Characterization, solution behavior, and microstructure of a hydrophobically associating nonionic copolymer

Article abstract of DOI:10.1007/s10953-008-9309-8

To obtain an oil-displacement polymer with good thermal stability and solution properties, self-assembling acrylamide (AM)/4-butylstyrene (BST) copolymers (PSA) were synthesized by the micellar copolymerization technique. The resulting polymer was charact

Full text of DOI:10.1007/s10953-008-9309-8

J Solution Chem (2008) 37: 1227–1243
DOI 10.1007/s10953-008-9309-8
Characterization, Solution Behavior, and Microstructure
of a Hydrophobically Associating Nonionic Copolymer
Chuanrong Zhong · Ronghua Huang · Jinyong Xu
Received: 28 February 2008 / Accepted: 7 April 2008 / Published online: 12 July 2008
©
Springer Science+Business Media, LLC 2008
Abstract To obtain an oil-displacement polymer with good thermal stability and solution
properties, self-assembling acrylamide (AM)/4-butylstyrene (BST) copolymers (PSA) were
synthesized by the micellar copolymerization technique. The resulting polymer was charac-
terized by elemental analysis and UV and FT-IR spectroscopy. Conventional DSC measure-
ment was used successfully to characterize the hydrophobic microblock structure of PSA,
and two glass transition temperatures were found in the polymer: at 203 °C for the AM
segments and at 106 °C for the hydrophobic BST segments. The initial decomposition tem-
perature (234 °C) of the polymer is higher than that of polyacrylamide (210 °C). The DSC
and TG results suggest that incorporation of BST into PSA enhances the molecular rigid-
ity and thermal stability of the polymer. The apparent viscosity of a PSA solution greatly
depends on the amount of BST in the polymer, and the polymer exhibits salt-thickening,
temperature-thickening, thixotropy, pseudo-plastic behavior, anti shearing, and good anti-
aging properties at 80 °C. In addition, the apparent viscosities of PSA solutions are increased
remarkably by the addition of a small amount of surfactant. AFM measurements show that
large block-like aggregates and small compact aggregates are formed in aqueous solutions
−
1
of 0.4 g·dL PSA because of strong intermolecular hydrophobic associations, despite the
low molecular weight, and their sizes increase upon addition of a small amount of salt.
Keywords Acrylamide · Atomic force microscopy (AFM) · Differential scanning
calorimetry (DSC) · Hydrophobic association · Micellar copolymerization · Solution
properties · 4-Butylstyrene
C. Zhong (ꢀ) · J. Xu
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Energy Resources College,
Chengdu University of Technology, Chengdu 610059, Sichuan, China
e-mail: zhchrong2006@yahoo.com.cn
R. Huang
State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute, Sichuan
University, Chengdu 610065, Sichuan, China

1228
J Solution Chem (2008) 37: 1227–1243
1
Introduction
Partially hydrolyzed polyacrylamide (HPAM) is widely applied to increase the viscosity of
the aqueous phase in order to reduce the fluidity ratio of water-to-oil and to improve sweep
efficiency during the enhanced oil recovery (EOR) process [1]. The viscosification of the
traditional polymer relies on its high molecular weight, the expansion and physical entan-
glement of polymer chains due to the repulsion of carboxylate groups and hydrogen bonds
of the amido groups. However, when high shear rates are applied they cause breakage of the
polymer chains, resulting in an irreversible decrease of viscosity. In addition, an increase of
salinity causes the solution viscosity of HPAM solutions to decreases abruptly because of
shielding of ionic groups, and the amido groups in HPAM hydrolyze rapidly to form car-
◦
boxylate groups above 70 C. Therefore, HPAM is commonly used in the oil reservoirs of
low temperature and salinity.
Hydrophobically associating acrylamide-based copolymers with a small number of hy-
drophobic groups have been extensively studied in recent years [2, 3]. When dissolved
∗
in an aqueous solution above the critical concentration (C ), the polymer molecules ag-
p
gregate to form reversible supermolecular structures via intermolecular association of hy-
drophobic groups, resulting in a dramatic increase in the apparent viscosity, and degradation
upon shearing does not occur because of the low molecular weight [4]. The hydrophobic
monomers are mainly derivatives of acrylamide, acrylic esters and their derivatives [5–9],
which are easy to hydrolyze under acidic or basic conditions at high temperature, which then
results in an abrupt decrease in the viscosity of the polymer solution. For the polymers, the
viscosities at low polymer concentration are not high enough for some applications, espe-
cially in EOR that involves high temperature and a non-neutral underground oil-layer envi-
ronment [10]. In addition, acrylamide/styrene copolymers were reported previously [11–13].
However the viscosity of these polymer solutions was low, because of weak interactions of
the phenyl groups used as hydrophobic units. Consequently, the technical applications of
hydrophobically modified water-soluble polymers have been restricted.
To obtain a polymeric thickener with high viscosity and good thermal stability in aqueous
solutions, we herein synthesized a novel acrylamide (AM)/4-butylstyrene (BST) copolymer
(PSA) by micellar polymerization. The PSA polymer was characterized by UV and FT-IR
spectroscopy and elemental analysis. The influence of BST on the thermal stability of the
PSA polymer was studied by TG and DSC, and the solution viscosities of the PSA polymers
were investigated as functions of the amount of BST in the polymer, temperature, shear rate,
salt concentration and surfactant. Atomic force microscopy (AFM) was used to observe the
microstructures of PSA in aqueous and brine solutions to reveal the association of BST as a
hydrophobic monomer.
2
Experimental
2.1 Reagents
AM was recrystallized twice from chloroform. 4-Butylstyrene was synthesized according
to the procedure of reference [14]. Other reagents were analytically pure and used without
further purification.
2.2 Instrumentation
The UV spectrum was obtained with a UV-240 spectrophotometer (Shimadzu, Japan). The
purified PSA polymer was dissolved in pure water and the polymer concentration was
−
1
0.1 g·dL
.

