(509b) Ultraviolet Absorber Function in Polymers: Multiscale Simulation Starting at the Molecular Level

It is very common for the underlying mechanisms behind polymer properties to span wide ranges of time and length scales. This typically necessitates a multiscale modeling approach for simulating properties of interest. We are particularly interested in polymer matrix effects on how ultraviolet light absorber molecules prevent chain chemical degradation via converting incoming UV radiation into thermal energy. Our hypothesis is that its conversion to heat is related to the accessible volume that surrounds an in-situ UV absorber additive molecule. Calculations on three scales were conducted to address this hypothesis for a simple UVA (oxybenzone) in atactic polypropylene. First, the conformation dependence of the absorption energy was estimated using configuration interaction singles calculations. Next, the fraction of absorber molecules found in a conformation likely for absorption were calculated using classical molecular dynamics simulations (explicit atoms, classical OPLS force field with in-house additions). Varying the environment (isolated UVA compared to full polymer environment) revealed the effects of polymer solvation and nonbonded interactions. finally, relationships between absorber position and inter-chain packing were assessed using coarse-grained geometric analysis calculations. Relatively little accessible volume surrounded absorber molecules in polymers. Within its local environment, the UVA also modified the extent of polymer chain dynamics.