Role of thermodynamic and kinetic interaction of poly(vinylidene fluoride) with various solvents for tuning phase inversion membranes

Abstract

An extensive study of thermodynamics and kinetics of non‐solvent induced phase separation was carried out for poly(vinylidene fluoride)/solvent/water system for four different solvents. Literature available on semicrystalline polymers was mostly based on experimental cloud points, obtained to a narrow range of polymer concentration (<10 wt%), much less than the working range for membrane preparation (20–25 wt%). Aim of this work was to model the thermodynamic phase diagram using extended Flory–Huggins theory which was used as a tool, along with the kinetic data to obtain tailor‐made membranes with desired morphology and properties. Interaction parameters involving solvent, nonsolvent, and polymer played an important role to tune the porosity of the membrane. Thermodynamic calculation showed solvent N,N‐dimethyl acetamide resulted in the most porous membrane (permeability 5.4 × 10−11 m Pa−1 s−1) followed by N,N‐dimethyl formamide (permeability 4.2 × 10−11 m Pa−1 s−1), N‐methyl pyrrolidone (permeability 3.8 × 10−11 m Pa−1 s−1), and acetone (impermeable to water even at 1380 kPa), which was the densest one. Prepared membranes were characterized in terms of surface morphology, molecular weight cut‐off, tensile strength, pore volume distribution, crystallinity, and surface roughness, which were correlated to inferences based on thermodynamic and kinetic calculations. POLYM. ENG. SCI., 58:1062–1073, 2018. © 2017 Society of Plastics Engineers

Effect of mixed solvents on phase inversion of polymeric membranes

“…The binodal curve for the Ac0 system (DMF as solvent) has been validated using cloud point data from the literature. 25 For the systems containing acetone, the cloud point could not be determined because of the high volatility of acetone. Cloud point determination involves titration with the nonsolvent under continuous stirring.…”

“…25,[48][49][50] The non-polar conformation of the ⊍ form is relatively more stable compared to the polar ⊎ form and is observed to be a major constituent. 25,48 The characteristic diffraction peak at 19.5°represents the 110 plane reflection of the ⊍ phase whereas the peak corresponding to the reflections of the ⊎ and γ phase are not observed. [48][49][50][51] PVDF membranes are reported to have a small quantity of ⊎ form crystals along with ⊍ form.…”

“…Viscous solvents hinder reorientation of polymer chains resulting in decreased crystallinity. 25,52 As the proportion of acetone increases in the solvent mixture, the solubility increases until Ac40 and the polymer chains swell to a larger extent. This is also reflected as increased viscosity until Ac40.…”

“…3,11,22,[24][25][26] Modeling of the thermodynamic phase diagram for a ternary system has been studied extensively for various membrane-forming polymers, such as polysulfone, polyethersulfone, cellulose acetate, polyvinylidene fluoride (PVDF), polyacrylonitrile etc. 3,11,22,25,26 A few authors have attempted to model the thermodynamics for a quaternary system of polymer blends, organic and inorganic additives and complex materials. [27][28][29][30][31] However, the study of the solution thermodynamics and phase inversion kinetics for a mixture of solvents has still not been attempted.…”

Novel behavior in a polymer solution: the disappearance of the melting temperature (Tm) and enthalpy change (ΔHm) of the solvent

“…A single polymer chain is usually in a coil form in solution due to balanced interactions with the solvent and the polymer itself 1 . Molecular interactions with solvents and polymers have typical properties that can be applied to many industrial processes, and they play important roles in applications, such as membrane casting and formation [2][3][4][5][6] , battery separator preparation [7][8][9] , and polymer synthesis 10,11 , as well as in determining solubility parameters [12][13][14][15] . Recently, a 30 wt% polystyrene (PS) solution in dimethylformamide (DMF) was studied in an ultralow temperature region by Samitsu et al 16 .…”