Langmuir 1991, 7, 608-609
608
Gels from Dilute Polymer/Surfactant Solutions
SDS SOLUTION
+
5o00-
1 % POLYMER JR 400
P. S. Leung and E. D. Goddard’ Union Carbide Chemicals and Plastics Company, Inc., Specialty Chemicals Division, Tarrytown, New York 10591
Introduction In a previous publication we drew attention to the remarkable increases in viscosity that can occur on adding a small quantity of anionic surfactant to a solution of a cationic cellulose polymer.1 See Figure I. Such viscosity increases were not observed when a polycation based on vinyl chemistry replaced the cellulosic. We argued that “structure”development in the former case involved crosslinking via surfactant ions which are bound to the relatively “stiff” cellulosic chains. Recently we demonstrated2 that, when the highest available molecular weight grade of cationic cellulosic polymers was used in the experiments, gels of considerable strength rather than viscous solutions were formed and the manifestation seemed to be relatively indifferent to the particular anionic surfactant used. Gels are viscoelastic materials, most properly examined by oscillatory rheological measurements. The second publication referred to above reports the results of such measurements. In the present Note we provide data and attempt a preliminary analysis of the rheology behavior observed.
0
2
I - ’-’
3000-
I
g
,
II -125125/
$
0 L
!=
loo-
PRECIPITATION:
I
I
!
w
I
-I O.&l
0.&1
0.k
I 0.1
(1) Leung, P. S.; Goddard, E. D.; Han, C.; Glinka, C. J. Colloids Surf. 1985, 13, 47.
(2) Goddard, E. D.; Leung, P. S.; Padmanabhan, K. P. A. 16 Intl. Congr., IFSCC, NY, NY 1990. (3) Ferry, J. D. Viscoelastic Properties of Polymers; John Wiley and Sons: New York. 1980.
0743-7463/91/2407-0608$02.50/0
10.0
10
5
Lo
m
2 .
LL
. E
m
1 % U
The methods and chemicals are all the same as previously described.2 In brief the polymer used, Polymer J R 30M from Union Carbide Corp., is a high molecular weight cationic cellulosic. The anionic surfactant used was SDS, sodium dodecyl sulfate, from EM Industries. Rheological measurements were done with a Bohlin VOR Rheometer a t 25 “C.
havior.
1.o
SDS CONCENTRATION Figure 1. Relative viscosity of 1% Polymer J R 4 W S D S mixtures as a function of SDS concentration (%).
Experimental Section
Results and Discussion As will be seen, the gels formed from dilute solutions of the polymer, especially in the presence of interacting surfactant anion, show many types of viscoelastic behavior within the wide range of time and frequency space we examined. The objective is to try to formulate a mechanical model, for example based on a combination of Maxwell elements, which will approximate the behavior of each type of system and yield corresponding viscoelastic and relaxation parameters. We start with results obtained with a lower molecular weight homologue of the cationic cellulosic polymer: a t 2 c( concentration this polymer (JR 400) showed simple viscoelastic (Maxwellian) behavior inasmuch as a typical “terminal zonen was encountered in which the slopes of the log G’and log G”versus log w plots approached values of 2 and 1, re~pectively.~ The cross-over point of G’ and G” yields a single relaxation time. The higher molecular weight polymer (JR 30M) at 1 % concentration demonstrated similar, if somewhat more complicated, behavior inasmuch as the slopes of log G’/log w and log G”/log w were no longer 2 and 1 within the measured frequency range, and cross-over occurred at a lower frequency, viz., ca. 5 Hz. See Figure 2. It is likely that a higher level of chain entanglement accounts for this difference in be-
I
75-
0.5
0.2
Frequency
Hz
Figure 2. Viscoelastic parameters: elastic modulus, G’, loss modulus, G”, phase angle, 6, and dynamic viscosity, v’, versus oscillation frequency for 1% Polymer J R 30 M.
