Preparation And Study On Ultra-High Molecular Weight Polyethylene Microporous Membranes

As the linear polyethylene with planar zigzag structure, the mechanical strength and impact resistance of ultra-high molecular weight polyethylene (UHMWPE) were proved to be outstanding owing to its long and smooth chain. UHMWPE microporous membranes are expected to be used in some industrial fields such as gas-liquid separation, sewage filtration and so on depending on their excellent comprehensive performance. In this thesis, the structure and properties of UHMWPE microporous films prepared under different conditions were investigated.At first, the structure and performance of various membranes with different molecular weight, compounding concentration and thickness were discussed. It was found that only solid-liquid separation occurred under the cooling rate of1℃/min owing to the strong interaction between UHMWPE and liquid paraffin, both have the similar solubility parameters. With increasing molecular weight and concentration of UHMWPE, membranes with smaller pore structure were sbtained because it was harder for diluent to diffuse and gather together in a system with larger viscosity. In addition, mass fraction played a more important role in influencing the system viscosity compared with molecular weight of UHMWPE. Water flux reduced significantly due to bigger mass transfer resistance in dense surface and longer resistance channel when thickness ncreased from100μm to200μm. The tensile strength of various microporous membranes with different molecular weight were between4～5MPa in the same concentration of25wt%. While the nechanical strength increased to6.95MPa along with UHMWPE occupied30wt%in system.Secondly, the structure and properties of UHMWPE microporous membranes prepared in different cooling conditions were conducted a study combined with non-isothermal crystallization:inetics. It showed that it was easier for UHMWPE to crystallize at a lower temperature owing to crystallization rate was controlled by nucleation process. The crystallization rate constant (Zc) went up generally at a faster cooling rate by means of the Jeziorny method. With Mo method, it was found that crystallization rate of UHMWPE decreased linearly along with the increase in crystallinity of films during isothermal crystallization process. Thus, the pore structure in surface or cross section as well as the pure water flux of membranes all turned to decrease along with the faster cooling rate. Furthermore, the mechanical strength and elongation of UHMWPE membranes were weaker at a slower cooling rate because big hole might form flaws and films ruptured easily in the stretch process.Finally, the structure and property of films modified by PEG20000were investigated. It was found that the pore structure and porosity of modified membranes increased and the retention rate reduced prominently. With the increase in modifier content, the hydrophilicity of films showed ascendant trend in general. However, the attenuation of hydrophilicity would occur due to the loss of PEG20000with prolongation of soaking time in water. Compared with unmodified UHMWPE films, the notable rise in pure water flux or its recovery rate as well as the significant decline in attenuation of protein flux all meant better water permeation and fouling resistance of UHMWPE membranes after modification. Furthermore, the appearance of layered structure in cross section brought about reduction in permeation capability of membranes when PEG20000accounted for20wt%. The compatibility of PEG20000and UHMWPE turned to be optimum and the crystallinity increased significantly when the modifier occupied I5wt%. Meanwhile, the physical crosslinking of molecular chains between modifier and UHMWPE also contributed to improve the tensile strength of membranes up to near15MPa.