Crystal structure of isotactic polypropylene

Abstract

To account for the observed diffraction pattern of a commercial specimen of oriented fibers of isotactic polypropylene in the a modification, the crystal structure is described as belonging to the space group P 21/c but having a certain disorder in the distribution of “up” and “down” chain direction. As a result of this disorder, reflections having the (h + k) sum odd are weaker than in the ideal P 21/c space group, and the structure thus resembles partially the situation in the C 2/c space group where no such order is required or possible. In the specimen studied in this work, best agreement between theory and experiment (R = 0.158) was achieved if it was assumed that in about 25 % of matter the chains were placed in the wrong direction.