MOLECULAR-DYNAMICS SIMULATIONS COMBINED WITH LARGE-ANGLE X-RAY-SCATTERING TECHNIQUE FOR THE DETERMINATION OF THE STRUCTURE, CONFORMATION, AND CONFORMATIONAL DYNAMICS OF POLYPHOSPHAZENES IN AMORPHOUS PHASE - STUDY OF POLY[DI(4-METHYLPHENOXY)PHOSPHAZENE]

Suitable parameter sets for the CHARMm force field were derived using the Dinur-Hagler energy second-derivative procedure, on the basis of S CF calculations at the 6-31G level, for the uncommon structural units in poly(phenoxyphosphazenes) [P=N, P-N, P-X (X=aryloxy)]. It is shown that application of molecular dynamics (MD) simulations, in combinati on with experimental energy dispersive X-ray diffraction (EDXD) measur ements, provide unambiguous structural and conformational information on amorphous polymers. The procedure for the analysis of the EDXD data involves comparison of computed atom-atom radial distribution functio n (RDF) curve from MD simulations for the various polymer backbone con formations, with the RDF obtained from experimental X-ray scattering d ata. The applicability of this combined experimental/computational met hodology is illustrated on the amorphous poly[di(4-methylphenoxy)phosp hazene] (PMPP). The results showed that (i) the backbone conformation is safely [TC](n) rather than [T3C](n) and (ii) the computed RDFs are best assessed by using a MD simulation technique that avoids assumptio n of static chain conformation and the needed best fit of the distance dependent parameters s(jk). In this method of analysis, the RDF that to be compared with the experimental one is directly calculated from a ll microstates collected during the entire simulation period. Validati on of the polymer model provides a complete picture, otherwise experim entally inaccessible, of the internal fluctuations of the polymeric ha ins. The computational protocol delineated for analysis of EDXD data i s general and its application specifically necessary when highly flexi ble amorphous polymers are involved.