Heterogeneity and Dynamics in Azobenzene Methacrylate Random and Block Copolymers: A Nanometer-Nanosecond Study by Electron Spin Resonance Spectroscopy

The dynamic response at nanometers and nanoseconds was investigated by electron spin resonance spectroscopy in random and block copolymers of an azobenzene methacrylate (MA4) and methyl methacrylate (MMA) with cholestane molecular tracer. The study evidenced the presence of different molecular sites for the reorientation at nanoscale, that were modulated by the amount of MA4 counits in the random copolymers and by the self-assembly in supramolecular structures in the block copolymers. A number of dynamics features was revealed, such as the presence of memory effects, the existence and the degree of coupling of the rotational dynamics to the structural relaxation of the matrix or to its viscosity, and the population of dynamic sites. This provided a detailed characterization of the mechanisms dictating the dynamic response of the structurally different copolymers. The influence of microstructure on the relaxation properties of the random copolymers was also discussed by considering the evaluated cooperativity coefficients, cooperativity indices, and packing lengths.