EFFECTS OF THE ASSOCIATION ON THE PHASE-BEHAVIOR OF MIXED-SOLUTIONS OF HYDROPHOBICALLY-MODIFIED AND SIMILAR UNMODIFIED POLYMERS

It is predicted theoretically and demonstrated experimentally that whe n solutions of hydrophobically modified associative and unmodified wat er soluble polymers are mixed, then over a certain range of compositio n the system phase separates into two distinct solutions. The modified polymers used in the experiments were the so called associative thick eners (AT) consisting of polyethylene-glycol (PEG), end-capped at both ends by alkane groups. Similar sized PEG homopolymers were employed a s the non-associative chains. Overall polymer concentrations of the bo th phase separated solutions were measured and found to be identical. In contrast the viscosity of the two phases were several orders of mag nitude different. This suggests that the polymer species in one phase are predominantly AT whereas those in the other phase are mainly of th e unmodified type. The two phase region was only observed when the ove rall polymer concentrations were above 2% and the mole fraction of the AT below 0.65. It is shown that these results can be explained quanti tatively by considering the effects of association within the Flory-Hu ggins scheme. In particular the critical mole fraction of AT is predic ted to be 0.68, in excellent agreement with the experimental observati ons. The theoretical calculations are further extended to mixed soluti ons involving single end-capped chains and also those consisting of mi xtures of hydrophobically modified and similar unmodified star shaped polymers. In the latter case, the tendency for phase separation betwee n the two species of polymers is found to increase rapidly as the numb er of hydrophobes on the associating polymer molecules are made larger .