Effect of Sodium Poly(styrene sulfonate) on
Thermoreversible Gelation of Gelatin
J. R. GILLMOR, R. W. CONNELLY, R. H. COLBY,* J. S. TAN
Imaging Research and Advanced Development, Eastman Kodak Company, Rochester, New York 14650-2116
Received 1 February 1999; accepted 21 April 1999
ABSTRACT: The effect of an added polyanion, sodium poly(styrene sulfonate) (NaPSS),
on the thermoreversible gelation and remelting of gelatin gels has been investigated by
polarimetry and rheology. The presence of NaPSS can either enhance or reduce collagenlike helix formation, depending on the polymer concentration relative to that of
gelatin and the gelation temperature. At temperatures , 20°C, the helical content is
reduced by increasing the amount of added NaPSS, demonstrating the disruption of
helical structure of gelatin by the polyanion. Synchronous measurements of optical
rotation and modulus at 25°C, in both gelation and remelting, indicate that the optical
rotation at the gel point for the pure gelatin is lowered on addition of NaPSS. At low
frequency, the storage modulus of gelatin is increased by the addition of a small amount
of NaPSS relative to that of gelatin, but decreased with excess NaPSS. The mechanical
properties of gelatin with and without NaPSS will be discussed in light of the competition between network junction formation by strands of triple helices among gelatin
chains and temporary ionic crosslinking between gelatin and the polyanion. © 1999 John
Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2287–2295, 1999
Keywords: thermoreversible gel; gelatin; polyelectrolyte; helix formation; optical
rotation; gel point; modulus; remelting; network formation; polyelectrolyte/gelatin complexation
INTRODUCTION
Gelatin is a well-known protein (denatured collagen) that is used extensively in the food and photographic film industries. Collagen is the major
component of bone, skin, and tendons. The native
collagen consists of three helical peptide chains (a
chains) held together by hydrogen bonding. On
thermal denaturation and degradation by physical treatment and mild hydrolysis, collagen is
transformed into gelatin. It is composed of predominantly a chains and smaller portions of
dimers and trimers, frequently referred to as b
Correspondence to: J. S. Tan (E-mail: jstan@kodak.com)
* Current address: Department of Materials Science and
Engineering, Pennsylvania State University, University Park,
Pennsylvania 16802
Journal of Polymer Science: Part B: Polymer Physics, Vol. 37, 2287–2295 (1999)
© 1999 John Wiley & Sons, Inc.
CCC 0887-6266/99/162287-09
and g chains, respectively.1,2 Each a chain has
about 1000 peptide residues with a specific known
sequence of 18 amino acids, some of which are
acidic and others which are basic, depending on
the degree of ionizations. There are 94 basic and
130 acidic groups per a chain (molecular weight
' 1 3 105).2 In aqueous solutions . 40°C, the
configuration of the gelatin chain resembles that
of a random coil polymer. When cooled to , 30°C,
the gelatin chain can partially renature, forming
inter- as well as intrachain collagenlike triplestranded helices. The thermal gelation mainly involving hydrogen bonding is associated with a
negative enthalpy of the order kT at room temperature.1 These helices act as junction points of a
gel network.3– 6 If subsequently heated to 40°C,
the helices remelt, and the gelatin reverts to its
random coil configuration. Gelatin is one of several examples of biopolymers that form thermor2287
2288
GILLMOR ET AL.
eversible gels.7 This article focuses on the gelation and subsequent remelting behavior of gelatin
gel.
The triple helix of gelatin is optically active,
which allows us to monitor the extent of helix
formation on gelation using a polarimeter.8 –11 We
first study gelation by quenching an aqueous solution of gelatin to a constant temperature, while
monitoring changes in optical rotation and modulus with time. The resulting growth of modulus
as triple helices are formed allows us to apply
percolation ideas to understand the gelation process. Once the gel network has formed, we then
study remelting by heating the gel at a constant
heating rate.
