Solution Viscosity Behavior of Complexes
of Poly(y-benzyl-L-glutamate) with Lightly Sulfonated
Polystyrene or Its Zinc Salt
1. SHAO,'rt R. A. WEISS,'** and R. D. LUNDBERG'
Department of Chemical Engineering and Polymer Science Program, University of Connecticut, Storrs,
Connecticut 06269-31 36; 'Exxon Research and Engineering Co., Annandale, New Jersey08801
SYNOPSIS
The rheological behavior of solutions containing blends of poly(y-benzyl-L-glutamate)
(PBLG) and either the free acid or the zinc salt of lightly sulfonated polystyrene (SPS)
was studied as a function of blend composition, polymer concentration, degree of sulfonation
of the SPS, and the polypeptide molecular weight. The zinc salt of SPS formed a transition
metal complex with the amine-end groups of the PBLG, and this resulted in an enhancement
of the solution viscosity relative to a weighted average of the viscosities of the individual
polymer solutions. The ZnSPS/PBLG solutions showed no anomalous time or shear dependencies. In contrast, solutions containing PBLG and the sulfonic acid derivative of
SPS also had enhanced viscosities, but in addition, they exhibited time-dependent viscosities
(thirotropic behavior) and shear thickening (dilutant behuuior). This was attributed to a
nonequilibrium structure of the interpolymer complex due to a competition between acidamine and acid-ester interactions. Although the acid-amine interaction is enthalpically
favored, when sufficient sulfonic acid groups were available, interactions between the sulfonic
acid and the glutamate ester side groups of PBLG developed and this interaction promotes
a helix-to-coil transition of the PBLG. 0 1995 John Wiley & Sons, Inc.
Keywords: ionomer molecular complexes solution viscosity polypeptide
-
INTRODUCTION
The solution viscosity behavior of ionomer solutions
is influenced by the persistence of intra and inter
molecular associations of the ionic groups.'-'' Aggregation of the ionic species is affected by the ionic
group concentration, the type of counterion, the polarity of the solvent, and whether or not specific
solvent-polymer interactions occur. When intermolecular interactions predominate, the associated
polymer network produces a highly viscous solution
or a physically associated gel. Dilatant behavior has
been reported for ionomer solutions at moderate
shear rates,"-13 though at sufficiently high shear
rates, shear-thinning viscosity behavior is usually
* To whom correspondence should be addressed.
' Dr. Phoebe Shao died in a car accident one week after finishing this article.
Journal of Polymer Science: Part B: Polymer Physics, Vol. 33.2083-2092 (1995)
8 1995 John Wiley & Sons, Inc.
CCC 0887-6266/9S/142083-10
observed. Witten and Cohn proposed that shearthickening behavior for associating polymers arises
from an increase in the number of interchain associations as a result of extension of the polymer
coils by the shear field.'* SANS experiments by
Pedley et al.," however, indicated that the ionomer
chains are aggregated even in dilute solution and
the single-chain dimensions do not change upon
shearing the solution. They attributed shear-thickening behavior to deformation of the aggregates or
association of the aggregates.
The solution behavior of ionomer-polymer
complexes is also of some interest. Molecular
complexes of a n ionomer with amine-containing
polymers may exhibit higher viscosities than solutions of either of the individual components.'6-20
Shear thickening has also been observed for solutions of transition-metal complexes involving
the zinc salt of sulfonated-poly(ran-ethyleneco-ran-propylene-co-ran-ethylidenenorbornene),
2083
2084
SHAO, WEISS, AND LUNDBERG
S-EPDM, a n d poly(run-styrene-co-run-4-vinylpyridine), PSVP.'6-1s
This article reports the solution behavior of complexes of a lightly sulfonated polystyrene ionomer,
SPS, and a n amine-terminated rigid rod polymer,
poly(y-benzyl-L-glutamate), PBLG. Zinc salts of
SPS, Zn-SPS, form transition-metal complexes with
the amine end-group of PBLG, which yield a comblike molecular architecture.21,22The free acid derivative of the ionomer, H-SPS, interacts with PBLG
by proton transfer to the amine end-group and by
hydrogen bonding t o the carbonyl oxygen of the glutamate side group, which produces a network-like
structure in
The different molecular
architectures of the two complexes were expected t o
yield different solution rheology. Solution viscosity
was measured as a function of composition, polymer
concentration, and shear rate for the two types of
complexes. Although PBLG is a lyotropic liquid
crystalline polymer, only solutions with relatively
low polymer concentration were studied and no birefringence was observed for any of the solutions.
