Crystal transition and structure of poly (γ‐n‐alkyl glutamate)s

Abstract

AbstractThe nature of the crystal transition of the α‐helical forms of poly (γ‐n‐alkyl glutamate)s (alkyl = ethyl, propyl, and butyl) is described. The transition is thermally reversible, and its temperature T2 is much higher than the glasslike transition temperature T1 associated with the side‐chain motion. The main chains undergo large‐scale motion (librational about the chain axis and translational along the axis) above T3 ≈ 200°C. The structure observed below T2 is anomalously disordered compared with that observed between T2 and T3. The crystal structure emerging above T2 is analyzed for a typical sample of poly(γ‐n‐propyl L‐glutamate). The trigonal unit cell contains three α‐helices so that each helix is surrounded by other helices in the same fashion, but the helices are not interrelated by a crystallographic symmetry element. The side chains suffer no particular change at T2. The main‐chain motion gives rise to the T2 transition by inducing attractive forces between interpenetrating side chains.