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Formation of Mesoglobular Phase of Amphiphilic Copolymer Chains in Dilute Solution: Effect of Comonomer Distribution
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 criticalsolution temperature (32 °C) in water. Its copolymerization with hydrophilic vinylpyrrolidone (VP) attemperatures higher and lower than its LCST respectively resulted in segmented and random VPdistributions. As the solution temperature increases, PNIPAM changes from hydrophilic to hydrophobicat 32 °C 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 differentcomonomer distributions, in water at temperatures higher than 32 °C was studied by laser light scattering.A limited number of neutral P(NIPAM-co-VP) chains can associate to form narrowly distributed stablemesoglobules existing between single-chain collapsed globules and macroscopic precipitates. The copolymerchains with a segmented VP distribution aggregate more readily to form larger mesoglobules than theircounterpart with a random VP distribution. The formation of mesoglobules is related to a competitionbetween intrachain contraction and interchain association. Besides thermodynamic consideration, theformation 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.