J Solution Chem (2008) 37: 1227–1243
1229
The FT-IR spectrum was measured with a NICOLET-560 FT-IR spectrophotometer
−
1
(
USA) using KBr disks. The spectroscopic resolution was 1 cm and the scanning number
was 32.
The NMR spectra of purified 4-butylstyrene in CDCl3 was measured with a 400 MHz
INOVA-400 instrument (America Varian Company, USA).
The glass transition temperature of PSA was measured with a NETZSCH DSC204 in-
strument (Germany). N2 was used as the experimental inert atmosphere, and the heating rate
◦
−1
◦
was 20 C·min for the temperature range of 25 to 300 C.
The thermogravimetric curve of PSA was obtained with a PE TGA7 thermogravimetric
analyzer (Perkin-Elmer Co.). N2 was used as an inert atmosphere, and the heating rate was
◦
−1
◦
1
0 C·min for the temperature range 40 to 700 C.
The apparent viscosities of polymer solutions were measured with a Brookfield DVIII
−
1
◦
R27112E viscometer at a shear rate of 6 s at 35 C and the measurement time for all sam-
ples was two minutes. The intrinsic viscosities were measured with a 0.6 mm bore Ubbelo-
◦
−1
hde capillary viscometer at (30.0 ± 0.1) C and 1 mol·L sodium nitrate was used as the
solvent.
The atomic force micrographs were made with a SPA400 AFM (Seiko, Japan), and all
measurements were performed in the tapping mode. All samples were covered with mica
sheets and dried naturally.
The carbon, nitrogen and hydrogen contents of the polymers were determined with a
CARLO ESRA-1106 Elemental Analyzer (Italy). The molar compositions of PSA were
determined using Eqs. 1 to 5, where A and B are the number of moles of AM and BST,
respectively, in 100 g of terpolymer. The coefficients are the numbers of carbons, nitrogen
and hydrogen atoms in each monomer unit. After obtaining A and B for each polymer
sample, mol-% compositions were calculated.
%
%
%
-C/12.01 = 3A + 12B
-N/14.01 = 1A
-H/1.01 = 5A + 16B
(1)
(2)
(3)
(4)
(5)
mol-%(AM) = 100A/(A + B)
mol-%(BST) = 100B/(A + B)
2
.3 Synthesis of 4-Butylstyrene
-Butylchlorobenzene (0.2 mole) and an equal number of moles of vinyltributyltin were
4
dissolved in 100 mL of dioxane and placed in a 500 mL flask. In the presence of 2.0720 g of
Pd2(dba)3 and 8.2880 g of P(t-Bu)3 as catalysts, and 2 g of CsF as an additive, the reaction
◦
mixture was stirred for 24 h at 100 C. 4-Butylstyrene was then distilled from the crude
product under vacuo. The purity and yield were 93% and 80%, respectively. The observed
1
H NMR (400 MHz) shifts δ (ppm) are: 4H (–CH of phenyl), 7.353; 2H (=CH2), 5.193 and
a
2
b
2
c
2
d
3
5
.709; 1H (=CH), 6.700; 9H (–CH CH CH CH ), 2.654 (2Ha), 1.739 (2Hb), 1.406 (2Hc),
and 0.907 (3Hd).
2.4 Synthesis of Copolymers
The PSA copolymers were synthesized by free radical micellar copolymerization [15].
A 100 mL three-necked round bottom flask was equipped with a mechanical stirrer, and