We can also directly assess the roles of various
additives, such as surfactants or polyelectrolytes,
on the gelation and subsequent remelting of thermoreversible gelatin gel. The polyanion, sodium
poly(styrene sulfonate) (NaPSS), is known to interact with gelatin,12,13 forming a soluble polyelectrolyte–protein complex in dilute aqueous solution. The surfactant, sodium dodecyl sulfate,
is also known to form micelles associated with
gelatin.14 –17 Here we focus our attention on understanding the effect of NaPSS on the thermoreversible process. Although the polyanion interacts strongly with gelatin through ionic interaction between the polyanion and the positively
charged amino acids, it is not obvious what effect
the polyanion will have on the network formation
of the thermoreversible gel. On the one hand, the
interaction between the polyanion and gelatin
may involve multiple gelatin chains and the interaction sites may provide junction points in the
network. On the other hand, the strong interaction may perturb the triple helix formation among
gelatin chains leading to reduced and delayed
gelation. We will show that both of these effects
occur, and which effect dominates over the other
depends on the quenching temperature and concentration of the added polyanion relative to that
of gelatin.
EXPERIMENTAL
Materials
The gelatin used was a type IV alkaline-processed
gelatin with an isoelectric pH 5 4.9 and a weightaverage molecular weight of ; 2 3 105, containing a mixture of the a, b, and g chains. An aqueous solution of a 3 wt % gelatin has a conductivity
of 0.3 mS/cm, corresponding to a salt content of
; 3 mM. For the alkaline-processed gelatin, the
net charge per gelatin1 with a molecular weight of
1 3 105 is approximately 220 at pH 5 5.65.
The homopolymer, NaPSS, was prepared in
water by free-radical polymerization of sodium
styrene sulfonate, using K2S2O8 as the catalyst.
Sodium styrene sulfonate (100 g) and water (600
mL) were introduced into a three-neck 1 L flask
equipped with a stirrer and a condenser. The
solution was bubbled with nitrogen for 30 min
and then placed in a 55°C bath. A small amount of
the catalyst (0.128 g) was added and the solution
was stirred under nitrogen for 16 h. The viscous
mixture was dialyzed for 2 days and concentrated
to yield a 9.2% solution. The polyanion was characterized by size-exclusion chromatography and
light scattering to yield a weight-average molecular weight of 1.4 3 106 and a polydispersity
index of 3.0.
All aqueous gelatin solutions were prepared at
45°C using slight agitation for 30 min. Appropriate amounts of an aqueous NaPSS stock solution
were then introduced to prepare the gelatin/
NaPSS mixtures to a final gelatin concentration
of 3 wt % with various NaPSS concentrations. The
value of pH changes from 5.75 for a 3 wt % gelatin
solution to 6.3 on addition of 0.6 wt % NaPSS.
Polarimetry
A Perkin–Elmer Model 241 Polarimeter (PerkinElmer Corporation, Newark, Connecticut) employing plane-polarized light from the sodium D
line (589 nm) was used to measure the optical
rotation of samples during gelation and remelting. A water-jacketed cell with 10-cm path length
was used. After filling the cell with the solution
and equilibrating at 45°C, water of the desired
temperature (between 10 and 30°C) was circulated through the water jacket to quench the sample within 1 min and the optical rotation was
monitored as a function of time at a fixed temperature. After incubating for 24 h, the temperature
of the circulating water was raised at a rate of
0.3°C/min and the optical rotation was measured
as the gel remelted. Although the precision of the
angle of optical rotation, a, measurement is about
60.01°, the reproducibility may be changed to
60.05°, because of the equilibration time required
for quenching, particularly for time-dependence
experiments.
NaPSS ON THERMOREVERSIBLE GELATION
2289
RESULTS AND DISCUSSION
Gelation of Gelatin
Figure 1. Sketch of the concentric cylinder cell design for the oscillatory shear rheometry measurements.
Rheology
A Rheometrics RMS-800 (Rheometric Science,
Inc., Piscataway, NJ) was used in the oscillatory
shear mode with a sealed concentric cylinder geometry. A standard 20-mL glass scintillation vial
was coupled to the motor and a cylindrical aluminum bob was coupled to the transducer, using
specially designed fixtures as shown in Figure 1.
