Synthesis and Properties of Azo-Based ABC Triblock Copolymers Owning Interaction and Composition Parameters That Influence Their Phase Behaviors

Abstract

Reversible addition–fragmentation chain transfer polymerization is employed for the preparation of a series of novel ABC-type azo triblock copolymers (TBCs) consisting of poly(CAEMA), poly(BMA), and poly(DOPAM) using AIBN as an initiator and anisole as a solvent. The TBCs are characterized by means of 1H nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). Their mesophase, photoresponsive, and morphological behaviors are examined using differential scanning calorimetry (DSC), optical polarizing microscopy (OPM), ultraviolet–visible spectrophotometry (UV–vis), atomic force microscopy (AFM), and grazing-incidence small-angle X-ray scattering (GISAXS). Molecular weights and polydispersities (≤1.38) increase slightly with the increase of the molar ratio of monomer to macroinitiator, confirming a controlled/living radical polymerization. All TBCs demonstrated endothermic and exothermic transition peaks corresponding to the smectic-to-nematic and nematic-to-smectic phases. Thermal investigation revealed that the TBCs containing 54 and 46 wt % of azo contents displayed batonnet textures of smectic phase, while the TBC with low azo block fraction of 39 wt % showed threaded texture of nematic phase. For TBC having the lowest azo content of 31 wt %, neither liquid crystalline texture is detected. TBC-2 with 46 wt % of azo block, 28 wt % of PBMA, and 26 wt % of PDOPAM produced a lamellar compared to TBC-1 and TBC-3 which formed a mixture of cylinder and lamellar morphologies. Contrastingly, TBC-4 containing the highest PDOPAM volume ratio of 50 wt % generated hexagonal cylinder-type morphology. All TBCs exhibited a reversible trans–cis–trans photoisomerization behavior in chloroform solution and in thin film under UV and visible light irradiation (or dark storage) at varied intervals of time.