EXPERIMENTAL
completion of the reaction, hexane was added dropwise to extract the BLG anhydride, which was the
monomer used to synthesize PBLG. Polymerization
of the BLG anhydride was carried out a t room temperature in dimethyl formamide ( D M F ) using hexylamine a s the initiator. M, was calculated from the
monomer/initiator molar ratio, assuming that each
amine initiates one chain. M , was calculated from
the intrinsic viscosity in a dichloroacetic acid (DCA)
~olution.'~
Mixtures of SPS and PBLG were prepared in solution using DMF, chloroform, or 1,2-dichloroethane
as solvents. Separate 5 g/dL solutions of SPS and
PBLG were first prepared, and then the PBLG solution was added slowly t o the SPS solution with
constant stirring. T h e solutions were stirred overnight. T h e composition of the final mixture was
varied by adjusting the amounts of the two solutions
used. In the discussion that follows, the composition
is expressed in terms of the stoichiometry of the
terminal amine groups and the sulfonic acid groups,
[ NH2]/ [ S 0 3 H ] , or by the ratio of amine groups t o
zinc ions, [ NH,] / [ Zn"] . The concentration of the
cation rather than the anion was used for the mixtures involving the zinc salt, because based on other
amine-transition metal complexes involving sulfonate ionomers, 16-'* it was expected that the complex
would involve one Zn2+per NH2 group.
Reduced viscosities were measured with a Ubbelohde capillary viscometer and the shear rate dependence of viscosity was measured with a Brookfield cone-and-plate viscometer. Temperature was
controlled to within +O.O5"C with circulating water.
For the capillary experiments, flow times greater
than 100 seconds were used t o avoid the necessity
for kinetic energy corrections. Viscosity versus shear
rate measurements were made over the shear rate
range from 11.25 s-' to 450 s-'.
Polystyrene (M,, = 100,000; M , = 280,000) was sulfonated following the procedure described by Makowski et aLZ3T h e reaction was carried out a t 50°C
in 1,2-dichloroethane ( D C E ) using acetyl sulfate as
the sulfonating agent. The sulfonated polymer was
isolated as the free acid derivative, H-SPS, and the
zinc salt was prepared by neutralizing the H-SPS
in a toluene/methanol (90/10 v / v ) solution with a
25% excess of zinc acetate. The degree of sulfonation
was determined by titrating the H-SPS in a toluene/
methanol (90/ 10 v / v ) solution with methanolic sodium hydroxide. The SPS samples used in this investigation had sulfonate concentrations of 1.2, 1.95,
and 5.2 mol % and are denoted a s MSPS1, MSPS2,
and MSPS5, respectively, where M denotes the acid
RESULTS AND DISCUSSION
( H ) or zinc salt ( Z n ) derivatives.
PBLG with Mu = 20,100 ( H P B L G ) was purThe complexation of HSPS and ZnSPS with PBLG
chased from Sigma Chemical Co. and PBLG with
was described in previous reports.21,22For ZnSPS/
M,, = 5000 and M , = 6800) (LPBLG) was synthePBLG blends, the sole interaction was the formation
sized by the method described by Daly and P o ~ h e . ~ ~of a transition-metal complex between the zinc sulIn the preparation of the latter, all solvents were
fonate groups of ZnSPS and the terminal amine
freshly distilled and stored over molecular sieves
groups of PBLG. This produced a comb-like graft
prior t o use. Benzyl-L-glutamate ( B L G ) was first
copolymer structure. For the HSPS/PBLG blends,
dispersed in dry tetrahydrofuran ( T H F ) at 5OoC. A
complexation involved proton-transfer from the
sulfonic acid groups to the amine terminal groups
one-third molar equivalent of bis-trichloromethyl
carbonate (triphosgene) was dissolved in T H F and
and hydrogen bonding between the sulfonic acid
groups and the carbonyl oxygen of the glutamate
then added to the stirred suspension of BLG/THF.