After equilibrating each cell containing the sample solution at 45°C, the sample cell was placed in
a 25°C water bath and stirred for 1–2 min to
reach equilibrium. The cell was then mounted in
the rheometer, thermostated at the same temperature, and the increase in modulus was monitored
as a function of time up to 16 h, by using a frequency of 1.0 rad/s and a strain amplitude of 0.1.
The frequency dependence of the complex modulus of the gel, after 24 h incubation at 25°C, was
then measured using a strain amplitude of 0.04.
The complex modulus of the gel at a frequency of 1
rad/s was measured during remelting at a fixed
heating rate of 0.3°C/min. The reproducibility of the
rheology measurement depends on how fast the
sample can be brought to the desired quenching
temperature (within 1–2 min). The measurement of
the complex modulus has a relative error of 63%.
Optical rotation is used to monitor the relative
extent of triple helix formed during gelation, although the proportionality constant between the
number density of helices and optical rotation is
expected to vary with temperature. The rate and
the extent of helix formation depends on gelatin
concentration and temperature.18 The effects of
temperature and time are shown in Figure 2 for
an aqueous 3 wt % gelatin solution, where a is
plotted against time for samples quenched and
equilibrated at various temperatures from 10 –
30°C. The helical structure as measured by the
optical rotation is built up more rapidly as the
quenching temperature is lowered. Above 40°C,
where the gelatin is in its denatured state, a
gelatin solution (3 wt %) rotates plane-polarized
light approximately 23.9° with a 10-cm path
length.1 At lower temperatures, the gelatin is partially renatured, forming a triple helix structure
that rotates the light even more, approaching 28°
after 1000 min at 10°C (see Fig. 2).
Gelation of Gelatin/NaPSS Mixture
The time dependence of a for gelatin (3 wt %
throughout the article) solutions with various
concentrations of NaPSS when quenched to 10
and 25°C are shown in Figure 3(a). For the 10°C
data, a decreases monotonically with increasing
Figure 2. Time dependence of optical rotation for
gelatin (3%) quenched to various temperatures. The
curves are drawn through the data points for visual aid
only.
2290
GILLMOR ET AL.
Figure 3. (a) Time dependence of optical rotation for
gelatin (3%)/NaPSS (E: 0%, h: 0.04%, ‚: 0.08%, ƒ:
0.17%, {: 0.33%) mixtures quenched to 10 and 25°C
(the symbols presented at time . 103 min are actual
data points), respectively; (b) NaPSS concentration dependence of optical rotation for gelatin (3%) and gelatin
(3%)/NaPSS mixtures quenched to 10 and 25°C, respectively. The curves are drawn through the data points
for visual aid only.
NaPSS concentration, C NaPSS [see Fig. 3(b), curve
for a’s at 60 min]. This is clearly a result of a
complex formation between gelatin and NaPSS
that disrupts and delays triple helix formation
among gelatin chains. Based on the estimated
overlap concentration of the NaPSS sample used
in the present work [(M w 5 1.4 3 10 6 , R g > 120
nm in 0.01M salt solution measured by light scattering), i.e., C* 5 M w /{(4/3) p R 3g N } ' 0.03 wt
%], the NaPSS chains in the concentration range
studied here overlap. In this NaPSS concentration range, some of the gelatin chains are bound
to single NaPSS chains and some of them are
acting as bridges between NaPSS chains.13 Ap-
parently, this binding disrupts the interchain helix formation among gelatin molecules.
For the 25°C data, on the other hand, changes
in a are smaller because of the smaller negative
enthalpy involved in gelation of gelatin.1 Since
there is a higher degree of mobility of gelatin
molecules at 25°C compared to that at 10°C, it is
harder to align the gelatin side-chains for helical
formation. The helix formation after 3 h at this
temperature does not show a monotonic decreasing function of C NaPSS [see Fig. 3(b), curve for a’s
after 3 h]. At low NaPSS concentration, a increases slightly with C NaPSS, possibly a result of
localization of gelatin chains along the polyanion
backbone. This localization effect causes enhanced neighboring intrachain, as well as interchain, triple helix formation of the gelatin chains.
