Synthesis of well-defined PS-based Azo-liquid crystals with control of phase transitions and photo-behaviors for liquid crystal networks with photomechanical deformation

Benefiting from their good mechanical processability and high tunability, Si–H and vinyl-functionalized polystyrenes (PSs), i.e., poly(4-vinylphenyl)-dimethyl-silane (PVPDMS/PS-SiH) and poly(4-vinylphenyl)-1-butene (PVSt/PS-Vinyl), produced by living anionic polymerization (LAP) were treated with four vinyl- and Si–H-terminated azobenzene mesogens (Azo-Vinyl, Azo-SiH, AzoB-Vinyl, and AzoB-SiH, referred to as M1, M2, M3, and M4, respectively) via hydrosilylation for the fabrication of PS-based side-chain liquid crystal polymers (SCLCPs) containing different spacers, i.e., PS-SiC/SiOSi-Azo/AzoB. As a strategy for basic molecular control in PS-based SCLCPs, the cooperative influences of the spacers and the degree of polymerization (DP) on the mesomorphic transitions were explored. The phase transitions could be tuned slightly based on the DP but dramatically by adjusting the spacers. For example, the temperature ranges of mesomorphic formation (ΔT) were in the order PS-[SiOSi]-Azo (ΔT = 34–49 °C) < PS-[SiC]-Azo (ΔT = 49–60 °C) < PS-[SiOSi]-AzoB (ΔT = 154–164 °C) < PS-[SiC]-AzoB (ΔT = 162–193 °C). Taking advantage of the quantitative structure-property correlation of PS-based SCLCPs, PS-based liquid crystal networks (LCNs) were further designed for the photomechanical deformation. PS-based LCNs show good mechanical properties with a storage modulus of 120 MPa, controlled phase transitions, and glass transition temperatures (Tg) around room temperature. Crosslinked PS-based LCNs of rigid PS homopolymers with selective crosslinking can exhibit directional bending upon irradiation with UV light at 50 °C after stretching, and this was shown to be a photomechanical deformation, which expands the applicability of these materials.