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

Abstract

ABSTRACT:The {32-form crystal ofpoly(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 /32-form crystal was studied. The linear thermal expansion coefficient for the intermolecular periodicity, 10-5 cc-1 ), was similar to that for the intersheet periodicity below 200°C, indicating that there is only ordinary thermal lattice vibration in the {32 -form crystal. This finding differs from that for the {3-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 /32-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. (direction of the side chain) and /3= 104°. They also found IR absorption bands differing from those for the ordinary /3-form crystals in silk fiber, PMDG and PLA and attributed the difference to the strong interaction between the hydrogen bonded peptide groups of the side chains in an adjacent sheet. Takahashi et a/. 6 • 7 prepared a well oriented fiber containing only the /32 -form crystal by spinning at a draw ratio of 2.7 from a concentrated aqueous solution ofPGA containing 43% H 3 P04 coagulant. They provided the orthorhombic unit cell with a= 9.26 A (interchain periodicity in one sheet), b = 7. 73 A (intersheet periodicity) and c = 6.90 A (fiber axis).In our preceding papers on unannealed silk fiber 8 • 9 and PLA, 10 the temperature dependence of the spacings corresponding to the interchain and intersheet periodicities was measured and it was found that the slopes of those spacings against temperature increased above about 160°C. It was then proposed that the main chain motion occurs in the crystal as a result of weakening of intermolecular hydrogen bonds. For fully annealed PMDG 11 and silk fiber, 12 we clarified that the chains are packed very stably and densely in the sheet owing to strong intermolecular hydrogen bonds and that slippage of the sheets rather than the motion of the chains themselves takes place above about 200°C. In this paper, a fully annealed /3 2 -form crystal of PGA was used to study the molecular motion in the /3-form crystal for poly(amino acid) having strong interaction between the hydrogen bonded peptide groups in one sheet and the side chain in an adjacent sheet. The spacings corresponding to the interchain 797

Dielectric Properties of Poly(γ-benzyl L-glutamate-co-L-leucine) and Poly(γ-methyl L-glutamate-co-L-leucine) in the Solid State

“…1 The molecular motions in a number of poly(L-glutamate)s have been investigated in detail by various methods including dynamic mechanical, dielectric, and wide-line nuclear magnetic resonance measurements. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] It has been reported that glutamate side chains have motion near room temperature, accompanied by some mechanical and dielectric relaxations, while the backbone chain remains in the rigid a-helical conformation.…”