Study On The Regulation Of Structure And Performance And Toughening Mechanisms Of Carbon Nanofiber-toughened Immiscible Blends

Polymer blending has the advantages of simple processing and short preparation cycle,and the controllable structure of the blend and the complementarity of properties between the components can be achieved,so it has become one of the main means to prepare new polymer materials.However,due to the fact that most of the polymer blends are immiscible and the interfacial bonding strength is very weak,the stress concentration of the dispersed phase can not induce the plastic deformation of the matrix under the external loads and the cracks are easy to propagate rapidly along the interface between the two phases,resulting in the poor toughness of blends,which greatly limits the application of blends in automotive,construction,etc.In addition,with the expansion of application requirements,it is required that the blends exhibit both high fracture toughness and certain functionality.Therefore,on the basis of toughening blends,it is of great significance to prepare polymer blends with structure-function integration.Compared with other toughening methods,the method of incorporating nanoparticles into blends and tailoring the selective distribution of nanoparticles at the blend interface can not only improve the fracture toughness of the blends,but also endow the blends wih new functional properties,therefore it has a great development prospect.However,at present,the research on nanoparticles toughening blends is mostly focused on the effect of nanoparticles on the condensed state of blends,and there is little in-depth research on the coupling relationship between nanoparticles and dispersed phase synergistically toughening blends.Furthermore,the dependence of the efficiency of nanoparticle toughened blends on the microstructure of the matrix is also rarely reported.In this thesis,carbon nanofibers(CNFs)were introduced into different polymer blends (taking into account the strength of the interfacial interaction between the blend components,the dispersed phase morphology of blends,the crystallization behavior of blends,etc.).The coupling mechanism of CNFs and dispersed phase synergistically toughened blends was systematically studied.And the effect of matrix microstructure on the toughening efficiency of CNFs was investigated by tailoring the molecular chain movement ability and crystalline structure of the matrix.Additionally,the extensional flow field was incorporated to induce the formation of multi-oriented microstructures in the composites,and the mechanisms of CNFs improving the mechanical properties and thermal conductivity of the blend composites with multi-oriented microstructures were explored.A series of studies were carried out on the effect of the synergistic coupling relationship between CNFs and blend components on the fracture toughness of blends and the relationship between the structure and performance of blends.The details are as follows:(1)In chapter 2,CNFs were incorporated into polypropylene/ethylene-propylene-diene terpolymer(PP/EPDM)blends by a two-step method.The effects of dispersion and distribution of CNFs on the processing fluidity,the crystallization structure of PP matrix,the phase morphology and the fracture behaviors of the blend composites were investigated.The results show that the incorporation of a small quantity of CNFs(0.2 wt%)has little effect on the processing fluidity of the blends,but can greatly improve the fracture toughness of the blends and induce the brittle-ductile transition behavior of the blend composites at lower EPDM content.The toughening mechanisms are mainly attributed to the decrease of the diameter of EPDM particles and the matrix ligament thickness during the migration of CNFs from EPDM phase to PP matrix,which promotes the superposition of stress field around EPDM particles and this is beneficial to the intense plastic deformation of matrix and dissipate a lot of energy.The heterogeneous nucleation of CNFs reduces the spherulite size of PP matrix,and this increases the crack propagation path to promote the energy dissipation.Furthermore,the straight CNFs exhibit oriented dispersion along the flow direction of melt,which inhibits the rapid propagation of cracks perpendicular to the orientation direction and promotes the improvement of the fracture toughness of the blend composites to some extent.This work confirms that the introduction of a small quantity of CNFs can greatly improve the fracture toughness of the blends without sacrificing the processing fluidity,strength and rigidity of the blends,and it is of great significance for the large-scale preparation of super-tough PP blend composites.(2)Efficiently designing the interfacial interaction between nanoparticles and components of the blends to achieve the controllable microstructure evolution and resultant performances is still greatly challenging.In chapter 3,CNFs were modified by different strategies and the CNFs before and after being modified were introduced into poly(_L-lactic acid)/thermoplastic polyurethane(PLLA/TPU)blend to explore the effect of the difference of interfacial interaction between CNFs and blend components on the morphological evolution and stress transfer.It is found that the pristine CNFs exhibit little toughening effect on the blend due to the poor interfacial interaction between pristine CNFs and the components of the blends.Hydrogen bonding interaction is formed between the hydroxylated CNFs and the components of the blends.CNFs play the role in stress transfer and induce the morphological transition of blend composites from the sea-island structure to the co-continuous structure,which facilitates the superposition of the stress field around the TPU phase,resulting in the improvement of the fracture toughness of the blend to a certain extent.While the epoxidized CNFs simultaneously exhibit the covalent bonding interactions with the two components of the blend.CNFs have the dual functions of compatibilization and stress transfer,which promotes the reduction of the diameters of the TPU particles and the matrix ligament thickness,and increases the degree of superposition of the stress field around the TPU phase,resulting in the blend with high fracture toughness.This work not only reveals the influence mechanisms of the interfacial interaction between CNFs and the components of the blend on the microstructure and macroscopic properties of the PLLA/TPU blend,but also provides an effective way for the preparation of the supertoughened PLLA-based blend composites.