1230
J Solution Chem (2008) 37: 1227–1243
a nitrogen gas inlet and outlet. AM (5.0 g, 0.0704 mole) and sodium dodecyl sulfate (SDS)
1.2274 g, SDS) were dissolved in 20.69 mL of distilled water and the solution was placed
in the flask. Hydrochloric acid was used to control the pH of the reaction solution to between
(
−
3
6
and 7. The mixture was stirred for 15 min., and BST (0.1716 g, 1.0724 × 10 mole) was
then added to the reaction flask. The flask was purged with N2(g) for half an hour until a
clear homogeneous mixture was observed. The reactant solution was then heated to 50 C,
◦
−
1
with stirring, in a tempering kettle under N2(g), and 5.720 mL of 0.005 mol·L K2S2O8
solution was then added to the reactant solution. After the polymerization proceeded for
◦
1
6 h at 50 C, the reaction mixture was diluted with five volumes of distilled water, and two
volumes of acetone were then added with stirring to precipitate the polymers. The polymers
were washed with acetone twice and extracted with ethanol using a Soxhlet extractor, for
two days to remove all traces of water, surfactant, residual monomers and initiator. Finally,
◦
the polymers were dried under vacuo at 50 C for three days.
In the absence of BST, polyacrylamide (PAM) was synthesized using the same reaction
conditions and purification method as mentioned above.
For the UV spectrum of PSA, there is a characteristic absorption peak at 240 nm at-
tributed to the phenyl group, indicating that the 4-butylstyrene hydrophobic units are in-
−
1
corporated into the polymer molecules. FT-IR peaks of PSA (cm ) are: –N–H stretch,
3
2
430.78; C=O stretch, 1654.50; –CH3, –CH2, and –CH stretches, 2928.60, 2861.89 and
788.61; –CH3, –CH2, and –CH bending, 1455.08, 1417.45 and 1323.28; =C–H phenyl
stretch, 3090.39; and =C–H phenyl bending, 827.75.
3
Results and Discussion
3.1 DSC Measurements
The block structure of the hydrophobic segments was determined previously by a fluorescent
probe [15]. In that paper, the chain structure of the hydrophobically associating polymers
was successfully characterized by the DSC technique. A high heating rate is favorable for
◦
−1
obtaining the fine glass transition. Therefore, in this study, the heating rate was 20 C·min
to observe the glass transition temperatures (T g) of the microblock BST segments. As shown
◦
in Fig. 1, where (a) is enlarged in the range of 80 to 120 C to obtain (b), there are two glass
◦
transition temperatures: one at 203 C for the AM hydrophilic segments and a second at
06 C for the microblock BST hydrophobic segments for PSA–BS1. The DSC result indi-
◦
1
cates that the BST units are incorporated into the PSA chains to form microblock structures
that favor intermolecular hydrophobic association [15]. In addition, the polymers are com-
pletely dissolved by water, and there is no white homopoly(4-butylstyrene) in the polymer
solution. The results show that the polymer is a copolymer of AM and BST, and not a mix-
◦
◦
ture of polyacrylamide and poly(4-butylstyrene). The T g (203 C) of the hydrophilic seg-
ments in PSA is higher than that of homo-polyacrylamide (163 C), which suggests that the
rigidity of the polymer chains has increased. This is due to the steric effect of the aromatic
ring of the BST units. On the other hand, the T g of the BST segments is dependent on the
length of the hydrophobic segments, which in turn is determined by the amount of BST sol-
ubilized in the SDS micelles during the reaction. The T g of homopoly(4-butylstyrene) with
◦
a high molecular weight is 118 C. The DSC result is consistent with the mechanism for
micellar copolymerization, which is that the hydrophobic monomer is solubilized within the
surfactant micelles and AM is dissolved together with the initiator K2S2O8 in the aqueous
phase [15].

J Solution Chem (2008) 37: 1227–1243
1231
Fig. 1 Glass transition
temperature of a PSA copolymer,
◦
−1
heating rate: 20 C·min
:
◦
−1
;
(
a) main scale of 20 C·min
(
b) enlarged graph of (a) in the
◦
range of 80 to 120 C
(a)
(b)
Fig. 2 TG analysis curve of a PSA copolymer
3.2 TG Measurements
The heat-resistant property of the PSA polymer was measured with the thermogravimetric
analyzer, and the curves are shown in Fig. 2 where the broken line is a differential ther-

