Preparation Of Cellulose Nanofiber-based Core-shell Aerogel Fibers From Coaxial Wet Spinning

Due to their advantageously low density,high porosity and large specific surface area,aerogels have been widely applied in the fields of thermal insulation,water remediation,air purification and drug delivery.Aerogels are commonly in powders or in monoliths,and till now,there are only few examples on aerogel fibers,mainly because their preparation is complicated and their mechanical property of the resulting fibers is poor.Therefore,it is of great significance to develop aerogel fibers with excellent mechanical properties using simple preparation methods.In this study,cellulose nanofiber(CNF)-based aerogel fibers have been fabricated from coaxial wet spinning,and the properties of the resulting aerogel fibers have been investigated.The main results are listed below.Pure CNF aerogel fibers have been prepared from extrusion templating.Results showed that the average pore size and the mechanical properties of the pure CNF aerogel fibers decreased with the decrease of freezing temperature.The mechanical properties of the pure CNF aerogel fibers were too low,with the tensile stress at break at 0.15 MPa and the elongation at break at 3.91%,and the low mechanical properties limit the practical application of the fibers.Cellulose acetate/polyacrylic acid/cellulose nanofibers(CA/PAA/CNF)aerogel fibers with core-shell structure have been successfully fabricated from coaxial wet spinning.The introduction of the shell layer significantly enhanced the mechanical properties of aerogel fibers,and the CA/PAA/CNF fibers have a tensile stress at break at 4.3 MPa and the elongation at break at 7.8%.Due to the leakage of CNF suspension,however,CA/PAA/CNF aerogel fibers were not full of CNF aerogels.In order to fill the aerogel fibers completely,step solidification coaxial wet spinning was developed to fabricate aerogel fibers with core-shell structure.The core layer of the resulting aerogel fiber was full of CNF aerogels using the method.The mechanical properties of the resulting fibers were mainly dependent on the height between the spinning port and the coagulation bath and the extrusion speed of the core layer spinning solution.Three types of aerogel fibers with core-shell structures,named as CA/PAA/CNF,PAN/CNF and TPU/CNF,were successfully prepared by using cellulose acetate,polyacrylonitrile and polyurethane as shell.CA/PAA/CNF had the maximum tensile stress at break at 5.83 MPa,while TPU/CNF had the maximum elongation at breaking at 256.3%.Moreover,CA/PAA/CNF had the best thermal insulating property,and could be used for many times.Graphene/CNF conductive aerogel fibers with core-shell structure have been developed from step solidification coaxial wet spinning using the mixture of CNF suspension and graphene dispersion as the core spinning solution.The addition of graphene endowed the aerogel fiber to have electrical conductivity,although it also reduced the porosity of the core layer.increasing graphene content,the electrical conductivity of the fibers was enhanced,but their tensile properties decreased slightly.When the mass ratio of graphene to CNF was fixed at 4:1,the conductivity of the resulting fibers was at 98.29 S/m.The fibers with Graphene/CNF and graphene aerogels as the core layers had relatively low response voltage at 3 V and 2.5 V,respectively,and faster response time at 4 s and 3 s,respectively.Both the fibers could be electrically heated to a high temperature at the low voltage in a short time.The electrical heating property could be repeated for many times.