Temperature- And PH-sensitive Polyurethane Membrane Materials: Preparation And Characterization

As the smart polymeric membrane has become one of the hot research topics in material science at present, countries around the world are actively engaged in researching and developing the new functional materials. The temperature- and pH-sensitive polyurethane(PU) membrane reported in this research not only having good performances that common PU possesses, such as excellent physical properties, good biocompability, etc., but also having the temperature and pH sensitivities can be very interesting and have wide application prospect in chemical industry, environmental protection, biomedical field, etc.The PU membrane with temperature and pH sensitivities were synthesized and prepared from a immersion-precipitation phase transformation method and a two-step block copolymerization from polycaprolactone diols(PCL), 4, 4′-diphenylmethane diisocyanate(MDI), dimethylol propionic acid(DMPA), N-Methyldiethanolamine(MDEA), etc. The chemical structure, phase state, surface and cross-section morphology, porosity, surface wettability, mechanical property, and permeate flux, etc., of the PU membranes were characterized. The temperature- and pH-sensitive mechanisms of the PU membrane were characterized. The application of this PU membrane in realizing controllable release of drugs,material separation, etc., were also preliminarily characterized by means of an amino-acid permeate experiment. The L-phenylalanine(L-Phe) was chosen as model amino-acids.Specifically, with PCL, MDI, BDO, DMPA, etc. used as main raw materials, a type of temperature- and pH-sensitive PU membranes were successfully prepared. This PU membrane, called “Type DMPA PU”, contains pH-sensitive components by using DMPA as the chain extender in PU synthesis. The structure and properties of PU membranes were characterized by differential scanning calorimetry meter(DSC), water contact angle tester,mechanical tester, porosity tests, water flux tests, water absorption tests, etc. Results showed that PU(1), PU(2), PU(3) and PU(4) membrane showed a similar crystalline melting transition in their soft segments at 48.2, 47.1, 45.9, and 44.0oC, respectively, meanwhile, the crystalline melting temperature(Tm) and relative enthalpy(△H) were successively trending downward in PU series. With increasing DMPA content, or –COOH group content, contained in PU macromolecules, the mechanical property of PU membranes was decreased, but the surface wettability and porosity of PU membranes were increased. When the environment temperature was raised from 35 oC to 55 oC, the soft segments of all PU membranes underwent a phase transition from crystalline state to amorphous state, and the water fluxes across these PU membranes changed markedly, showing the temperature sensitivity. Due to PU(2), PU(3)and PU(4) containing –COOH groups, their water fluxes and water absorption were obviously changed in the p H range from 4.0 to 5.5, showing the pH sensitivity.With PCL, MDI, MDEA, etc. used as main raw materials, another type of temperatureand pH-sensitive PU membranes were successfully prepared and are called “Type MDEAPU”, accordingly. The chemical structure, phase state, surface and cross-section morphology,porosity, surface wettability, mechanical property, and permeate flux, etc., of the PU membranes were characterized. Results showed that PU(a), PU(b), PU(c) and PU(d)membrane showed a similar crystal-melting endothermic peak, corresponding, respectively, to the crystalline melting transition of their soft segments. The section of PU membrane was composed of skin layer, finger-like pores and sponge-like structure, and the finger-like pores became larger in volume and the sponge-like structure became thinning with increasing content of-N(CH3)- group. Moreover, with increasing-N(CH3)- group content, contained in PU series, the surface wettability was increased, the porosity was increased but the mechanical property was decreased, for these PU membranes. When temperature was raised above the Tm of their soft segments, the water fluxes of all four PU membranes changed markedly, showing the temperature sensitivity. When the pH changed from 8.5 to 10, the-N(CH3)- groups, embedded in PU macromolecules, completed a transformation from ionization to protonation, the water fluxes of these PU membranes were also changed markedly, showing the pH sensitivity.The controllable permeability of this temperature- and pH-sensitive PU membrane on amino-acids was characterized. The L-phenylalanine(L-Phe) was chosen as model amino-acids and the PU(3) was chosen as model PU membranes. Results showed that the crystal structure of soft segments of the PU membranes had good reversibility and recoverability. The bottom surface of PU membrane was smooth, dense and crack-free, the top surface had some tiny pore structures with size scale lower 100 nm, and the cross-section was composed of skin layer, finger-like pores and sponge-like structure, among which the relative dense bottom surface mostly determined the permeate flux(J) and rejection coefficient(R) of membrane. The surface wetting property of this PU membrane showed an obvious pH response, and as the pH was increased from 4 to 5.5, the water contact angles decreased dramatically. J of the L-Phe across the PU membrane increased with increasing temperature while the R decreased as temperature rose, the both showed a sharp change when temperature was raised to the Tm of the soft segment of PU, showing the temperature response.With the increase of pH, the J was decreased with a sharp decrease in the pH range from 4.0to 5.5, the R behavior of L-Phe, however, increased with increasing pH, and exhibited two sharp increases in the pH ranges from 4.0 to 5.5 and from 8.5 to 10.0, showing the pH response.Hopefully, this research can provide reference and guiding significance to enrich and develop the PU material and smart polymeric membrane material.