Aqueous solution properties of poly(n-isopropylacrylamide)

Abstract

Aqueous solution properties of poly (N-isopropylacrylamide) (PNIPA) samples synthesized by radical and living anionic polymerizations were examined in detail. It is found that the cloud points in aqueous solutions of PNIPA samples synthesized by radical polymerization with azobis(isobutyronitrile) as an initiator are appreciably different from each other if the solvents used for the polymerization are different, even though the samples have the same weight-average molecular weight Mw and stereochemical composition specified by the fraction fx of racemo diads. From analyses of the mean-square radius of gyration, second virial coefficient, and intrinsic viscosity for those samples in methanol at 25.0°C, it is found that the PNIPA samples synthesized by radical polymerization have branched structure and that the number of branch points depends on the kind of solvent used for the radical polymerization. As for linear PNIPA samples synthesized by living anionic polymerization, it is shown that the cloud point in their aqueous solutions decreases remarkably with decreasing Mw. in the range of 5 × 103≲Mw≲7 × 104 due to effects of hydrophobic chain end groups of the samples. The transmittance of light passing through an aqueous solution of a linear PNIPA sample synthesized by living anionic polymerization was examined in detail in the vicinity of its cloud point. It is found that the transmittance approaches a constant value even at a temperature slightly higher than the cloud point, indicating that macroscopic phase separation does not take place at the cloud point in the solution and therefore the cloud-point curve does not correspond to the binodal. Static and dynamic light scattering measurements were then carried out for aqueous solutions of the linear PNIPA sample and one synthesized by radical polymerization at some temperatures considerably lower than the cloud point. It is shown that both PNIPA aqueous solutions contain aggregates even at such temperatures, and the number, size, and density profile of the aggregates depend on the kind of chain end group and also on the primary structure. © The Society of Polymer Science Japan.