Structure Design And Heat-moisture Comfort Property Investigation Of Waterproof And Breathable Nanofibrous Membranes

With economic growth and improved living standards,the masses tend to pursue the comfort and protection of clothes.Waterproof and breathable?W&B?membranes which could effectively resist the penetration of liquid water and allow water vapor to diffuse readily,have attracted a great deal of attention and are extensively applied in protective garments,field uniforms,and surgical gowns.Current commercial waterproof and breathable membranes mainly consist of thermoplastic polyurethane?TPU?hydrophilic nonporous membranes,polytetrafluorethylene?PTFE?hydrophobic porous membranes.TPU membranes possess good resistance to liquid water penetration with the aid of their nonporous structure and transfer water vapor to the opposite side through the adsorption-diffusion-desorption of hydrophilic groups.But TPU membranes exhibit poor breathable performance and fail to transfer air.PTFE membranes possess waterproofness and breathability owing to their numerous internal pores,which are far larger than the size of a water molecule and yet much smaller than the smallest water droplet.And PTFE membranes exhibit higher water vapor transmission rate,because the convection of water vapor in pores is much faster than the diffusion in the solid TPU membranes.However,PTFE membranes suffer from the low elasticity and complex fabrication process,and are hard to degrade once manufactured.Therefore,developing new technology to fabricate new W&B membranes with high hydrostatic pressure and water vapor transmission rate is still a principal issue.Electrospinning has considerable interest with simple operation and wide range of raw materials,and the obtained nanofibrous membranes exhibit with hydrophobic porous structure with high porosity and small pore size,which has arisen widely attention of domestic sociology scholars in recent years.Researchers have fabricated various nanofibrous W&B membranes with excellent waterproofness and breathability via electrospinning in one step.However,electrospun nanofibrous W&B membranes still face bottleneck problems for improving heat-moisture comfort: nanofibrous W&B membranes could only transmit the moisture to keep skin dry and dissipate heat passively,and fail to balance the temperature of the microclimate above the skin actively;and nanofibrous W&B membranes are apt to be blocked by perspiration outside liquid water,resulting in the decline in evaporative heat dissipation.Thus,more efforts should be devoted to developing W&B nanofibrous membranes with outstanding heat-moisture comfort.The thesis focused on the improvement of heat-moisture comfort of W&B nanofibrous membranes,and make structure design and mechanism investigation.Through the synergistic regulation of surface topography and surface chemical compositions,the superhydrophobic nanofibrous membranes were prepared and show good adhesion resistance to liquid water,resulting in good heat dissipation through the evaporation of moisture.Besides,with the incorporation of various thermoregulating dopants,many kinds of thermoregulating superhydrophobic electrospun W&B nanofibrous membranes were fabricated,including thermoconductive,heat-refective,and heat-retaining superhydrophobic membranes.Meanwhile,the criterion of constructing a superhydrophobic surface was also detailed investigated,and the effect of the porous structure on their waterproofness and heat-moisture comfort of the membranes was also revealed.The detailed research works are summarized below:?1?We presented the humidity-induced polyvinylidene fluoride?PVDF?/fluorinated polyurethane?FPU?coaxial electrospinning,and established the ternary phase diagram of the PVDF-DMAc-H₂O system to analyze the phase behavior of the employed PVDF solutions and regulated the humidity to construct a balsam-pear-like structure on the PVDF nanofibers,endowing the obtained membranes with stably integrated micro/nanoscale roughness.Meanwhile,we regulated the loading rate of FPU by adjusting the flow rate of FPU solution and constructed FPU “nano-armor” on PVDF nanofibers,thus the PVDF/FPU membranes showed durable superhydrophobicity with the water contact angle of 154 o after 50 cycles of loading friction,and the membranes exhibited the hydrophobic porous structure to resist water penetration with the hydrostatic pressure of 53 k Pa.The result established