(3)In generally,for nanoparticles-toughened blend composites,the nanoparticles exhibit toughening effect only when the content of toughening agent is high.In view of the problem of low toughening efficiency of nanoparticles,in chapter 4,CNFs were incorporated into the ethylene-octene copolymer(POE)-toughened PP blends and the annealing treatment was introduced to tailor the mobility of the chain segments of PP matrix.The effect of the change of the mobility of the matrix molecular segments on the fracture mode of the blend composites and the toughening efficiency of CNFs was systematically studied.The results show that for the unannealed samples,the fracture mode mainly involves the initiation and propagation of crazes,and CNFs play a role in hindering the propagation of crazes.For the annealed samples,due to the fact that the annealing treatment enhances the mobility of the molecular segments in the amorphous region of the matrix,the stress that inducing yielding in the matrix is smaller than the stress that inducing crazing,which promotes the change of the fracture mechanism of composites from crazing fracture to shear yielding-crazing fracture.Here,the role of CNFs is firstly to promote the stress transfer between the matrix and the dispersed phase,and secondly to increase the superposition degree of the stress field around the dispersed phase,and finally to inhibit the initiation and propagation of secondary cracks.Hence,for the annealed blend composites,CNFs exhibit the significant toughening effect on the blend even at low POE content(10 wt%).(4)The large accumulation of polymer materials has caused a negligible negative impact on the living environment of human beings.Therefore,it is of great significance to prepare supertoughened and biodegradable PLLA-based blend composites.To date,the role of crystalline structures in toughening modification of the nanoparticles incorporated PLLA based composites is very unclear.In chapter 5,CNFs incorporated PLLA/poly(butylene adipate-co-butylene terephthalate)(PBAT)blend composites were investigated.The crystallinity of the PLLA matrix were precisely tailored through annealing treatment.Interestingly,CNFs and PBAT particles self-assemble into the candied haws-shaped structure during processing.In addition,the annealing treatment facilitates the epitaxial crystallization of PLLA on the surface of CNFs that are not covered by PBAT due to the excellent heterogeneous nucleation effect of CNFs.The results show that the toughening efficiency of CNFs in the blend composites exhibits the obvious dependence on the crystallinity of the matrix.The toughening mechanisms are mainly connected with the greatly strengthened interfacial interaction between PLLA matrix and PBAT particles by CNFs through CNFs bridging effect and CNFs inducing PLLA crystallization,facilitating the transfer of stress under the load condition,especially at high PLLA crystallinity.Furthermore,it is found that the outstanding photothermal conversion effect of CNFs endow the blend composites with mechanical-damage-healing ability.This work provides a new way to prepare supertoughened,fully biodegradable PLLA-based blend composites.(5)In charter 6,CNFs were introduced into PLLA/PBAT blends,and combined the effect of extensional flow field to induce the blend composites to form multi-oriented microstructures.Explored the effect of the introduction of CNFs and extensional flow field on the molecular chain conformational transformation and crystalline morphology evolution of the blend composites,and expounded the mechanisms for the enhancement of thermal conductivity,strength and toughness of blend composites with multi-oriented microstructures.The main mechanism for the improved thermal conductivity is that the multi-oriented structure promotes the formation of CNF thermal conductive network in the composites and reduces the interfacial thermal resistance between CNFs and matrix.The strengthening mechanism is attributed to the orientation of both PLLA molecular chains and CNFs in the stretching direction restricting the movement of PLLA molecular segments around CNFs and the toughening mechanism is due to the transformation of PLLA molecular chains from low energy gt conformers to high energy gg conformers induced by extensional flow field,which lead to the decrease of the critical onset stress for shear yielding of PLLA.More interestingly,after the extrusion-stretched samples being annealed,the oriented PLLA molecular chains form the oriented crystal structures such as extended-chain lamellae,common“Shish-kebabs”and hybrid“Shish-kebabs”,which further enhance the thermal conductivity and heat resistance of the samples.This work not only reveals the mechanism of morphological evolution and conformational transformation of matrix molecular chain induced by CNFs and extensional flow field,but also provides a new way for the preparation of PLLA matrix composites with high thermal conductivity,high heat resistance and excellent mechanical properties.