Preparation and Properties of Reinforcements/ Polyacrylonitrile Composite Nanofibers
|Keywords||polyacrylonitrile multi-walled carbon nanotubes graphene electrospinning mechanical properties|
Polyacrylonitrile (PAN) fiber is one of the commerially important polymeric fibers. Nanofiber is one of the most versatile materials today. It is widely used in protective clothings, photosensors, biomedicines, solar-sails and so on. Electrospinning is an attractive approach to the fabrication of fibrous materials for variety of applications, including strength agents, fuel-cell catalysts, adsorbing agents, storage equipments and so on. Electrospun mats from ultrafine polymer fibres are drawing a great attention because of their unique properties such as high surface-to-volume ratio, high porosity and diameters in the nano-scale. Nowdays, modified PAN fibers with reinforcements have attracted considerable attention due to their unique properties and potential applications. Multi-walled carbon nanotubes (MWCNTs) are excellent reinforcing nanomaterials to improve the properties of various polymer materials. However, MWCNTs have high surface energy and are easy to form aggregates, which lead to the poor dispersion of reinforcements in polymer matrix. Reinforcements should have good interfacial interaction with polymer matrix as, which can make stress transferred to the reinforcements. How to disperse reinforcements uniformly in polymer matrix and increase the interfacial interaction between reinforcements and polymer matrix are the problems urgent to be solved.In this thesis, MWCNTs/PAN composite nanofibers were produced by electrospinning. The purpose of reducing the cast and without affacting the reinforcing effects were to adopt Graphene Nanosheets (GNS) to be reinforcement material. Structures and properties of nanofibers had been studyed in three following aspects:(1) Oxidation of MWCNTs by using Fenton reagents to generate an amount of hydroxyl groups and carbonyl groups on the surface of MWCNTs. PAN was covalently grafted onto the surface of MWCNTs through esterication of polymer with hydroxyl group on the surface of modified MWCNTs. The PAN-grafted MWCNTs was dispersed in PAN solution, which was electrospun to form a nanofiber sheet. The grafted CNTs exhibited good dispersion in the nanofiber sheet and well aligned along the fiber axis. At low loading of MWCNTs, the nanofiber sheet exhibited much higher tensile strength and modulus compared with the nanofiber membrane based on neat PAN, with only slight decrease in elongation at break.(2) The functionalized multiwalled carbon nanotubes (f-MWCNTs) were obtained by Friedel-Crafts acylation, which introduced aromatic amine groups onto the sidewall. The composite solutions containing f-MWCNTs and polyacrylonitrile (PAN) were then prepared by in-situ or ex-situ solution polymerization. The resulting solutions were electrospun into composite nanofibers. In the in-situ polymerization, morphological observation revealed that f-MWCNTs was uniformly dispersed along the axes of the nanofibers and increased interfacial adhesion between f-MWCNTs and PAN. Furthermore, two kinds of f-MWCNTs/PAN composite nanofibers had a higher degree of crystallization and a larger crystal size than PAN nanofibers had, so the specific tensile strengths and modulus of the composite nanofibers were enhanced. And the thermal stability of f-MWCNTs/PAN from in-situ method was higher than that of ex-situ system. And f-MWCNTs/PAN of in-situ system provided better mechanical properties than that of ex-situ system.(3) Graphene was prepared by Hummers. After that, PAN copolymer was covalently grafted on the graphene sheets by in situ living free-radical polymerization. Homogeneous dispersion of GNS in PAN matrix was successfully achieved. The resulting polymers were electrospun into composite nanofibers. The nanofiber mats exhibited much better mechanical properties.