Effect of comonomer distribution on the coil-to-globule transition of a single AB copolymer chain in dilute solution

Utilizing the difference between the chain conformations at two temperatures, respectively, below and above the lower critical solution temperature (similar to32 degreesC) of poly(N-isopropylacrylamide) homopolymer, we successfully prepared pairs of poly(N-isopropylacrylamide-co-vinylpyrrolidone) (NIPAM-co-VP) copolymers with a similar composition and chain length but different monomer distributions on the chain backbone. Using these copolymers, we investigated the temperature induced coil-to-globule transition of individual chains in dilute solution. In each case, we found that the copolymer chains could form stable single-chain globules at high temperatures, and as expected, the transition temperatures were higher than similar to32 degreesC because comonomer VP is hydrophilic in the temperature range studied. The laser light-scattering and calorimetry results showed that the chains prepared at 60 degreesC, presumably with a globular proteinlike segmented VP distribution, had a lower transition temperature and could form denser globules than those prepared at 30 degreesC with a random VP distribution. In other words, the folding of the chains prepared at higher temperatures was easier. In a sense, they could memorize their parent globular core-shell structure with a collapsed PNIPAM core and a swollen shell made of small VP loops, reflecting in a ratio of the radius of gyration to hydrodynamic radius (/) much smaller than the 0.774 predicted for a uniform sphere.