Studies On Preparation And Morphology Controlling Of IPP Hollow Fiber Microporous Membrane Via Thermally Induced Phase Separation

The last several decades have witnessed a tremendous growth of polymeric membranes for apromising separation technique. The R&D of polymer membrane with good performances and lowprice, such as isotactic polypropylene (iPP) membrane, are of crucial importance, which benefitmembrane technique competitive with classical separation techniques for large scale water treatment,organic separation, and so on. Consequently, the result is expanding of membrane applications andgrowth of membrane market. The iPP membrane via thermally induced phase separation (TIPS) has aseries of advantages than one via melt spinning–cold stretching, such as higher permeability, narrowerpore size distribution, and better mechanical properties. The domestic manufactories haven't producedthe membrane via TIPS. The membranes via TIPS from international market are expensive. There arefew investigations to prepare membrane by TIPS in literature. This study made attempts to optimizethe morphology and performances of iPP hollow fiber microporous membrane by some properapproaches.The screening of diluents to prepare iPP membrane was proposed via combining Hansen solubilityparameters in literature and equilibrium sorption measurements. An improved group contributionapproach was presented to predict Hansen solubility parameters of diluents. The Hansen solubilityparameter of iPP was obtained with equilibrium swelling data in literature. It is suggested that thescreening experiments of diluents base on predicting result with Hansen solubility parameters.Consequently, a lot of candidate diluents for iPP membrane were found. The optimum diluents toprepare iPP membrane are cosolvent of Di-n-butyl phthalate (DBP) and Dioctyl phthalate (DOP).The thermodynamics of thermally induced phase separation of the ternary solution of iPP, DBP andDOP was studied by pseudo-binary approach, considering the cosolvent of DBP and DOP to be onecomponent. The mathematics model to calculate pseudo-binary temperature-concentration phasediagrams was proposed. The cloud point data of iPP/cosolvent systems were obtained with Opticalmicroscopy. The melting points and dynamic crystallization data of iPP/cosolvent systems weremeasured with DSC. The interaction parameter χ between iPP and cosolvent is the function oftemperature and cosolvent composition. χ was determined from cloud point data of iPP/cosolventsystems. Good agreement was obtained in pseudo-binary phase diagrams between the theoreticallypredicted from χ and experimentally generated. With the increase of DBP fraction of cosolvent, thepattern of the phase separation of the ternary solution changes to liquid-liquid phase separation withsubsequent polymer crystallization from polymer crystallization only. Varying cosolvent compositioncan successfully control pseudo-binary phase diagrams. DBP mass fraction in cosolvent (α), iPP mass fraction in casting solution (β) and air quenchertemperature tAir were designed as the variables in the spinning process. It is suggested that themorphology and performances of iPP hollow fiber microporous membrane are originated from thecompetition between liquid-liquid phase separation and polymer crystallization in casting solution. Forsufficiently high β and tAir near room temperature, the resulting membrane presents a typical spheruliticparticulate structure, and a quite weak mechanical strength as α=1;In contrast, the resulting membraneexhibits a typical cellular structure, excellent mechanical properties, but a quite low permeability asα=1;The mixed membrane structures combing cellular and particulate are obtained as 0.20 ≤ α ≤ 0.50.It is noted that cellular structure is favorable with higher α, lower β and higher tAir. It is indicated thatfor crystalline polymer such as iPP, the hollow fiber microporous membrane with good performancesmust exhibits a mixed membrane morphology that is basically cellular but with particulate boundaries,in which the liquid-liquid phase separation precedes polymer crystallization. The optimum membraneexhibits an open cellular structure, has a mean pore size of 0.20μm and a narrow pore size distribution.Its mechanical properties are superior to the polysulfone hollow fiber membrane or PVDF one. Its purewater flux is 285 L/(m2 ?h ?bar) at a trans-membrane pressure of 1 bar. In addition, it is provided with anexcellent resistance to compaction and chemical solvents.The effect of both TH3988 and adipic acid as nucleating agent on TIPS of iPP solution,themorphology and performances of the resulting hollow fiber membranes were investigated. It is notedthat the addition of nucleating agent changes nucleating temperature, crystallization temperature,nucleating rate, crystallization rate, and degree of crystallinity. The morphology type of the resultingmembrane can't change when there is TH3988 or tiny adipic acid in casting solution. On the contrary,The morphology type of the resulting membrane changes when there is sufficiently high adipic acid incasting solution. The pure water flux of the resulting membrane increases with subsequently decreaseswith increasing TH3988. The pure water flux of the resulting membrane decreases with increasingadipic acid. The mechanical strength of the resulting membrane increases with increasing bothTH3988 and adipic acid. It is suggested that nucleating agent can adjust the competition betweenliquid-liquid phase separation and polymer crystallization in casting solution. As a result, the poreconnectivity of the membrane can be improved when appropriate nucleating agent is selected. Atappropriate amount of TH3988, the pure water flux of the nucleating iPP membrane is 362L/(m2 ?h ?bar),more than 150% of those of the no-nucleating iPP membrane. Moreover, other performances of theresulting membrane are improved.