Superelastic And Fire-Retardant Nano/Microfibrous Sponges For High-Efficiency Warmth Retention

When the humans stay out in cold environment for a long time,it is easy to catch motionless frostbite and even cause death in severe situations.Warmth retention materials can store a large amount of still air to reduce heat loss,thereby playing the role of cold-proof and warmth-keeping.Natural warmth retention materials mainly include cotton and hemp,which have the disadvantages of strong moisture absorption and poor strength,so they are gradually replaced by hydrophobic synthetic fiber assemblies.However,the diameter of synthetic fiber is relatively thick（>10μm）,which makes it difficult to greatly enhance its warmth retention performance.Electrospinning has been widely used to prepare micro/nano fiber materials,which has the virtues of small aperture,controllable morphology,and tunable pore structure,showing great application prospects in warmth retention field.However,electrospun fibers usually assemble into close-packed two-dimensional membranes,exhibiting a mediocre warmth retention performance due to the low fluffiness（the thickness usually<100μm）and limited porosity（usually<85%）.In recent years,researchers have prepared three-dimensional fluffy electrospun fiber materials with good warmth retention properties,but they are extremely likely to burn and produce melt droplets,leading to safety hazards.Meanwhile,their structural stability is weak,which greatly restricts the practical applications in the domain of warmth retention.Therefore,it is of vital significance to develop warmth retention materials with fluffy structure,robust mechanical and flame retardant properties.In this paper,the polymers of polyphenylene sulfone（PPSU）and polyurethane（PU）were firstly chosen to fabricate three-dimensional（3D）fluffy PPSU/PU microfibrous sponges with high thermal insulation properties by accurately regulating the relative humidity（RH）.When the RH is75%,the fibrous sponges exhibit mean fiber diameter of 1.79μm,low bulk density of 5.94 mg cm^-3,and high porosity of 99.53%.Besides,the tensile strain and tensile stress could reach 64.2%and133.5 k Pa,respectively.Most importantly,the fibrous sponges show excellent heat preservation ability with thermal conductivity of 26.49 m W m^-1K^-1.In order to enhance the mechanical properties and flame retardant,the high-strength polyamide-imide（PAI）nanofibers with high limit oxygen index（LOI）are creatively introduced into the PPSU/PU fibrous sponges.The results show that the interlocked dual-network structure and bonded structure are formed,highly helpful for enhancing mechanical properties.The tensile stress reaches 148.6 k Pa,and there is nearly no plastic deformation after 100 compression cycles.The fibrous sponges could recover to their original shape quickly without any deformation after removing the external force,demonstrating the robust compression resilience property.At the same time,the introduction of PAI nanofibers improves the flammability of the fibrous sponges with LOI value of 26.2%,an average damage length of 75 mm and no melt drops during the combustion process,showing excellent flame retardant properties.In addition,its thermal conductivity is as low as 24.68 m W m^-1 K^-1 and could still maintain 27.31 m W m^-1 K^-1 after 100 times of compression cycles.Under different RH of 40-90%,its thermal conductivity maintains 25.24-26.09 m W m^-1 K^-1,indicating stable and efficient warmth retention performance.Finally,the comprehensive performances of PPSU/PU/PAI nano/microfibrous sponges and three commercial warmth retention felts were compared.The consequences show that the three commercial warmth retention felts all generate melt drops during the combustion process,and the damage lengths are much greater than the fibrous sponge.Also,the fibrous sponges exhibit better warmth retention performance than that of PET felts(30.91 m W m^-1 K^-1),but its weight is only 42.8%of PET felts,showing light-weight warmth retention properties.The successful preparation of the fibrous sponges provides novel opinions and prospects for the fabrication of new flame-retardant warmth retention materials,and the sponges have spacious application foreground and practical significance.