Preparation And Characterization Of Intelligent Polymer Membranes With Controllable Water Vapor Permeability

Shape memory polyurcthanes (SMPUs) and temperature-sensitive hydrogels were synthesized and prepared. The structure and properties of these two kinds of smart polymers were tested and analyzed. Then these two kinds of polymers were assembled to prepare smart composite membranes, and the water vapor permeability of the membranes could be controlled by temperature.Segmented shape memory polyurethanes were synthesized by solution synthesis method. Poly(butylenes adipate) glycol (PBAG), polycaprolactone polyol (PCL) and polyethylene glycol (PEG) were used as soft segment, toluene-2,4-diisocyanate (TDI) as hard segment, and 1,4-butanediol (BDO) as chain extender in order to prepare segmented SMPUs. The influence of soft segment molecular weight and hard segment content on the thermal properties, crystallization structures and water vapor permeability was investigated. The phase separation of soft/hard segment of SMPUs was enhanced when the higher hard segment content and larger molecular weight of soft segment was used, which resulted in larger water vapor permeability of shape memory polyurethane membranes. Meanwhile, the trigger temperature T_s.m (the crystalline melting temperature) shifted to higher temperature region with the increase of molecular weight of soft segment and the decrease of hard segment content. Two representative PBAG-based SMPUs were mixed according to different weight ratios in order to adjust the trigger temperature and water vapor permeability of the obtained mixture. The results showed that the shape recovery temperature of mixed polyurethanes based on the same soft segment with differing molecular weight and hard segment contents, changed linearly as a function of the content of either polyurethane. This occurred within the range of the shape recovery temperature of the two kinds of simplex polyurethanes, providing a practical method to control the shape recovery temperature of shape memory polyurethanes. Hydrophilic polymer PEG was used as soft segment with PCL, which could induce a trigger temperature around 30℃. The water vapor permeability of PCL/PEG based SMPUs was enhanced greatly.Poly(N-isopropylacrylamide-co-sodium acrylate) gels with N,N-methylene bisacrylamide (BIS) as crosslinker, ammonium persulfate (APS) and Tetramethylethylenediamine (TEMED) as initiator and accelerator were prepared by free radical polymerization method. The swelling behavior of Poly(NiPAAm-co-SA) hydrogels was investigated systematically by varying the SA contents, crosslinker concentrations, the pre-gel solution concentrations ( total monomer content in water and also in different swelling solution medium) for obtaining higher swellabilities to study the stimuli-sensitive behavior in the solutions with different pH and temperature. The results indicated that the swelling rate of poly(NiPAAm-co-SA) hydrogel with small content of SA can changed about 50% which proved that the content of SA should be controlled within 10% (mol/mol) in order to obtain the hydrogels with distinctive thermo-sensitivity. The introduction of hydrophilic comonomer SA did greatly enhance the swelling ability of poly(NiPAAm-co-SA) hydrogels, which can improve the swelling rate of poly(NiPAAm-co-SA) hydrogels 20-40 times when compared with pure PNiPPAm hydrogels. Phase separation technique was employed to prepare fast response and strong swelling hydrogels.Finally, two methods were used to prepare smart composite membranes. Membrane A was made using dry process by multi-coating poly(NiPAAm-co-SA) hydrogel dry micro-particles and non-ionic shape memory polyurethane. Membrane B was made using wet process by multi-coating poly(NiPAAm-co-SA) hydrogels swollen micro-particles and ionic shape memory polyurethane. The mechanism of water vapor transmission of the membranes is as follow. In the composite system, the water vapor permeability of shape memory polyurethanes can increase dramatically at the trigger temperature, which is provided by the enhanced micro-brown motions of polyurethane molecules around the soft segment crystalline melting temperature (?)s.m· Meanwhile, the hydrogels micro-particles assembled with polyurethane exhibit a reversible volume transition at a transition temperature or lower critical solution temperature (LCST), that is. the hydrogel particles could swell and deswell dramatically with the change of temperature. When the composite membranes were heated to the temperature above the LCST with the presence of water, the hydrogel particles dispelled greatly and pores were created around the particles. As a result, the water vapor permeability of composite membranes was enhanced greatly. When the temperature decreases, the hydrogel particles swelled and the pores disappeared. The water vapor permeability of the composite membranes decreases. The results showed that the water vapor permeability of composite membranes is superior to the single component membranes, and the "double on-off" effect of water vapor permeability of the composite membranes can be achieved.