Natural and simulated weathering of polystyrene: A molecular view of the polymeric interface

This work presents the changing abundance of surface functional groups (SFGs) on polystyrene (PS) upon weathering within one or a few molecular monolayers from a molecular point of view. PS particles were aged by exposing it to a gas flow of typically (5 %) O₃ in O₂ (PS_O3), UV radiation using a solar simulator under controlled conditions in the laboratory (PS_SS) and to the water/air interface immerged in a freshwater lake for 2 months (PS_L). The chemical composition of the interface of weathered, compared to pristine (virgin or PS_V) material was established using a titration technique that probed the chemical composition of the molecular interface of the polymer. The main conclusions of this exploratory study are: (a) The interface of PS changes significantly compared to ATR-FTIR spectra that do not show additional absorptions in the mid-IR spectrum over a penetration depth of more than hundred monolayers at 10 μm; (b) The average surface functionalization of the gas-solid interface, corresponding to the sum of all examined types of SFG, increases from 20 % of a monolayer for PS_V to 40, 50 and 84 % for PS_L, PS_O3 and PS_SS, respectively; (c) in all cases the most important SFG was surface -OH ranging from 11.2 to 64 % for PS_V and PS_SS, respectively; (d) each PS sample shows a characteristic SFG pattern or fingerprint using several probe gases; (e) O₃ interaction led to interface acidification; (f) UV treatment leads to the highest degree of surface -OH functionalization compared to PS_O3 and PS_L. The accumulation of SFG's renders the interface more reactive towards adsorption of probe gases.