A much studied, “simple” gelling system is that based on cetyltrimethylammonium bromide (CTAB) with added sodium salicylate4 and was included in our experiments for comparison. Many investigators have shown the formation of rodlike aggregates in this ~ y s t e m . ~Figure -~ 3 presents a modulus/frequency plot of 5 % CTAB/5% sodium salicylate solution. The same terminal zone frequency dependence can be seen in this gelling solution. Within the frequency space of the measurements, G” shows a maximum and G’ a plateau with a cross-over, which is again consistent with single Maxwell element behavior. The frequency of cross-over of G’ and G” yields a T value of 2.5 s, which is in good agreement with the value obtained from the maximum in stress decay rate of the measured relaxation spectrum (not shown). The above systems represent rather simple viscoelastic behavior in which the stabilizing network is characterized by relatively few cross-links or by relatively weak crosslinking bonds. As will now be demonstrated in Figures 4 and 5 the rheological behavior of the two-component (4) Gravsholt, S. In Polymer Colloids I I ; Fitch, R. M., Ed.; Plenum: New York, 1980; pp 405-417. (5) Rehage, H.; Hoffmann, H. Rheol. Acta 1982, 21, 256. (6) Skikata, T.; Hirata, H. Langmuir 1989,5, 398. (7) Strivens, T. A. Colloid Polym. Sci. 1989, 267, 269.
0 1991 American Chemical Society
Notes
Langmuir, Vol. 7, No. 3, 1991 609
250
200 v1
p. 150
. E
m + m
100
-0
50
0
Frequency Hz Figure 3. Viscoelastic parameters versus oscillation frequency for 5 70 cetyltrimethylammonium bromide-5% sodium salicylate mixture.
polymer, surfactant systems is much more complicated than that of the simple polymer or cetyltrimethylammonium salicylate solutions and now represents more pronounced levels of cross-linking. When SDS was added to the more dilute (0.2 % 1 solution of the polymer, in amount to realize maximum gel strength, minima in G" and in tan 6 were found in the measured frequency range; these indicate at least two regions with relaxation processes of longer and shorter times as could be represented by more complex arrangements of springs and dashpots, for example one that includes two Maxwell elements in parallel.6 The above mentioned regions would correspond to relaxations through local configuration adjustment and longer range rearrangements of couplings through entanglement and cross-linking. It is safe to assume that these factors are playing an increasingly important role in the two-component solute system. When the concentration of polymer is increased to 1% (together with that of SDS to realize maximum gelling) much stronger gels are encountered. Figure 5 shows the frequency dependence of the rheological parameters. The elastic modulus G' is now significantly higher than the loss modulus G" over the entire measured frequency range. Such behavior indicates elastic behavior with little stress relaxation and little energy dissipation during the periodic deformation. It is consistent with a relatively high crosslink density in the network which, in principle, is determinable from the plateau m o d ~ l u s .A~ Maxwell model with a continuous relaxation spectrum would seem well suited to fit the above frequency dependencies. Work on model development for the above systems is currently in progress. (8)Leaderman, H. Viscoelasticity Phenomena in Amorphous High Polymeric Systems. In Rheology Vol 2; Eirich, F. R., Ed.; Academic Press: New York, 1958.
Frequency Hz Figure 4. Viscoelastic parameters versus oscillation frequency for 0.2% Polymer JR 30M-0.01% SDS mixture. m
I
,
I
1
0.001
."
'
'
0.01
'
0.1
Frequency
'
'
'!
'
'
\10
11
Hz
Figure 5. Viscoelastic parameters versus oscillation frequency for 1%Polymer JR 30M-0.15% SDS mixture.
It should be mentioned that concurrent with our research, work on related gelling systems comprising nonionic or anionic polysaccharides plus added ionic surfactants has been carried out by the Lindman team in S ~ e d e n . ~Finally, J~ we refer to unpublished work carried out in this laboratory on gelling systems consisting of cellulosic polymer/surfactant pairs charged in the opposite sense to the above systems; these are based on combinations of carboxymethylcellulose and DTAB." (9) Thalberg, K.; Lindman, B.; Karlstrom, G. J.Phys. Chem. 1990,94, 4289. (10) Karlstrom, G.; Carlsson, A.; Lindman, B. J.Phys. Chem. 1990,94, 5005.
(11) Padmanabhan, K. P. A.;Leung, P. S.; Goddard, E. D. Unpublished work.
