Building Aqueous Viscosity through Synergistic PolymerPolymer

Chapter 17

Building Aqueous Viscosity through Synergistic Polymer-Polymer Interactions James V. Gruber and Peter N. Konish

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Downloaded by EAST CAROLINA UNIV on August 29, 2013 | http://pubs.acs.org Publication Date: August 20, 1999 | doi: 10.1021/bk-1999-0737.ch017

Amerchol Corporation, 136 Talmadge Road, Edison, NJ 08818-4051

The rheology of aqueous solutions containing a hydrophobically -modified, cationic cellulose ether, 2, blended with amylose, 3, a linear polyglycan obtained from potato starch, was examined. The viscosity of solutions containing 0.5, 1.0 and 2.0 weight percent 2 optimally blended with amylose concentrations of 0.12, 0.25 and 0.5 weight percent, respectively, were investigated under controlled shear and stress. The mixtures show indications of building yield at increasing concentrations of 2 and are pseudo-plastic at high shear. The storage and loss moduli were examined as well and indicate that 2 and 3 interact synergistically to form gels at the higher concentrations of 3 employed. The data continues to support a hypothesis reported earlier suggesting that mixtures of 2 and 3 interact through a non-covalent crosslinking mechanism where the hydrophobic groups attached to 2 intercalate into the interior of the helix formed by the amylose, a so -called "Lariat Complex".

It is the nature of polymers to interact either synergistically or antagonistically with other polymers when they are intimately mixed. That is, when one polymer species is blended with another, the two materials may either have a molecular affection for one another, or they may be repelled by one another. In truth, this description is simplistic as portions of various copolymers may attract while other regions repel. It is often difficult to predict these interactions in advance. Part of the difficulty arises from the fact that polymers can assume various configurations, e.g., random, helical, loose or tight coils, pleated sheets, folds, etc. It thus becomes necessary to predict in a more 31

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Current address: Mane, USA, 60 Dermarest Drive, Wayne, NJ 07479.

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© 1999 American Chemical Society

In Polysaccharide Applications; El-Nokaly, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

253 dimensional sense whether interactions will be cooperative or repulsive. When interacting polymers are dissolved in a solvent, these attractions and repulsions are further compounded by solvation effects. Water is a particularly tricky solvent which can control and manipulate the 3-dimensional structure of polymers through strong hydrogen bonding and hydrophobic effects. In addition, water can transmit electrostatic effects and polymer counter-ions which can further influence polymer morphology and polymer/polymer interactions. Regardless of these difficulties, efforts to better understand aqueous polymer/polymer interactions have continued to develop and several classic aqueous polymer/polymer synergisms have attracted research attention. These include, for example, the interaction of xanthan gum with gluco- or galactomannans, kcarrageenan with galactomannans and poly(acrylic acid) with poly(ethylene oxide). Cairns, et al., in describing the synergistic study of binary polysaccharide gels, developed conceptual models of various types of polysaccharide/polysaccharide interactions, Figure l . Within a polymer/polymer network, synergistic interactions can be described as either: a) single polymer networks containing a second polymer, b) interpenetrating networks, c) phase-separated networks or d) coupled networks. Distinguishing one type of interaction from another is not always straightforward. Coupled networks are a particularly interesting form of polymer networking because in a solvated system certain portions of one of the polymer chains must have a particular affection (stronger than the powerful forces of hydration) for a portion of the other polymer chain. Sometimes these effects can be a manifestation of a common ion shared by both polymers. More intriguingly, the effect can be driven by hydrophobic properties inherent in certain regions of each polymer. Regardless of the source of the synergism, the points of attachment are referred to as junction zones. When junction zones develop in aqueous polymer solutions, the effects can be quite pronounced. A particularly well known example of a synergistic aqueous rheological viscosity response can be seen in the interaction of xanthan gum with guar, a polygalactomannan. The nature of the junction zone in this classic example is still being debated. Recently, we reported on a new type of synergistic polysaccharide/polysaccharide interaction which is an extension of Cairns' coupled network. This network occurs when a water-soluble, hydrophobically-modified, cationic cellulose ether (Polyquaternium-24), 2, is blended with solujtilized potato amylose, 3, a nearly linear polyglucan. The synergistic interaction of these two polysaccharides has potential applications in the development of surfactant-free emulsions and skin lotions. We wish to expand our discussion on this unique synergistic polymer/polymer system by examining some of the rheological properties of solutions containing these two polymers. This discussion, we feel, further supports our hypothesis that these two polymers interact through a newly described junction zone which develops when the amylose coils in a helical configuration around the hydrophobic groups of the cationic cellulose ether.


2

3

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5

5

6,7

8

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Experimental Materials. The formation of the Polyquaternium-24/amylose complex has been described elsewhere. In summary, an aqueous solution of Polyquaternium-24 heated 8



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(c)

(d)

Figure 1. Types of binary polysaccharide gel-structure. Polysaccharide A [•], polysaccharide B [—]: (a) single polymer network containing the second polymer within the gel, (b) interpenetrating networks, (c) phase-separated networks, (d) coupled network "Reprinted from Carbohydrate Polymers, 160, Cairns, P.; Miles, MJ.; Morris, VJ.; Brownsky, GJ, X-Ray Fibre-Diffraction Studies of Synergistic, Binary Polysaccharide Gels, 411-423, 1987, with permission from Elsevier Science".


