Study On Phase Separation And Membrane Preparation Of Ultra-high Molecular Weight Polyethylene

It is one of challenging work that how to controlling the different phaseseparation and preparing polymer microporous membrane with various structures andproperties for the binary strong interaction system in the membrane formation processof thermal induced phase separation. We know that most of papers have beenpublished for phase separation of binary strong interaction system, however, it wasfocused on the solid-liquid phase separation. There are still many questions thatwhether there is liquid-liquid phase separation, the effect of nucleating agent onliquid-liquid phase separation and membrane, the preparation of high water flux. Infact, the complex phase separation mechanism is the underlying challenge of theseproblems. How to study the complex phase separation and further preparemicroporous membrane with the different these mechanism? This doesn’t only has theengineering background but also the theoretical significance. In this thesis, theresearches mainly are focused on the binary strong interaction system of ultra-highmolecular weight polyethylene (UHMWPE) and liquid paraffin (LP) throughrheology and optical microscopy methods and so on. Firstly, we distinguishedliquid-liquid phase separation from liquid-solid phase transition, and obtainedliquid-liquid phase separation temperature by experimental design. Secondly,dibenzylidene sorbitol (DBS) could self-assemble into fiber in the UHMWPE/LPmatrix, the self-assembly of DBS raised liquid-liquid phase separation and themechanism under “self-assembly promoted phase separation” was proposed. Finally,we designed a membrane with high water flux under the control of flow field. Themain content and the results are as followed:1. Phase separation behavior of binary strong interaction system, UHMWPE/LP, wasinvestigated through combination of differential scanning calorimetry, rheologyand optical microscopy. A new rheological method was designed to detectliquid-liquid phase separation temperature, and further obtained phase diagram inorder to guide the fabrication of UHMWPE membrane. The experimental resultsindicated that the rheological method is more sensitive than other testing instrument, and showed a clear evidence of liquid-liquid phase separation, which was further justified by optical microscopy. The liquid-liquid phase separation temperature was obtained more accurately by the "inverse quenching" method in rheological time sweeps. Based on the phase diagram, UHMWPE membranes with different porous structures were prepared through thermal induced phase separation with the control of the liquid-liquid phase separation and crystallization. Compared to solid-liquid phase separation, liquid-liquid phase separation dominated process would produce membranes with the larger porosity and water flux.2. The gel behavior of DBS in different solvents was studied by rheology, optical microscopy and FESEM. It was found that solvent polarity and DBS concentration have important influences on gel morphology, dissolution temperature and viscoelasticity property. The experimental results indicated that the gel dissolution temperature is extremely sensitive to DBS concentration and solvent polarity, which could be explained by Gibbs-Thomson equation through the interfacial stress. The critical gel concentration was found to increase as the difference in the polar and hydrogen-bonding components of solubility parameter (△δPh) between gelator and solvents decreases. Meanwhile, the difference of the critical gel concentration in different solvents could well explain the difference in the relaxation exponent and the gel strength at the gel point. The phase diagram, which classified the sol, the gel and the cluster regime in different solvents, was determined by rheology and optical microscopy. In the stable gel state, the plateau modulus depended on the gelator concentration via a power-law scaling, GNo∝c2, which was consistent with entanglement theory and independent of the type of solvents. However, the gelator/solvent interaction and its temperature dependence were found to affect the stability of gel substantially under large amplitude oscillatory shear. The critical strain would increase as△δPh decreases, and show more evident temperature dependence on solvent with smaller△δPh.3. DBS was chosen as an in situ forming nucleating agent to study UHMWPE/LP physical gel, phase separation behavior and the property of UHMWPE membrane.The experimental results indicated that the gel dissolution temperature has astrong dependence of DBS concentration. The self-assembly of DBS induced thelocal concentration fluctuation and promoted phase separation. By controlling therelative quenching depths, the grow rate of the characteristic length showed acrossover from phase separation to crystallization, which was further justified byWAXD and FESEM. Consequently, we proposed a new mechanism of“self-assembly-promoted phase separation”. Meanwhile, DBS nucleating agentaccelerated UHMWPE crystallization. Finally, UHMWPE membrane wasfabricated by thermal induced phase separation and found that water fluxdecreases sharply due to the drop the microporous sizes.4. The effect of flow field on DBS fiber network and UHMWPE membrane werestudied by rotary rheometer. The experimental results indicated that the relaxationtime of DBS fiber network is longer than UHMWPE chain, which shows a greatsensitive to flow field. A modest shear flow, performed in the UHMWPE/LP/DBSgel state, is sufficient to deform DBS fiber network and results in a decrease ofstorage modulus. During flow, shear causes alignment of DBS fibrils and leads tothe formation of primary nuclei in the flow direction. These nuclei facilitate thegrowth of oriented crystal lamellae that aligned perpendicular to the flow direction.This peculiarity was used to design thermal and mechanical histories for obtaininghighly oriented UHMWPE membrane. In comparison to the isotropic UHMWPEmembrane, the highly oriented UHMWPE membrane provides low tortuous pathsacross the membrane, thereby limiting pressure drop and maintaining high waterflux and mechanical strength.5. The phase separation behavior of UHMWPE/DIDP binary weak interactionsystem was studied, phase separation mechanism under different quenchingtemperatures and effect of cooling rate and UHMWPE concentration on thestructure of membrane were discussed. The experimental results indicated thatphase separation mechanism changes from spinodal decomposition to nucleationgrow with quenching temperature decreases, which is similar with the growth rate and sizes of droplet. In the non-isothermal process, the drop size decreases withincreasing cooling rate because the period of liquid-liquid phase separationdecreases, which lead to the decrease of microporous size. Meanwhile, the dropletdiameter and growth rate decrease with increasing UHMWPE concentration dueto the control of liquid-liquid phase separation and viscosity.6. UHMWPE membranes were prepared by solvent mixture of DIDP/LP. Meanwhile,the effect of ratio of DIDP/LP, cooling rate and polymer concentration onUHMWPE membrane were studied. The experimental results indicated that thephase diagram could be controlled successfully by varying the composition of thesolvent mixture. With increasing LP concentration, liquid-liquid phase separationmove to the low temperature region. It is also shown that the membranemorphology can be controlled with different diluent content when cooling rateconditions are kept constant.