Nanoporous Crystalline Aerogels of Syndiotactic Polystyrene: Polymorphism, Dielectric, Thermal, and Acoustic Properties
Vipin G. Krishnan, Angel Mary Joseph, Surendran Kuzhichalil Peethambharan, E. Bhoje Gowd
Abstract
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.