Mn,GPC

Mn,theoretical

Entry 14, Table 2

Mo

lec

ula

r w

eig

ht

(g/m

ol)

Conversion (%)

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0

PDI

Po

lyd

isp

ers

ity

in

de

x (

PD

I)

Figure 3.3: Dependence of the Mn and PDI on the conversion of AM, entry 1 & 14

(Table 3.2); dotted lines serve as guides

It is crucial, for determining the architectural purity of the comb-shaped

polymers, to establish the initiation efficiency of the system. This has been

performed via 1H-NMR for the branched polymer (see below), but also

through the use of a model compound, 3-chloro-1-propanol (entry 2, Table

3.2). It was confirmed with GC-MS (of the reaction mixture) that no initiator

(below the detection level of the GC-MS) was present after the ATRP with

AM. This is strong evidence for high initiation efficiency.


74

Comb polymerizations. Comb PAM has been prepared according to Scheme

3.2.

Scheme 3.2: Synthesis of the comb PAMs

The presence of many halogen atoms on a relatively short polymeric chain

(Mn of the macro-initiator is 2797 g/mol) might lead to steric hindrance in the

addition of the first AM units to the C-Cl bonds. To determine whether the

PAM chains grow on each halogen of the macroinitiator (PK30-Cl12) a 1H-NMR

spectrum was recorded after the reaction (Figure 3.4).

4 3 2 1

PK30-Cl12

-graft-PAMPK30-Cl12

A

BB

PK30-Cl12

-graft-PAM

ppm

PK30-Cl12

A

Figure 3.4: 1H-NMR spectra of the PK30-Cl12 (macro-initiator) and the PAM grafted

product (PK30-Cl12-graft-PAM)

Chapter 3

75

Given the low monomer/macro-initiator ratio (150:1), in theory, only a few

acrylamide units should be present on the polyketone backbone. The

spectrum of the corresponding polymeric material (PK30-Cl12-graft-PAM) is

compared with the one of the corresponding macro-initator (PK30-Cl12),

taken here as reference. The resonance at 3.5 ppm corresponds with the two

-hydrogens next to the chlorine functionality in the PK30-Cl12 macro-

initiator. In the spectrum of the product this resonance disappears (at least

within the experimental error of 1H-NMR), thus confirming the reaction on the

halogen. The appearance of the resonance at 4.3 ppm in the product

spectrum, corresponding with the –hydrogen of the chlorine functionality

attached at the acrylamide chain end, further confirms the AM polymerization

at the halogen initiation point. This in combination with the model compound

(entry 2, Table 3.2) confirms that the average number of arms is equal to the

average number of halogens per chain.

Table 3.2: Characteristics of the (co)polymers

Architecture Entry [M]0:[I]0:[CuCl]0:

[Me6TREN]0a

M/s1/s2b

(w:v:v);

T; Time (min)

Conv.

