Hypothesis: The porosity and pore size of polystyrene (PS) porous films prepared by immersion precipitation can be adjusted by varying the composition of the ingredients. The effect of composition variations on the characteristics of porous films can be elucidated by thermodynamic parameters.
Methods: A PS/chloroform solution with additives (polystyrene-block-poly(acrylic acid), polyethylene glycol (PEG), and glycerol) and/or non-solvent (2-propanol) was prepared. Then, the solution was cast and immersed in a 2-propanol/chloroform coagulation bath. The resultant porous film was imaged by scanning electron microscopy (SEM) and investigated by image analysis. The phase diagram of system was plotted through experimental measurements (cloud point) and theoretical method. The thermodynamic properties of the solution and coagulation bath were evaluated using non-solvent osmotic pressure difference (ΔΠ_NS) and solvent osmotic pressure difference (ΔΠ_NS) between the solution and coagulation bath. Moreover, the approaching ratio of solution (AR_Solution) and bath (AR_Bath) to the binodal curve was also obtained.
Findings: The porosity and pore size of the films increased with decreasing the polymer concentration and adding the additives to the casting solution. By decreasing polymer concentration from 17% wt to 12% wt, the film structure changed from dense to porous (35% porosity and average pore diameter 2.8 μm). Addition of small amounts of PEG400 and glycerol to the solution (20% by weight relative to PS) significantly increased the porosity of the films from 35% to 54% and 66%, respectively. The porosity and pore size decreased first and then increased with adding non-solvent to the solution. Adding solvent to the bath helps increase porosity. Addition of non-solvent to the solution and addition of solvent to the bath were accompanied by decreasing solvent quality (AR_Solution increase) and decreasing precipitation power of coagulant (AR_Bath increase) for the polymer, respectively. The increase in porosity and pore size was mainly associated with increasing ΔΠ_NS and decreasing ΔΠ_S.