Separation of NaCl, glycin from collagen solution by the thermal sensitive polyurethane membrane

A novel thermal sensitive polyurethane (TSPU) membrane with a thermal switch was prepared via wet phase inversion technique and used for separation of NaCl, glycin from collagen solution. From Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD), it was found that the TSPU showed segmented structure (i.e. the hard segment and soft segment) and independent phase transition temperatures (the phase transition temperature of the soft segment defined as switch temperature, Ts). Scanning Electron Microscope (SEM) and Atom Force Microscope (AFM) were employed to study the morphology structures of membranes, the results indicated that the membrane showed relative dense surfaces (or skin) and porous cross sections, which controlled their selectivity and permeation. The porosity of membrane measured by the mass loss of wet membrane after drying revealed that when the temperature varied ± 10 °C around the Ts, the porosity of membranes increased from 51.7% to 73.3%, showing a significant improvement to thermal stimuli. When this TSPU membrane was used for separation of NaCl, glycin from collagen solution, we found the ions of Na+ and Cl− could permeate the TSPU membrane at any temperature, and higher temperature resulted in higher penetration rate. Whilst the penetration of glycin relied on the temperature variation, that is, a barrier effect at lower temperature (T < Ts) and higher permeation fluxes at higher temperature were observed. Typically, when the temperature exceeded the Ts, the permeation flux of glycin increased markedly, showing sensitivity to thermal stimuli. Collagen, due to its large molecule size, could not permeate the TSPU membrane in all temperature range. As a result, molecules of NaCl, glycin and collagen with different size could be selectively separated by TSPU membrane driven by the temperature.