T h e reaction was carried out under N2,
and a t the
groups on the PBLG side-chain. Because the latter
BEHAVIOR OF COMPLEXES OF PBLG
F’
0
1
2
3
4
5
c (g/dl)
Figure 1. Reduced viscosity vs. concentration in chloroform at 25°C for ( 0 )PS, ( A )HSPSI, and (m)HSPSS.
intermolecular interactions may involve multiple
sites on both blend components, a physically crosslinked network is formed. The hydrogen bond interactions also perturbed the PBLG conformation
and induced a helix-to-coil transition of the PBLG
chain adjacent to the site of the complex.
The different intermolecular interactions and
conformational transitions are expected to affect the
solution behavior of blends in different ways. The
rheology of solutions of the neat polymers and their
blends are discussed in the following sections.
Effects of Blend Composition and Aging Time
of the Solution
The effect of polymer concentration on the reduced
viscosity for the HSPS ionomers and their polystyrene precursor in a relatively low polarity solvent,
chloroform, is shown in Figure 1. Above a polymer
concentration, c , of ca. 2 g/dL, the viscosity of the
solutions was considerably enhanced by the presence
of the sulfonic acid groups. Below c = 2 g/dL, however, the viscosity decreased with increasing sulfonation. This crossover in the viscosity of SPS ionomers relative to the nonionic precursor has been previously reported by a number of investigators, ‘-lo
and it is believed to result from changes in the aggregation of the ionic groups from predominantly
intramolecular interactions below a critical overlap
concentration to intermolecular interactions above
that concentration. For HSPS, aggregation results
from intra- and intermolecular hydrogen bonding of
the sulfonic acid groups. The rapid rise in the viscosity
of HSPSS above c = 3 g/dL represents gelation of
2085
the solution as a consequence of the interchain hydrogen bonding between sulfonic acid groups. The
viscosity of the solution of HSPS1, which has a lower
sulfonic acid concentration, began to increase at a
higher polymer concentration and is expected to “gel”
at a higher polymer concentration.
High-molecular weight PBLG possesses an a-helix conformation in solvents such as chloroform and
THF.26The helix is stabilized by intramolecular hydrogen bonding and intermolecular hydrogen bonding occurs between the end groups. The addition of
small quantities ( < 5% by volume) of a coil-supporting solvent, such as dichloroacetic acid, DCA,
effectively solvates the intermolecular self-association of PBLG without disrupting the helix conformation. The addition of more than 5 vol % DCA
causes a helix-to-coil transition of the PBLG conformation. The effect of the addition of 1%DCA to
PBLG solutions in chloroform is shown in Figure
2. The addition of DCA markedly reduces the viscosity of HPBLG solutions, which is consistent with
the disruption of interpolymer associations. DCA,
however, had little effect on the viscosity of LPBLG
solutions, which is probably because oligomeric
PBLG primarily adopts a coil conformation even in
a helicogenic solvent.
Like DCA, the SPS ionomers also disrupt selfassociation of PBLG. The addition of 3 wt % HSPSZ
(i.e., 3% of the total polymer in solution) lowered
the reduced viscosity of HPBLG in THF to the same
value as that obtained by adding 1 vol % DCA.
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
0
1
2
3
c (g/dl)
Figure 2. Reduced viscosity vs. concentration at 25°C
for (0)HPBLG in chloroform, ( 0 )HPBLG in 99% chloroform/l% DCA, (A)LPBLG in chloroform, and (A) in
99% chloroform/l% DCA.
2086
h
SHAO, WEISS, AND LUNDBERG
(total polymer concentration, c = 3 g / d L ) for two
different sulfonation levels are shown in Figure 4.
For relatively low concentrations of the HPBLG,
i.e., I20 wt % ( [ NH2]/ [ Zn2'] < 0.35),the reduced
viscosity of the blend was close to that calculated
by eq. ( 1) . However, for higher concentrations of
HPBLG, a positive deviation from the weight-average viscosity was observed, which was due to the
development of strong intermolecular interaction
between the two polymers.
The deviation of the solution viscosity from a
simple weighted average of the viscosities of the
component polymers may be characterized by a viscosity enhancement factor, defined by eq. ( 2 ) .
