Cholesteric–azobenzene liquid crystalline copolymers: design, structure and thermally responsive optical properties

Abstract

We report the synthesis of new functional cholesteric–azobenzene side-chain liquid crystalline copolymers and terpolymers, their temperature-responsive mesophase behavior, and stimuli responsive photonic properties from 400 nm to the near infra-red region of the electromagnetic spectra. Polynorbornene copolymers are synthesized using norbornene based n-alkyloxy cholesteryl (NBCh9) and norbornene based n-alkyloxy azobenzene moiety bearing different terminal functional groups (NBAz12-X, X = H, CN, CH₃O, and NO₂) by ring opening metathesis polymerization with modified second-generation Grubbs catalyst. Polynorbornene terpolymers of NBCh9, NBAz12-X, and norbornene bearing poly (ethylene glycol) (NBMPEG) are synthesized in a similar way. The copolymers and terpolymers self-assemble to form cholesteric mesophases (N*) primarily from cholesterol molecules which forms the major phase and within which (1) azobenzene moieties serve as a molecular switch to tune the helical pitch of the cholesteric mesophase leading to wavelength modulation from the visible to near infrared range and (2) PEG provides internal plasticizing ability and mechanical integrity for easy processing of free-standing films. Unlike other azo based homopolymers and azo/cholesterol-based copolymers, this new copolymer and terpolymer platform exhibits unprecedented temperature-responsive optical properties for applications in thermochromic materials, thermal/photoresponsive switches, sun protecting glazing, photopatterning and temperature sensing labels.