Experimental and Theoretical Studies on the Phase Behavior of Aqueous Solutions of Structurally Controlled Hyperbranched Poly(N-isopropylacrylamide)s

Abstract

Structurally controlled hyperbranched poly(N-isopropylacrylamide)s (HB-PNIPAMs) having different branching densities were systematically synthesized, and their phase behavior, i.e., lower critical solution temperatures (LCSTs), in water was studied. The branching significantly affects LCSTs, and they decrease with an increase in the branching density. The origin of the experimental results was theoretically analyzed on the basis of the concept of cooperative hydration, which is the sequential formation of hydrogen bonds between polymer chains and water molecules. Assuming that the cooperativity of hydration does not propagate beyond the branching points, we calculate the spinodal curves, binodal curves, and critical points and construct the phase diagrams. The degrees of hydration of the subchains, branching points, and end groups are also calculated to examine their hydration effect on the phase behavior. In this investigation, we consider the following two situations: (i) only subchains can be hydrated, and (ii) every part of HB polymers, including branching points and end groups, can be hydrated. We compare the theory with the reported observations of the cloud points of structurally well-controlled HB-PNIPAMs in water. Qualitative agreement is obtained by using the same value of cooperativity as the study of aqueous solutions of linear PNIPAMs. We also show that the hydration behavior of the end groups and the branching points strongly influences the phase behavior and illustrate that the binodal temperature is determined by the balance between the effect of promoting hydration by their hydration and suppressing hydration due to the inhibition of cooperative hydration by the branching points.