Multiporosity in dried poly(vinylidene fluoride)-camphor systems: effect of drying method on porosity

Abstract

Extraction of camphor from poly(vinylidene fluoride) (PVF₂)-camphor systems prepared at two different compositions (10% and 40% w/w) indicates multiporosity. Two methods were applied for the extraction process : (i) by applying vacuum (10⁻³ mmHg) (VD) and (ii) by replacement of the host matrix by cyclohexane followed by drying (CD). The morphology, porosity, structure, and thermal properties of these samples were studied by field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), mercury intrusion porosimetry (MIP), nitrogen adsorption porosimetry (BET adsorption isotherm), wide-angle X-ray scattering (WAXS), and differential scanning calorimetry (DSC). High-pressure (20-34 000 psi) MIP histograms for all the samples indicate the presence of meso- and macropores, and low-pressure (0.5-50 psi) MIP histograms indicate the presence of large macropores in the samples. Analysis of pore size distribution from nitrogen adsorption studies by the Horvath-Kawazoe (HK) and Barett-Joyner-Halenda (BJH) models indicates the presence of micro- and mesopores produced from single, double, or multiple camphor layers between the PVF₂ strands particularly for CD samples. Pore diameter values nearly equal to the model value of 2:1 and 4:1 compounds were detected for both the single- and double-camphor layer complexation in the dried samples. However, vacuum-dried samples (P-10VD and P-40VD, the numerical numbers indicate weight percent of polymer in the gel from where it is derived) do not exhibit existence of any micropore. Mesopores were observed in all the samples although the pore size distribution is broader in VD samples than that in CD samples. The macropores are larger in size in the P-10VD samples compared to that in P-10CD samples, but in the P-40VD sample the macropore size is either reduced or lost. The hysteresis loops indicate the presence of interconnectivity between the pores, and it is lesser in the vacuum-dried samples. The surface area has greatly decreased in VD samples compared to that in CD samples. This suggests that on vacuum drying a large amount of porosity and interconnectivity is lost than those of CD samples. A schematic model is presented for the loss of porosity in vacuum drying. The crystal structure is independent of drying method, and the melting point of the porous surface increases in VD samples compared to that in CD samples, indicating that thicker porous surface is produced on vacuum drying. The collapsing of pores decreases the porosity in VD samples due to mechanical jerking. Honeycomb-type pores are observed in the P-40CD sample, and the faster crystallization rate of camphor than that of PVF₂ has been attributed to the honeycomb-type pore formation. The sheetlike morphology of PVF₂ obtained from dried dilute gels has been attributed to its crystallization through the niches of camphor crystals followed by extraction of camphor.