Local Structure Contributions to Surface Tension of a Stereoregular Polymer

We have used all-atom molecular dynamics (MD) simulations to calculate the surface tension of melt poly(methyl methacrylate) (PMMA) as a function of tacticity. Computation of surface tension using the Kirkwood-Buff approach required hundreds of nanoseconds of equilibration. The computed slopes of surface tension versus temperature are in very good agreement with reported experimental values. Using a rigorous treatment of the true interface, which takes into account the molecular roughness, we find that isotactic PMMA, in comparison to syndiotactic and atactic PMMA, shows a larger surface concentration of polar ester-methyl and carbonyl groups on the surface versus nonpolar α-methyl groups. A mechanistic hypothesis based on the helical nature of the isotactic PMMA chains, their relative flexibility, and their reported conformational energies is proposed to explain the trends in composition near the surface. We highlight here how surface composition and surface tension are controlled by both polarity and steric constraints imposed by tacticity.