Construction Of Green-solvent-based Nanofibers And Investigation Of Their Waterproof And Breathable Performance

Waterproof and breathable（W＆B）materials can resist liquid water penetration while transmitting water vapor,which are highly demanded in various applications such as protective clothing,medical hygiene,electronics,seawater desalination,building walls,etc.W＆B materials used for textiles usually contain three different categories:tightly woven fabrics,coated fabrics,and laminated fabrics.Tightly woven W＆B fabrics feature high breathability owing to their highly porous structure,but their large pores lead to poor waterproofness.Although coated W＆B fabrics usually exhibit superior waterproof property,they suffer from unsatisfactory breathable property induced by the dense structure.Currently,laminated W＆B fabrics are the mainstream in the market of W＆B textiles benefiting from their desirable waterproof and breathable performances.Particularly,W＆B membranes play a decisive role in these laminated fabrics,which can be divided into two types:hydrophilic nonporous membranes and hydrophobic porous membranes.Thereinto,hydrophilic nonporous membranes possess high waterproof property due to their solid（i.e.,nonporous）structure,and they transmit water vapor molecules via hydrophilic groups based on the absorption-diffusion-desorption process,but they cannot transmit air.Conversely,hydrophobic porous membranes can resist the liquid penetration by the Laplace’s pressure in the hydrophobic capillary,and their substantial microchannels contribute to transmitting water vapor and air,thus arising tremendous interest among consumers.However,polytetrafluoroethylene W＆B membranes,as the most popular hydrophobic microporous W＆B membranes,not only suffer from poor elasticity and slow decomposition,but also involve toxic perfluorooctanoic acid and perfluorooctanesulfonate during their production process.Therefore,it is urgently needed to develop novel hydrophobic microporous membranes with high W＆B performance and environmental friendliness.As the cutting edge of nanofiber fabrication techniques,the electrospinning technique features the facile process,ease of large-scale manufacture,and diverse spinnable raw materials.The prepared nanofibrous membranes have small pore size,high porosity,and tunable wettability,thus holding great promise in constructing high-performance and environmentally friendly hydrophobic microporous W＆B membranes.So far,many researchers from all parts of the world have developed several electrospun nanofiber W＆B membranes using different raw materials including polyurethane,poly（vinylidene fluoride）,polyacrylonitrile,polyamide and so on.Although these membranes show a promising potential application in W＆B textiles,there are still some critical bottlenecks to be addressed.Firstly,these nanofiber W＆B membranes are usually prepared on the basis of the toxic solvents（e.g.,tetrahydrofuran,formic acid,N,N-dimethylacetamide,etc.）which result in serious environmental issues such as air/water pollutions and human health concerns such as skin irritation and liver toxicity.Secondly,there are few studies on analyzing the moisture transmitting behavior through nanofiber W＆B membranes on a microscopic scale,and the nanofibrous membranes’resistance to the penetration of other important liquids such as oil and blood is practically important but remains unexplored yet.Finally,although nanofiber membranes for W＆B textiles would be stretched frequently in practical applications,there have been no relevant studies focusing on the stretchability and elasticity of nanofiber W＆B membranes.In this paper,to solve the bottlenecks mentioned above,we developed environmentally friendly nanofiber W＆B membranes by electrospinning based on green solvents.This paper emphasized the construction of ethanol/water-solvent-based nanofiber hydrophobic microporous membranes.The mass transfer mechanism of water vapor molecules across nanofiber membranes was explored.Moreover,we investigated the nanofibers’resistance to the oil/blood penetration and proposed the multilevel tensile mechanism of stretchable nanofiber W＆B membranes.Finally,we examined the impacts of the structure of green-solvent-processed nanofiber membranes on the waterproof,breathable,and mechanical properties.The main