Crystallisation-driven self-assembly of poly(2-isopropyl-2-oxazoline)-block-poly(2-methyl-2-oxazoline) above the LCST

Abstract

The solution behaviour in water of a polyoxazoline-type block copolymer, namely poly(2-isopropyl-2-oxazoline)-block-poly(2-methyl-2-oxazoline), denoted as P(iPrOx-b-MeOx), above the lower critical solution temperature (LCST) of the PiPrOx block was exploited to induce a temporary or permanent self-assembly. Spherical micelles were first obtained and could be disassembled in a reversible manner when kept for a short period of time (i.e. t < 90 min) above the LCST, and cooled down to room temperature. In contrast, annealing the copolymer solution for more than 90 min at 65 °C induced the crystallisation of the PiPrOx block, as evidenced by wide angle X-ray scattering (WAXS) experiments. This crystallisation-driven self-assembly phenomenon resulted in different morphologies, including spherical and distorted crystallised micelles and micron-size fibers, their relative proportion varies with the annealing time. Formation of micron-size range fiber-like structures might be explained by the re-organization of parent crystallised micelles. The crystal structure, as determined by WAXS, appeared to be identical to that of the PiPrOx homopolymer.