(289d) Predicting Nematic Phases for Semiflexible Polymers from Simulations

Authors: 
Zhang, W., Penn State University
Gomez, E. D., The Pennsylvania State University
Milner, S. T., The Pennsylvania State University

The nematic phase is important for semiflexible polymers. For example, the nematic phase affects the properties of polymers used in many applications, including displays, high strength fibers, and biomedical devices. The existence of nematic phases can also result in better processing of functional semiflexible polymers such as semiconducting conjugated polymers. Crystallization from the nematic phase promotes the formation of ordered final morphologies, which can in turn enhance the electric properties and the resulting device performance for conjugated polymers. Directly observing the nematic phases, however, is challenging for many semiflexible polymers. The narrow temperature window of the nematic phase and the low isotropic-to-nematic (IN) transition enthalpy can prevent the observation of nematic phase using techniques such as differential scanning calorimetry (DSC). Equilibrating molecular dynamic simulations of semiflexible polymers near the phase transition can be also difficult. We develop a method for predicting the nematic phases of semiflexible polymers by first estimating the nematic coupling constant α, a material parameter quantifies the orientational coupling between backbone segments. The coupling constant α, together with the chain stiffness κ, governs the IN transition temperature TIN and the chain alignment in mesophases and at interfaces. Because crystallization or thermal degradation can preclude the IN transition, TIN cannot always be used to predict α. We combine self-consistent field theory (SCFT) with atomistic molecular dynamics (MD) simulations of semiflexible chains under external tension in the isotropic phase to predict the nematic coupling constant α. In simulations, we apply external tension to the polymer backbones to induce uniaxial alignment, which is amplified by the presence of nematic coupling. Using SCFT, we compute the order parameter q for the same semiflexible chains under the same tension with a given α. The value of α is determined by fitting the order parameter q(α) obtained using SCFT to the simulation results. Using the predicted α, we obtain the variational free energy, from which the IN transition temperature TIN is determined. We demonstrate our method by predicting α and TIN of a commonly studied conjugated polymer, poly(3-hexylthiophene) (P3HT). The estimated TIN suggests that oligomeric 3-hexylthiophene is nematic after melting from crystal. Using our mean field free energy, we also predict the IN transition transition enthalpy is much smaller than the enthalpy of fusion for P3HT. Our predictions are consistent with experiments, in which a nematic phase is reported based on polarized optical microscopy (POM) and only a single crystal melting peak is observed using DSC for P3HT.