Controllable Preparation Of Waterproof And Breathable Polyurethane Composite Membranes Based On The Porous Structure And Hydrophobic Surface

In recent years,waterproof and breathable microporous membranes have attracted much attention due to its unique properties of resistance to water permeation and the admittance to vapor transmission.Hence,it is widely applied in protective clothing,filtration,and membrane distillation,especially in fabricating protective clothes.In general,the waterproof and breathable performances of membranes are obtained from the synergistic effect of hydrophobic surface and interconnected porous structure,which could block water droplet penetration and provide an interconnected passageway for vapor transmission at the same time.Currently available techniques for fabricating waterproof and breathable membranes such as melt blown,coating,lamination and electrospinning have been widely used.However,these methods are confronted with many disadvantages including complicated preparation processes and the difficulty in regulating porous structure.Thus,it is imminent to design a novel and facile route for fabricating waterproof and breathable microporous membranes.Recently,electrospinning has been proved to be an available strategy for fabricating macroporous membranes,the nonwoven fiber web with adjustable and interconnected porous structure by random accumulating of fibers.The as-prepared electrospun microporous membranes has some features such as the controllable composition and structure,simple preparation process and low cost.Based on these characteristics,a series of electrospun waterproof and breathable fibrous membranes,fabricated by using various polymers as raw materials,such as polylactic acid(PLA),polyvinylidene fluoride(PVDF),polyurethane(PU),polyacrylonitrile(PAN)and so on,could be applied in filtration,distillation,biological scaffolding.Among these studies,PU is regarding as an potient raw materials for preparing waterproof and breathable fibrous membranes because of the unique block macromolecular structure IV and excellent physicochemical properties such as high elongation and good abrasion resistance.Therefore,we would use PU as a raw material to prepare waterproof and breathable microporous membrane by electrospinning.In this paper,electrospun PU fibrous membranes were prepared at different polymer concentrations(16,18 and 20 wt%).The influence of different PU concentrations on the fiber morphology,porous structure,wettability,mechanical properties,and waterproof/breathable properties were investigated.The results showed that the as-prepared PU fibrous membranes has some microstructure features such as uneven fiber structure and large pore size.Meanwhile,the fibrous membranes possessed great mechanical properties and breathability while the waterproof property was very poor.Significantly,the waterproof and breathable performance relied on the synergistic effect of hydrophobic surface and porous structure.In order to improve the waterproofness and breathability of PU fibrous membranes,the two-step method would be adopted to optimize the fibrous membranes.First of all,Na Cl was introduced to tune the fiber construction and porous structure of PU fibrous membranes.Furtherly,the as-prepared fibrous membranes were modified by FAS to improve hydrophobicity.As aresult,a novel FAS-PU/Na Cl fibrous membranes was prepared by two-step method.The synergistic effect of the optimized porous structure and the robust hydrophobic surface endowed the PU fiber membranes with great waterproofness(hydrostatic pressure of 126.1 KPa),breathability(WVT rate of 9.1 kg m-2d-1)and enhanced mechanical properties(breaking strength of 10.5 MPa).Considering that the two-step process takes a long time and needs additional modification instruments,and PU/Na Cl fibrous membranes possesses a great breathability.Thus,we then fabricated a novel hydrophobic PVDF-CNTs/PU/PVDFCNTs sandwich-structured composite membrane by sequential electrospinning based on our previous study.The composite fibrous membranes consist of PU fibrous membranes as moisture-permeable layers,PVDF fibrous membranes as waterproof layers,and CNTs as inspiring additive.In addition,the fibrous construction,porous structure and surface wettability of composite fibrous membranes have been facilely regulated by tuning the thickness ratio of PVDF and PU fibrous membranes and the contents of CNTs.The synergistic effect of porous structure and hydrophobic surface on the waterproofness,breathability and oil-water separation ability of the composite membranes was also investigated.The as-prepared sandwich-structured composite fibrous membranes showed robust waterproofness(hydrostatic pressure of 38 KPa)and breahtability(WVT rate of 8.6 kg m-2d-1)as well as oil-water separation capacity.Meanwhile,the hydrophobic-lipophilic PVDF fibrous membranes also endowed the composite membranes with oil-water separation ability.