Frustration and single crystal morphology of isotactic poly(2-vinylpyridine)

The crystal structure of isotactic poly(2-vinylpyridine) (iP2VP) established in 1977 by Puterman et al. is shown to conform to a recently proposed frustrated packing scheme which involves three isochiral three-fold helices packed in a trigonal unit-cell, and observed in a number of polymers and biopolymers. Single crystals of iP2VP grown from thin films at high temperature (Tc ≈ 200°C) display highly unusual morphologies with six (100) growth sectors but a three-fold overall symmetry apparent through different lamellar thicknesses in adjacent growth sectors, and oblique growth facets of three of the sectors. These features are morphological manifestations of the frustration of the helix packing, and support the P3₁ or P3₂ space group of the unit-cell. The different lamellar thicknesses are mainly due to an isothermal thickening process, which has different impacts depending on the nature of the growth faces. Analysis of the growth features of frustrated structures can be of help when dealing with finer details of theories of polymer crystal growth.