Polysynthetic twinning in poly(vinylcyclohexane) single crystals and "fractional" secondary nucleation in polymer crystal growth

The internal structure of single crystals and twinned crystals of isotactic poly(vinylcyclohexane) (PVCH) has been investigated by electron diffraction and dark-field imaging following a procedure introduced by Pradère et al. (Macromolecules 1988, 21, 2747). Crystals produced in thin film at high T _c are single crystalline. At somewhat lower T_c, they display large twinned sectors bounded by (200) and (020) planes only. PVCH crystals produced from squalane or phenyldecane solutions display very small, elongated twinned domains oriented normal to the growth faces: the crystals are made of polysynthetic twinned growth sectors. A specific and growth sector dependent streaking indicates that the lattices are shifted by one-quarter of the unit-cell edge across the twin planes: the growth faces are jagged. These features are discussed in the general context of polymer nucleation and growth. Deposition of the initial stem in the growth face takes advantage of the existing step associated with the twin plane, a process that can be described as a "fractional" secondary surface nucleation process. Lateral spread, i.e., completion of the growth face, is limited by the jagged surface. Since both secondary nucleation and lateral spread are determined or limited by structural features, the usual distinction of different growth regimes is blurred.