Effect of solvent interactions with cellulose acetate on porous structure of beads formed in aqueous phase
Abstract
Cellulose acetate (CA) beads with ununiformed porous structure can be fabricated from CA / dimethyl sulfoxide (DMSO) / acetone solution by immersion precipitation in water. It is a complicate process involving skin formation, solvent-nonsolvent exchange by diffusion and convective flow. It also involves temperature and volume change due to phase separation. Most of previous studies had focused on diffusion and convection flow across the skin of membrane, due to concentration gradients. The thermodynamic properties and change of solvent composition during CA complex formation and demixing that affects the diffusion and convective flow of solvent and nonsolvent have not been discussed. The composition of the CA / DMSO / acetone solution and the order and rate of adding CA into the solution resulted in different formation of CA / solvent complexes as evidenced by the different viscosity of the prepared solutions. Also, the evaporation time during the spraying, before droplets reach the surface of the coagulation bath, could result in the morphology, pore size and porosity alterations. With a constant evaporation time, the beads examined using scanning electron microscopy (SEM) and size exclusion chromatography (SEC) techniques showed that different CA / solvent complexes have finger-like or sponge-like porousstructures with surface layer and subsection in the center. Longer evaporation duration time facilitated synthesis of a firm dense skin on the surface layer, a large void in the center of the bead and decreased pore size. The order of component addition resulted in generation of two types of beads with molecular exclusion limits at either 200Da or 20,000Da and without significant variation in porosity (about 80%). Pore size and porosity were both decreased with an increase of cellulose acetate content in solution. With the increase of CA concentration, the beads became more spherical and suitable for packing a chromatography column. Meanwhile, increasing the ratio of DMSO / acetone decreased the density at the bead surface.
Degree
M.S.
Advisors
Chen, Purdue University.
Subject Area
Food Science
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