Cracking from drying stresses

The question of the original location of the cracks must be adressed. Cracks during drying occur from microflaws expected to be present on the surface of the gel. Regarded as a whole, the network is under compression stress. However, Scherer [22] demonstrated that at the extremity of the flaw tips the solid part remains in tension. 

Table 5.91 shows the flexural stress for variously modified transparent polyamide grades (obtained from dry specimens by the bend strip method) in which the first stress-cracks appear after one minute of contact with the particular chemical. Media concentration is 100%. At lower concentrations, stress-cracking resistance is generally higher. 

It was also important to choose an appropriate substrate to coat the active polymer material onto. Hence, TFC membrane modules prepared with SPPOBr as the active layer coated on PEI hollow fibers spun from a PEI/NMP/GBL solution showed superior performances compared to those prepared from PEI/DMAc/LiN03 solutions. Coating on the lumen side of the fibers gave better performance than coating on the shell side. This could be due to the fact that the skin layer of the fibers was formed on the lumen side. Stability of the coated layer was greatly improved when a wet feed stream used instead of a dry feed stream. Water vapor in the feed stream most likely prevented the active layer from stress cracking on drying. Moreover the presence of water vapor in the membrane may have enhanced the permeance of CO2. 

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