Depolymerization dynamics study of poly-alpha-methylstyrene

Abstract

Polymers are macromolecus composed of repeated monomer subunits, which construct the fabulous world of DNA, protein, cellulose, rubber, plastic, etc. and possess crucial positions in fields like life science, chemistry industry and material preparation with special demands. However, the performance of polymer would decay because of the influence of environment effects and even be depolymerized. While under particular scenarios, such a decomposition process might be valuable. Poly-alpha-methylstyrene (PAMS) is the substrate materials for preparing glow discharge polymer (GDP) shell which coats the fuel target for inertial confinement fusion. The key step for its involvement is to depolymerize into single molecules under high temperature so that it can escape through the space among GDP molecules which has deposited on it, and then leaves only the GDP shell. Besides of this special application, the atomic level understanding of depolymerization processes is also at the key position to modulate other reactions involving polymer materials. Here, through molecular dynamic simulation based on density functional tight-binding methods containing dispersion correction (DFTB-D), we demonstrate the typical dissociation of sequential poly-alpha- methylstyrene (PAMS) tetramer fragments with unsaturation on both ends, on the C- end and on the CH2- end, respectively. DFTB method we used here is an approximation based on the second order expansion of the density functional theory (DFT) total energy with respect to charge density variation relative to a chosen reference density. It promotes the efficiency of DFT method around two to three orders meanwhile remains acceptable precision for electronic structure calculation and large-scale quantum dynamic simulations. The reliability of the method has been admitted in massive researches, especially those on carbon-based molecules and organic systems. Our results show, with the temperature of 500 and 600 K, the dissociation of PAMS fragments is implemented by depolymerization processes, where monomers separate from one of the unsaturated ends one by one, and rising temperature could reasonably accelerate the reaction. Further simulations of a longer hexamer PAMS fragment under 600 K indicates the length effect won’t cause qualitative influence on the depolymerization process. The electronic structures of these three fragments indicate both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are localized at the bond-breaking unsaturated ends, which supports the prediction of dynamic simulations for reaction sits. Furthermore, the DFTB-D electronic structure calculations following the C—C bond breaking steps at different possible sites along the backbone chain also show the preference of the reaction to unsaturated end sites, from the view of potential energy surface. Spin distribution analysis also reflects the energy curves respect to the different depolymerization processes are closely related to the spin-polarized electronic structures of corresponding products. These results give a representative atomic-level prospect about the decomposition process of sequential PAMS under high temperature with detailed interpretation on the reaction sit preference and order, which may be informative for preparation of relative device. Besides, we hope this theoretical study could offer a fundamental reference for understanding the mechanism of decomposition for polymer materials beyond PAMS and spread across the wonderful microscopic world.