1.o
en
3
W
F"
0.5
0.0
0
20
40
60
80
100
Wt.% PBLG
Figure 3. Reduced viscosity vs. composition at 25°C
for solutions ( c = 3 g/dL) of mixtures of PS and HPBLG
in T H F containing < 2% DCA.
Noninteracting Mixtures (PS/PBLG)
In the absence of specific intermolecular interactions, the solution viscosity of a mixture of two
polymers is often a weighted average of the viscosities of the individual components.
where qII and qn are the reduced viscosities of the
two component polymers a t a concentration c = c1
c2. For blends of polystyrene and HPBLG, for
which no specific interaction occurs, the reduced
viscosity followed eq. (1)as shown in Figure 3. T h e
viscosities of HPBLG and the P S / H P B L G solutions were measured in T H F containing a small
amount ( < 2 vol % ) of DCA to minimize self-association of the PBLG.
+
ZnSPS/ PBL G Mixtures
where vCalcis calculated from eq. ( 1) . A value of R
= 0 indicates no enhancement and positive and negative values of R denote positive and negative viscosity enhancements, respectively. The data from
Figure 4 are replotted a s R versus the ratio of the
interacting functional groups, [ NH2]/ [ Zn2'] in
Figure 5. For the ZnSPS/HPBLG mixtures, the solution viscosity enhancement increased nearly linearly with increasing [ NH2]/ [ Zn2+],and the slope
of the enhancement rate was greater for the mixtures
employing the higher sulfonated ionomers. Those
results were not surprising in that increasing
[ NH2] / [ Zn2+]promotes more intermolecular complexation and the higher sulfonated polymer has
more sulfonate groups per chain, which increases
the apparent molecular weight of the complexed
mixture for the same polymer composition. Rheology
1.2
1
.o
-a
h
/T---l
0.8 -
1
Because of the poor solubility of the ZnSPS ionomers in low polarity solvents such as chloroform, DMF
was used as the solvent for the studies involving
solutions of the ZnSPS /PBLG mixtures. Although
DMF is very effective at solvating interactions
between H S P S and PBLG, intermolecular interactions between ZnSPS and PBLG persisted in
DMF solutions and strongly influenced the solution
behavior.
When specific interactions occur between two
polymers, the solution viscosity of the mixture generally is higher than that predicted by eq. ( 1).The
reduced viscosities of the ZnSPS / HPBLG mixtures
ba
W
0.6 -
-0
e
F 0.4
-
0.2 -
0.0
0.2
0.4
0.6
0.8
1.0
XPBLG
Figure 4. Composition dependence of the reduced viscosity at 25OC for solutions (c = 3 g/dL) of (0)ZnSPSl/
HPBLG blends and (@) ZnSPS5/HPBLG blends in DMF.
BEHAVIOR OF COMPLEXES OF PBLG
1
spectively. For both solvents, the viscosity not only
exhibited a substantial positive deviation from the
weighted average viscosity, but it also increased with
increasing aging time of the solution. The enhanced
viscosity was a consequence of intermolecular interactions between the polymers, and a maximum
viscosity was observed for both solvents at ca.
[ NH2]/[ SO3H] = 0.35. In this case, the maximum
viscosity does not coincide with the expected 1 : 1
.o
0.8
R
2087
0.6
0.4
0.2
0.0
-0.2'
0
'
'
" '
1
'
" '
2
.
" '
3
'
.
'
4
Figure 5. Viscosity enhancement factor, R , vs. blend
composition, [NH2]/[Zn2'] at 25OC for solutions ( c = 3 g/
dL) of (0)
ZnSPSl/HPBLG blends and (0)ZnSPS5/
HPBLG blends in DMF.
INH, 14 SO,H I
>
0
0.39
1.03
2.31
6.13
rn
0.0
0.2
0.4
0.6
0.8
1.0
5
n
5M
v
studies of similar transition metal complexes involving ZnSPS indicated that the stoichiometry of
the amine-Zn2+complex is 1 : l.'6*'7*27
If this were
so for the system under consideration in this study,
we would expect a maximum in R at [ NH,] / [Zn2+]
= 1.0. However, no maximum in R was observed for
the ZnSPSl /HPBLG solutions, which suggests that
either this system has a different stoichiometry for
the amine-ion complex or, more likely, that the efficiency of complexation was less than 100%. The
composition range probed for the ZnSPS5 /HPBLG
solutions was insufficient for determining whether
a maximum occurred in R at [NH2]/[Zn2+]= 1.0
(see Fig. 5 ) .
