Polyisocyanates and the Interplay of Experiment and Theory in the Formation of Lyotropic Cholesteric States

We have made the first observations of the lyotropic cholesteric liquid crystal state of polyisocyanates, in which a synthetically controlled range of the dynamic excesses of one helical sense offer new opportunities for study in the area. In poly((R)-2,6-dimethylheptyl isocyanate) we have generated the cholesteric lyotropic state and studied the pitch as a function of solvent, temperature, and concentration. These data were broken down into entropie, S_q, and enthalpic, H_q terms and were compared to theoretical predictions. The data for the polyisocyanate were compared to those already known for poly(γ-benzyl L-glutamate) and schizophyllaii. For all three helical polymers, the sign of S_q agrees with Straley's screw model, allowing an absolute assignment of macromolecular helical sense from liquid crystal properties. The nematic phase of the dynamically racemic poly(n-hexyl isocyanate) was doped with a derivative of a steroid and of menthol, and the resulting cholesteric pitch was studied as a function of temperature and dope concentration. The failure to fit these data to theoretical expectations applicable to intrinsically chiral polymers may be due to a sensitivity of the helix sense ratio of the macromolecule to the chiral characteristics of the cholesteric phase.