1232
J Solution Chem (2008) 37: 1227–1243
mogravimetric curve that displays the decomposition rate of PSA. The thermogravimetric
◦
curve consists of five sections: (1) a weight loss of 9.885% from 41 to 102 C, which re-
sulted from volatilization of water molecules from the polymer. (2) The weight is constant
◦
◦
from 102 to 233.75 C. (3) The polymer begins to decompose at 233.75 C, and the imine
reaction occurs for a small number of amido groups and the weight loss is 12.967% between
◦
◦
2
4
33.75 to 315.18 C. The polymer decomposes fastest at 268.31 C. (4) Between 315.18 to
51.77 C, a large number of amido groups decompose and the main chains of the polymer
◦
degrade, leading to the large loss of weight (54.021%). The decomposition rate is the high-
◦
◦
est at 364.03 C. (5) In the range of 451.77 to 681.09 C, the loss of weight is 21.707%.
The decomposition rate then becomes slow, and the degradation of the polymer chains and
the decomposition of (4-butyl)phenyl groups dominate. Finally, only 0.845% of the polymer
◦
◦
remains at 681.09 C. The initial decomposition temperature (233.75 C) for the polymer is
obviously higher than that (210 C) of the homo-polymer polyacrylamide [16], indicating
◦
that the thermal stability of the hetero-polymer is good because of the incorporation of the
rigid benzene ring into the polymer.
3.3 Solution Behavior of PSA
The polymerization conditions, compositions and properties of some of the samples are
◦
shown in Tables 1 and 2. The reaction temperature of all samples was 50 C, and the pH of
the reacting solutions is between 6 and 7. For micellar polymerization, the viscosity of the
polymer solution is greatly influenced by both the length and number of hydrophobic blocks
in the polymer chains, which is determined by the hydrophobe/surfactant ratio. As shown in
Table 1, the maximum viscosity is attained at 4 wt% SDS when the total monomer concen-
tration is 15 wt%, but at a lower surfactant concentration of 3 wt% it was observed that many
Table 1 Synthesis conditions, apparent viscosity and yield of PSA copolymers
Sample
Reaction conditions
Apparent
viscosity
(mPa·s)
Yield
(%)
a
b
c
M1: M2
SDS
wt%)
Monomers
(wt%)
(
PSA-SD1
PSA-SD2
PSA-SD3
PSA-SD4
PSA-M1
PSA-M2
PSA-M3
PSA-BS1
PSA-BS2
PSA-BS3
PSA-BS4
98.8 : 1.2
98.8 : 1.2
98.8 : 1.2
98.8 : 1.2
98.8 : 1.2
98.8 : 1.2
98.8 : 1.2
98.5 : 1.5
98.2 : 1.8
98.0 : 2.0
97.5 : 2.5
3.0
4.0
5.0
6.0
2.7
5.6
7.2
5.6
5.6
5.6
5.6
15
15
15
15
10
20
25
20
20
20
20
38.2
127.0
78.7
70.5
84.5
81.6
76.3
66.7
87.2
91.5
89.2
88.4
85.5
82.0
28.0
23.5
208.0
42.0
296.0
96.0
53.0
33.0
−
1
Polymer concentration in aqueous solution: 0.30 g·dL
a
M1, acrylamide; M2, BST feed molar compositions
b
mass percent composition in water
cMonomers = total monomer concentration, the mass percent composition in water

J Solution Chem (2008) 37: 1227–1243
1233
Table 2 Composition and properties of PSA copolymers
◦
Sample
Elemental composition
Molar
T g/ C
Intrinsic
C
N
H
composition
Hydrophilic
segments
Hydrophobic
segments
viscosity
a
−1
(
wt%)
(wt%)
(wt%)
M1: M2
(dL·g
)
PSA-SD1
PSA-SD2
PSA-SD3
PSA-SD4
PSA-M1
PSA-M2
PSA-M3
PSA-BS1
PSA-BS2
PSA-BS3
PSA-BS4
51.39
51.86
51.93
51.58
51.31
51.89
52.03
52.14
52.41
52.69
53.17
19.41
19.17
19.14
19.31
19.45
19.16
19.09
19.03
18.90
18.75
18.51
7.16
7.19
7.20
7.18
7.16
7.20
7.21
7.23
7.24
7.26
7.30
99.27 : 0.73
98.72 : 1.28
98.64 : 1.36
99.05 : 0.95
99.36 : 0.64
98.69 : 1.31
98.53 : 1.47
98.40 : 1.60
98.08 : 1.92
97.74 : 2.26
97.17 : 2.83
192.0
201.4
199.1
197.6
186.4
202.3
204.5
203.0
204.1
204.7
206.2
/
2.70
3.48
3.01
2.44
2.85
3.71
3.20
4.05
3.91
3.35
2.90
103.6
102.8
102.0
/
104.0
107.8
106.0
106.8
107.5
108.1
a
M1, acrylamide; M2, BST molar compositions in copolymers
droplets of hydrophobic monomer are dispersed in the reacting solution, which greatly dis-
turbs the micellar polymerization. However, excessive surfactant produces a large number
of bubbles, resulting in a low hydrophobic monomer content and low yield of the polymer.
−
1
Compared with the intrinsic viscosity of HPAM, which is generally higher than 15 dL·g
,
the intrinsic viscosities of the PSA polymers are lower. This result shows that the thicken-
ing property of PSA solutions mainly depends on intermolecular hydrophobic associations.
By increasing the amount BST in the feed, the intrinsic viscosity of the polymer decreases
because of steric obstruction of the hydrophobic monomer, indicating that the molecular
weight of the polymer is reduced, and the glass transition temperatures of the hydrophilic
and hydrophobic segments increase because of the increase in the length of the microblock
BST segment. The molar compositions of the polymers are higher than the corresponding
monomer feed compositions for BST (except for PSA-SD1, PSA-SD4 and PSA-M1), which
is due to the local concentration effect of BST solubilized within the surfactant micelles.
3.3.1 Effect of the Amount of BST on Viscosity
The thickening ability of the hydrophobically associating water-soluble polymer is greatly
dependent on both the length and number of hydrophobic blocks in the polymer chains as
determined by the amount of hydrophobe. As shown in Fig. 3, the optimum BST amount is
1
.5 mol-%. For PSA-BS1, the solution viscosity increases sharply with increasing polymer
−
1
∗
concentration above 0.1 g·dL , which is the critical association concentration (C ). For
p
PSA-BS2, PSA-BS3, and PSA-BS4, the solution viscosities are lower because of the larger
amount of BST. For comparison, the concentration dependence of the solution viscosity
for PAM is also shown in Fig. 3, but the PAM polymer displays only a linear relationship
between solution viscosity and polymer concentration. At a constant surfactant concentra-
tion but with increasing amount of BST from 1.2 to 1.5 mol-%, intermolecular hydrophobic
∗
associations are reinforced above C , resulting in the obvious increase in solution viscos-
p
ity. However, higher BST contents in the copolymers resulted in poor solubility in water and
strong intramolecular associations. Consequently, solution viscosities decreased remarkably.

