Research On In Situ Fibrillated PTFE-reinforced Composites And Its Microcellular Injection Molding

In this dissertation,in situ fibrillated polytetrafluoroethylene（PTFE）-reinforced composites via twin-screw extruder and its microcellular injection molding were studied.Because polymer blending is a cost-effective,easy-to-use,and environmentally-friendly as well as highly efficient polymer modification technology.Compared with polymer grafting,cross-linking,copolymerization and other chemical modification,melt blending is economical,mass-produced due to its easy to control and high output as well as excellent characteristics and shape of the product.In the melt-blend system,the variability of the disperse phase shows a variety of morphologies,and the morphology and the size of the disperse phase can significantly affect the performance of the blend.Therefore,the properties of polymer blends can be changed by varying the morphology and types of the dispersed phase.In-situ fibrillation technology means that the reinforcing phase in the blend is produced during the melt blending not before melt blending,which means the deformation and orientation of the dispersed phase in the continuous phase were caused by stretching,shearing,etc.due to the external force.Micron-scale or even nano-scale fibrils were formed in the original place to reinforce the matrix material.The use of in-situ fibrillation technology in microcellular injection molding can greatly improve the microcellular foaming performance of the polymer,thereby broadening the application of in situ fibrillation enhancement technology.For the microcellular injection molding technology,first,the gas（N₂ or CO₂）in a supercritical state is mixed with the polymer melt,then form extremely small pores with homogeneous distribution and uniform size inside the polymer melt;moreover,with different polymer matrix and applications,the size of the formed pores is usually 0.1-10μm.Polytetrafluoroethylene（PTFE）in situ reinforced poly（lactic acid）（PLA）was studied.PLA/PTFE composites containing different amounts of PTFE were prepared by melt blending via twin-screw extruder.The average elongation-at-break of the composite increased by 72%compared to pure PLA.The PTFE elongated into fibrils during blending and formed a physical network of entanglements in the melt and had a significant nucleation effect on polymer crystals and greatly increased the crystallinity of PLA.Moreover,PTFE dramatically enhanced the melt viscosity and foamability of PLA.1 wt%PTFE had the most notable heterogeneous nucleation effect on foamed pores,with the pore size decreasing from81.46μm for neat PLA to 25.2μm,and the pore density increasing from 1.34×10⁸ cells/cm³to 2.53×10⁹ cells/cm³.The effect of PTFE on amorphous polymer TPU was studied.A drastic transformation of oval or rounded PTFE particles into highly stretched,sub-micron fibrils in the TPU matrix via melt compounding was revealed.Substantial improvements in the dynamic mechanical and tensile properties of the TPU/PTFE composites was observed.The entangled PTFE fibers significantly changed the viscoelasticity of the TPU matrix.SEM,and micro-computed tomography（Micro-CT）confirmed that the presence of PTFE fibrils dramatically improved the foamed structure,resulting in a two-order-of-magnitude increase in pore density in comparison with neat TPU.Moreover,the fibrillar PTFE dramatically enhanced the hydrophobicity and decreased the coefficient of friction of the TPU/PTFE composites.Finally,the effect of in situ fibrillated PTFE on PLA/TPU/PTFE ternary composite system was studied.The immiscibility between PLA,TPU,and PTFE were confirmed by SEM.The fibrillated PTFE in the PLA domain enhanced the distribution of dispersed spherical TPU particles.Significant enhancements in the crystallization behavior of PLA/TPU/PTFE were observed.The ductility increased 20 times with a brittle–ductile transition,and the viscoelastic properties of PLA matrix were dramatically changed by TPU and fibrillated PTFE.The local surface nanomechanical property variation of the PLA/TPU blend was examined by peak force quantitative nanomechanics（PQNM）based on atomic force microscopy（AFM）.The average elastic modulus and deformation at the PLA/TPU interface varied from 28.2±1.1 GPa for PLA to 2.3±0.2 GPa for TPU,and from 7.3±0.5nm for PLA to 61.3±3.8 nm for TPU,respectively.Furthermore,the thickness of the interface between PLA and TPU was around 208.3±14.8 nm.MIM experiments confirmed that the presence of PTFE fibrils dramatically improved the foamability behavior of PLA.