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

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 (δ e and ε e ). 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 δ e 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 ε e 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 crystallinenanoporous 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.

Molecular Features Behind Formation of α or β Co-Crystalline and Nanoporous-Crystalline Phases of PPO

“…Along with another commercial thermoplastic polymer (i.e., syndiotactic polystyrene) ( De Rosa et al, 1997 ; Petraccone et al, 2008 ; Gowd et al, 2009 ; Acocella et al, 2015 ; Itagaki et al, 2017 ), PPO is the only other polymer known to give NC phases. NC polymers (mainly as films, fibers, and aerogels) ( Daniel et al, 2013a ; Wang and Jana, 2013 ; Daniel et al, 2016 ; Krishnan et al, 2021 ) can be helpful in many applications as molecular separation membranes ( Daniel et al, 2011 ; Galizia et al, 2012 ; Daniel et al, 2013b ; Galizia et al, 2013 ) for the purification of air ( Daniel et al, 2021 ) and water ( Cozzolino et al, 2021 ) from organic compounds, molecular sensors ( Pilla et al, 2009 ; Lova et al, 2016 ), and catalysts ( Vaiano et al, 2014 ; Litta et al, 2021 ). Moreover, the CC phases with active guest molecules can be useful for many different kinds of applications ( Guerra et al, 2012 ), mainly as antimicrobial ( Albunia et al, 2014 ; Rizzo et al, 2019a ; Golla et al, 2021 ).…”

High-Strength, Flexible, Hydrophobic, Sound-Absorbing, and Flame-Retardant Poly(vinyl alcohol)/Polyelectrolyte Complex Aerogels

“…Interestingly, these aerogels were highly flexible and could be made in or cut into any shape (Figure S4a). However, during compression experiments, these aerogels exhibited an irreversible buckling behavior and were converted to dense solids without any disintegration of the samples, similar to syndiotactic polystyrene aerogels. , Figure S4b gives the compressive stress–strain curves for neat PVA and PVA-PEC aerogels, and their deformation behavior is similar to that of a typical honeycomb-like open-cell foam . The compressive moduli of the samples were calculated from the slope of the linear elastic region of the stress–strain curves and are tabulated in Table .…”

“…Being an outstanding class of lightweight materials, aerogels, first reported by Kistler, , are the go-to materials for the future as they possess properties like extremely low densities, high porosities, high surface-to-volume ratios, low thermal conductivities, low dielectric constants, low sonic velocities, etc . These unique traits qualify them as thermal and acoustic insulators, − low dielectric substrates, − supercapacitor materials, − etc. Polymer-based, particularly biodegradable/biopolymer-based aerogels, are trending nowadays, thanks to their superior mechanical properties over conventional inorganic aerogels and sustainability.…”

Ambient Drying Route to Aramid Nanofiber Aerogels with High Mechanical Properties for Low-k Dielectrics

“…For example, polyimide aerogels with a porosity of approximately 90% exhibit low dielectric constants ranging from 1.29 to 1.33 and low loss tangents ranging from 0.001 to 0.004 . Krishnan et al prepared highly porous and crystalline syndiotactic polystyrene aerogels with an extremely low dielectric constant of 1.02 ± 0.02 . However, their fabrication process typically requires supercritical drying or freeze-drying, involving harsh operating conditions and high cost, thus hindering the potential for large-scale manufacturing. − Ambient drying is an alternative approach to fabricating aerogels, which uses a solvent exchange process to prevent the significant collapse of the nanoporous structures by reducing the capillary force generated during drying. , However, the porosity of the aerogels, especially nanoporous aerogels, is often low (typically <60%) using ambient drying, since the capillary force is inversely proportional to the pore size. , Furthermore, the mechanical properties of aerogels can significantly decrease when introducing a large number of pores into the matrix, furthering limiting the practical application of these aerogels as dielectric materials in microelectronics.…”

“…15 Krishnan et al prepared highly porous and crystalline syndiotactic polystyrene aerogels with an extremely low dielectric constant of 1.02 ± 0.02. 16 However, their fabrication process typically requires supercritical drying or freeze-drying, involving harsh operating conditions and high cost, thus hindering the potential for largescale manufacturing. 17−19 Ambient drying is an alternative approach to fabricating aerogels, which uses a solvent exchange process to prevent the significant collapse of the nanoporous structures by reducing the capillary force generated during drying.…”