Computer Simulation of Cyclic Siloxane-Based Liquid Crystals: Molecular Dynamics and X-ray Scattering.

摘要

The molecular dynamics technique is employed in the present study to investigate molecular models of a liquid crystal (LC) based on cyclic penta(methylsiloxane) with combinations of cholesteryl4'-allyloxybenzoate and biphenyl-4'allyloxybenzoate mesogens, which is of technological importance because of attributes related to light processing media. The packing and structure exhibited by these commercially available cholesteric LC's show unusually well-defined interlayer order giving rise to high order X-ray reflections. The purpose of this investigation was to: (1) Investigate the conformational flexibility of the molecular arrangements and organization of this cyclic siloxane-based LC at room temperature, providing insight into the relative stability of the various models proposed. (2) Serve as an instructional reference describing the basic theory and use of molecular mechanics and dynamics for the study of large molecular systems. Results from the molecular dynamics calculations indicate the degree of conformational flexibility among the pendant mesogens of this molecular system, and its effects on interdigitation. In particular, the isolated disk model exhibits the most flexibility and greatest stability as indicated by mean dihedral angles and range for certain key torsions. The dynamics simulation of the cylinder pairs suggests a large conformational flexibility of the siloxane rings. Also, movements of the interdigitated mesogens were much higher for a fixed rings system than for the case where the ring atoms were free to move. X-ray scattering pattern calculations for the structures generated during the dynamics run demonstrate higher order than for the initial models.