X~~~~
INH,l/ISO,Hl
0
0.39
1.03
2.31
6.13
Q)
010
0.2
0.4.
0.6
0.8
1O:
HSPS/PBLG Mixtures
HSPS and PBLG interact by proton transfer and
hydrogen bonding."*" Hydrogen bonding between
the sulfonic acid groups and the glutamate side
groups can occur at multiple sites on both the
HPBLG and HSPS chains. This generally resulted
in a physically crosslinked network and a gelatinous
precipitate when the sulfonic acid concentration was
greater than 2.5 mol 3'6 and when a relatively nonpolar solvent such as chloroform or dichloroethane
was used. Gelation could be avoided by using a low
sulfonation level ( HSPSl ) and the lower molecular
weight LPBLG.
Figures 6 and 7 show the reduced viscosity as a
function of composition and aging time after preparing the solution of HSPSl /LPBLG mixtures ( c
= 3 g/dL) in chloroform and dichloroethane, re-
R
XmLG
Figure 6. (a) Reduced viscosity and (b) viscosity enhancement factor at 25°C vs. composition of solutions ( c
= 3 g/dL) of HSPSl/LPBLG blends in chloroform for
different times after solution preparation: (0)
1 day, (0)
7 days, (V) 12 days, (V)21 days.
2088
SHAO, WEISS, AND LUNDBERG
tions more effective at increasing viscosity than the
end-group
complexation. That is, they yield a higher
0
0.39
1.03
2.31 6.13
m
3,
I
I
I
I
apparent
molecular
weight for the blend and, thus,
I
a higher solution viscosity.
The number of experimental compositions used
to
construct Figures 6 and 7 is insufficient to pinv- \
point
the exact location of the viscosity maximum,
n
but the data for the dichloroethane solutions, Figure
I
M
7, indicate that the location of the maximum was
W
not constant. The viscosity maximum for the fresh
dichloroethane solution was close to [NH2]/[S03H]
1
= 1, which is what would be expected if the ionic
interaction were predominant. With increasing aging time
of the solution, the maximum shifted to'
I
;
,
ward [NH2]/[S03H]= 0.35. It is significant that the
largest viscosity-aging effects occurred for [NH2]/
1
.
1
.
1
.
1
.
0
[S03H] < 1(i.e., when there was an excess of sulfonic
acid groups relative to amine groups). When there
was an excess of amine groups, very little if any viscosity enhancement or aging effect was observed (see
Figs. 6 and 7). The excess acid groups were free to
hydrogen bond with the glutamate side groups and
as discussed in a previous article," that interaction
induced a helix-to-coil transition of the PBLG chain
adjacent to the complex site. Transformation of the
chain conformation from helix to coil is also expected to increase the viscosity because of less hydrodynamic shielding of the molecule, and it also
should increase the accessibility of additional glutamate groups to the HSPS. The latter effect is
probably responsible for the time dependence of the
viscosity-that is, during aging of the solution, more
coil form of the PBLG is formed and the concentration of hydrogen bond interactions increase. The
rate of the viscosity increase upon aging increased
with an increase in the excess sulfonic acid concen0.0
0.2
0.4
0.6
0.8
1.0
tration for the limited amount of data available, and
(b)
this
result is consistent with the explanation for the
xpsuj
time effects given above. No time dependence of the
Figure 7 . (a) Reduced viscosity and (b) viscosity ensolution viscosity was observed when [NHz]/[S03H]
hancement factor at 25°C vs. composition of solutions (c
>
1 or when ZnSPS was used in place of HSPS.
= 3 g/dL) of HSPSl/LPBLG blends in dichloroethane
for different times after solution preparation: (0)1 day,
"H2l/lS03Hl
?
