Structure Control And Properties Research Of Polyethylene Membrane Via Thermally Induced Phase Separation

High density polyethylene (HDPE) is one of the widely used membrane materials due to its good mechanical strength, thermal and chemical stability, and radiation resistance. In this paper, we aimed at studying the influences of various technical conditions on phase separation behaviors and membrane structures during HDPE membrane formation by thermally induced phase separation (TIPS) method. The formation mechanism of HDPE membrane was studied and the relations among each factor, phase separation behaviors and membrane structures were obtained. So the membrane structures can be controlled during preparation process. The experiments and conclusions are summarized as follows.First, the influences of diluent, polymer concentration, cooling rate and polymer molecular weight on phase separation behaviors and membrane structures were investigated in HDPE/DPE systems. The thermodynamic and kinetics properties of membrane formation process were characterized by DSC and optical microscope. It was found that the pore size decreased with increasing either the polymer concentration or the cooling rate. Furthermore, the effect of polymer molecular weight on the phase diagram and the droplet growth rate was investigated. The cloud point curve shifted to the lower temperature region as the polymer molecular weight decreased, while the dynamic crystallization temperature did not change obviously. The membrane prepared with higher molecular weight HDPE showed smaller cellular pore structure.Microporous HDPE membranes were prepared by TIPS process of a ternary solution of HDPE, liquid paraffin (LP) and diphenyl ether (DPE). The results showed that the phase diagrams could be controlled successfully by varying the composition of the solvent mixture. With increasing LP concentration, liquid-liquid phaseseparation points move to the low temperature region. It is also shown that the membrane morphology can be controlled with different diluent content when cooling rate conditions are kept constant.The kinetics of droplet growth under non-isothermal conditions was studied via optical microscopy and this work is intended as a starting point for future work on non-isothermal droplet model. The kinetics non-isothermal droplet growth partly obeys some rules of revised coalescence-induced coalescence modeling. The effects of cooling rate, polymer concentration and polymer molecular weight on the growth exponent ξ follow the same trends for the experimentally observed non-isothermal growth as for modeling predictions. The prediction made by revised model that the value of the growth exponent ξ would change with the time was not consistent with the experimental result.Hydropilic HDPE membranes with enhanced fouling resistance were prepared from the blends of polyethylene and an amphiphilic diblock copolymer by thermally induced phase separation. Chemical and morphological changes as well as anti-foul property on membrane surface were analyzed by Fourier Infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, water contact angle and protein adsorption. The results revealed that the substantial surface segregation of amphiphilic copolymer takes place at the surface and internal pore channel of membrane during the membrane fabrication. Compared with the pure PE membrane, the membranes modified by amphiphilic diblock copolymer show the higher surface porosity, lower water contact angle and better foul resistant.Ultra-high molecular weight polyethylene (UHMWPE) based porous membrane has been attracting a great deal of attention due to its excellent physical properties. The effects of polymer concentration, cooling rate and polymer molecular weight on the phase diagram and membrane morphology were investigated in UHMWPEmembrane formation via the thermally induced solid-liquid phase separation. The supercritical CO₂ (SCCO₂) fluid was used to extract the diluent from the perform abtained via TIPS process. The extraction efficiency is intensively affected by the SCCO₂ pressure and temperature. It was found that the extraction efficiency of SC-CO₂ was obviously superior to that of the traditional organic solvent extraction.The kinetics of the isothermal and non-isothermal crystallization of UHMWPE in liquid paraffin was studied by differential scanning calorimetry. Several kinetic analyses were used to describe the crystallization process. The commonly used Avrami equation and the one modified by Jeziorny were used, respectively, to describe the primary stage of isothermal and non-isothermal crystallization. The crystallization half-time of UHMWPE increased with crystallization temperature while crystallization rate constant decreased with crystallization temperature. The Avrami exponent was evaluated to be in the range of 2.6-2.8 for isothermal crystallization, and around 5 for non-isothermal crystallization. The Ozawa method failed to describe the non-isothermal crystallization behavior, whereas Mo' and Jeziorny' equation could fit the non-isothermal crystallization kinetics successfully.