Variation of crystallite orientation with degree of crystallinity: Isotactic polystyrene

The orientation of the crystallographic c axis (chain axis) was followed by x‐ray diffraction during the crystallization of four samples of isotactic polystyrene differing in elongation ratio. The crystallite orientation can be expressed by 〈cos 2 χ c 〉, where χ c is the angle between the c axis and the stretching direction. The degrees of crystallinity w were estimated from the diffraction data by using density for calibration. It was found that 〈cos 2 χ c 〉 decreases in a linear manner with crystallinity, the rate of decrease being very small when the elongation ratio α is 5, but becoming progressively larger as α is decreased toward unity. A qualitative measure suggests that amorphous orientation decreases during crystallization at a rate which is nearly independent of α. The variation of 〈cos 2 χ c 〉 with w is therefore governed by the orientation of the statistical chain segments prior to crystallization. If the elongation ratio is small, the supply of well oriented statistical segments is limited, and 〈cos 2 χ c 〉 will decrease at a rapid rate during crystallization. A treatment due to Krigbaum and Roe permits evaluation of the ratio, ν/ N , where ν and N are the average numbers of statistical segments in the crystallization nucleus of critical size, and in a network chain, respectively. Our polystyrene samples were not crosslinked, so chain entanglements must serve as junction points. Values of ν could not be obtained, since N was unknown. However, the (ν/ N ) ratio for isotactic polystyrene decreases slowly with α, and the values agree reasonably well with those obtained in a previous study of oriented polychloroprene networks. After nearly complete crystallization (ω ca. 0.30), the long period spacing measured by low angle diffraction was approximately 135 Å, and varied only slightly with elongation ratio in the range α = 1 to 5. It therefore appears that chain folded lamellae are present in both drawn and undrawn samples of isotactic polystyrene.