Crystallization of swollen, crosslinked polymers
Innovations in porous materials have driven technological advances in many fields. Catalysis, separations, sound absorption, thermal and electrical insulation, transport through porous materials and biomedical applications all benefit from the ability to imbue materials with particular combinations of pore structure, surface area, and physiochemical properties in a variety of macroscopic geometries. Current methods of producing porous materials are limited by their reliance on hazardous solvents and limitations in the shape and mechanical properties of the porous materials.
The process of crystallization from swollen, crosslinked states, or the CSX process, is a novel and environmentally friendly method for producing durable porous polymeric materials in a wide variety of shapes. The CSX process evolved from the existing technologies of thermally induced phase separation, supercritical fluid processing, and chemical crosslinking. The general CSX process is applicable to polymers which are crystallizable, chemically crosslinkable, and capable of being swollen with an appropriate fluid. Preshaped, crosslinked polymers are swollen in a suitable solvent at elevated temperatures and pressures; isobarically crystallized from the swollen state; and recovered as porous materials. Articles produced by these method retain their pre-processing shape and, due the presence of crosslinking, have enhanced mechanical properties. A working hypothesis of pore formation mechanism is that polymer chains crystallize from swollen state to form a bicontinous porous structure in the final material. Structural integrity is derived from both crystalline structures of the polymer as well as chemical crosslinking.
A series of radiation crosslinked polyethylenes of various crystallinities were crystallized from supercritical propane to determine the impact of key material and process parameters on final properties obtained by CSX processing. Pore sizes in the range of 0.1–10 μm are obtained. The effect of degree of crosslinking, swelling temperature, and swelling pressure were investigated. Additional aspects and opportunities of CSX processing are considered, including combination of extraction and impregnation processes during the swelling step of the process. Techniques of high pressure rheology are reviewed and a novel high pressure rheometer design is presented.