Since PVA is compatible with most hydrophilic polymers, its crystallinity , and the permeability of the membrane can be improved. According to current research, PVA can be blended with polymers such as polyethylene glycol (PEG), sodium alginate (SA), chitosan (CS), polyacrylic acid (PAA), polyvinylpyrrolidone (PVP), etc. Preparation of a new type of separation membrane. The steps of preparing PVA blend film are shown in Figure 1-10:
Factors Affecting PVA Membrane Separation Performance
The operating parameters of pervaporation and the structural properties of the membrane will all affect the separation performance of the membrane. This section discusses the effect of membrane crosslinking agent on membrane structure and the influence of support layer in PVA composite membrane on membrane separation performance.
①Influence of crosslinking agent
Cross-linking can improve the stability of PVA in water, improve its mechanical properties, optimize the selectivity of the membrane, enhance the permeability of the membrane, thereby preventing the excessive swelling of the membrane in the feed solution, resulting in unstable transfer ability and polymer crystallization phenomenon.
In general, the cross-linking process can improve the selectivity of polymer membranes, but will reduce the permeation flux , mainly because the solubility of the feed liquid components in the membrane and the membrane The mobility of the polymer chains in the medium is weakened, resulting in a decrease in the free volume between the polymer chains. Therefore, cross-linking reduces the solubility and diffusivity of membrane components during pervaporation , which leads to a decrease in permeation flux compared with that before cross-linking.
The increase in the selectivity of the cross-linked membrane is mainly due to the fact that the size of the water molecules is smaller than that of the organic molecules in the feed solution, so the water molecules are easier to diffuse into the cross-linked membrane. With the increase , the selectivity of the crosslinked membrane to water also increases.
The type and concentration of crosslinking agent have obvious influence on the performance of PVA film. For PVA, the more commonly used crosslinking agents are polyaldehydes and polyacids, which use the hydroxyl groups on their side chains to react with aldehyde groups or carboxyl groups to change the original chain structure of PVA into a body structure and enhance the stability of the polymer. The commonly used cross-linking agents are glutaraldehyde , maleic acid, etc.
②Influence of supporting membrane in composite membrane
PVA composite membranes are usually made of polyacrylonitrile (PAN), polyethersulfone (PES), cellulose acetate (CA), etc. as membrane materials for the support layer. In order to prevent factors that are unfavorable to membrane production, such as porosity and seepage, the material of the support layer is often chosen to be different from the hydrophilicity and hydrophobicity between the casting solution. There are differences in properties, so that the membranes made often have pores , resulting in different degrees of defects in the PVA thin layer, which in turn leads to differences in the pervaporation separation rate.
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