Tacticity Effects on the Bulk Modulus of Poly(methyl methacrylate) Explored by Coarse-Grained Simulations

It has long been realized that stereochemistry is quite important to the mechanical properties of relevant polymers, but it still remains far from complete understanding. In this work, the tacticity dependences of the bulk modulus (B) of two stereoregular poly(methyl methacrylate) bulks have been extensively studied. Both equilibrium (EQ) and nonequilibrium (NE) molecular dynamics (MD) simulations are employed with chemically specific coarse-grained models. The fluctuation equation seems to be a simple route to calculate the B, but the result can significantly depend upon the barostat mass. The thermodynamics relation is valuable to disclosing the entropy contribution, but the result can suffer from great errors in nonlinear fitting. In particular, the instantaneous pressures and volumes during the EQ MD simulations in the "true" NPT ensemble are thus proposed to calculate the B via the definition formula. It is demonstrated in both case-by-case and averaged ways that the effects of tacticity on the B of the polymer are very slight. The identified trend is well explained as one competitive consequence of intermolecular interactions and conformational flexibility.