Molecular dynamics simulation of semirigid polymers

Abstract

AbstractThe chain rigidity of poly(p‐hydroxybenzoate) was estimated through the theoretical evaluation of its persistence length (Lp). A non‐Brownian molecular dynamics (MD) simulation of an isolated chain with 20 monomeric units was performed. The sampled conformational population was analyzed and the orientational correlation function between monomeric units along the chain was calculated. An algorithm based on the worm‐like chain model was applied to evaluate the persistence length. The results were compared with those obtained from equilibrium models like the freely‐rotating‐chain and the rotational‐matrix method with fluctuations. Equilibrium models give different results depending on the degree of accuracy used in describing the monomeric unit. The inclusion of thermal fluctuations is crucial to obtain realistic results. These coincide with those given by MD simulation when only nearest‐neighbour orientational correlations are taken into account: inclusion of higher‐order correlation terms leads to lower values of the persistence length. The origin of this discrepancy was investigated. The MD simulation results are characterized by an overrepresentation of conformations with a short end‐to‐end distance resulting from an anomalous energy concentration in the first bending mode of the chain. In analogy with previous simulation results from systems characterized by a week coupling amoung their degrees of freedom, failure in the energy equipartition is proposed as a likely explanation of the anomalous dynamical behaviour.