The application of a photochromic probe to monitor the self-assembly of thermosensitive block copolymers

Abstract

Analysis of the thermal decolouration kinetics of a spirooxazine dye (SOX) incorporated into various thermally responsive, amphiphilc block copolymers, poly(N-isopropyl acrylamide)-block-poly(N-acryloyl morpholine), was used to probe their self-assembly behaviour in water. The Arrhenius plots of the thermal decolouration kinetics for each of the block copolymers displayed a deviation from linearity and this occurred within a temperature range that correlated with their self-assembly into micelles, as measured by dynamic light scattering (DLS). The block copolymers containing SOX units within the PNIPAM section displayed the most notable deviations. The results were interpreted in terms of a change in the local viscosity and polarity sensed by the photochromic probe and its effect on the transition state of the open form on progression to the closed form. The incorporation of the free dye, SOX–PROP, within micelles assembled from amphiphilc poly(n-butyl acrylate)-block-poly(N-acryloyl morpholine), was also apparent from the measured decolouration kinetics.