Multiporous polymeric materials from thermoreversible poly(vinylidene fluoride) gels

Abstract

Thermoreversible gels of poly(vinylidene fluoride) (PVF₂) in organic diesters [(CH₂)_n-(COOEt)₂] were dried by replacing the high boiling diesters with a guest solvent cyclohexane and followed by leaching with methanol. The porosity of the samples was measured using mercury intrusion porosimetry (MIP) for pore size > 6 nm and by nitrogen adsorption porosimetry using the Barett-Joyner-Halenda (BJH) technique for pore size 3-6 nm. Samples dried from gels in diethyl adipate (n = 4), diethyl suberate (n = 6), and diethyl azelate (n = 7) were studied. Porosity in different dimensions, e.g., micro, meso, and macro sizes, was observed in the same sample making them as multiporous materials. Both nano- and macroporosity increase with increasing "n" of diesters, and pore volumes and surface areas were also found to increase in a similar fashion. Field emission scanning electron micrographs also support the above points. The differential scanning calorimeteric thermograms at higher heating rate (40 °C/min) show two peaks; the higher melting peak increases with decreasing "n". The porous PVF₂ materials have a-polymorphic structure as evidenced from X-ray diffraction. Both nano- and macroporosity decrease with increasing polymer concentration in the gel. The materials show hysteresis loop in the intrusion and extrusion histograms of MIP and nitrogen adsorption porosimetry, indicating ink bottle or interconnected channel structure in the material. Samples dried from increased polymer concentration in the gel have lower nanopore concentration, lower surface area, and lower pore volume.