(%) Mn,tot Mn,GPC PDIc Mn,SPAN

Linearf

1d 479:1:1.5:1.5 1:6; 25 °C; 60 76.6 28 623 21 100 1.47 28 623

2e 9511:1:1.5:1.5 1:3; 25 °C; 30 19.1 129 124 84 692 1.72 129 124

3 966:1:1.5:1.5 1:6; 25 °C; 60 75.3 51 703 38 310 1.57 51 703

4 1 625:1:1.5:1.5 1:6; 25 °C;120 84.7 97 833 69 100 2.18 97 833

5 4 354:1:1.5:1.5 1:6; 25 °C; 60 69.1 213 852 108 800 2.30 213 852

6 8 790:1:1.5:1.5 1:6; 25 °C; 25 59.5 371 752 131 660 3.23 371 752

7 14 399:1:1.5:1.5 1:6; 25 °C; 15 50.8 519 928 210 200 2.25 519 928

Star

8 1 965:1:6.0:6.0 1:6; 25 °C;180 77.5 108 246 79 680 2.06 54 123

9 2 884:1:6.0:6.0 1:6; 25 °C;180 76.4 156 670 107 800 1.92 78 335

10 5 811:1:6.0:6.0 1:6; 25 °C;120 62.6 258 567 216 500 2.01 129 284

Combg

11 1 197:1:1.5:1.5 1:6:1/3;25 °C; 60 77.7 66 109 72 020 2.86 13 815

12 2 395:1:1.5:1.5 1:6:3.0;25 °C; 60 74.8 127 337 104 900 2.31 24 020

13 6 006:1:1.5:1.5 1:8:1.5;25 °C; 60 72.5 309 507 206 400 2.33 54 382

14 9 003:1:1.5:1.5 1:6:1.0;25 °C; 60 47.6 304 608 188 800 1.88 53 565

15 12 025:1:1.5:1.5 1:6:1/3;25 °C; 60 68.8 587 766 271 600 1.97 100 758

a. Molar ratio

b. M/s1/s2 = Monomer / solvent 1 / solvent 2 = Acrylamide / water / acetone

c. The PAM polymers are prepared solely in water (except the comb were some acetone is used as

a cosolvent for the macroinitiator)

d. Initiator = chloro acetate

e. Initiator = 3-chloro-1-propanol

f. Initiator = methyl 2-chloropropionate

g. Comb PAMs with varying arm molecular weight and relatively low dispersities can be readily

prepared by changing the monomer-initiator ratio. The dispersities of the comb PAMs decrease as

the Mn,tot increases.


76

The 1H-NMR spectrum of the PK30-g-PAM shows that the halogen atoms are

reactive towards AM insertion. This enables the preparation of comb-like

polymers with a controlled number of branches as well as branch length. This

has been achieved by systematically changing the monomer/initiator ratio

(Table 3.2). The characteristics of the corresponding linear and star-shaped

PAM (for comparison of the rheological properties in aqueous solutions) are

also provided in Table 3.2.

Comb copolymerization, synthesis of PK30-g-(PAM-b-PNIPAM).

To further demonstrate the control of the polymerization (i.e. no loss of the

halogen end group), block copolymers of PK30-g-(PAM-b-PNIPAM) were

prepared. The 1H-NMR spectra of samples of the reaction mixture at different

times are displayed in Figure 3.5. As can be observed in Figure 3.5, the

resonance (2) of the methyl groups of NIPAM increase in relation to the

resonances (1) corresponding to the backbone of the copolymer.

5 4 3 2 1

2

macroinitiator

1440 min

480 min

360 min

240 min

ppm

120 min

MeOH

1

Figure 3.5: 1H-NMR spectra of the block copolymer at different reaction times

Chapter 3

77

The NIPAM blocks increase in size as the reaction proceeds. This is strong

evidence for the controlled character of the reaction.

Rheological properties. Early studies4, 30 on solution properties of long

chain branched PAM demonstrated that the hydrodynamic volume of a

branched PAM is lower than for its linear analogue (of same molecular

weight). A lower hydrodynamic volume is synonymous to a lower solution

viscosity in dilute solutions. The influence of the molecular architecture on

the rheological behavior of polymers has already been investigated for

different polymers, mostly in the melt. 31-38 It was demonstrated that for

polyisoprenes31, 39, polypropylene36, 40-42, polyethylene37, 43-47 and

polystyrene35, 48, 49 an enhancement of the zero shear rate viscosity (0) can

be achieved by changing the architecture (linear compared to star, long chain

branched, comb, and H-shaped) of the polymers. In particular, several

experiments 31 display an exponential increase in the 0 with an increase in

the arm molecular weight (Mw,arm). At relatively low total molecular weights

(Mw < 10000 g/mol for HDPE 50, Mw < 100000 g/mol for polybutadienes32, Mw

< 600000 g/mol for polystyrene49) the η0 of the branched (comb, long chain

branched, and H-shaped) polymers is lower compared to their linear

analogue. However, as the molecular weight increases (above the

aforementioned values) the η0 of the branched polymers rapidly surpasses

(given its exponential dependence on the Mw,arm) the value of the linear ones.

Solution viscosity. The molecular weight determination with GPC is

based on the hydrodynamic volume. The comparison between linear, star

and comb-shaped PAM at similar Mn,tot (entries 4, 8 and 12 in Table 3.2)

using the GPC data show that the hydrodynamic radius of the comb PAM is

larger. This suggests a more extended nature of the arms of the comb PAM in

water solution. The PAM side chains originate from a small backbone (Mn =

2093 g/mol) and therefore steric hindrance might lead to extended PAM side

arms in comparison to linear PAM. Similar results have been reported for

poly(acrylic acid) grafts on a polydextran backbone.51 When the solution

viscosity is plotted against the polymer concentration (Figure 3.6) a markedly

different behavior can be observed for the branched/comb polymers

compared to their linear analogues.

