A new route to stabilize the smectic C phase in a series of laterally attached side-chain liquid crystalline polynorbornenes with a one-carbon spacer
Macromolecules, 33(24), pp 8983-8991, 2000-11-28
A series of polynorbornenes (PNBEs) with 1,4-bis [(3'-fluoro-4'-n-alkoxyphenyl)ethynyl] benzene mesogens (n = 9-12) laterally attached to the polymer backbone through a one-carbon spacer were previously synthesized by ring-opening metathesis polymerization of the corresponding norbornene-based monomers. Wide-angle X-ray diffraction (WAXD) experiments demonstrate that the mesogens organize into the tilted layer structure of a smectic C (Sc) phase at room temperature, and polarized light microscopy demonstrates that the highest temperature ordered phase is a nematic (N) phase. Upon heating above room temperature, the tilt angle of the Sc phase of all of the PNBEs (n = 9-12) continuously decreases, especially at temperatures above 70 degreesC for the PNBEs (n = 9-11) and 60 degreesC for the PNBE (n. E = 12) However the S-C phase of the PNBEs (n = 9-11) transforms to a N phase before the tilt angle reaches zero. That is, the SA phase is never actually achieved in the PNBEs (n. = 9-11), although the decrease in the tilt angles corresponds to a broad endotherm in the differential scanning calorimetry (DSC) experiments in the same temperature region. This speculation is proven by the stable SA phase of PNBE (n = 12) observed by WAXD at, temperatures above 83 degreesC. The transition between the Sc and Sg phases is not a first-order transition according to DSC measurements and is instead a second-order transition based on the gradual decrease to 0 degrees in the tilt angle, which serves as an order parameter.