Molecular Dynamics Simulations of a Main-Chain Liquid Crystalline Polyether in the Crystalline State. 1. Chain Conformation and Dynamics of the Spacer Methylene Sequences

Abstract

Molecular dynamics simulations have been performed for the main-chain thermotropic liquid crystalline polyether (EDMB-10), which is composed of the 3,3′-dimethyl-4,4′-biphenylene mesogens and 10-methylene spacers, in order to compare the spacer conformation and dynamics with those revealed by solid-state 13C NMR spectroscopy. A three-dimensional periodic cell that contains 4 or 16 molecular chains composed of four mesogen and five spacer units are employed, the central CH2 sequences of the five spacer sequences being focused on for four chains in the 4-chain model or for the central four chains in the 16-chain model. For both models, the initial structure stays unchanged below 150 K. At 200 K, three dimensional structure begins to change and the molecular arrangement and the cell parameters are finally varied without any significant change in conformation. Below 250 K the torsion angles of the O–CH2 bonds at the spacer ends are ±90° and all the C–C bonds adopt the trans conformation for both structural models. At 300 K, the O–CH2 bond and its second neighboring C–C bond are found to undergo the cooperative counter rotation of ±90°tt↔ttg± keep the periodic length between the neighboring mesogen units along the chain almost constant. At 450 K, the CH2 sequences start to adopt a conformational pair of g+tg−, which is frequently called the 2g1 kink, for the 4-chain model, but the introductions of the kinks are restricted to the alternate four C–C bonds. This result is qualitatively in good accord with the result previously obtained for EDMB-10 by the solid-state 13C NMR analysis, but the t/g probabilities at the alternate C–C bonds for the simulations significantly disagree with the experimental results. In contrast, for the 16-chain model, a pair of the g+ and g− conformations are allowed to introduce to the alternate four C–C bonds at almost equal probabilities without any restriction of the forms of the kinks, resulting in the good qualitative and quantitative agreements with the experimental results. The time evolution of the torsion angles for the respective C–C bonds of the spacer CH2 sequences are also described for the 16-chain model to reveal cooperative conformational transitions.