Solvation-Shell-Induced Entropic Repulsion and Insights into Solvent Quality of Crystalline Conjugated Polymer Solutions

Free-energy analysis based on the potential of mean force (PMF) on a representative series of model crystalline conjugated polymers (P3ATs) reveals that while the thermodynamic feature reflected in the well depth of PMFs is practically no different for four solvent media investigated, two better-solvent media give rise to moderate entropic repulsions at intermediate separation distances that can significantly impact the (kinetic) features of pi-pi packing and nominal solubility as observed in early experiments. Similar observations apply when varying the side-chain lengths (C-0-C-12), whose effects remain controversial in prior studies. The defining long-range feature of PMFs is noted to be governed by solvation shell stability, whose intimate dependencies on solvent mobility -and not just polymer-solvent interactions, as usually assumed -shed new light on the effects and origins of solvent quality for crystalline conjugated polymers.