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to 90 °C is carefully blended with a solution of potato amylose also at 90 °C (the amylose must be hydrated at higher temperatures, near 110 °C in a pressure tube or a pressure cooker prior to blending), such that the resultant mixture composes 1.0 part Polyquaternium-24, 0.25 part amylose, and enough deionized water to bring the mixture to 100 parts. The hot mixture is then place into a 2-gallon water bath heated to 90 °C and the temperature is allowed to cool slowly overnight by removing the power to the bath. This slow cooling allows the polymer complex to form its optimum solution viscosity. Xanthan gum was obtained from Aldrich and was used as received. Equipment. A l l rheological studies were conducted on either a Bohlin V O R rheometer using a C25 cup-and-bob or a Double Gap (DG) 24/27 cup-and-bob, or a Bohlin CS rheometer using a CP 4/40 cone and plate or a Double Gap (DG) 40/50 cup and bob. The temperature was maintained at a constant 30 °C during the measurements. The viscosity measurements were averaged over 40 seconds with a small 0.1 second delay between measurements. To establish the linear viscoelastic region of the polymer complexes in a dynamic test, the solutions were subjected to a stress sweep at a fixed frequency of 1 Hz. The resulting spectra indicated that a controlled stress of 20 Pa would be sufficient to run the resulting dynamic analysis. A l l the rheological data was subsequently transferred to Microsoft Excel 7.0 for graphing. Results and Discussion A summary of our initial empirical observations on the complex between amylose and Polyquaternium-24 can be found in Table I. From these observations, we proposed a 8

Table I. Summary of empirical data on interactions of Polyquaternium-24 and amylose.

•

Observation No effect from Polyquaternium-10,1

•

• •

Synergism develops on cooling Pseudoplastic

• •

• •

Inhibits amylose retrogradation Ratio of amylose to hydrophobe is approximately 1:2

• •

Implication Hydrophobe required for synergism Thermoreversibility Typical for polysaccharide solutions Amylose is not "free" to complex Two hydrophobic groups per amylose molecule

mechanism of junction zone formation which requires the amylose to wrap itself around two adjacent hydrophobic groups on the Polyquaternium-24, Figure 2. We euphemistically refer to this type of junction zone as a "Lariat Complex". The empirical data in Table I is consistent with the proposed junction zones, but does not



Figure 2. Proposed junction zones for the complex between Polyquaternium-24 and amylose, a "Lariat Complex" (Reprinted with permission from Macromolecules 1997, 30, 5361-5266. Copyright 1997 American Chemical Society).


257 establish this as the only possible mechanism. The possibility of phase separation could not be entirely ruled out from these studies. In addition, recent work employing the Polyquatemium-24/amylose complex to develop surfactant-free emulsions suggested that the combination of these two polymers in aqueous solution imparts a yield point to the aqueous solution. Attempts to build such formulations from Polyquaternium-24 alone failed in a matter of days, whereas emulsion formulations which contained both polymers were stable at elevated temperatures for up to six months. Rheological investigation of gelation phenomena is a useful technique to glean additional information about the characteristics of polymer blends. We examined the rheological behavior of solutions containing Polyquaternium-24, 2, amylose, 3, and the combination of 2 and 3. A graph of viscosity and shear stress verses shear rate is


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shown in Figure 3. From the graph it is immediate apparent that the combination of the two polymers creates significant viscosity (solutions of 0.25% amylose are very low in viscosity, nearly equivalent to unadulterated water-data not shown). The appearance of the small deflection at low shear rate in the viscosity curve of the combined polymers is indicative of the formation of a network between the two polymer components. In addition, in examining the resulting shear stress curve for the polymer combination, one notes two apparent peaks. We suggest the peak corresponding to point A represents the maximum extension of the amylose helices with increasing shear rate and that point B may represent the eventual linear comingling of the two polymer systems under high shear. As can be seen in Figure 4, however, the system is thixotropic suggesting that the amylose helices never completely detach from the cellulose ether hydrophobes. The synergistic complex of the amylose and Polyquaternium-24 requires more time to recover from stress than a typical 0.5% xanthan solution, indicated by the separation of the forward and reverse viscosity curves. Once the shearing force is removed from the system, the amylose helices recoil and viscosity rebuilds. Figure 5 demonstrates that, as expected, the rheological properties of solutions containing 2 and 3 are concentration dependent. A complex formed using 0.5% Polyquatemium-24 and 0.12% amylose (i.e., half the typical concentrations)


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Figure 3. Viscosity (closed points) and shear stress (opened points) verses shear rate for 1% Polyquaternium-24, 0.25% amylose and the combination of 1% Polyquatemium-24/0.25% amylose. 1.00E+03 1.00E+02 1.00E+01

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Figure 4. Thioxtropy curve for a 1% Polyquaternium-24/0.25% amylose complex (A-up, A-down) and a curve of a 0.5% xanthan solution (•-up, • down). In Polysaccharide Applications; El-Nokaly, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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Figure 5. Viscosity verses shear stress curves for a 2% Polyquaterniurn-24/0.5% amylose mixture (•), a 1% Polyquaternium-24/0.25% amylose mixture ( • ) , a 0.5% Polyquaternium-24/0.12% amylose mixture ( A ) and the three polymer concentrations without added amylose (corresponding open symbols).