In Figure 3.6 three different PAM are compared, a linear, a (4-arm) star

and a comb-like (12-arm). The solution viscosity at = 10 s-1 is similar for all

the polymers at low concentration. As the concentration of the polymeric

solution increases the observed behavior depends on the architecture of the

polymer. The star polymer displays lower solution viscosity compared to their

linear analogue. This can be attributed to the lower hydrodynamic volume of

star polymers.29


78

0 2 4 6 8 10 12 14 16

0

10

20

30A

Vis

co

sit

y (

Pa

.s)

Concentration (wt%)

comb, entry 12

linear, entry 5

linear, entry 4

star, entry 8

0 2 4 6 8 10 12 14 16

0

20

40

60

80B

comb, entry 13

linear, entry 6

star, entry 10

linear, entry 5

Vis

co

sit

y (

Pa

.s)

Concentration (wt%)

Figure 3.6: Variation in the solution viscosity (measured at = 10 s-1) as a function of

the polymer concentration and molecular weight. A: linear (2), star and a comb PAM at

a Mn,tot ~ 105000 g/mol and B: linear (2), star and a comb PAM at a Mn,tot ~ 230000

g/mol

The higher solution viscosity of the 12-arm comb-like PAM (Figure 3.6 A and

B) can be attributed to its higher Mn,tot (approximately 25% higher [3.6A] or

10% [3.6B]). However, the differences in solution viscosity are too high to be

attributed solely to the higher Mn,tot. To verify this hypothesis two linear PAMs

(entries 5 & 6) with a higher Mn,tot compared to that of the comb PAMs are

also displayed in Figure 3.6 A and B and as can be seen the solution

viscosities of both linear PAMs are lower than that of the comb. Nevertheless,

one would expect the linear polymer to display the highest solution viscosity

given the more compact structures of the star/branched polymers in

Chapter 3

79

solution.29 However, as can be observed, the comb-like PAM displays a

solution viscosity higher than both the linear analogues of similar (and

higher) molecular weight. In the semi-dilute regime entanglements are

present, and therefore melt like rheological properties can be the explanation

for the observed behavior.

The comparison between the polymers at similar Mn,tot is justified for

industrial applications. However, the three architecturally different polymers

can also be compared using a different approach, where the span molecular

weights (Mn,SPAN) of the star/branched polymers are similar to the molecular

weight of the linear one (Figure 3.7).31

0 2 4 6 8 10 12 14 16

0

20

40

60

80

A

A-Zoom

linear, entry 3

star, entry 8

comb, entry 13

Vis

co

sit

y (

Pa

.s)

Concentration (wt. %)

0,0 0,5 1,0 1,5 2,0 2,5 3,0

0,00

0,01

0,02

0,03

0,04

0,05

0,06

linear, entry 3

star, entry 8

comb, entry 13

A-Zoom

Vis

co

sit

y (

Pa

.s)


0 2 4 6 8 10 12 14 16

0

10

20

30

40

50

60

linear, entry 4

star, entry 10

comb, entry 15

B

Vis

co

sit

y (

Pa

.s)

Concentration (wt. %)B-Zoom

0,0 0,5 1,0 1,5 2,0 2,5 3,0

0,00

0,10

0,20

0,30

0,40

linear, entry 4

star, entry 10

comb, entry 15

Vis

co

sit

y (

Pa

.s)


B-Zoom

Figure 3.7: Viscosity (measured at = 10 s-1) as a function of the polymer

concentration and molecular weight. A; linear, star and a comb PAM with a similar

MN,SPAN (MN,SPAN ~ 52000 g/mol) and A-Zoom; zoom in of the dilute region. B; linear,

star and a comb PAM with a similar MN,SPAN (MN,SPAN ~ 105000 g/mol) and B-Zoom;

zoom in of the dilute region


80

As can be observed in Figure 3.7, the increase in solution viscosity with

concentration is dependent on the span molecular weight of the samples and

the molecular architecture. At the lowest molecular weight studied (Figure

3.7A) the solution viscosity of the star polymers increases in a similar fashion

(although slightly more pronounced) as the linear one whereas the comb-like

displays a more pronounced increase towards higher concentrations. At a

higher span molecular weight (Figure 3.7B) both the star and comb-like

polyacrylamides display a more pronounced increase in solution viscosity with

concentration than to the linear one (with similar Mn,SPAN), with the comb-like

one showing the highest viscosity. This is in line with the theory that

stipulates that the η0 increases exponentially with increase in the Mw,arm for

star/branched polymers31 (compared to a power law for linear polymers52).