1234
J Solution Chem (2008) 37: 1227–1243
Fig. 3 Influence of the amount
of BST on the apparent viscosity
of aqueous PSA solutions
Fig. 4 Influence of the amount
BST on the apparent viscosity of
PSA in brine solutions where the
NaCl concentration is
−
1
0.137 mol·L
Intermolecular hydrophobic associations and the viscosification effect of the polymers in
brine solution are strengthened because of the increase in solution polarity from addition
of NaCl (Fig. 4). The phenyl group from BST can induce van der Waals interactions due
to its planar and polarizable structure, and improve the rigidity of the hydrophobic unit. In
addition, the length of the hydrophobic carbon chain in BST is longer than that of the phenyl
group in styrene. Hence, the incorporation of (4-butyl)phenyl groups into the PSA polymer
leads to strong intermolecular hydrophobic associations and high viscosities for the polymer
solution at low polymer concentrations.
3.3.2 Effect of NaCl on Viscosity
The apparent viscosity is plotted as a function of NaCl concentration for the PSA poly-
mers and PAM in brine solutions (Fig. 5). With increasing NaCl concentration, for PSA-
BS1 and PSA-SD2, the apparent viscosities first increase and then decrease at a polymer
−
1
concentration of 0.3 g·dL . This is due to an increase in solution polarity induced by elec-
trolytes, which results in reinforced intermolecular hydrophobic associations and good salt-
thickening behavior. However, when the salt concentration further increases, the hydropho-
bic microstructures become more compact, and then the condensed aggregates associate to
form larger aggregates, resulting in the phase separation of gels and a decrease in the appar-
ent viscosity. The salt resistance of the PSA-SD2 polymer is higher than that of PSA-BS1
−
1
because of the lower hydrophobe content. For the 0.1 g·dL PSA-BS1 solution, there are
few aggregates, and monomeric molecules mainly exist in the dilute solution. Thus, the
solution viscosity varies only slightly with an increase of salt concentration.

J Solution Chem (2008) 37: 1227–1243
1235
Fig. 5 Effect of the NaCl
concentration on the apparent
viscosity of PSA in brine
−
1
solutions: ꢀ 0.3 g·dL
PSA-BS1; ꢁ 0.3 g·dL
PSA-SD2; ꢂ 0.1 g·dL
PSA-BS1
−
1
−
1
Fig. 6 Influence of the SDS
concentration on the apparent
viscosity of PSA in dilute
aqueous solutions where the
polymer concentration is
.1 g·dL 1
−
0
3.3.3 Effect of Surfactant on Viscosity
Figure 6 shows the influence of the SDS concentration on the apparent viscosity of the poly-
−
1
mers in dilute solution at the concentration 0.1 g·dL . By addition of SDS, the apparent
viscosity of PAM without BST hardly varies in aqueous solution, and the solution viscosity
of PSA-SD1 is also almost constant because very little of the hydrophobe is present in the
−
1
molecules. For 0.1 g·dL PSA-BS1, with increasing SDS concentration, the solution vis-
−
1
cosity first decreases below 4 mmol·L , then rises, and finally tends to become constant. In
−
1
a 0.1 g·dL PSA-BS1 solution, most of polymer chains exist in the form of the monomeric
molecule, and intramolecular hydrophobic associations easily occur. The intramolecular as-
sociations are strengthened by addition of a small amount of surfactant, and molecule coils
become more compact, leading to the decrease of solution viscosity. When the concentra-
−
1
tion of SDS reaches 4 mmol·L , intramolecular associations become strongest, but then
−
1
weakens with increasing the SDS concentration. However, above 8 mmol·L , which is the
critical micellar concentration (CMC) of SDS, hydrophobic segments are surrounded in SDS
micelles, the intramolecular association is destroyed, and mixed micelles involving only hy-
drophobic segments are formed with further increasing of the SDS concentration (Fig. 7).
Therefore, the viscosity becomes almost constant. In addition, electrostatic repulsions of
2
4
−
SO in mixed micelles acts to extend the polymer chains, contributing to the viscosity
increase.
The effect of the SDS concentration on the apparent viscosities of copolymers in semi-
dilute solution is shown in Fig. 8. In the studied concentration range of surfactant for PSA-
BS1 and PSA-M2, the solution viscosities first increase remarkably, reach maxima (attribut-
able to inter-polymer crosslinking through the bridging of the surfactant molecules with hy-
drophobic groups), and then decrease. Finally, the solution viscosities are almost invariant