conclusions are as follows:（1）The polyamide nanofiber membranes were prepared by using ethanol as the solvent and ethanol-soluble polyamide（EPA）as the raw polymer.Subsequently,the EPA nanofibers were homogeneously coated with the eco-friendly water-based low surface energy fluorinated polyacrylate with short perfluoroalkyl(-C₆F₁₃)chains via the impregnating method,thus endowed with superior amphiphobicity,smaller pore size（0.64μm）,and stable bonding structure.The resultant green polyamide nanofiber membranes could resist the penetration of water and oil,exhibiting excellent water and oil intrusion pressures of 101.2 and 32.4 k Pa,respectively.Although the stacking structure of EPA nanofiber membranes became dense and the porosity decreased after the impregnating treatment,the membranes still maintained the 3D interconnected channels and porous structure（porosity=55.4%）,showing a good water vapor transmitting（WVT）rate of 11.2kg m^-2 d^-1.By calculating the Knudsen number（0.25）,we proposed that the diffusion process of water vapor molecules in fibrous membranes was driven by molecule collisions as well as by molecule reflections to the wall of pores.（2）The spinnable ethanol-soluble polyurethane（EPU）with polar carboxyl groups was synthesized by a stepwise polymerization route.Moreover,ethanol-resistant polyurethane（PU）nanofiber W＆B membranes were developed using ethanol as a solvent and trimethylolpropane tris（2-methyl-1-aziridine propionate）（TTMA）as a crosslinker by the emulsion electrospinning technique.TTMA was introduced into the nanofibers to construct the crosslink structure by post thermal treatment,resulting in exceptional ethanol resistance.The resulting nanofiber membranes maintained stable structure and properties after 60 min immersion in the ethanol solvent.Besides,fluorinated polyurethane（FPU）was introduced in spinning emulsions to result in the increment of the water contact angle from 97.8^o to 134.5^o.The developed nanofiber membranes exhibited a robust hydrostatic pressure of 86.2 k Pa,high WVT rate of 13.1 kg m^-2 d^-1,good air permeability of 5.5 mm s^-1,and desirable elasticity with a strain recovery ratio of 79.2%after 100 cyclic strain-release tests.（3）Furthermore,water was used as the green solvent,for the first time,to prepare polyurethane W＆B membranes by the unique waterborne emulsion electrospinning technique combined with heating treatment.The aziridine-based crosslink agent was introduced into waterborne polyurethane（WPU）nanofibers to frame branched networks,thus improving the heat resistance and structure stability.The porosity of crosslinked nanofiber membranes was maintained at 52%after being heated at 160 ^oC.More importantly,the water-based fluoropolymer was introduced into spinning emulsions,and the electrospun nanofibers were heated to achieve the surface enrichment of fluoroalkyl segments,thus endowing the water-processed nanofibers with robust hydrophobicity（140^o）.Benefiting from the hydrophobic channels with small pore size and high porosity,the water-processed PU nanofiber W＆B membranes exhibited high water and blood intrusion pressures of74.3 and 44.9 k Pa,respectively,WVT rate of 12.8 kg m^-2 d^-1,and air permeability of 9.3 mm s^-1.（4）On the basis of waterborne nanofiber W＆B membranes developed above,we further prepared environmentally benign fluorine-free waterborne nanofiber membranes with waterproof and breathable capabilities using long-chain alkyl polymer（LAP）as the fluorine-free dopant and polycarbodiimide（PCD）as the aziridine-free crosslinker.The WPU membranes possessed stable interconnective porous structures due to the crosslink networks within nanofibers,and the nanofiber membranes exhibited robust hydrophobicity through the surface enrichment of long-chain alkyls.The fluorine-free waterborne membranes showed the acceptable waterproofness of 35.9 k Pa,desirable WVT rate of 4885 g m^-2 d^-1,and excellent air permeability of 19.9 mm s^-1.The nanofiber membranes possessed better moisture permeability befitting from the larger water vapor pressure deficit at the higher temperature and lower humidity.Benefiting from the plausible multilevel tensile mechanism,the membranes showed the impressive tensile elongation of 372.4%at break and high elasticity of 56.9%after being stretched for 100 cycles at a large stain（300%）.