Photoinduced Mass-Migration Behavior of Two Amphiphilic Side-Chain Azo Diblock Copolymers with Different Length Flexible Spacers

This article reports the synthesis and photoinduced mass-migration behavior of two amphiphilic side-chain azo diblock copolymers (PEG-<i>b</i>-P2CN, PEG-<i>b</i>-P6CN), which contain ethylene and hexamethylene as flexible spacers between main chains and pendent azobenzene functional groups. The copolymers were synthesized by atom transfer radical polymerization (ATRP) to obtain precursor block copolymers from a PEG-based macromolecular initiator and postpolymerization azo-coupling reaction of the precursors to introduce the azo chromophores. Spectroscopic analyses, GPC, and thermal analyses indicated that the diblock copolymers possessed well-defined structures and narrow molecular weight distributions. The photoinduced mass-migration behavior was studied through two different approaches, that is, investigating photoinduced deformation of colloidal spheres of the copolymers and surface relief grating (SRG) formation on the polymer thin films. The colloidal spheres used for the light irradiation experiments were prepared by solvent-induced phase separation and set on the substrates (such as TEM copper grids) as solid particles. Upon irradiation with a linearly polarized Ar<sup>+</sup> laser beam (488 nm, 150 mW/cm<sup>2</sup>), the colloidal spheres of both copolymers were gradually elongated in the polarization direction of the laser beam, which evidenced the photoinduced mass migration on the micrometer scale. In the process, the PEG-<i>b</i>-P2CN colloids showed a significantly larger deformation rate and degree compared with the PEG-<i>b</i>-P6CN colloids. The mass-migration ability was also tested by inscribing surface relief gratings on PEG-<i>b</i>-P2CN and PEG-<i>b</i>-P6CN films with interfering Ar<sup>+</sup> laser beams. The PEG-<i>b</i>-P2CN showed a significantly faster rate of grating formation and larger modulation depth in comparison with the PEG-<i>b</i>-P6CN. The observations suggest that the flexible spacer could play an important role in transferring the light-driving force from the azo chromophores to the polymer backbones, where the shorter spacers showed a more significant effect to cause the mass migration.