Comparative study of temperature-induced association of cyclic and linear poly(N-isopropylacrylamide) chains in dilute solutions by laser light scattering and stopped-flow temperature jump

We have comparatively studied the association of cyclic- and linear-poly(N-isopropylacrylamide) (c-PNIPAM and l-PNIPAM) chains with varying chain lengths and end groups in dilute aqueous solutions by laser light scattering (LLS) and stopped-flow temperature-jump measurements. Dynamic and static LLS results reveal that the heating leads to a microphase transition. The resultant structures of interchain aggregates depend on the heating rate and the chain topologies. In comparison with l-PNIPAM chains, a slow heating of c-PNIPAM chains in the solution results in stable mesoglobules with a lower average aggregation number, a looser structure, and a smaller average size (similar to 290 nm). The temperature-jump-induced association of c-PNIPAM chains in the stopped-flow measurement reveals two kinetic growth stages, which were tentatively ascribed to the loose packing of contracted c-PNIPAM chains and further contraction-induced fragmentation of initially packed c-PNIPAM chains due to the lack of interchain entanglements. On the other hand, for l-PNIPAM chains, the intrachain contraction and interchain penetration/entanglement simultaneously occur as the temperature increases, leading to larger and more compact aggregates whose size increases with the solution temperature.