ε Form Gels and Aerogels of Syndiotactic Polystyrene

High Modulus, Strut-like poly(ether ether ketone) Aerogels Produced from a Benign Solvent

Poly(ether ether ketone) (PEEK) was found to form gels in the benign solvent 1,3-diphenylacetone (DPA). Gelation of PEEK in DPA was found to form an interconnected, strut-like morphology composed of polymer axialites. To our knowledge, this is the first report of a strut-like morphology for PEEK aerogels. PEEK/DPA gels were prepared by first dissolving PEEK in DPA at 320 °C. Upon cooling to 50 °C, PEEK crystallizes and forms a gel in DPA. The PEEK/DPA phase diagram indicated that phase separation occurs by solid-liquid phase separation, implying that DPA is a good solvent for PEEK. The Flory-Huggins interaction parameter, calculated as χ12 = 0.093 for the PEEK/DPA system, confirmed that DPA is a good solvent for PEEK. PEEK aerogels were prepared by solvent exchanging DPA to water then freeze-drying. PEEK aerogels were found to have densities between 0.09 and 0.25 g/cm3, porosities between 80 and 93%, and surface areas between 200 and 225 m2/g, depending on the initial gel concentration. Using nitrogen adsorption analyses, PEEK aerogels were found to be mesoporous adsorbents, with mesopore sizes of about 8 nm, which formed between stacks of platelike crystalline lamellae. Scanning electron microscopy and X-ray scattering were utilized to elucidate the hierarchical structure of the PEEK aerogels. Morphological analysis found that the PEEK/DPA gels were composed of a highly nucleated network of PEEK axialites (i.e., aggregates of stacked crystalline lamellae). The highly connected axialite network imparted robust mechanical properties on PEEK aerogels, which were found to densify less upon freeze-drying than globular PEEK aerogel counterparts gelled from dichloroacetic acid (DCA) or 4-chlorphenol (4CP). PEEK aerogels formed from DPA were also found to have a modulus-density scaling that was far more efficient in supporting loads than the poorly connected aerogels formed from PEEK/DCA or PEEK/4CP solutions. The strut-like morphology in these new PEEK aerogels also significantly improved the modulus to a degree that is comparable to high-performance crosslinked aerogels based on polyimide and polyurea of comparable densities.

Poly(vinylidene fluoride) Aerogels with α, β, and γ Crystalline Forms: Correlating Physicochemical Properties with Polymorphic Structures

Strategic customization of crystalline forms of poly(vinylidene fluoride) (PVDF) aerogels is of great importance for a variety of applications, from energy harvesters to thermal and acoustic insulation. Here, we report sustainable strategies to prepare crystalline pure α, β, and γ forms of PVDF aerogels from their respective gels using a solvent exchange strategy with green solvents, followed by a freeze-drying technique. The crucial aspect of this process was the meticulous choice of appropriate solvents to enable the formation of thermoreversible gels of PVDF by crystallization-induced gelation. Depending on the polymer-solvent interactions, the chain conformation of PVDF can be modulated to obtain gels and aerogels with specific crystalline structures. The crystalline pure α-form and piezoelectric β-form aerogels were readily obtained by using cyclohexanone and γ-butyrolactone as gelation solvents. On the other hand, the γ-form aerogel was obtained using a binary solvent system consisting of dimethylacetamide and water. These aerogels with distinct crystalline structures exhibit different morphologies, mechanical properties, hydrophobicities, acoustic properties, and electrical properties. Measurement of thermal conductivity for these aerogels showed exceptionally low thermal conductivity values of ∼0.040 ± 0.003 W m-1 K-1 irrespective of their crystal structures. Our results showcase the fabrication approaches that enable PVDF aerogels with varied physicochemical properties for multifunctional applications.