The longer the branches are, the more pronounced the differences between

the linear and branched polymers should be. These predictions are based on

experiments performed in the melt (i.e. fully entangled chains).

Nevertheless, the general parameters that affect the viscosity can also be

applied to polymers in solutions where entanglements are present.53, 54

As can be observed in Figure 3.7, the solution viscosities of the comb

and star-shaped PAMs at low polymer concentration are close to each other.

As the polymer concentration increases the solution viscosity of the comb

and star PAMs increase more rapidly than the linear PAMs.

Clear differences in the solution viscosity can be observed when

comparing the architecturally different polymers at high concentration, i.e.

above the overlap concentration. However, as can be observed in Figure 3.6

and 3.7, at low polymer concentration the differences are rather small and

therefore difficult to detect. In order to gain deeper insight, dilute polymer

solutions are compared, and experiments aimed at demonstrating

hydrophobic associations are performed.

In the dilute region of a polymeric solution, where no entanglements are

present, the viscosity can be described using the “free draining” chain model.

The solution viscosity is determined by the solvent viscosity and the excess

viscosity caused by the energy consumption of a tumbling polymer coil under

flow. According to Stokes and Evans55 the excess viscosity of a solution

(containing Nav·C / Mn macromolecules) is:

(3.4)

where is the solvent viscosity, is the zero shear rate viscosity, is the

degree of polymerization, is the friction factor per segment, is the

hydrodynamic radius as determined by light scattering measurements, is

Chapter 3

81

the Avogrado constant, is the polymer concentration and is the

molecular weight of the polymer. The viscosities at vanishing shear rate ( )

are determined from the low-frequency loss moduli.53

Equation 3.4 relates the excess viscosity ( ) to the friction factor ( )

per segment. The latter can be easily evaluated (Figure 3.8A) by determining

the slope of the plot of vs. .

1E24 1E25 1E26

1

10

100

1000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Slo

pe

linear, entry 1

star, entry 6

comb, entry 11

B

A

0 (

mP

a.s

)

NpR

2

g[(N

avC)/M

w] (nm

-1)

Figure 3.8: Plot of vs. (A) and the corresponding friction

factor per segment (B) for a linear, star and comb PAM

The corresponding values (Figure 3.8B) are clearly not a function of the

molecular architecture since all differences are well within the experimental

error. This is quite important since it strongly suggests that the differences in

the solution viscosities (both at low and higher concentration) cannot be

attributed to differences in the segmental friction factor. The behavior

observed for the star PAM can be then attributed to the increase in

entanglement density as a result of the architecture. The comb PAM however

possesses a hydrophobic backbone and can therefore display hydrophobic

aggregations. Therefore, it is important to investigate whether or not

hydrophobic associations arise in solution. The comb-like PAM possesses

pyrrole units in the backbone making it possible to probe the solution

structure with fluorescence spectroscopy. The critical aggregation

concentration (CAC) can be determined from the corresponding spectra (data


82

not shown for brevity). The CAC values are 3 wt.% and 2 wt.% for entry 13

and 15 respectively. In Figure 3.7 (A-Zoom & B-Zoom) the upward trend of

the solution viscosity of entries 13 and 15 starts at lower concentrations than

their respective CAC. We can therefore conclude that the higher viscosity of

the comb polymers below the CAC is due to the molecular architecture

(longer relaxation time and thus a higher solution viscosity, similar to the

melt31 compared to a linear polymer) and above the CAC a combination of

the molecular architecture and hydrophobic associations.

Viscoelastic behavior. The elastic response of an aqueous polymeric

solution is dependent on the molecular weight56, the concentration56 and the

architecture/chemical composition (presence of hydrophobic groups) of the

polymer.56, 57 In Figure 3.9 two different comparisons are presented.

