Novel electrospun fibres of amphiphilic organic-inorganic graft copolymers of poly(acrylonitrile)-graftpoly( dimethylsiloxane) for silicone composite reinforcement

[摘要] ENGLISH ABSTRACT: Novel silicone nanocomposites were prepared using poly(acrylonitrile) (PAN) based reinforcingfibres as well as multi-walled carbon nanotubes (MWCNTs). Compatibility of the fibre fillers withthe silicone matrix required the synthesis of novel amphiphilic, organic–inorganic graftcopolymers of PAN and poly(dimethylsiloxane) (PAN-g-PDMS). These fibre precursor materialswere synthesised via the 'grafting through technique using conventional free radicalcopolymerisation. The PDMS macromonomer content in the feed was varied from 5 wt% to 25wt% and the molecular weights of the macromonomer were 1000 g.mol-1 and 5000 g.mol-1. Thesolvent medium of the precipitation reaction was optimised at a volume ratio of 98% benzene to2% dimethylformamide (DMF). Successful incorporation of PDMS yielded graft copolymer blendmaterials of PAN-g-PDMS, blended with PAN homopolymer and unreacted PDMSmacromonomer. A gradient elution profile was developed to track the successful removal of thePDMS macromonomer via hexane extraction. The gradient profile showed that as the PDMScontent in the feed increased, the number of graft molecules in the blend increased relative tothe number of PAN homopolymer molecules. The crystallisability of the PAN segments wasshown to decrease as the PDMS content increased. The synthesised polymer was used asprecursor material for the electrospinning of fibre fillers. The electrospinning of the precursormaterial was successfully achieved using 100% DMF as electrospinning solution medium. Theamphiphilic nature of the precursor material in DMF resulted in self-assembled aggregatestructures in the electrospinning solution. An increasing PDMS content was shown to affect theaggregation of the precursor material, and resulted in an increase in the solution viscosity. The'gel-like solutions limited the achievable fibre morphological control when altering conventionalelectrospinning parameters such as voltage, tip-to-collector distance, and solutionconcentrations. The rapid evaporation and stretching of the solution during electrospinning,combined with the phase segregated amphiphilic molecules in solution and the crystallisation ofthe PAN segments resulted in (non-equilibrium morphology) fully porous fibres. The crystallinitywas shown to decrease after electrospinning of the fibre precursor materials. Successfulincorporation of surface oxidised MWCNTs into the electrospun fibres was achieved. Thecontent of nanotubes was varied from 2 wt% to 32 wt%. The MWCNTs reduced the mean fibrediameters by acting as cross-linkers between the PAN segments and increasing the solutionconductivity. The nanotubes dispersed well throughout the porous structure of the fibres andaligned in the direction of the fibre axis. Fabrication of silicone composites containing nonwovenand aligned fibre mats (with 8 wt% MWCNTs in the fibres, and without) was successfully achieved. The compatibilisation of the PDMS surface segregated domains allowed excellentdispersion and interaction of the PAN based fibre fillers with the silicone matrix. Mechanicalanalysis showed improved properties as the PDMS content in the fibre increased. The highestPDMS content fibres did, however, exhibit decreased properties. This was ascribed to increasedPDMS (soft and weak) content, decreased crystallinity and increased fibre diameter (lowerinterfacial area). Dramatic improvements in strength, stiffness, strain and toughness wereachieved. The most significant result was an increase in strain of 470%. The mechanical resultscorrelated with results of SEM analysis of the fracture surfaces. The dramatic improvements inproperties were a result of the fibre strength and ductility, as well as the mechanism ofcomposite failure.

[发布机构] Stellenbosch University