Apparently this localization effect overrides the
disruption effect on helix formation as observed
earlier at 10°C in the low NaPSS concentration
region. At high NaPSS concentration above the
overlap concentration, bridging by a gelatin molecule between polyanion chains is prevalent.
Therefore, the disruption of interchain helix formation among gelatin molecules may dominate
and reduce the overall optical rotation. The results in Figure 3(b) suggest that the quenching
temperature is an important factor for determining not only the magnitude of optical rotation but
also its NaPSS concentration dependence.
The kinetics of triple helix formation in gelatin
has been claimed to be pseudo-first order based on
roughly linear plots of ln(d a /dt) versus time.19
The curvatures of similar plots derived from our
data for the pure gelatin and gelatin/NaPSS mixtures, shown in Figure 4, for solutions quenched
to 10°C, demonstrate that the helix formation is
more complicated than a simple first-order process. Figure 4 demonstrates that the qualitative
character of helix formation kinetics is not profoundly affected by addition of NaPSS.
To study the correlation between modulus and
optical rotation and the effect of added NaPSS, we
made synchronous measurements of rheology and
polarimetry. Figure 5(a,b) shows the time dependence of the modulus (G9 at 1 rad/s with strain
amplitude 0.1) and optical rotation, respectively,
for the gelatin and the gelatin/NaPSS (0.08%)
mixture at three different temperatures (10, 20,
and 25°C). Figure 5(a) clearly shows that the
modulus is enhanced by the addition of the polyanion, to a degree that decreases as the temperature is lowered. The relative importance of the
contribution to modulus by ionic junctions de-
NaPSS ON THERMOREVERSIBLE GELATION
Figure 4. First-order kinetic analysis for optical rotation for gelatin (3%)/NaPSS (E: 0%, h: 0.04%, ‚:
0.08%, ƒ: 0.17%, {: 0.33%) mixtures quenched to 10°C.
The curvature seen for all samples indicates that the
kinetics are not simple first order, but the rates are
independent of NaPSS concentration.
2291
mately 1-min sample loading time in the rheometer does not adversely affect the results. We find
that a wide range of values of agel and t can
describe our data reasonably. Figure 6(a) shows
the combinations of agel and t that fit our data for
all time points at 25°C [shown in Fig. 5(a,b)].
Regardless of the value of t used, agel is always
smaller when NaPSS is present. Thus the polyanion contributes effective crosslinks that lower
the amount of helix needed to make a gel. This
conclusion is robust as long as the exponent t does
not change when NaPSS is added. Three parameter fits of our data to eq. (1) yield an optimum
value of t 5 2, in reasonable agreement with
literature determinations of t 5 1.82 and 1.9 for
gelatin22,23 and other biopolymer physical gels.27
creases, possibly because the contribution by helix formation of gelatin at the lower temperatures
is much higher.
In contrast, NaPSS lowers the amount of helix
formation [see Fig. 5(b)], and the extent of lowering increases slightly as temperature is lowered
from 20 to 10°C. Furthermore, at 25°C the effect
of NaPSS on optical rotation is reversed. Taken
together, Figure 5(a,b) shows that NaPSS participates in the network in two ways. The polyanion
contributes effective crosslinks that raise the
modulus but also interferes with helix formation
at lower temperatures.
These results may be interpreted in terms of
the percolation model of gelation. According to
theories,20 G9 is related to the number of
crosslinks by a power law. Here, we assume that
the measured optical rotation is proportional to
the number density of crosslinks. The fact that
percolation ideas apply to physical gels, and gelatin gels in particular, is well documented.3,4,21–26
We use eq. (1) to determine the optical rotation at
the gel point (agel),
G9 5 A~ a 2 a gel! t
(1)
In this equation, A is an empirical constant and t
is the critical exponent relating to the growth of
the modulus beyond the gel point.20 The utility of
eq. (1) is limited to data at 25°C, where the helix
formation is sufficiently slow that the approxi-
Figure 5. Time dependence of (a) the storage modulus at a frequency of 1 rad/s with strain amplitude 0.10
for gelatin (3%) (open symbols) and gelatin (3%)/NaPSS
(0.08%) (filled symbols) at 10, 20, and 25°C, respectively; (b) optical rotation (accuracy 60.05°) for gelatin
(3%) (open symbols) and gelatin (3%)/NaPSS (0.08%)
(filled symbols) at 10, 20, and 25°C, respectively.
