RIS Metropolis Monte Carlo studies of poly(l-lactic), poly(l,d-lactic) and polyglycolic acids

Single chain properties (characteristic ratio and persistence length) have been calculated for poly(l-lactic), poly(l,d-lactic) and polyglycolic acids (PLLA, PLLA/PDLA and PGA, respectively) utilizing the RIS (rotational isomeric state) Metropolis Monte Carlo (RMMC) method. The polymer consistent force field (PCFF), that previously was modified by reoptimizing some of its torsion parameters to data of a few model molecules for the current polyesters, was used in the generation of chain conformations in Monte Carlo steps. The same single chain properties have also been calculated using the original PCFF to study the effects of the force field modifications on the results. Calculations were carried out with two different chain lengths (DP=50 or 100) and cut-off limits for nonbonded interactions (Max_Bonds=4 or 6, and also 9 for PLLA) due to the sensitivity of the RMMC method to these run parameters. The modified PCFF predicted the PLLA/PDLA chains to be the most flexible, the characteristic ratio being 6.5 for the copolymer with alternating l,d-diads, and 6.8 for the copolymer with random combination of l,d- and d,l-diads (using Max_Bonds=4 and DP=100), whereas the PLLA chains were found to be the least flexible (with a characteristic ratio of 7.7). On the other hand, using the original PCFF the order of flexibility was totally reversed, and the characteristic ratios became 8.0 and 8.5 for the PLLA/PDLA and 5.6 for the PLLA chains. For the selected chain lengths the characteristic ratios were found to depend significantly on the cut-off parameter for nonbonded interactions, but only slightly on chain length. The dependence of the characteristic ratios on the cut-off limit was also found to be different in the calculations with the modified and original PCFFs.