X-Ray Scattering and Thermal Characterization of α,β-Dimethyl Stilbene Polycarbonates

Abstract

Structural and thermal characterization is reported for a series of recently synthesized¹ liquid crystalline polycarbonates based on di-methyl substituted stilbene mesogen and methylene-containing flexible spacer of length n. In this work, we focus on the α, β-dimethyl stilbene (DMS-n) polycarbonates and on the comparison between them and α-methyl stilbene (HMS-n) polycarbonates^2–4, especially those with heptane spacer. Wide angle X-ray scattering patterns show that DMS polycarbonates are able to form a nematic liquid crystalline phase. However, the liquid crystalline phase is generally not stable with respect to the crystalline phase. In the DMS polycarbonates, no distinct isotropic-to-nematic (i → lc) transition is seen either by polarizing optical microscopy or by differential scanning calorimetry (DSC). We suggest that the steric effect of the second lateral substituent results in a less stable, virtual mesophase. For DMS-n, an odd-even effect is observed in the crystallization and melting temperatures for n values between four and eight. The transition temperatures associated with even parity n (n = 4,6,8) are greater than those for odd parity n (n = 5,7,9) and all transition temperatures drop when n exceeds eight. Small angle X-ray scattering studies were performed on the DMS polycarbonates to examine the structure of the crystalline phase. For n in the range from 5–8, DMS polycarbonates generally have higher values of linear crystallinity and crystal thickness than corresponding HMS polycarbonates.