2292
GILLMOR ET AL.
tentative at best, because more experimental data
at different temperatures and with different
NaPSS concentrations are needed for such a conclusion.
Mechanical Properties of Gelatin Gels with and
without Added NaPSS
Figure 6. Percolation theory analysis of gelation. (a)
Dependence of optical rotation at the gel point on the
critical exponent t. These points were obtained by fixing t and using a two-parameter fit to eq. (1) for the
25°C data shown in Figure 5 to obtain agel and A; (b)
demonstration of universality if t 5 2 is assumed, by
plotting the storage modulus against relative extent of
helix formation, « 5 (a 2 agel)/agel. In both plots, the
open symbols are for gelatin (3%) and the filled symbols
are for gelatin (3%)/NaPSS (0.08%).
Gels prepared in solution22,28,29 seem to generally
have t 5 1.9, consistent with the prediction of de
Gennes.30 At t 5 2, as shown in Figure 6(a), we
find that agel 5 24.06° for the gelatin solution and
23.78° on addition of NaPSS (0.08%).
Figure 6(b) compares the power laws using t
5 2 for the gelatin and the gelatin/NaPSS (0.08%)
mixture quenched to 25°C, where the relative extent of helix formation is « 5 (a 2 agel)/agel. This
plot apparently demonstrates the universal behavior (with and without NaPSS) in the scaling of
modulus with the extent of helix formation « relative to the gel point. This universal behavior is
Djabourov9,31,32 used optical rotation measurements to show that network formation in gelatin
is not complete even after 1000 h. For practical
purposes, we chose a gel formation time of 16 h for
our experiments, at which point the network formation is no longer rapid. Whereas network formation is by no means complete, it is sufficiently
slow that we can obtain reproducible data in oscillatory shear at frequencies as low as 0.01 rad/s.
The frequency dependence of storage modulus
with strain amplitude 0.04 is plotted in Figure
7(a) for gelatin and gelatin with various amounts
of added NaPSS. Shown in Figure 7(b) is the plot
of G9 (at frequency 5 0.02 rad/s and strain amplitude 5 0.04) versus C NaPSS.
The helix formation of gelatin can either be
enhanced or reduced depending on the concentration of the added NaPSS relative to that of gelatin, as shown in Figure 3(a,b). Similarly, the network formation process can also be enhanced or
reduced on addition of NaPSS. At low frequency
(0.02 rad/s), the maximum appears around
C NaPSS ' 0.3%, corresponding to a weight ratio of
r 5 gelatin/NaPSS ' 10. Earlier light scattering
data13 showed that in the low C NaPSS region (gelatin-rich region, i.e., r $ 10), the polyanion added
is saturated with the bound gelatin. In the low
NaPSS concentration region, as NaPSS is added
to gelatin, two competing effects result. NaPSS
chains participate in the network, owing to the
fact that about 90 –100 gelatin chains are ionically bound to each NaPSS molecule (i.e., each
parent NaPSS with a molecular weight of 1 3 106
will build up a complex with a molecular weight of
2 3 107).13 This effect increases the gel modulus
as well as the helix formation, but saturates
around r ' 10.
In the high C NaPSS region (gelatin-starved region, i.e., r # 10) above the overlap concentration
of NaPSS, there are unoccupied binding sites on
NaPSS that lead to bridging by the gelatin between polyanion chains. As a consequence, the
interchain gelatin helix formation is disrupted, as
shown by the decrease in the optical rotation in
Figure 3(b). Hence, the two competing effects
cause the appearance of the maximum in Figure
NaPSS ON THERMOREVERSIBLE GELATION
2293
squares, corresponding to the right-hand-side
axis) clearly indicate that triple helices melt over
a wide range of temperatures between 25 and
40°C. This broad transition behavior is similar to
those reported for other biopolymers.33,34 In contrast, the modulus data (open triangles, corresponding to the left-hand-side axis) drop sharply,
even on a linear scale, between 27 and 32°C, also
similar to the sharp transition in G9 observed for
other biopolymers.35 The modulus of pure gelatin
indicates liquid behavior at 32°C. The optical rotation at 32°C is about 24.3°. This value is similar to the gel point of agel 5 24.06° in the gelation
process as described in Figure 6(a).
