X-RAY-SCATTERING AND THERMAL CHARACTERIZATION OF ALPHA,BETA-DIMETHYL STILBENE POLYCARBONATES

Structural and thermal characterization is reported for a series of re cently synthesized(1) liquid crystalline polycarbonates based on di-me thyl substituted stilbene mesogen and methylene-containing flexible sp acer of length n. In this work, we focus on the alpha, beta-dimethyl s tilbene (DMS-n) polycarbonates and on the comparison between them and alpha-methyl stilbene (HMS-n) polycarbonates(2-4), especially those wi th heptane spacer. Wide angle X-ray scattering patterns show that DMS polycarbonates are able to form a nematic liquid crystalline phase. Ho wever, the liquid crystalline phase is generally not stable with respe ct to the crystalline phase. In the DMS polycarbonates, no distinct is otropic-to-nematic (i --> lc) transition is seen either by polarizing optical microscopy or by differential scanning calorimetry (DSC). We s uggest that the steric effect of the second lateral substituent result s in a less stable, virtual mesophase. For DMS-n, an odd-even effect i s observed in the crystallization and melting temperatures for n value s 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. Sma ll angle X-ray scattering studies were performed on the DMS polycarbon ates to examine the structure of the crystalline phase. For n in the r ange from 5-8, DMS polycarbonates generally have higher values of line ar crystallinity and crystal thickness than corresponding HMS polycarb onates.