0,1 1 10 10010

-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

A

G' G

" (

Pa

)

Frequency (rad/s)

} comb, entry 13

star, entry 8

linear, entry 3

}

}

= G"

= G'

0,1 1 10 1000

10

20

30

40

50

60

70

80

90

B

comb, entry 13

star, entry 8

linear, entry 3

Ph

as

e a

ng

le

Frequency (rad/s)

0,1 1 10 10010

-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

C

G' G

" (

Pa

)

Frequency (rad/s)

} comb, entry 12

star, entry 8

linear, entry 4

}

}

= G"

= G'

0,1 1 10 1000

10

20

30

40

50

60

70

80

90

comb, entry 12

star, entry 8

linear, entry 4

D

Ph

as

e a

ng

le

Frequency (rad/s)

Figure 3.9: G’ & G” (A) and phase angle (B) as a function of the frequency for a 4-

arm star, 12-arm comb-like and linear at similar Mn,SPAN and a polymer concentration of

10.71 wt.% and G’ & G” (C) and phase angle (D) as a function of the frequency for a

4-arm star, 12-arm comb-like and linear at similar Mn,tot and a polymer concentration

of 10.71 wt.%

Chapter 3

83

The comparison between a linear, star and comb PAM of similar MN,SPAN

demonstrates that the comb PAM exhibits a more pronounced elastic

behavior, especially at low frequency (Figure 3.9B). When comparing a

linear, 4-arm star and comb at similar Mn,tot only a small difference is

observed at low frequency, i.e. a slightly more elastic behavior for the 4-arm

star and comb compared to the linear PAM (Figure 3.9D). However, at

relatively higher frequencies (> 1 rad/s) the differences become more

significant with the star PAM showing the highest elastic behavior (elastic

response 4-arm star > 12-arm comb > linear). The arms of the 12-arm comb

are shorter compared to the arms of the 4-arm star. At higher frequencies

(higher deformations) the disentanglement of the arms will occur more easily

for the comb given its shorter arms. It is also evident (Figure 3.9C) that the

transition from viscous to elastic behavior occurs at lower angular frequency

for the 4 arm star. Similar results were reported for polyethylene in the

melt.47

The model developed for the viscoelasticity of monodisperse comb

polymer melts50 predicts that the highest 0 (in the melt) for comb polymers

is obtained with combs having long arms but few branches (≤ 12). In

addition, an exponential dependence of the 0 on the molecular weight of the

arms is obeyed. The comparison between a regular 3-arm star and combs

polymers (at least the ones included in the comparison in the paper) show

that the 3-arm star possesses the highest 0. However, the model also

predicts that for a specific range of molecular weights (20000 < MW < 80000

g/mol) a comb polymer possessing 6 arms has a higher 0 compared to a 3-

arm star.50 For polyisoprene the 0 of a 3-arm star is lower than that of a 4-

arm star.31 Our data suggest that comb polymers in aqueous solution can

have a higher solution viscosity than a 4-arm star.

3.4. Conclusion

The controlled synthesis of linear, star and comb-shaped PAM by ATRP in

water has been achieved. All the initiation sites on the macroinitiator seem to

react during the ATRP, as strongly evidenced by 1H-NMR and the use of

model compounds. GPC analysis demonstrates that the comb polymers

display a higher hydrodynamic volume in dilute water solution compared to

their linear and star analogues, preliminarily explained by the more extended

nature of the arms in the comb polymers. Rheological measurements in

(semi)dilute water solution demonstrated that the solution viscosity of comb-

like PAM is higher (whilst maintaining the concentration constant) than its

linear and star-shaped analogues both at equal Mn,SPAN and Mn,tot. In addition


84

the elastic response of water solution containing the comb-like PAM is more

pronounced than for the linear and star-shaped PAM (both at equal Mn,SPAN

and Mn,tot). The controlled synthesis of PAM with different architectures allows

the manipulation of the rheological properties of aqueous solution thereof. By

simply changing the architecture of the polymer, a significantly different

behavior, i.e. higher solution viscosity and more pronounced elastic response

at equal Mn,SPAN and Mn,tot, is obtained. The obtained results pave the way for

application of these polymeric materials in EOR.

3.5. Acknowledgement

This work is part of the Research Programme of the Dutch Polymer

Institute DPI, Eindhoven, the Netherlands, projectnr. #716.

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