(648d) Predicting Nematic Coupling of Polybutadiene Using Atomistic Molecular Dynamic Simulations

Various experiments suggest the presence of nematic interactions in flexible polymer melts and networks. These interactions affect how these polymers respond to external fields like stress fields, magnetic fields and electric fields, even when liquid crystalline phases are not apparent. Such interactions can be quantified by nematic coupling constant, and these interactions affect the molecular ordering in polymer systems. To determine the value of the nematic coupling constant in flexible polymers, we combine self consistent field theory (SCFT) with atomistic molecular dynamic simulations of chains under external tension in an isotropic melt; we have previously used this approach to predict the nematic coupling parameter for a semiflexible polymer, poly(3-hexylthiophene-2,5-diyl). We apply this method to determine the nematic coupling constant of poly-butadiene (PB) and determine the scalar order parameter for a melt of strained PB chains and for free PB chains embedded in such a strained melt. The ratio of these order parameters at various temperatures is then compared with the experimental results. The ratio obtained using our method and extrapolated to experimental conditions predicts a value of 0.8 and is consistent with experimental results (~0.9). Our simulations corroborate theory by indicating that coupling between orientation of neighboring segments exists in PB.