( 0 )7 days, (V) 14 days, (V)21 days.
stoichiometry of the ionic intermolecular complex
( i.e., -SO; - H2N-) ,because hydrogen bonding between the sulfonic acid species and the glutamate ester also occurs. Although the ionic interaction is the stronger and more favorable of the two,
the concentration of glutamate ester groups is much
greater than amine end groups. A single PBLG molecule can interact with more than one HSPS molecule by hydrogen bonding, making those interac+
Shear Rate and Time Effects
Because of the network-like structure formed in the
HSPS/PBLG solutions, those complexes may have
potential as rheology modifiers. As discussed in the
introduction to this report, shear can perturb the
interactions in associating polymer systems and
shear-thickening behavior has been observed with
ionomer solutions. The HSPSl/LPBLG solutions
were evaluated for shear rate and time effects. The
solutions were aged two weeks before the shear viscosity measurement, which was thought to be suf-
BEHAVIOR OF COMPLEXES OF PBLG
ficient time for achieving equilibrium, or at least
reproducible behavior.
Figure 8 shows the time dependence for HSPS1/
LPBLG solutions in chloroform for two different
compositions and concentrations at 1°C and a shear
rate of 90 s-'. The low temperature was used to minimize evaporation of the solvent. Thixotropic behavior was observed for both solutions. The rate of
the viscosity rise was greater for the more concentrated solution, which also had the lower excess of
sulfonic acid groups. A precipitate formed for both
samples during the experiment, which indicates that
the formation of the physical network in solution
was enhanced by the shear field. That is consistent
with the idea that the deformation of the polymer
molecules by the shear field makes specific sites on
the chain more accessible and promotes increased
intermolecular association.
A comparison of the viscosity changes upon aging
the chloroform and dichloroethane solutions (cf.,
Figs. 6 and 7) suggests that the extent of hydrogen
bonding complexation was lower for the mixtures
in dichloroethane. That conclusion is supported by
the lack of precipitate formation when the dichloroethane solutions were sheared. Dichloroethane is
also the less volatile solvent, and therefore, it was
used to investigate the effect of shear rate on viscosity of the HSPSl/LPBLG solutions. To eliminate the influence of thixotropy, all viscosities were
recorded after shearing the solutions for a fixed time
period of 15 min.
10
0
10
20
30
40
50
60
Time (min)
Figure 8. Time dependence of the viscosity of chloroform solutions of (0)60/40 (w/w) HSPSl/LPBLG (c = 5
g/dL) and ( 0 )80/20 HSPSl/LPBLG (c = 3 g/dL); T
= 1"C, shear rate = 90 s-'.
2089
v
'"p$i3g1
,
, .
I
0
0
50
100
150
200
81
250
Shear Rate ( 1/s)
Figure 9. Viscosity vs. shear rate for dichloroethane
solutions (c = 5 g/dL) of (0)HSPS1, ( 0 )LPBLG, (V)
80/20 PS/LPBLG, and (V)80/20 HSPSl/LPBLG; T
= 15°C.
Figure 9 compares the viscosity-shear rate behavior for solutions ( c = 5 g/dL) of HSPS1, LPBLG,
an 80/20 (w/w) mixture of HSPSl and LPBLG, and
an 80/20, noninteracting mixture of PS/LPBLG
blend. The total polymer concentration for each solution was c = 5 g/dL and the temperature was 15°C.
Over the shear rate range studied the solutions of
HSPS1, LPBLG, and PS/LPBLG appeared to be
Newtonian, though there may have been a small increase in the viscosity of the LPBLG solution at the
lower shear rates used. The viscosity of the PS/
LPBLG blend measured was between those of
HSPSl and LPBLG, consistent with eq. (1). The
viscosity of the HSPSl/LPBLG complex, however,
was significantly greater than that of either of the
component polymers, and a pronounced shear
thickening was observed up to a shear rate of at
least 250 s-'.
Figure 10 shows the concentration dependence of
the viscosity-shear rate behavior for the 80/20
HSPSl/LPBLG blends in dichloroethane. Shear
thickening was not observed when c = 1 g/dL, and
for 1 < c < 4 g/dL only a very weak increase in the
viscosity was seen between shear rates of 10 - 100
s-'. For c 2 4 g/dL, the shear-thickening behavior
was much clearer.