In the case of the gelatin/NaPSS mixture [Fig.
8(b)], the modulus (filled triangles, corresponding
to the left-hand-side axis) indicates liquid behav-
Figure 7. (a) Frequency dependence of storage modulus (with strain amplitude 0.04) for gelatin (3%) with
various amounts of added NaPSS quenched to 25°C for
24 h; (b) NaPSS concentration dependence of storage
modulus at a frequency 0.02 rad/s with strain amplitude 0.04 for gelatin (3%) with various amounts of
added NaPSS quenched to 25°C for 24 h. The curve is
drawn through the data points for visual aid only.
7(b). It is also noted that the network junctions
that result from the gelatin/NaPSS interaction
are only temporary, as evidenced by the strong
frequency dependence of G9 in Figure 7(a) at high
NaPSS concentration.
Remelting
The modulus and optical rotation data collected
during melting (heating at 0.3°C/min) of a gelatin
gel and a gelatin/NaPSS (0.08%) mixture equilibrated at 20°C for 24 h are presented in Figure
8(a,b), respectively. In the case of pure gelatin
[Fig. 8(a)], the optical rotation data (open
Figure 8. Remelting at 0.3°C/min for (a) gelatin (3%)
(open symbols) and (b) gelatin (3%)/NaPSS (0.08%)
(filled symbols) after quenching to 20°C for 24 h. Triangles are storage modulus (left axis) at a frequency 1
rad/s with strain amplitude 0.10 and squares are optical rotation (right axis). The optical rotation at the gel
point from remelt, estimated at the temperature where
the storage modulus extrapolates to zero, are shown.
2294
GILLMOR ET AL.
Table I. Optical Rotations at the Gel Point for
Gelation and Remelting Processes
Gelatin (3%)
Gelatin (3%)
/NaPSS (0.08%)
The authors acknowledge the synthetic support from
Wayne Bowman. We also appreciate discussions with
Drs. C. P. Lusignan, M. Rubinstein, and T. H. Whitesides.
agel
(Gelation)
agel
(Remelting)
24.06°
24.3°
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23.78°
23.75°
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ior at 33.5°C with the corresponding optical rotation (filled squares, corresponding to the righthand-side axis) of 23.75°. Table I shows the comparison of the optical rotation data for pure
gelatin and the gelatin/NaPSS mixture for both
the gelation and the remelting processes. This
result confirms the conclusion from Figure 6(a)
that the gel point for the gelation process is lowered by NaPSS imparting effective junction
points.
CONCLUSIONS
The extent of helix formation and storage modulus of gelatin are affected by the addition of
NaPSS, depending on temperature and the relative amount of the added NaPSS compared to
gelatin. At low temperatures (#20°C), helix formation is reduced because the added NaPSS disrupts gelatin interchain helix formation. The lowfrequency modulus of gelatin at 25°C is increased
when a small amount of NaPSS relative to gelatin
is added, but decreased when excess NaPSS is
added. The maximum modulus corresponds to the
stoichiometric amount of gelatin that is just
enough to bind to NaPSS. For NaPSS concentration below this stoichiometric point, there is excess free gelatin, and the contribution of gelatin/
NaPSS ionic binding raises the modulus as
NaPSS is added because the ionic binding sites
act as the temporary network junctions. For concentration above the stoichiometric point, gelatin
chains bridge between different NaPSS chains,
severely hampering helix formation, thereby
causing the modulus to decrease on the addition
of excess NaPSS. Combining optical rotation and
modulus measurements, both during gelation and
remelting, we have shown that the amount of
helix formation required to reach the gel point is
reduced by the addition of NaPSS at 25°C. This
finding is consistent with the ionic binding sites
acting as temporary network junctions.
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