Crystallization and shape emulation in atactic poly(vinyl chloride) and polyacrylonitrile

Abstract

Molecular modelling calculations have been used to explore the unexpectedly large values of crystallinity measured in atactic poly(vinyl chloride) (PVC) and polyacrylonitrile (PAN). Using MOPAC V:AM1 (distributed by QCPE), it was found that not only can a chain of isotactic PVC (and PAN) emulate the shape and axial repeat of an equivalent (planar zigzag) syndiotactic molecule, but that the shape-emulating conformation has its own energy minimum. This conformation is similar to that first suggested by Juijn. It appears that the dipole interactions between adjacent hydrogen and chlorine atoms are able to stabilize this conformation even though there is no minimum apparent on a two-dimensional energy map corresponding to one repeat unit. Calculations on longer sequences show that this conformation has a subsidiary energy minimum. Similar behaviour has been found in the case of PAN. Wide-angle X-ray diffraction from PVC crystals composed of syndiotactic and isotactic monomer units (but using established unit cell dimensions) was simulated using CERIUS (Molecular Simulations Ltd, Cambridge, UK). The crystalline component of the experimental diffraction profile was found to correspond to that calculated for a mixed tacticity lattice. Furthermore, the relative intensities of the 200 and 110 diffraction maxima have been shown to give a good indication of the mean chain tacticity within the crystallites which can be related to the overall tacticity of the polymer. Calculations of crystallinity content based on mixed tacticity lattices (where dyad tacticity = 0.55) were found to be consistent with the crystallinity and crystallite thickness observed experimentally (10% and 33 Å), whereas these observations are quite inexplicable if the only crystallizable sequences are syndiotactic. A similar predicted relationship between lattice tacticity and relative intensity of the 200 and 110 peaks could not be confirmed for PAN as the almost pseudo hexagonal chain packing means that the 200 and 110 peaks are all but superimposed in the experimental plots. It is noted that extra low angle reflections are found in the simulations of mixed tacticity crystallites where the models are not large enough to ensure a completely random distribution of the isotactic, shape-emulating sequences, if they have been arranged with any degree of periodicity. Low angle reflections have also been observed experimentally. Hence, although PAN does not have the 200 110 WAXS fingerprint of PVC, there is, in addition to the general observation of high crystallinity for the atactic polymers, experimental evidence for co-crystallization of the configurational isomers.