1236
J Solution Chem (2008) 37: 1227–1243
Fig. 7 Schematic representation of the effect of surfactant on intramolecular hydrophobic association
Fig. 8 Effect of the SDS
concentration on the apparent
viscosity of PSA polymers in
semi-dilute aqueous solutions
where the polymer concentration
is 0.3 g·dL 1
−
when the surfactant concentration is higher than the CMC of SDS. At low concentrations of
SDS, aggregates, which involve the surfactant molecules and the hydrophobic groups from
different copolymer molecules, can form and this leads to the observed increase in solution
viscosity. However, by addition of excess SDS, micelle-like aggregates with an individual
hydrophobic segment form that disrupt the intermolecular association. The viscosity max-
imum of PSA-BS1 is higher than that of PSA-M2 because it contains more hydrophobic
groups. For PSA-SD1 with increasing concentration of SDS, the solution viscosity varies
only slightly because the hydrophobe content of the polymers is not high enough to form
mixed hydrophobic microdomains.
3.3.4 Effect of Temperature on Viscosity
As shown in Fig. 9 for PAM, as the temperature increases the solution viscosity regularly
decreases because of the faster movement of the polymer chains and weaker hydrogen bond-
−
1
−1
ing. For 0.3 g·dL PSA-BS1 and 0.3 g·dL PSA-SD2 in aqueous solutions, the solution
◦
viscosities increase with increasing temperature from 20 to 40 C, followed by a decrease
above 40 C. Compared with PSA-SD2, the enhancement of the solution viscosity of PSA-
BS1 between 20 to 40 C is higher because of the stronger inter-chain associations. These
◦
◦
results indicate that an increase of temperature is favorable for inter-polymer hydrophobic
association within a certain temperature range.
Hydrophobic hydration is exothermic whereas hydrophobic association is an endother-
mic process with an entropy increase in a low temperature range [17]. Water molecules

J Solution Chem (2008) 37: 1227–1243
1237
Fig. 9 Influence of temperature
on the apparent viscosity of PSA
aqueous solutions where the
polymer concentration is
.3 g·dL 1
−
0
Fig. 10 Schematic
representation of hydrophobic
association
surrounding the hydrophobic groups are arranged orderly by their hydrogen bonds, which
are called iceberg structures (Fig. 10). The structures are not stable because of their higher
energy states, and hydrophobic groups are easily extruded from the aqueous environment
and aggregate to form a stable thermodynamic state via van der Waals interactions. This
is hydrophobic association. The iceberg structures collapse, which is endothermic, results
in an increase of disordered states. Hydrophobic groups in hydrophobic micro domains are
freer than in the aqueous environment, their aggregations with each other occur exother-
mically, and the energy is lower than that absorbed by the thaw of the iceberg structures.
Thus, hydrophobic association is an endothermic process with an entropy increase in the
low-temperature region. At higher temperatures, the iceberg structure decreases or becomes
weaker because of the faster movement of water molecules and polymer chains, and values
of entropy increase and the endothermic energy obviously decreases, resulting in exother-
mic hydrophobic association. Therefore, the solution viscosity decreases with increasing
temperature.
3.3.5 Effect of Shear Rate on Viscosity
The plots of viscosity versus shear rate are displayed in Fig. 11 for PSA-BS1 in aqueous
◦
solutions at 35 C. With increasing shear rate (curve 1), the viscosity decreases sharply, the
rate of reduction then slows, and finally the viscosity tends to be constant because of the bal-
ance between intermolecular association and disassociation, which exhibits pseudo-plastic
behavior. Upon gradually reducing the shear rate (curve 2), the viscosity becomes slightly
lower than the primary viscosity at a given high shear rate and does not recover immediately,
indicating that the intermolecular hydrophobic associations take places at a finite rate. After
the first measured sample was allowed to set for 2 h, the remeasured viscosity (curve 3)
was higher than the first measurements. This solution behavior is attributable to the expan-
sion of polymer chains and the transition from intramolecular associations to intermolecular
associations upon shearing, leading to more hydrophobic domains and an increase in the
solution viscosity. These results imply that the intermolecular associations are reversible, all

