Crystal structures of α and β forms of poly(tetramethylene succinate)

Abstract

Crystal structures of the α and β modifications of poly(tetramethylene succinate) (PTMS) were analyzed by X-ray diffraction: the β form appeared with an application of stress. These two modifications belonged to the monoclinic system with the space group of P21/n. In both cases, a unit cell included two chemical repeating units. For the α form, the cell dimensions were a=0.523nm,b=0.912nm,c (fiber axis)=1.090nm, and β=123.9°; for the β form, a=0.584nm,b=0.832nm,c (fiber axis)=1.186nm, and β=131.6°. The difference in the fiber periods of the two crystalline forms was attributed mainly to the conformational difference in the tetramethylene unit, i.e. TGTḠT of the α form and TTTTT of the β form. It was also found that in PTMS, the packing coefficient, K, which was defined by the ratio of the intrinsic volume with respect to the true volume of the α form was almost equal to that of the β form. This observation could be contrasted to those obtained in poly(butylene terephthalate) (PBT), where the K of the α form was considerably greater than that of the β form. The difference between PTMS and PBT was attributed to the difference between the unit cell volumes of the α and β forms of these polymers.