Thermal-and PH-sensitive Polyurethane Membrane:Preparation And Controllable Separation Performance

The environmentally sensitive polyurethane membrane,one of polyurethane membranes,could response to the variation of external environment.Accordingly,temperature and pH are closely related to the human physiological condition,thermal-and pH-sensitive polymer membranes belong to important branches of environmentally sensitive polyurethane membranes,which have been widely used in medical materials,pharmaceuticals,food industry,and water treatment,etc.Due to the block structure of polyurethane materials,thermal-and pH-sensitive polyurethane membranes can be obtained by introducing temperature monomer or pH sensitive monomers during the copolymerization process.In this research,polyethylene glycol 6000（PEG,Mn=6000）,4,4’-diphenylmethane diisocyanate（MDI）and trimethylolpropane（TMP）were used to prepare thermal-and pH-sensitive polyurethane hydrogel by three-step block copolymerization method.Then,a series of thermal-and pH-sensitive hydrogel and thermoplastic polyurethane separation（PU）membranes was prepared by blending the polyurethane hydrogel with thermoplastic polyurethane material;we also systematically discussed the influence about cross-section morphology,chemical structure,phase transition temperature,crystallization of PU membranes by different composition.In order to investigate thermal and pH-sensitive properties,the equilibrium swelling rate and water flux of polyurethane membranes were detected in variation of temperature and pH values.The results showed that morphology of polyurethane membranes exhibited porous structure in the surface and cross section.From the DSC and XRD experiments,as increaseing hydrogel content,phase transition temperature（T_s）and melting enthalpy（ΔH）of the polyurethane membranes increaseed,which indicate the crystallinity improved significantly.When the temperature exceeded the phase transition temperature,the polyurethane membranes showed obvious phase change process.Thermal-and pH-sensitive polyurethane hydrogel have been successfully introduced during the preparation of polyurethane membrane.When the temperature reached the phase transition temperature,the swelling ratio and water flux of the polyurethane membrnaes showed a significant decrease or increase trend respectively,which was ascribe to disappeared the crystallinity of polyurethane membrane and shrunk the polyurethane segment,result in the hydrogen bond between the water molecules was broken,and the hollow structure inside the polyurethane membranes expanded.Moreover,when the external pH exceeded 8.5,the functional group in the polyurethane segment rapidly transformed from the ionized state to the protonated state（-N⁺H（CH₃）-+OH-→-N（CH₃）-+H₂O）.Due to the shrunk polyurethane segments,the hydrogen bond between the polyurethane segments and the water molecule broke and porous strutures of polyurethane membranes expanded,which leads the swelling rate to decrease,water flux to increase evidently,expressing thermal-and pH-sensitivity features.The polycaprolactone polyurethane were obtained by block copolymerization using polycaprolactone 4000（PCL,Mn=4000）,4,4’-diphenylmethane diisocyanate（MDI）and 1,4butanediol.A series of thermal-and pH-sensitive hydrogel and polycaprolactone polyurethane separation membranes were prepared by mixing thermal-and pH-sensitive polyurethane hydrogel and polycaprolactone polyurethane.The effects of different compositions of polyurethane membranes on the surface and cross-section morphology,chemical structure,phase transition temperature,crystallinity,hydrophilicity and thermal-and pH sensitivity of polyurethane membranes were investigated.As the content of polyurethane hydrogel increased,the surface and cross-sectional hollow structure of the polyurethane membranes raised significantly,which showes thinner cortex and larger finger-shaped mesopores in cross section.According to the DSC and XRD results,increasing crystallinity of the polyurethane membranes and increasing phase transition temperature and melting enthalpy were attributed to the content of polyurethane hydrogel increaseed.When temperature rose from 35 ^oC to 55^oC,the hydrogen bond between the polyurethane segment and water was gradually broken and polyurethane segment shrunk,which brought about reduced equilibrium swelling ratio and increased water flux.When the external pH value rose from 8.5 to 10,the pH-sensitive group in the polyurethane segment converted from the ionized state to the protonated state,led to weaked the hydrogen bond between the polyurethane segments and enlarged the porous structure of polyurethane membrane,showing reducing equilibrium swelling and increasing water flux respectively.The membrane（a）was selected from thermal-and pH-sensitive hydrogels and thermoplastic polyurethane separation membranes to investigate the influence of different temperature and pH on the permeability of the polyurethane membrane,using K⁺,glycine,and bovine serum albumin as mimic drugs.Similarly,membrane（A）and membrane（B）were also be detected to research the effect on separating performance of PU membranes by different temperature and pH value.The results indicated that increaseing temperature and pH value could have a positive effect on the permeability of glycine.Concretely,permeability of glycine increased from 66.1%to 83.1%in the temperature range from 35 ^oC to 55 ^oC,rose from 66.2 to 90.0%in the pH range of 5.5 to 8.5%.In addition,Permeability of methylene blue in membrane（A）and membrane（B）expressed identical trend.In temperature range from 15^oC to 65^oC,permeability of methylene blue in membrane（A）increased from 8%to36%,while the permeability of methylene blue in membrane（B）rose from 4.1%to 21.5%.When the external pH was lower than 7,the permeability of methylene blue of membrane（A）and membrane（B）merely only keeped at 8%and 4%respectively.However,the permeability of methylene blue increased to 36.2%and 26.3%respectively in pH range from 7 to 11.In this paper,thermal-and pH-sensitive polyurethane hydrogels were prepared by block copolymerization method,and then thermal-and pH-sensitive polyurethane hydrogels were blended with thermoplastic polyurethane or polycaprolactone polyurethane,Finally,a series of thermal-and pH-sensitive polyurethane membranes were obtained by immersion phase transformation,further explored the effects of thermal-and pH sensitivity on the separation performance of polyurethane membranes,which could provide a reference for the research and application of environmentally sensitive polyurethane membranes.