1238
J Solution Chem (2008) 37: 1227–1243
Fig. 11 Influence of shear rate
on the apparent viscosity of
aqueous PSA solutions where the
polymer concentration is
−
1
0
.3 g·dL PSA-BS1: curve 1,
increasing shear rate; curve 2,
decreasing shear rate; curve 3,
measured 2 h after the first
measurement
◦
Table 3 Apparent viscosities of PSA copolymers in brine solution after ageing at 80 C (mPa·s)
Sample
Ageing time (d)
0
1
5
15
30
45
60
90
PSA-BS1
PSA-M2
109.3
87.6
95.7
70.9
84.1
63.3
73.8
58.5
66.4
52.4
60.3
48.1
57.1
43.8
53.8
39.3
Polymer concentration: 0.2 g·dL−1, NaCl concentration: 0.137 mol·L−1
aggregates break at high shear rates, and molecular chains orientate in the fluid flow field. In
addition, the results also show that the polymer does not degrade and the molecular structure
is stable upon shearing.
3.3.6 Aging Effect
−
1
−1
Table 3 shows the aging effect of 0.2 g·dL PSA-BS1 and PSA-M2, in 0.137 mol·L NaCl
◦
◦
solution saturated with oxygen, at 80 C. After aging for 90 days at 80 C, the viscosity
retention ratio of a PSA-BS1 brine solution is 49.2%, which is higher than that (22.2%)
−
1
−1
of 0.15 g·dL for poly (acrylamide/N-(4-butyl)phenyl acrylamide) in 0.256 mol·L NaCl
◦
solution after aging for 7 days at 80 C [18]. This result shows that the anti-aging property of
◦
the polymer is good in brine solution at 80 C because of the incorporation of BST into the
copolymer chains. The rigid phenyl groups of BST are linked directly to main chains that
can stabilize the hydrophobic units and effectively interfere with the hydrolysis of amido
groups in the polymers. This aging result is consistent with the DSC and TG results.
4
The Associating Microstructure of PSA in Solutions
4.1 Aggregation in Aqueous Solutions
The aggregation morphology of a 0.4 g·dL− PSA-BS1 aqueous solution is shown in Fig. 12,
where (a) is locally enlarged to obtain (b), which is then locally enlarged to form (c). Numer-
ous aggregates with different shapes and sizes are formed via intermolecular hydrophobic
associations, and small compact aggregates associate with each other through van der Waals
interactions and hydrogen bonding. These results indicate that there is a heterogeneous dis-
tribution of the hydrophobic groups in the copolymer molecules, and that small compact
1

J Solution Chem (2008) 37: 1227–1243
1239
Fig. 12 AFM images of
−
1
0
.4 g·dL PSA-BS1 in aqueous
solution: (a) two-dimension
image; (b) local enlarged image
of (a); (c) local enlarged image
of (b)

1240
J Solution Chem (2008) 37: 1227–1243
Fig. 13 AFM image of
−
1
0
0
(
.4 g·dL PSA-BS1 in a
−
1
.034 mol·L NaCl solution:
a) two-dimension image;
(b) local enlarged image of the
top section in (a); (c) local
enlarged image of the right
section in (a)
aggregates are formed by the polymer chains with more hydrophobic groups. Although the
intermolecular hydrophobic associations of (4-butyl)phenyl groups are strong, continuous
three-dimensional network structures are not observed in Fig. 12. This is attributed to there
being no ionic groups to extend the molecules in nonionic polymer chains, leading to a
coiled molecular conformation.
4.2 Aggregation in Brine Solutions
As shown in Fig. 13 for 0.4 g·dL− PSA-BS1 in a 0.034 mol·L NaCl solution, larger asso-
ciative blocks are formed by the addition of a small amount of salt, resulting in an increase of
the solution viscosity. In addition, a small amount of more compact aggregates with smaller
sizes are exhibited in Fig. 13b, resulting from the formation of more condensed hydrophobic
micro domains. As observed in Fig. 14, when increasing the NaCl concentration from 0.034
1
−1
−
1
to 0.513 mol·L , the polymer morphologies change from huge blocks to small compact
aggregates that associate to form gels. The results indicate that when the polarity of a brine

J Solution Chem (2008) 37: 1227–1243
1241
Fig. 14 AFM images of
−
1
0
0
(
.4 g·dL PSA-BS1 in a
−
1
.513 mol·L NaCl solution:
a) two-dimension image;
(b) local enlarged image of (a)
solution is large, intermolecular associations of hydrophobic groups are strong enough to
result in the phase separation of gels and the decrease in apparent viscosity. The AFM result
is in good agreement with the plot of the apparent viscosity versus the NaCl concentration.
5
Conclusions
Hydrophobic acrylamide-modified copolymers (PSA) with 4-butylstyrene (BST) were syn-
thesized by the micellar copolymerization technique. The block structures of PSA were
conveniently measured by DSC, indicating that the incorporation of BST enhances molecu-
◦
lar rigidity. TG analysis shows that the initial decomposition temperature of PSA is 234 C,
higher than that of the homo-polyacrylamide, suggesting that the copolymer has better ther-
mal stability than homo-polyacrylamide. The amount of BST remarkably affects the appar-
ent viscosity of PSA solutions because of strong intermolecular hydrophobic associations,
and with increasing concentration of polymer with the optimum BST content, the solu-
∗
tion viscosities increase dramatically in aqueous and brine solutions above C despite the
p

