Superelastic,Ultralight,and Washable Electrospun Fibrous Sponges For Effective Warmth Retention

If human stays at low temperature for a long time,it will be harmful or even life-threatening to human.The warmth retention equipment with favorable features of storing enormous amounts of still air and reduce heat loss,has been widely applied in various fields ranging from garments,household textiles,buildings to the aerospace industries.Natural warmth retention materials had been widely used,but due to their shortcomings such as strong moisture absorption and moderate heat preservation capability,they are gradually replaced by synthetic warmth retention materials.However,due to the limitation of fiber fineness,it is difficult to improve the warmth retention performance of synthetic materials.Electrospun fibrous materials for the advantage of thin fiber diameter,small aperture,and tunable pore structure,show great application prospects in the field of warmth retention.Nevertheless,electrospun fibers were always assembled into close-packed two-dimensional membranes with low fluffiness,which restricting the application.The three-dimensional（3D）sponges prepared by direct electrospinning suffered unsatisfying mechanical properties,which easily caused the collapse of the fluffy structures under external forces.Therefore,it is urgent to develop high efficient warmth retention materials with excellent mechanical properties and washable properties.In this paper,polysulfone（PSU）and polyamide acid（PAA）were used as the main polymers to form 3D structure,and polysulfone/polyimide（PSU/PI）fibrous sponges with ultralight and superelastic properties was prepared by imidization.The effect of imidization process on the morphology and chemical structure of fibrous sponges was studied,and the optimal imidization temperature was determined to be 160/180/200°C.The effect of polymer ratio on the structure and properties of the fibrous sponges was studied.The results showed that when the mass ratio of PSU/PAA was 8/2,the porosity was up to 99.7%,and the volume density was only 3.6 mg cm^-3.At the same time,the bond structure between the fibers gives the sponges good mechanical properties and structural stability,and the material could recover its original shape under the compressive stress of 500 Pa,and possess excellent warmth retention performance(thermal conductivity of 25.6 m W m^-1 K^-1).Further,low surface energy fluorinated polyurethane（FPU）was added to the polymer to improve the hydrophobicity of the sponges,and the effects of FPU content on the morphological structure and various properties of the sponges were investigated.The results showed that the addition of FPU would not damage the fluffy structure.The obtained PSU/PI/FPU sponges show excellent mechanical properties and structural stability with only4%minor plastic deformation after 100 compression cycles（strain of 60%）;the volume density of PSU/PI/FPU sponges was as low as 3.6 mg cm^-3 and the porosity was as high as 99.7%,demonstrating ultralight characteristics,and the sponges present excellent warmth retention performance with thermal conductivity of 25.5 m W m^-1 K^-1.After 100 cyclic compression,the thermal conductivity could remain below 25.7 m W m^-1 K^-1,and it could also remain stable thermal conductivity under different relative humidity（RH）;in addition,the PSU/PI/FPU fibrous sponges still displayed the lowest thermal conductivity of 25.7 m W m^-1 K^-1 after cyclic washing,indicating stable and efficient warmth retention performance.Finally,the structure and performance of the PSU/PI/FPU fibrous sponges were compared with existing insulation materials.The experimental results show that PSU/PI/FPU sponges has much smaller fiber diameter（3.4μm）,smaller average pore size（15.08μm）and lower thermal conductivity than commercial warmth retention felts;the PSU/PI/FPU fibrous sponges could almost maintain their original shape after washing,and there is almost no increase in thermal conductivity(25.7 m W m^-1 K^-1).The result shows the excellent characteristics of the PSU/PI/FPU fibrous sponges and its spacious application prospects in the warmth retention field.