Thermal and X-Ray Structural Studies of Structural Changes in the Active Layer of Polytetrafluoroethylene Ultrafiltration Membranes

An analytical review on the study of structural changes in membranes is presented. Ultrafiltration films of polytetrafluoroethylene membranes in the temperature range from 30 to 500°С are investigated by thermogravimetry. An analysis of the thermal data is carried out, which indicates that in the temperature range from 100 to 215°C, the first stage of destruction occurs, where the value of the lost mass for the air-dry sample is 33%, and for the water-saturated sample, the mass loss is 10%, which is 3 times less than the first sample. For the working sample, a slow process of mass change is observed in a wide temperature range from 100 to 410°C, which with total weight loss is about 9%. The second stage of destruction occurs in the temperature range from 410 to 500°C with a loss of mass for an air-dry sample of 61 and 85% for a water-saturated sample, and the working sample loses 86% of its mass. The data on X-ray crystallography (X-ray crystallography) allow us to conclude that the effect of water and transmembrane pressure leads to a significant change in the morphology of polymer molecules at the supramolecular level. If for an air-dry sample of a polytetrafluoroethylene ultrafiltration membrane, a diffuse nature of the reflection with a maximum at 2θ = 12° is observed, then for a water-saturated sample an intense diffraction peak appears at 2θ = 17° against the background of an amorphous halo. For the working sample, the reflection has a compact structure with a sharp asymmetric peak at 2θ = 13.8°. Complementary methods of TG (thermogravimetry), DSC (differential scanning calorimetry), X-ray diffraction analysis (XRD), and gravimetry have established that conformational changes in molecules in the polytetrafluoroethylene-membrane matrix occur as a result of the detachment of side monomers or oligomers and their sublimation from the film volume under the influence of transmembrane pressure and the interaction of water molecules.