university of groningen synthesis and evaluation of novel ...reversible addition-fragmentation chain...
TRANSCRIPT
University of Groningen
Synthesis and evaluation of novel linear and branched polyacrylamides for enhanced oilrecoveryWever, Diego-Armando Zacarias
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.
Document VersionPublisher's PDF, also known as Version of record
Publication date:2013
Link to publication in University of Groningen/UMCG research database
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.
CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.
Download date: 19-05-2020
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
Synthesis of branched polyacrylamide
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.
Synthesis of branched polyacrylamide
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.
Synthesis of branched polyacrylamide
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
Synthesis of branched polyacrylamide
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.
Synthesis of branched polyacrylamide
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.
Synthesis of branched polyacrylamide
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
Synthesis of branched polyacrylamide
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)
Concentration (wt. %)
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)
Concentration (wt. %)
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
Synthesis of branched polyacrylamide
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
Synthesis of branched polyacrylamide
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
Synthesis of branched polyacrylamide
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.
3.6. References
1. Wever, D. A. Z.; Picchioni, F.; Broekhuis, A. A. Prog. Polym. Sci. 2011, 1558. 2. Shalaby W. Shalaby; Charles L. McCormick; George B. Butler Water-Soluble
Polymers: Synthesis, Solution Properties, and Applications; American Chemical Society: Washington DC, 1991; .
3. Huang, S.; Lipp, D. W.; Farinato, R. S. Encyclopedia of Polymer Science and Technology; John Wiley & Sons, Inc.: 2002; .
4. Kulicke, W. M.; Horl, H. H. Colloid Polym. Sci. 1980, 7, 817. 5. Gleason, E. H.; Miller, M. L.; Sheats, G. F. Journal of Polymer Science 1959, 133,
133.
6. Fanood, M. H. R.; George, M. H. Polymer 1988, 1, 128. 7. Fanood, M. H. R.; George, M. H. Polymer 1988, 1, 134. 8. Fanood, M. H. R. Iranian Polymer Journal 1998, 1, 59. 9. Guha, S. Journal of the Indian Chemical Society 2008, 1, 64. 10. Jewrajka, S. K.; Mandal, B. M. Journal of Polymer Science Part A-Polymer
Chemistry 2004, 10, 2483. 11. Jewrajka, S. K.; Mandal, B. M. Macromolecules 2003, 2, 311. 12. Tan, Y.; Yang, Q.; Sheng, D.; Su, X.; Xu, K.; Song, C.; Wang, P. E-Polymers
2008, 25. 13. Ito, M.; Ishizone, T. Journal of Polymer Science Part A-Polymer Chemistry 2006,
16, 4832. 14. Wang, W.; Wang, D.; Li, B.; Zhu, S. Macromolecules 2010, 9, 4062. 15. Boyer, C.; Bulmus, V.; Davis, T. P.; Ladmiral, V.; Liu, J.; Perrier, S. Chem. Rev.
2009, 11, 5402. 16. Matyjaszewski, K.; Xia, J. H. Chem. Rev. 2001, 9, 2921. 17. Wever, D. A. Z.; Raffa, P.; Picchioni, F.; Broekhuis, A. A. Macromolecules 2012,
10, 4040. 18. Appel, E. A.; del Barrio, J.; Loh, X. J.; Dyson, J.; Scherman, O. A. Journal of
Polymer Science Part A-Polymer Chemistry 2012, 1, 181. 19. Matyjaszewski, K.; Miller, P. J.; Pyun, J.; Kickelbick, G.; Diamanti, S.
Macromolecules 1999, 20, 6526. 20. Neugebauer, D.; Zhang, Y.; Pakula, T.; Sheiko, S. S.; Matyjaszewski, K.
Macromolecules 2003, 18, 6746. 21. Qin, S. H.; Matyjaszewski, K.; Xu, H.; Sheiko, S. S. Macromolecules 2003, 3, 605.