Carbonized Syndiotactic Polystyrene/Carbon Nanotube/MXene Hybrid Aerogels with Egg-Box Structure: A Platform for Electromagnetic Interference Shielding and Solar Thermal Energy Management

Functional materials for electromagnetic interference (EMI) shielding are a consistently hot topic in the booming communication engineering, proceeding the development that tends to the multifunctional EMI shielding materials. Herein, a series of carbonized syndiotactic polystyrene/carbon nanotube/MXene (CsPS/CNT/MXene) hybrid aerogels were fabricated for EMI shielding and solar thermal energy conversion purposes. To fabricate the hybrid aerogels, a porous CNT/MXene framework was initially prepared using freeze-casting. Subsequently, sPS was infused into the porous structure, followed by hyper-cross-linking and carbonization of sPS under an inert atmosphere. The resulting aerogels exhibited a distinctive egg-box structure, comprising numerous nanofibrous carbon microspheres embedded within the lamellar framework. The mass ratio between CNT and MXene was regulated to identify an optimum aerogel, that is, the CCM-4-6, which exhibited impressive properties including Young's compression modulus of 0.67 MPa, a water contact angle of 137.6 ± 4.1°, a specific surface area of 110 m2 g-1, an electrical conductivity of 43.0 S m-1, and an EMI SE value of 40 dB. Meanwhile, phase-change composites were fabricated through encapsulating paraffin wax within the hybrid aerogels. For the CCM-4-6 aerogel, a noteworthy encapsulation ratio was achieved at about 76.7%, along with remarkable latent heat, good thermal reliability, and commendable solar thermal energy conversion capacity. This study presents a facile route to prepare multifunctional EMI shielding materials.

A Liquid Crystal Elastomer-Based Unprecedented Two-Way Shape-Memory Aerogel

With the advantage of reversible shape-morphing between two different permanent shapes under external stimuli, the two-way shape-memory aerogel is expected to become a preferred aerogel for developing practical applications in actuators, sensors, robotics, and more. Herein, the first two-way shape-memory liquid crystal elastomer (LCE)-based aerogel is prepared by an orthogonal heat and light curing strategy coupled with an intermediate mechanical stretching step. The differential scanning calorimetry, temperature-varied wide-angle X-ray scattering, and polarizing optical microscope results indicate that the aerogel possesses a liquid crystal phase and the insider mesogens are well-oriented along the stretching direction. In addition to having superior compressibility and excellent shape stability, this LCE-based aerogel can perform a reversible shape deformation during the heating/cooling cycles with a shrinkage ratio of 37%. The work, that is disclosed here, realizes a truly two-way shape-memory behavior rather than the one-way shape deformation of traditional polymer aerogel materials, and may promote potential applications of this novel LCE-based aerogel material in control devices, soft actuators, and beyond.

Polymerization of Aniline in Tubular Cavities of the Crystalline Phase of Syndiotactic Polystyrene: Proposal of a Preparation Method of Sophisticated Polymer Composites

Syndiotactic polystyrene (SPS) has a unique crystalline phase, named ε, that has a tubular cavity inside. We connected these cavities by optimizing the solvent used to process the films and to polymerize aniline within these channeling cavities only by immersing the films in degassed hydrochloric acid solutions of ammonium persulfate. We succeeded in fixing pentameric to octameric oligoaniline in the cavities of the SPS ε crystalline phase. This is a new and sophisticated preparation method of polymer composites between SPS and linear oligomers/polymers within its long-connected tubular cavities.

Thermoreversible Gelation of Poly(ether ether ketone)

Solutions of poly(ether ether ketone) in dichloroacetic acid have been shown to form monolithic, thermoreversible gels at temperatures ranging from 10 to 140 °C. A phase diagram was constructed over broad concentration and temperature ranges, and the phase boundary suggests an upper critical solution temperature (UCST) behavior. Furthermore, poly(ether ether ketone) (PEEK) gels were solvent-exchanged with water to form hydrogels and subsequently lyophilized to form PEEK aerogels. The PEEK aerogels of density 0.2 g/mL were found to be highly porous and composed of uniform 200 nm morphological features. The crystal structure of the PEEK hydrogels and aerogels was found to be identical to that of melt-crystallized PEEK. The mechanical properties of the PEEK aerogels (in compression) were found to be superior to conventional silicate aerogels of comparable density. This report is the first example of a monolithic, thermoreversible gel of PEEK and the first demonstration of PEEK hydrogels and aerogels.