Properties of Materials and their Molecular Bases. New Approaches towards Understanding Liquid Crystals and Polymers

Abstract

The fundamental bases of liquid crystals and polymers–both widely employed in, and critically important to, modern society–apparently remain poorly understood. Hence, the need to explore alternative explanations to current paradigms. Thus, liquid crystals are viewed as partially molten solids that retain a semblance of the order of their crystalline precursors. However, this seems unviable as the long-range order in the resulting mesophase cannot survive at temperatures higher than the melting point of the crystal itself. In polymers, the non-covalent intermolecular forces are believed to be additively amplified along the length of the macromolecule. However, this ignores the fact that the said forces remain minuscule at the sub-unit level, so a collection of macromolecules would be continually associating and dissociating at each contact point. It is doubtful that this can explain the observed mechanical strength of polymers (leading to the “polythene enigma”).  It is argued herein that liquid crystals arise via the entanglement of long chains and U-shaped moieties in the incipient crystalline melt, a process essentially facilitated by proximity effects in the crystal. Thus, the entanglements are not easily reversed once the proximity effects are lost in the mesophase, which is likely a nanoparticle aggregate possibly composed of quasi-rotaxane and quasi-catenane species. Furthermore, liquid crystals–even those derived from achiral molecules–display optical activity, which is critical to their application in display devices. Although this symmetry breaking remains enigmatic, a chiral mechanochemical effect or even parity violation are possible explanations. In the case of macromolecular association, it is argued that the van der Waals force is inherently strong in enthalpy terms, but is stymied by entropic effects which dominate in the weak forces (generally). However, the entropic effects are possibly “damped” in the macromolecule (although in a subtle manner), so association is much greater than currently estimated. These lead to interesting theoretical insights into enthalpy-entropy relationships in atomic and molecular interactions, a sigmoid relationship possibly being indicated.