Stiffening of Semirigid Polymer Chains Before the Isotropic-Aniostropic Transition

The phase behavior of a semirigid polymer manifested by the polypeptide such as poly(β-phenethyl L-aspartate)(PPLA) in a mixed solvent system containing a denaturant acid was studied. The obtained results suggest that there is a marked enhancement of the propensity for the semirigid polymer chain to adopt a rigid conformation upon increasing polymer concentration before the critical value for the incipience of a liquid crystal phase, which has been firmly confirmed both theoretically and experimentally. In consideration of the first order estimation of polymer-polymer interaction, a theoretical approach was attempted within the framework of a helix-coil transition model proposed by Rajan and Woo. According to the analysis, the semirigid chains could become rather rigid before reaching liquid crystal phase. Such a kind of chain stiffening effect was verified by 1H NMR and 13C NMR measurements. The experimental results show that the PPLA molecules convert gradually from the random coil to the rigid rod conformation as polymer concentration turns higher in the isotropic phase.