Composition of the coagulation bath

Figure 3.35 Depending on the bore fluid and the composition of the coagulation bath, the selective skin layer can be formed on the inside, the outside or both sides of the hollow fiber membrane...

From the morphology and mechanical properties of these as-spun PANI fibers, it is obvious that the size and number of macrovoids must be reduced. Therefore, the effects of the temperature and composition of the coagulation bath were subsequently investigated. The coagulation bath temperature should affect the morphology of the as-spun PANI fibers. From theoretical calculations, a high bath temperature is expected to accelerate the diffusion and the demixing processes. From the above study, we... 

The main spinning parameters are the extrusion rate of the polymer solution the bore fluid rate the tearing-rate the residence time in the air-gap and the dimensions of the spinneret. These parameters interfere with the membrane-forming parameters such as the composition of the polymer solution, the composition of the coagulation bath, and its temperature. Figure ID - 7 shows a spun fiber in the air gap. 

No. Solution concentration Composition of the coagulation bath Observations ... 

Parameters connected to the coagulation include temperature, composition and injection rate of the bore fluid (BF), and temperature and composition of the coagulation bath. 

In wet spinning, a polymer solution is extruded through a spinneret directly into a coagulating bath. The spinning speed is slower than in the dry method, and the composition and temperature of the coagulating bath should be carefully regulated to set up the freshly spun filaments and to ensure that the proper amount of solvent is removed from them. 

Brunetti et al. [107] produced integrally skinned asymmetric membranes from PEEK-WC and studied the influence of different preparation parameters, such as the composition and temperature of the coagulation bath and casting knife gap set, on the membrane morphology and transport properties, permeance, and selectivity. Membranes were prepared by immersion precipitation, using THE as solvent. Three different coagulation baths were tested a mixture of methanol/water 70/30, pure methanol (MeOH), or pure IPA. 

As coagulation proceeds into the center of the forming fiber, the outside regenerates to cellulose at a rate dependent on the temperature and composition of the bath. 

Fibers spun by this method may be isotropic or asymmetric, with dense or porous walls, depending on the dope composition. An isotropic porous membrane results from spinning solutions at the point of incipient gelation. The dope mixture comprises a polymer, a solvent, and a nonsolvent, which are spun into an evaporative column. Because of the rapid evaporation of the solvent component, the spinning dope solidifies almost immediately upon emergence from the spinneret in contact with the gas phase. The amount of time between the solution s exit from the spinneret and its entrance into the coagulation bath has been found to be a critical variable. Asymmetric fibers result from an inherently more compatible solvent/nonsolvent composition, ie, a composition containing lower nonsolvent concentrations. The nature of the exterior skin (dense or porous) of the fiber is also controlled by the dope composition. 

FIGURE 3.8 Schematic representation of the composition path of the cast film immediately after immersion in the coagulation bath for (a) instantaneous and (b) delayed demixing systems, and the scanning electron microscope (SEM) picture of the resulting membranes (c) and (d). 

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