Tuning the Upper Critical Solution Temperature of the Polystyrene-Terpineol System toward a Low-Temperature Membrane Formation

The upper critical solution temperature (UCST) of binary polymer–solvent blends can create porous structures for many applications, including filtration. This study investigates how UCST can be tuned in a non-aqueous system of polystyrene and terpineol. The addition of small molecules, γ-valerolactone, oleic acid, and limonene, was tested to modify the terpineol-polystyrene phase separation temperature. The resulting porous structures were studied to determine the pore size, porosity, water flux, and rejection of test molecules to better understand the impact of the additives on both the processing temperature and porous materials' properties. The study found that the hydrogen bond propensity and miscibility of the additives significantly affected the UCST. A change of over 35 °C was observed when the additive concentration varied from 0 to 15 wt %, and the transition temperature increased or decreased depending on the additive solvent strength. The surface pore diameter was significantly altered, but the bulk pore diameter remained similar in all cases with the exception of oleic acid. The addition of small-molecule addives offers a way to control the UCST by tunning addities' solubilities and hydrogen bonding abilities in similar blends, toward lower energy use porous film synthesis.