1242
J Solution Chem (2008) 37: 1227–1243
low molecular weight of PSA. The polymer exhibits good salt-thickening behavior below
a NaCl concentration of 0.137 mol·L− , temperature-thickening, thixotropy, pseudo-plastic
behavior, and anti-shearing. The solution viscosities of the polymers increase, apparently
through the crosslinking of a small amount of surfactant with the hydrophobic groups. The
1
PSA polymer with a rigid benzene ring shows good anti-aging property in brine solutions at
◦
8
0 C, which is consistent with the DSC and TG results. AFM measurements show that large
−
1
block-like aggregates are formed in an aqueous solution of 0.4 g·dL PSA. The microstruc-
tures of the polymer become much larger by addition of small amounts of salt, resulting in
an increase in solution viscosity. However, by addition of larger amounts of NaCl, small
compact aggregates are formed and phase separation of gels occurs, leading to the decrease
of the apparent viscosity. The AFM results reveal the relationship between associating mi-
crostructure and thickening property of the polymer solutions.
Acknowledgements This work was supported by the Open Fundation of State Key Laboratory of Oil and
Gas Reservoir Geology and Exploitation (contract grant number: PLC200601), and National 973 Project
(contract grant number: G1999022502).
References
1
2
3
. Taylor, K.C., Nasr-El-Din, H.A.: Water-soluble hydrophobically associating polymers for improved oil
recovery: a literature review. J. Pet. Sci. Eng. 19, 265–280 (1998)
. Hutchinson, B.H., McCormick, C.L.: Water-soluble copolymers: 15. Studies of random copolymers of
13
acrylamide with N-substituted acrylamides by C n.m.r. Polymer 27, 623–626 (1986)
. McCormick, C.L., Kramer, M.C., Chang, Y., Branham, K.D., Kathmann, E.L.: Synthesis and aqueous
solution properties of responsive polyelectrolytes and polyampholytes. Polym. Prep. 34, 1005–1006
(1993)
4
. Kramer, M.C., Steger, J.R., Hu, Y.X., McCormick, C.L.: Water-soluble copolymers: 66. Phase transfer
studies of structural and environmental effects on domain organization in aqueous solutions of hydropho-
bically modified poly(sodium maleate-alt-ethyl vinyl ether)s. Polymer 37, 4539–4546 (1996)
. Ye, L., Luo, K.F., Huang, R.H.: A study on P(AM-DMDA) hydrophobically associating water-soluble
copolymer. Eur. Polym. J. 36, 1711–1715 (2000)
5
6
7
8
. Ye, L., Mao, L.J., Huang, R.H.: Synthesis and aqueous solution behavior of hydrophobically modified
polyelectrolytes. J. Appl. Polym. Sci. 82, 3552–3557 (2001)
. Ma, J., Huang, R.H., Zhao, L., Zhang, X.: Solution properties of ionic hydrophobically associating poly-
acrylamide with an arylalkyl group. J. Appl. Polym. Sci. 97, 316–321 (2005)
. McCormick, C.L., Middleton, J.C., Cummins, D.F.: Water-soluble copolymers, 37. Synthesis and char-
acterization of responsive hydrophobically-modified polyelectrolytes. Macromolecules 25, 1201–1206
(1992)
9. Busse, K., Kressler, J.: Synthesis of amphiphilic block copolymers based on tert-buty1 methacrylate and
2-(N-methylperfluorobutanesulfonamido) ethyl methacrylate and its behavior in water. Macromolecules
35, 178–184 (2002)
1
1
1
1
0. Stahl, G.A., Schulz, D.N.: Water-Soluble Polymers for Petroleum Recovery. Plenum, New York (1988)
p. 259
1. Valint, P.L. Jr., Bock, J., Schulz, D.N.: Synthesis and characterization of hydrophobically associating
polymer. Polym. Mater. Sci. Eng. 57, 482–486 (1987)
2. Lacik, I., Selb, J., Caudau, F.: Compositional hetero-geneity effects in hydrophobically associating water-
soluble polymers prepared by micellar copolymerization. Polymer 36, 3197–3211 (1995)
3. Biggs, S., Hill, A., Selb, J., Caudau, F.: Copolymerization of acrylamide and hydrophobic monomer in
an aqueous micellar medium: effect of the surfactant on the copolymer microstructure. J. Phys. Chem.
96, 1505–1511 (1992)
1
4. Littke, A.F., Schwarz, L., Fu, G.C.: Pd/P(t-Bu) : a mild and general catalyst for Stille reactions of aryl
3
chlorides and aryl bromides. J. Am. Chem. Soc. 124, 6343–6348 (2002)

J Solution Chem (2008) 37: 1227–1243
1243
15. Hill, A., Candau, F., Selb, J.: Properties of hydrophobically associating polyacrylamides: influence of
the method of synthesis. Macromolecules 26, 4521–4532 (1993)
16. Yan, R.X.: Water-Soluble Polymers. Chemical Industry, Beijing (1998) p. 86
1
7. McCormick, C.L., Nonaka, T., Johnson, C.B.: Water-soluble copolymers: 27. Synthesis and aqueous so-
lution behavior of associative acrylamide/N-alkylacrylamide copolymers. Polymer 29, 731–739 (1988)
8. Cui, P.: Study on synthesis and properties of hydrophobically associating water-soluble copolymer PABA
1
(master thesis), p. 30. Sichuan University, Chengdu (2001)