Effect of molecular weight on conformation of poly(β-benzyl L-aspartate) in films

In order to study the effect of the molecular weight on the crystallinity and conformational changes of poly(β-benzyl L aspartate) in films, a previous study on high molecular weight samples has been extended to included polymers of low molecular weight, about 3.3 × 10³. Films were prepared from chloroform solution by quick or slow evaporation at room temperature. The conformation and the thermal behavior were studied by means of infrared spectroscopy and differential scanning calorimetry. All films dried quickly are composed of polymer in the left-handed α-helical form. All samples studied which have molecular weights above 2.3 × 10⁴ are similar in crystallinity and the left-handed α-helices in them crystallize to ω-helices during slow evaporation. In the low molecular weight region, however, the left-handed α-helices reverse to right-handed α-helices during slow evaporation, and the right-handed α-helices, in turn, reverse and crystallize to highly ordered ω-helices upon heat treatment, although there is some simultaneous conversion to the β-form. The transition temperatures of the quick-dried films for conversion from the left-handed α-helix to the ω-helix and from the ω-helix to the β-form increase linearly with increasing molecular weight up to about 2 × 10⁴, but no large molecular weight dependence is observed beyond that region.