Molecular modeling and large-angle X-ray scattering studies of the structure of semicrystalline poly[bis(phenoxy)phosphazene]

The structure and conformation of semicrystalline poly[bis(phenoxy)phosphazene] (PBPP) was studied using molecular mechanics including ad hoc quantum mechanically derived force field (FF) parameters, in combination with the energy-dispersive X-ray diffraction (EDXD) technique. The atom-atom radial distribution function (RDF) curves were calculated for the various models of backbone conformations and for the structure in the crystalline a-form of PBPP. On the basis of comparison between theoretically calculated RDFs with the RDF obtained from the EDXD experiment, a model was proposed. The structural features of this model are (i) the polymer backbone adopts a low-energy, planar, trans-cis [TC], conformation, (ii) each chain is, on average, comprised of 16 monomeric units, and (iii) the unit cell may be restricted to contain two such chains running antiparallel to each other and with their backbones aligned parallel to the (1, 1, 0) Miller plane. The results demonstrate the capability of the combined use of computational chemistry (molecular modeling) and X-ray diffraction techniques such as EDXD to provide insights, otherwise experimentally inaccessible, into the conformational and structural features of semicrystalline polyphosphazenic materials.