The effect of the mixture composition on the viscosity-shear rate behavior for solutions of HSPS1/
LPBLG is shown in Figure 11. The total polymer
concentration for each solution was c = 3 g/dL.
Shear thickening became more pronounced as the
HSPS/PBLG ratio increased (i.e., as [ NH,]/[SOBH]
2090
SHAO, WEISS, AND LUNDBERG
70,.
I
,
,
.
,
,
-
60-
a
h
8-O
50 -
/ O :
0
v
.-A
m
8
.-CA
c,
>
40
.
L
A
A
8
'
30 -0A
20
10
:
-
o~'o-o
OW'
-
-
-
0 -
I
a
=
,
.
'
'
' 1
Figure 10. Viscosity vs. shear rate for solutions of 80/
20 HSPSl/LPBLG at 15OC for different concentrations
in dichloroethane: (0)5 g/dL, (A) 4 g/dL, (0)3 g/dL, (m)
2 g/dL, and ( 0 )1 g/dL.
decreased). This result is consistent with the conclusion that a n excess of sulfonic acid groups relative
to the amine group concentration is responsible for
the unusual time and shear effects in this system
(i.e., as more acid groups are available for complexation with the glutamate ester groups). For the 60/
40 HSPSl/LPBLG solution, the shear-thickening
behavior was preceded by shear-thinning up to a
shear rate of ca. 50 s-'. T h e 40/60 mixture also
showed shear thinning at low shear rates, and the
limited data in Figure 11 indicate that this solution
may also have exhibited weak shear thickening between shear rates of ca. 50-100 s-l, though that is
not a s evident as it was for the solutions with higher
ionomer composition. The solution of the 20/80
mixture exhibited only very weak shear thinning and
no shear thickening. T h e absence of shear thickening for this mixture suggests that all the sulfonic
acid groups were complexed with terminal amine
groups from PBLG and therefore none were available for interaction with the glutamate ester groups.
For the 8O/ZO composition (i.e-, where the excess
sulfonic acid group concentration was the greatest)
only shear thickening behavior was observed.
It is not apparent why the viscosity of the 60/40
HSPSl/LPBLG solution is greater than for the 80/
20 blend a t a shear rate of 10 s-l (see Fig. ll),though
several explanations come t o mind. First, this may
simply be a n artifact due t o a failure to completely
account for the differences in the thixotropy of the
two solutions (e.g., see Fig. 6). The unusual behavior
of a shear-thickening region following a shear-thin-
ning region may, however, arise from dynamic
changes in the structure of the interpolymer complex. This is discussed further below, but the essence
of this argument is t h a t the structure progresses
from one of predominantly acid-amine complexes
t o one containing a significant amount of acid-ester
associations. The latter promote conversion of parts
of the PBLG chain from helical to coil conformation,
which should increase the solution viscosity. One
other possible explanation is that the solution of the
80/20 mixture did exhibit shear thinning a t shear
rates less than 10 s-'.
Although the qualitative trends in the viscosityshear rate data were reproducible upon cycling the
shear rate up and down, the viscosities systematically increased for each cycle. This is probably a
consequence of the thixotropic nature of the solutions (i-e., due t o the increasing development of sulfonic acid-glutamate interactions with time a s discussed earlier). This nonequilibrium characteristic
of these solutions probably also accounts for some
of the scatter in the viscosity-shear rate data.
In contrast to the solution rheology of the mixtures containing HSPS, solutions made with ZnSPS
exhibited no shear-thickening behavior (Fig. 12).
Shear-thinning behavior was observed for all mixture compositions studied up t o a shear rate of ca.
100 s-', and above that shear rate the viscosity remained relatively constant. The solution viscosities
exhibited no time-dependence. These results further
support the origin of the complex rheological behavior described for the solutions containing the
F
0
0
50
100
150
200
250
Shear Rate (Us)
Figure 11. Viscosity vs. shear rate for dichloroethane
solutions of HSPSl/LPBLG (c = 3 g/dL) at 15OC as a
function of composition: (V)80/20 w/w, (V)60/40, ( 0 )
40/60, and (0)
20/80.
