Morphological observation and annealing effect of single crystals of poly(γ-methyl D-glutamate) in α-helical form

Single crystals of poly (γ-methyl D-glutamate) (PMDG) were obtained in a form of hexagonal platelet by isothermal crystallization from a mixed solution of N, N-dimethylformamide (DMF) and propionic acid (1:1 in volume). The electron diffraction pattern revealed that the single crystal is composed of α-helical molecular chains oriented perpendicularly to the plane surface. Comparison of the single crystal thickness with the length of the α-helical molecular chain calculated from the viscosity-average molecular weight showed that the molecular chains might fold at the lamellar surface. The fold formation was supported by the observation that fibrils were pulled out from the fracture surface when the single crystal was drawn on a substrate. Since the α-helical molecular chain cannot fold itself due to the helical conformation with intramolecular hydrogen bonds, it seems that the fold region is composed of non-α-helical segments. The surface of PMDG single crystals was considerably rougher than that of polyethylene single crystals. The surface state of PMDG single crystals was investigated on the basis of annealing and gold decoration. Thickening of single crystals was not observed when they were annealed up to 513°K but the lamellar thickness decreased with increasing annealing temperature upon annealing above 453°K. This annealing effect on the lamellar thickness was explained by the rearrangement of molecular chains caused by the transition from the α-helix to the non-α-helix on the surface and edge region of the single crystal.