Title: “Dynamic Bonds to Enhance Polymer Compatibilization”
This project uses molecular dynamics simulations to explore how dynamic bonds affect the miscibility of polymer blends. Polymer compatibilization is of great interest in the recycling industry because mixed plastics are made of different polymer types, such as Polethelene and Polypropelene (1). Polymers with different interaction strengths will have a tendency to phase-separate (Fig.1), which weakens the tensile strength of recycled materials. It is desirable to find a method for compatibilization that allows polymers to physically mix and de-mix, thus increasing material strength over multiple recycling processes. This project simulates the formation and dissociation of dynamic bonds between polymer types, which literature suggests may improve compatibilization (2). This project explores two systems, one where bond reactions occur directly between A-type and B-type polymers, and one where bonds form through a reagent additive. We find that dynamic bonds reduce the critical temperature of phase separation in both systems, where the strength of this effect depends on the chemical potential of the system. We also evaluated the surface tension of the mixture using two different techniques developed by Irving-Kirkwood (1949) and Gloor (2005). Both methods confirmed that surface tension decreases linearly toward zero as the system approaches the critical temperature. Systems with more dynamic bonds reached a value of zero surface tension at lower temperatures. However, dynamic bonds only produce such a compatibilizing effect if the bonds form across the two immiscible polymer types, either directly or through a reagent. In systems where dynamic bonds formed internally between A-type and B-type polymers, the critical temperature of phase separation was not significantly reduced for most chemical potentials. This project produces a guideline for a range of chemical potentials where compatibilization is possible in several different systems.
(1). TA Instruments, “Mixing Rules for Complex Polymer Systems.”
(2) Clarke et al. (2022). “Dynamic Crossinking Compatibilizes Immiscible Mixed Plastics”