Chapter 3
85
22. Borner, H. G.; Beers, K.; Matyjaszewski, K.; Sheiko, S. S.; Moller, M. Macromolecules 2001, 13, 4375.
23. Drent, E.; Keijsper, J. J. US Pat. 5225523, 1993. 24. Mul, W. P.; Dirkzwager, H.; Broekhuis, A. A.; Heeres, H. J.; van der Linden, A. J.;
Orpen, A. G. Inorg. Chim. Acta 2002, 147. 25. Zhang, Y.; Broekhuis, A. A.; Stuart, M. C. A.; Picchioni, F. J Appl Polym Sci 2008,
1, 262. 26. Silverstein, R. M.; Webster, F. X.; Kiemle, D. J. Spectrometric Identification of
Organic Compounds; John Wiley & Sons Inc.: 2005; , pp 512. 27. Braunecker, W. A.; Matyjaszewski, K. Progress in Polymer Science 2007, 1, 93. 28. Goto, A.; Fukuda, T. Progress in Polymer Science 2004, 4, 329. 29. Burchard, W. Branched Polymers II 1999, 113. 30. Kulicke, W. -.; Kniewske, R.; Klein, J. Progress in Polymer Science 1982, 4, 373. 31. Fetters, L. J.; Kiss, A. D.; Pearson, D. S.; Quack, G. F.; Vitus, F. J. Macromolecules
1993, 4, 647. 32. Kraus, G.; Gruver, J. T. J. Polym. Sci. Part A 1965, 1PA, 105. 33. Mykhaylyk, O. O.; Fernyhough, C. M.; Okura, M.; Fairclough, J. P. A.; Ryan, A. J.;
Graham, R. Eur. Polym. J. 2011, 4, 447. 34. Robertson, C. G.; Roland, C. M.; Paulo, C.; Puskas, J. E. J. Rheol. 2001, 3, 759. 35. Graessley, W. W.; Roovers, J. Macromolecules 1979, 5, 959. 36. Auhl, D.; Stange, J.; Munstedt, H.; Krause, B.; Voigt, D.; Lederer, A.; Lappan, U.;
Lunkwitz, K. Macromolecules 2004, 25, 9465. 37. Gabriel, C.; Munstedt, H. Rheol. Acta 2002, 3, 232. 38. Münstedt, H. Soft Matter 2011, 6, 2273. 39. Frischknecht, A. L.; Milner, S. T.; Pryke, A.; Young, R. N.; Hawkins, R.; McLeish, T.
C. B. Macromolecules 2002, 12, 4801. 40. Gotsis, A. D.; Zeevenhoven, B. L. F.; Tsenoglou, C. J. J. Rheol. 2004, 4, 895. 41. McCallum, T. J.; Kontopoulou, M.; Park, C. B.; Muliawan, E. B.; Hatzikiriakos, S. G.
Polym. Eng. Sci. 2007, 7, 1133. 42. Islam, M. T.; Juliani; Archer, L. A.; Varshney, S. K. Macromolecules 2001, 18,
6438. 43. Gabriela, C.; Munstedt, H. J. Rheol. 2003, 3, 619. 44. Wood-Adams, P. M.; Dealy, J. M. Macromolecules 2000, 20, 7481. 45. Lohse, D. J.; Milner, S. T.; Fetters, L. J.; Xenidou, M.; Hadjichristidis, N.;
Mendelson, R. A.; Garcia-Franco, C. A.; Lyon, M. K. Macromolecules 2002, 8, 3066.
46. Zamponi, M.; Pyckhout-Hintzen, W.; Wischnewski, A.; Monkenbusch, M.; Willner, L.; Kali, G.; Richter, D. Macromolecules 2010, 1, 518.
47. Gabriel, C.; Kokko, E.; Lofgren, B.; Seppala, J.; Munstedt, H. Polymer 2002, 24, 6383.
48. Roovers, J.; Graessley, W. W. Macromolecules 1981, 3, 766. 49. Roovers, J. Macromolecules 1984, 6, 1196. 50. Inkson, N. J.; Graham, R. S.; McLeish, T. C. B.; Groves, D. J.; Fernyhough, C. M.
Macromolecules 2006, 12, 4217. 51. Kutsevol, N.; Guenet, J. M.; Melnik, N.; Sarazin, D.; Rochas, C. Polymer 2006, 6,. 52. Degennes, P. G. J. Chem. Phys. 1971, 2, 572. 53. Ferry, J. D. Viscoelastic properties of polymers; John Wiley & Sons: New York,
1980; , pp 641. 54. Nielsen, L. E. Polymer rheology; Marcel Dekker Inc.: New York, 1977; , pp 207. 55. RJ, S.; DF, E. Fundamentals of Interfacial Engineering; Wiley-VCH: United States
of America, 1997; , pp 701. 56. Raju, V. R.; Menezes, E. V.; Marin, G.; Graessley, W. W.; Fetters, L. J.
Macromolecules 1981, 6, 1668. 57. Volpert, E.; Selb, J.; Candau, F. Macromolecules 1996, 5, 1452.
Synthesis of branched polyacrylamide
86
This page intentionally left blank