Formation of mesoglobular phase of amphiphilic copolymer chains in dilute solution: Effect of comonomer distribution

Poly(N-isopropylacrylamide) (PNIPAM) is a thermally sensitive polymer with a lower critical solution temperature (similar to32 degreesC) in water. Its copolymerization with hydrophilic vinylpyrrolidone (VP) at temperatures higher and lower than its LCST respectively resulted in segmented and random VP distributions. As the solution temperature increases, PNIPAM changes from hydrophilic to hydrophobic at similar to32 degreesC so that copolymer P(NIPAM-co-VP) becomes amphiphilic in water at higher temperatures. The association of two pairs of such copolymers with a similar chain composition and length, but different comonomer distributions, in water at temperatures higher than 32 degreesC was studied by laser light scattering. A limited number of neutral P(NIPAM-co-VP) chains can associate to form narrowly distributed stable mesoglobules existing between single-chain collapsed globules and macroscopic precipitates. The copolymer chains with a segmented VP distribution aggregate more readily to form larger mesoglobules than their counterpart with a random VP distribution. The formation of mesoglobules is related to a competition between intrachain contraction and interchain association. Besides thermodynamic consideration, the formation of a stable mesoglobular phase is also affected by chain viscoelasticity inside each aggregate, even in dilute solution. A proper variation of the chain structure and association temperature can result in mesoglobules with different sizes, molar masses, and structures.