Abstract
Molecular dynamics (MD) simulations have been performed for an oligomer of poly (dichlorophosphazene) (PDCPN) containing 56 repeating units i.e. (PCl2N)56, seeking for the orientations of the rotational angles over the P–N skeletal bonds. Two different molecular systems were simulated. In the first one, the chain was packed into a cubic lattice (i.e. a cubic box with periodic boundary conditions) having a side length of 22.1Å in order to reproduce a density of about 1g/cm3, while in the second one, the chain was assumed to be alone in vacuo. In both systems, the allowed orientations for the skeletal bonds are cis (φ=0) and trans (φ=180) with a slight preference for this second orientation. However, a more detailed analysis shows that the lattice produces more extended conformations, and therefore larger molecular dimensions, than the isolated molecule. Thus, the chain in the lattice is formed by sequences of two or three bonds in the trans conformations separated by trans–cis (the average number of bonds in the all-trans conformation is 〈nt〉≈2.7) with negligible incidence of cis–cis conformations. The trans sequences are shorter (i.e. 〈nt〉≈1.7) in the isolated molecule and the presence of cis–cis orientations is noticeable. The a priori probabilities for all the allowed orientations of each pair of bonds obtained by MD simulations can be reproduced by a very simple RIS model that was employed for the evaluation of the unperturbed dimensions of long chains. The values obtained for the characteristic ratio were C∞≈15.7 in the case of the lattice and 8.0 for the isolated molecule, the former value being in better agreement with experimental results than the later one. Long range interactions, mainly due to van der Waals forces, may be responsible for the differences observed in the two systems simulated here, and could also explain the widely different experimental values of molecular dimensions reported for this kind of polymers.
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