260 builds very little viscosity, being only slightly greater in viscosity than 0.5% Polyquaternium-24 alone. A solution of 2.0% Polyquaternium-24 and 0.5% amylose has even greater viscosity (on the order of kilopascals) and the stress range over which the complex displays Newtonian behavior is also increased. Perhaps the most significant information on the complex can be obtained from examination of the complex under dynamic mechanical conditions. Figure 6 shows that for both the 1.0% Polyquatemium-24/amylose complex and the 2.0% Polyquaternium-24/amylsoe complex, the storage modulus, G' (a measure of the elastic component of the system) exceeds the loss modulus, G " (a measure of the viscous component of the system). Such dynamic modulus curves are indicative of gelation although the crossover of G' with G " for the 1% polymer solution suggests that this mixture is more characteristic of a pseudogel. In the case of the 2% polymer mixture, the storage modulus exceeds the loss modulus over the whole range of frequencies examined. This type of behavior can be ascribed to strongly crosslinked gels. However, covalent crosslinking in this system is 10,11

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261 unlikely so the crosslinking must be attributable to a non-covalent mechanism. Because such viscosity enhancement was not noted for a cationic polymer similar to Polyquaternium-24 that does not contain hydrophobic substitutents, i.e., Polyquaternium-10, 1, we feel that the mechanism is not electrostatic in nature and must, therefore, be related to some type of interaction between the amylose and the hydrophobes on the Polyquatemium-24. It is well known that amylose will complex hydrophobic molecules via an inclusion mechanism, therefore, the junction zone proposed in Figure 2 seems to be the most likely possible candidate. 8


Conclusions The data presented above continues to support our observation that Polyquaternium-24 and amylose interact synergistically to form weak gels. While the data does not prove conclusively that this synergistic interaction occurs via the helical crosslinking complex we propose, it does lend further evidence to this suggestion. Additional proof will need to come from more extensive x-ray diffraction studies or perhaps from intermolecular NOE N M R experiments which may demonstrate either the presence of the amylose cc-helix in the complex, or intermolecular NOEs between the amylose glucose monomers and the cellulose ether hydrophobes. Such information would be useful to further establish the nature of the interaction between these two unique polysaccharides. Establishment of the nature of the interaction, however, has not prevented us from exploring the use of the binary polymer synergism in the development of novel surfactant-free cosmetic formulations. Literature Cited 1) Gruber, J. V. In Polysaccharide-Based Polymers in Cosmetics; Goddard, E. D.; Gruber, J. V. eds., Principles of Polymer Science and Technology in Cosmetics and Personal Care. Marcel Dekker, New York. 1999 (In press), pp 325-389. 2) Schorsch, C.; Gamier, C.; Doublier, J-L. Carbohyd. Polym. 1997, 34, 165-175. 3) Turquois, T.; Rochas, C.; Taravel, FR. Carbohyd. Polym. 1992, 17, 263-268. 4) Back, D. M.; Clark, E.M.; Ramachandran, R. In Polyethers, Ethylene Oxide Polymers. Kroschwitz, J.I.; Howe-Grant, M . eds., Kirk-Othmer Encyclopedia of Chemical Technology 4 ed., Vol. 19. New York: John Wiley & Sons, 1996, pp 700-722. 5) Cairns, P.; Miles, M . J.; Morris, V. J. Brownsky, G. J. Carbohyd. Polym. 1987, 160, 411-423. 6) Goycoolea, F. M . ; Richardson, R. K.; Morris, E. R.; Gidley, M . J. Macromolecules 1995, 28, 8308-8320. 7) Chandrasekaren, R.; Radha, A. Carbohyd. Polym. 1997, 32, 201-208. 8) Gruber, J. V.; Konish, P. N. Macromolecules 1997, 30, 5361-5366. 9) Konish, P. N.; Gruber. J. V. J. Cosmet. Sci. 1998, 49, 335-342. 10) Clark, A. H.; Ross-Murphy, S. B. Structural and Mechanical Properties of Biopolymer Gels. Clark, A. H.; Kamide, K.; Ross-Murphy, S. eds., Adv. Polym. Sci. 1987, 83, 57-192. 11) Kavanagh, G. M.; Ross-Murphy, S. B. Prog. Polym.Sci.1998,23, 533-562. th


Building Aqueous Viscosity through Synergistic PolymerPolymer

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