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Molecular Motion in Poly(amino acid) VIII. β2-Form Crystals of Poly(D-glutamic acid)

The β2-form crystal of poly(D-glutamic acid) has a strong interaction between the sheets, in which chains are bound side by side by intermolecular hydrogen bonds. In this work, the molecular motion of poly(D-glutamic acid) in the fully annealed β2-form crystal was studied. The linear thermal expansion coefficient for the intermolecular periodicity, 10−5 (°C−1), was similar to that for the intersheet periodicity below 200°C, indicating that there is only ordinary thermal lattice vibration in the β2-form crystal. This finding differs from that for the β-form crystal having no interaction between the sheets. Above 200°C, the side chains began to move as a result of weakening of the strong interaction between the sheets, leading to disordering and degradation of the β2-form crystal. In the amorphous region, two segmental micro-Brownian motions occurred at 120°C and above 140°C. The former was attributed to free amorphous chains which have no hydrogen bonding between the carboxyl groups of the side chains, and the latter to the weakening of hydrogen bonds.