BEHAVIOR OF COMPLEXES OF PBLG
6
5
h
34
W
.g
3
0
2
-5
2
01
0
"
50
'
I
100
"
'
150
I
200
'
I
250
Shear Rate (Us)
Figure 12. Viscosity vs. shear rate for DMF solutions
of ZnSPS5/HPBLG ( c = 3 g/dL) at 25°C as a function
of composition: ('I
80/20
) w/w, (V) 60/40, ( 0 )40/60, and
(0)20/80.
sulfonic acid derivative as being hydrogen-bonding
between the sulfonic acid and the glutamate ester,
since that interaction does not occur with the ionomer salt.22
CONCLUSIONS
Complex formation between the SPS ionomers and
PBLG was manifested by an enhancement of the
solution viscosity uiz Ci viz a weighted average of the
viscosities of the individual component polymer solutions. For ZnSPS/PBLG solutions, only a single
type of interaction occurred-that being a transition
metal complex of the Zn2+with the terminal amine
group of the PBLG. In this case, the viscosity enhancement increased monotonically with increasing
PBLG concentration over the range of compositions
evaluated. A maximum in the viscosity enhancement
was not observed at the expected stoichiometry of
the ion-amine complex (i.e., [ NH2]/ [ Zn2+] = 11.
That result suggests that the efficiency of complexation was not 100%and an excess of the polypeptide
was required to maximize the extent of complex formation. The ZnSPS / PBLG solutions exhibited
neither thixotropic nor dilatant rheological behavior.
For all compositions between 20-80% PBLG, these
solutions exhibited shear-thinning behavior up to
shear rates of about 100 s-l, above which the viscosity was relatively insensitive to increasing shear
rate.
2091
In contrast, solutions containing complexes of
HSPS and PBLG were thixotropic and exhibited
shear-thickening behavior. These two polymers may
interact through proton transfer between the sulfonic acid group from the ionomer and the amine
end group of PBLG, as well as by hydrogen bonding
between the sulfonic acid and the glutamate ester.21,22For blend compositions corresponding to
[ NH2/S03H] > 2, the solution viscosity was neither
time-dependent nor dilatant, or only weakly so. The
as-prepared solutions showed a maximum viscosity
enhancement at the stoichiometric composition of
the acid-amine complex, [ NH2/SOBH]= 1, though
with increasing aging time of the solution the maximum in the viscosity enhancement shifted to a
lower ratio of [ NH2/S03H]. For this system, the
acid-amine interaction is the more enthalpically favored one, and as long as there was a suitable excess
of amine groups, all of the sulfonic acid groups were
probably complexed with amine groups. In that case,
the solution viscosity was stable with time and the
solutions exhibited only shear-thinning behavior.
When [NH2/S03H] < 2, the HSPS/PBLG solutions were thixotropic and shear-thickening behavior was observed. For the composition corresponding to [ NH2/S03H] 1,the solution viscosity
decreased with increasing shear rate up to ca. 50
s-l, above which dilatant behavior occurred. All of
these observations are consistent with a time- and
shear-dependent complexation between sulfonic acid
groups and the glutamate ester side group of PBLG.
In addition, that association promotes a helix-tocoil transition of the part of the PBLG chain adjacent to the complex,21and coil-formation may provide an entropic driving force for further acid-ester
association. As a result, a highly nonequilibrium
structure of the interpolymer complex results, which
explains the time and shear dependencies of the viscosity for these solutions.
Eventually, as a network-like structure of these
solutions develops from random acid-ester associations, either gelation or precipitation is expected
to occur. Gel or precipitate was promoted when the
sulfonation level of the ionomer was ca. 5 mol ?6 or
when the higher molecular weight PBLG was used.
In some cases, the dynamics of the precipitate formation was very slow and it was not unusual to observe precipitation continuing for weeks or even
months following mixing of some of these solutions.
Although there remain many unanswered questions
as to the structure of the complexes formed and their
rheological behavior, it is clear from this study that
they exhibit complicated and unusual rheological
properties as a consequence of nonequilibrium as-
-
2092
SHAO, WEISS, AND LUNDBERG
sociations, which may have applicability for viscosity
modification or time-release phenomena.
This research was supported by a grant from the Exxon
Education Foundation.
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Received January 19, 1995
Revised April 19, 1995
Accepted May 5, 1995