Structural Differences Between Two Crystal Modifications of Poly(y-benzyl L-glutamate) zy JUNJI WATANABE, KAZUMICHI IMAI, RAMESH GEHANI, and ICHITARO UEMATSU, zyxwvu Department zyxw of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, zyx 152 Japan Synopsis X-ray diffraction patterns were obtained for as-cast and oriented films of poly(y-benzyl L- glutamate) and a comparison was made of the molecular packing of the a-helices in forms B and C. Form B showed Bragg reflections on the layer lines as well as on the equator. The spacings were explained by a monoclinic unit cell comprising two chains, with zyxw a zyxwv = 29.06 A, b = zyx 13 20 A, zyx c = 27.27 A, a = y = 90°, and /3 = 96O. The chains contained in this unit cell and consequently alternating in the crystal have opposite chain directions. Form C showed continuous scattering on the layer lines and reflections on the equator. This form, therefore, is a nematiclike paracrystal in which the packing of a-helices is periodic in the direction lateral to the chain axis (a = 14.8-115.2 %., b = 14.3-14.8 A, c = 27 A, and y = 118°-1200), but the relative levels of the chains along the chain axes are displaced. The formation of form C may be attributed to random placement of two chains with mutually opposite chain directions. INTRODUCTION Solid-state modifications of a-helical poly(y-benzyl L-glutamate) (PBLG) have been so far classified into three forms A, B, and C.1-3 These forms have been prepared by casting films from different solvents, and the corresponding two-dimensional crystal lattices have been reported. Form A is a poorly ordered material and its x-ray diffraction pattern was tentatively interpreted using a hexagonal unit cell with dimensions of a = b - 30 A, in which four chains are 10cated.~ This form is derived from the so-called “complex phase” which was investigated in detail by Elliott et al.4,5 On the other hand, form B is a well- defined form composed of a-helices packed on an accurate two-dimensional lattice with unequal cell edges (a = 15.9 A, b = 13.0 A, and y = 114°).3 In form C, the a-helical molecules are packed on a rather irregular pseudohexagonal lattice (a = 15.2 A, b = 14.3 A, and y = 11B0).l Between forms B and C, there is a clear difference with respect to the molecular packing of a-helices in the lateral direction. No one, however, has discussed this interesting structural difference. Most likely, this difference can be attributed to a difference in the side-chain conformation, which might have been induced by interaction between the side-chains and solvent. However, even if the films are annealed at a temperature of 150°C, at which the rotation of the entire side chain has already occurred in solid the molecular packing does not change appreciably from that in the original structure.* Thus, it is doubtful that the structural differences can be attributed to differences in side-chain conformation alone. In order to consider the packing properties of a-helices, it is important to clarify the other factors contributing to the structural differences which are retained even at high temperatures above 150OC. Journal of Polymer Science: Polymer Physics Edition, Vol. 19,653-665 (1981) 0 1981 John Wiley & Sons, Inc. CCC 0098-1273/81/040653-13$01.3O