Temperature and Pressure Effects on Local Structure and Chain Packing in cis‐1,4‐Polybutadiene from Detailed Molecular Dynamics Simulations

Summary: We present results for the temperature and pressure dependence of local structure and chain packing in cis‐1,4‐polybutadiene (cis‐1,4‐PB) from detailed molecular dynamics (MD) simulations with a united‐atom model. The simulations have been executed in the NPT statistical ensemble with a parallel, multiple time step MD algorithm, which allowed us to access simulation times up to 1 µs. Because of this, a 32 chain C₁₂₈ cis‐1,4‐PB system was successfully simulated over a wide range of temperature (from 430 to 195 K) and pressure (from 1 atm to 3 kbar) conditions. Simulation predictions are reported for the temperature and pressure dependence of the: (a) density; (b) chain characteristic ratio, C_n; (c) intermolecular pair distribution function, g(r), static structure factor, S(q), and first peak position, Q_max, in the S(q) pattern; (d) free volume around each monomer unit along a chain for the simulated polymer system. These were thoroughly compared against available experimental data. One of the most important findings of this work is that the component of the S(q) vs. q plot representing intramolecular contributions in a fully deuterated cis‐1,4‐PB sample exhibits a monotonic decrease with q which remains completely unaffected by the pressure. In contrast, the intermolecular contribution exhibits a distinct peak (at around 1.4 Å^?1) whose position shifts towards higher q values as the pressure is raised, accompanied by a decrease in its intensity.

3D view of the simulation box containing 32 chains of C₁₂₈ cis‐1,4‐polybutadiene at density ρ = 0.849 g · cm^?3 and the conformation of a single C₁₂₈ cis‐1,4‐PB chain fully unwrapped in space.