the criteria to fabricate superhydrophobic nanofibrous membranes,namely the synergistic regulation of surface topography and surface chemical compositions.The obtained PVDF/FPU membranes exhibited robust superhydrophobicity and excellent breathability(water vapor transmission rate of 13.1 kg m^-2d^-1)with good heat dissipation through the evaporation of moisture.?2?We performed the blend electrospinning with the incorporation of highly thermally conductive boron nitride?BN?nanosheets and hydrophobic FPU.BN nanosheets engendered nanoscale roughness on the membranes,and FPU could reduce the surface energy of the membranes.Due to the synergistic effect of the hydrophobic polymeric matrix and improved roughness,the PU/FPU/BN membranes showed robust superhydrophobicity with the water contact angle of 153 o and good waterproofness with the hydrostatic pressure of 32 k Pa.Meanwhile,we studied the effect of the composition of polymer-solvent-nonsolvent on the aggregation structure of membranes,and controlled the loading rate of BN nanosheets and ambient relative humidity?RH?to regulate their porous structure.The strategy caused BN to be linked with each other along nanofibers,and thus the membranes contained a well interpenetrated BN network and remained porous structure simultaneously,improving their thermal conductivity without sacrificing the moisture permeability.The obtained PU/FPU/BN membranes not only could dissipate heat through the evaporation of moisture(water vapor transmission rate of 11.6 kg m^-2d^-1),but also conduct heat to the surrounding directly?ultrahigh in-plane thermal conductivity of 17.9 W m-1 K-1 and cross-plane thermal conductivity of 0.29 W m-1 K-1?,endowing the wearer with cooling and comfort.?3?We presented the combination of electrospinning and electrospraying to fabricate heatreflective superhydrophobic PU/FPU/TiO₂ nanofibrous membranes.High-concentration PU/FPU solution was electrospun to generate porous nanofibrous membranes,and PU/FPU dilute solution was electrosprayed to generate the porous microsphere layer.Through the regulation of PU/FPU/TiO₂ solution composition,TiO₂ nanoparticles were encased by PU/FPU matrix and then bonded with each other,generating the porous framework with small pore size.Thus the obtained bilayer membranes showed stable superhydrophobicity and good waterproofness with the hydrostatic pressure of 51 k Pa.Besides,TiO₂ nanoparticles showed a high refraction coefficient and could effectively reflect sunlight,we studied the effect of the diameter of TiO₂ nanoparticles on the heat reflection of the bilayer membranes.And the PU/FPU/TiO₂ layer and PU/FPU nanofibrous layer both exhibited connected porous structure,resulting in high water vapor transmission.Ultimately,the obtained bilayer membranes showed good breathability with 11.8 kg m^-2d^-1 and excellent heat-reflective function with solar reflectivity of 85.7 %,namely the membranes could effectively reflect sunlight radiation heat and reduce the temperature rise in human skin,which were very applicable to the outdoor cooling fabrics.?4?We presented a robust methodology to construct phase-changeable nanofibers by incorporating active heat storage-release n-octadecane phase change capsules?PCC?and terminal FPU,and obtained corncob-like PU/FPU/PCC nanofibers.PCC engendered nano-protuberance on the surface of nanofibers,thus the membranes showed stable superhydrophobicity.The incorporation of PCC also reduced the pore size and improve the porosity of the membranes,and the resulting PU/FPU/PCC membranes presented prominent waterproofness with the high hydrostatic pressure of 84 k Pa.PCC exhibited the reversible solid-liquid phase change behavior,endowing the composite membranes with high phase-changeable capacity for thermal storage/release.And encapsulation structure of PCC and the strong binding force between PCC and PU,endowed the membranes with stable thermal regulation,preventing the abscission of PCC and the leakage of the phase change ingredient?n-octadecane?.The obtained composite membranes showed good breathability with a water vapor transmission rate of 11.4 kg m^-2d^-1 and outstanding heat-retaining with a high phase change enthalpy of 74 J g^-1 after 50 heating/cooling cycles,revealing the membranes could adjust the temperature of human body in a changeable environment,which can not only cool down in the hot environment,but also keep warm when the weather becomes cold.