Cellulose Nanocrystal Modified Polyacrylonitrile Fibers

At the present,polyacrylonitrile(PAN)fiber is an important precursor material for carbon fiber(CF)in domain,although there is a huge gap between the theoretical strength and the highest commercial one of PAN based CF.This gap was mainly attributed to the structural defects such as heterogenous radial structure of CF.The modification and structure controlling of PAN precursor were extremely important since the radial structure of CF was inherent from the precursor and gradually evolved during the manufacturing process.Nowadays,some biologically renewable nanomaterials with high mechanical properties,especially those with excellent physical and chemical properties such as rodlike particle cellulose nanocrystal(CNC),have been utilized as nanoreinforcements and templates for structural controlling.The addition of CNC could significantly influence the crystallization behavior of PAN,which is a typical semi-crystalline polymer material.Although PAN/CNC composites have been reported,the transformation mechanism,influence of CNC on the crystallization structure during the thermal stabilization and the radial structure of PAN fibers have not been studied in detail.In this thesis,a series of PAN/CNC complex films and fibers with different CNC contents were prepared.Firstly,PAN/CNC/DMSO complex solutions with different concentration of CNC were prepared and the rheological properties were measured by rheometer.The results showed that the viscosity of solutions increased with higher concentration of CNC.On the other hand,the initial temperature at which the mutation of viscosity decreased with the increase of CNC content.In order to investigate the crystallization behavior of CNC/PAN,a series film sample were prepared.It was proved that CNCs could be intercalated into the space between PAN lattice with the newly formation hydrogen bonding.Simultaneous temperature-dependent DSC and WAXD measurements were applied to trace the crystalline structural evolution of PAN/CNCs complex during heat treatment.Temperature dependence FTIR spectra was also utilized to investigate the chemical structural evolution.The results indicated that the secondary crystallization of PAN could be delayed by the addition of CNCs.Thermal stabilization of PAN/CNCs complex would occur earlier and more gently compared to neat PAN.The destruction of the crystal unit cells was later than the collapse of the PAN crystallites,which was attributed to the stronger inter-molecular force between CNCs and PAN."Hydrogen-bonding cage"conception was proposed and a reasonable structural evolution model was concluded to simulate the phase transition of PAN/CNCs complex during thermal stabilization.In addition,in order to investigate the properties of complex fiber,a series of PAN/CNC fibers with different drawing ratios with different concentration of CNC were prepared by wet spinning.The results showed that the addition of CNC could control and modify the aggregation structure of PAN fiber.Although the concentration of CNC showed no obvious effect on the crystallite size of PAN,the heterogenous radial structure was improved with the addition of CNC.The results of SEM to characterize the morphology also showed the same tendency that the heterogenous structure distribution gradually disappeared.Besides,the orientation of PAN microcrystals increased first and then decreased with the increasing of CNC content.The nematic characteristics and high crystallinity of CNC would lead to the optimal alignment of the PAN crystalline regions along the direction of the drawing force.Hence,the orientation degree of crystalline region was increased.However,superabundant CNC would retard the orientation of the crystalline regions.Finally,the orientation factor of molecular chains was also studied.The varied tendency was similar with that of crystalline structure.This phenomenon was attributed to the optimization of CNC lubrication and the synergy as a physical entanglement point.However,cellulose is also the precursor of carbon fiber,and the graphite crystallite obtained by carbonization could be oriented better.Therefore,the addition of CNC can not only control the components of PAN precursor to modify its aggregated structure,but also play the important role of template regulation in carbonization process.