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

Area of Science:

• Polymer Science
• Materials Science
• Computational Chemistry

Background:

• Stereochemistry significantly influences polymer mechanical properties, though a complete understanding is lacking.
• Poly(methyl methacrylate) (PMMA) is a widely studied polymer with varying tacticity.

Purpose of the Study:

• To investigate the dependence of bulk modulus on tacticity in stereoregular PMMA.
• To compare different methods for calculating bulk modulus in polymers.

Main Methods:

• Equilibrium (EQ) and nonequilibrium (NE) molecular dynamics (MD) simulations were performed using chemically specific coarse-grained models.
• Calculations of bulk modulus were explored using fluctuation equations, thermodynamic relations, and a proposed method utilizing instantaneous pressures and volumes from EQ MD simulations in the "NPT" ensemble.

Main Results:

• The effects of tacticity on the bulk modulus (B) of PMMA were found to be very slight.
• The fluctuation equation's accuracy for bulk modulus calculation is sensitive to barostat mass.
• Thermodynamic relations can introduce significant errors in nonlinear fitting for entropy contributions.

Conclusions:

• The proposed method using instantaneous pressures and volumes from EQ MD simulations provides a reliable way to calculate bulk modulus.
• The minor influence of tacticity on PMMA's bulk modulus is attributed to a balance between intermolecular interactions and conformational flexibility.