Internal Associations in Comb Polymers Bearing Multiple Internal Hydrophobes
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INTERNAL ASSOCIATIONS IN COMB POLYMERS BEARING MULTIPLE INTERNAL HYDROPHOBES Maria M. Santore, Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015
ABSTRACT The internal associations of comb-shaped associative thickeners are modeled by Gaussian statistics to predict the probability of chain collapse and that for various types of internal associations. In addition to the idealization of the polymer backbone to a random walk, the hydrophobic groups at the ends of each tooth of the comb are treated as point "stickers." The model considers all the ways by which stickers can form clusters of two or more teeth. The number of teeth per chain is not limited by the model, but rather by computation facilities. It is predicted that even with weak pairwise sticker interactions (2kT) adding short teeth to a long backbone drastically increases the probability of some internal associations, though the probability of complete chain collapse decreases significantly as teeth are added, due to the effect of backbone entropy. INTRODUCTION Recently, there has been much interest in specialty polymer architectures to gain insight into the fundamentals of polymer physics. An example of a specialty architecture that has practical applications is the associative thickener, a watersoluble backbone chain containing 1-2 wt% hydrophobic groups such as short alkane arms that interact reversibly in solutions to form micelles and networks, thus generating unusual properties such as shear thickening. An analogous system, soluble in organic solvents, are ionomers whose associations stem from dipole rather than hydrophobic interactions. These may find application in enhanced oil recovery and lubrication technologies. In a previous work 1 we approached the issue of solubility for associative polymers through a statistical mechanical treatment of the pairwise interaction potential between ideal associative chains, each of which contained two hydrophobic end groups. These end groups were treated as point stickers (per chain) of variable association energy. A statistical mechanical perturbation theory employing the pair potentials led to the limit of solubility, the concentration beyond which one might expect micellization or insolubility. A new twist in the development of water-soluble associative thickeners is the addition of spacer segments (figure 1) between the backbone and the hydrophobe, leading to a comb architecture. It is thought that this manner of hydrophobe attachment may promote associations and network formation. 2 The purpose of his work is to develop a statistical model for the associative behavior of the more complicated comb architectures, which follows the spirit of the previous work, but can account for multiple hydrophobes per chain and various comb shapes as in figure 1. This paper presents the first step in the theoretical treatment: the prediction of complete and partial chain collapse through internal associations for various comb topologies and hydrophobic adhesion or "sticker" cnergies. The model present
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