Photoinduced optical anisotropy in azobenzene methacrylate block copolymers: Influence of molecular weight and irradiation conditions

The photoinduced anisotropy in a series of azomethacrylate block copolymers with different molecular weights and azo contents has been investigated under several irradiation conditions. Depending on molecular weight and composition, different microstructures (disordered, lamellar, spherical) appear, due to block microsegregation. Measurements of birefringence (Δn) induced with linearly polarised 488 nm light show that the highest (and stable) ΔnN values (birefringence normalised to the azo content) are achieved in copolymers with a lamellar structure. Lower ΔnN are obtained in the copolymers in which azobenzene units segregate to nano spheres and the lowest (and less stable) ΔnN values, appear in disordered systems not showing any defined microstructure. Besides, higher ΔnN is obtained in the copolymers with larger molecular weight of the poly (methyl methacrylate) and the azo polymer blocks, both in the microspheres segregated polymers as well as in those without a clear microstructure. This behaviour is consistent with that of the photoinduced order of azobenzene units obtained from dichroism measurements. Irradiation temperature (from 30 to 90 °C) and light power (from 100 to 500 mW/cm2) also influence the photoinduced response. Photoinduced ΔnN growth rate is faster when both temperature and irradiation power increase. Furthermore, birefringence is only induced at temperatures up to 90 °C, the maximum value being obtained at about 70 °C. No clear dependence of the final ΔnN value with light power has been found.