Molecular Simulations and Mechanistic Analysis of the Effect of CO2 Sorption on Thermodynamics, Structure, and Local Dynamics of Molten Atactic Polystyrene.

Eleonora Ricci,Georgios G Vogiatzis,Maria Grazia De Angelis,Niki Vergadou,Doros N Theodorou

doi:10.1021/acs.macromol.0c00323

Abstract

A simulation strategy encompassing different scales was applied to the systematic study of the effects of CO2 uptake on the properties of atactic polystyrene (aPS) melts. The analysis accounted for the influence of temperature between 450 and 550 K, polymer molecular weights (Mw) between 2100 and 31000 g/mol, and CO2 pressures up to 20 MPa on the volumetric, swelling, structural, and dynamic properties of the polymer as well as on the CO2 solubility and diffusivity by performing molecular dynamics (MD) simulations of the system in a fully atomistic representation. A hierarchical scheme was used for the generation of the higher Mw polymer systems, which consisted of equilibration at a coarse-grained level of representation through efficient connectivity-altering Monte Carlo simulations, and reverse-mapping back to the atomistic representation, obtaining the configurations used for subsequent MD simulations. Sorption isotherms and associated swelling effects were determined by using an iterative procedure that incorporated a series of MD simulations in the NPT ensemble and the Widom test particle insertion method, while CO2 diffusion coefficients were extracted from long MD runs in the NVE ensemble. Solubility and diffusivity compared favorably with experimental results and with predictions of the Sanchez–Lacombe equation of state, which was reparametrized to capture the Mw dependence of polymer properties with greater accuracy. Structural features of the polymer matrix were correctly reproduced by the simulations, and the effects of gas concentration and Mw on structure and local dynamics were thoroughly investigated. In the presence of CO2, a significant acceleration of the segmental dynamics of the polymer occurred, more pronouncedly at low Mw. The speed-up effect caused by the swelling agent was not limited to the chain ends but affected the whole chain in a similar fashion.

Highlights

Supercritical CO2 has found many applications in the fabrication and processing of polymers to selectively control and manipulate their physical properties by changing temperature and pressure, taking advantage of the fact that its critical point (304.2 K and 7.4 MPa1) allows operating at mild conditions
A coarse-grained initial configuration is generated by growing chains within the primary simulation box by following the quasi-Metropolis bond-by-bond growth scheme of Theodorou and Suter,[51] and meso and racemo identities are assigned to every coarsegrained site by sampling a Bernoullian distribution
Molecular simulations were applied to the study of a polymeric system containing a plasticizing agent in full atomistic detail, and an extensive mechanistic analysis of the underlying microscopic phenomena was conducted

Summary

Introduction

Supercritical CO2 has found many applications in the fabrication and processing of polymers to selectively control and manipulate their physical properties by changing temperature and pressure, taking advantage of the fact that its critical point (304.2 K and 7.4 MPa1) allows operating at mild conditions. Supercritical CO2 is used as a reaction solvent and as an extraction medium, especially in the food and pharmaceutical industries, because of its nontoxic nature. It is applied in the impregnation of polymers to introduce dyes, antibacterial or antioxidant substances, or other types of additives into the matrix. A detailed understanding of the effects of CO2 on polymer properties is of great interest in a wide range of industrial sectors

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Conclusion