Nanoporous Crystalline Aerogels of Syndiotactic Polystyrene: Polymorphism, Dielectric, Thermal, and Acoustic Properties

Hierarchically porous crystalline nanoporous aerogels of syndiotactic polystyrene (sPS) received much attention because of their unique nanoporous structures along with meso- and macroporosity. Depending on the difference in the packing of polymer chains within the crystal lattice, sPS has two nanoporous crystalline forms, namely, δ and ε forms (δe and εe). In this study, we have prepared high-purity nanoporous δ and ε forms of sPS aerogels from their respective gels using a solvent exchange strategy with green solvents, followed by an environmentally friendly freeze-drying technique. Using these highly porous aerogels, the phase transition behavior of sPS at higher temperatures was investigated. The δe form showed a complex phase transition behavior on heating, and at a higher temperature, the γ form (obtained through an intermediate helical phase) transformed into a mixture of α and β forms. On the other hand, the εe form transformed directly into the γ form, and on further heating, the γ form transformed exclusively into the α form. The dielectric, thermal, and acoustic properties of crystalline-nanoporous aerogels were promising with an ultralow dielectric constant (1.02 ± 0.02), thermal conductivity (λ) as low as 0.04 W m–1 K–1, and a high sound absorption coefficient (close to 1). Moreover, these aerogels exhibited excellent oleophilicity, which was demonstrated in oil/organic solvent separation experiments. These multifunctional aerogels of sPS can, therefore, find a multitude of applications, especially in thermal and acoustic insulation and molecular sorption of oil/organic solvents.