Predicting chi for polymers with different chain architectures using simulation*

The Flory Huggins interaction parameter chi governs phase behavior in polymer blends and block copolymers. Chain architecture affects how chains pack in the melt, which can significantly influence chi. To explore this, we investigate chi for two different architectures of flexible bead-spring chains, using molecular dynamics simulations and our recently developed “morphing” method to compute the excess free energy of mixing. In the first case, we consider an idealized bead-spring model of polyethylene and polypropylene, in which all beads have the same interactions, but polypropylene chains have a side bead on every other backbone bead. In the second case, we examine blends in which both chain species have the “polypropylene” bead-spring structure, but one species has beads with a slightly weaker interaction – either the side beads (case 1), main chain beads (case 2) or branch point beads (case 3). We use our method to find c for all three cases, for which random mixing models would give identical results. Finally, we compare our values with predictions from PRISM, with correlation functions from simulations as input, as a purely simulation-based test of PRISM.