Study On Crystallization And Phase Separation Behaviors And Dielectric Properties Of PVDF/PMMA/CNT Composites

Among numerous polymer modification methods,blending modification is one of the most simple and practical way to obtain new polymer materials.As one kind of polymer blends,the crystalline/amorphous miscible polymer blend is miscible in the melt state while phase-separated in the solid state.Since it has abundant phase behavior,it is of great significance to study the crystallization and phase separation behaviors of this blend.The crystallization and phase separation behaviors of the blend during its solidification process can be influenced by many factors,including composition,temperature,the molecular weights of each component and the interactions between the components.Therefore,diverse phase morphologies and microstructures of polymer blends by regulating the above factors can be obtained.Poly(vinylidene fluoride)/poly(methyl methacrylate)(PVDF/PMMA)binary blend belongs to crystalline/amorphous miscible blend,which has the characteristics of homogeneous melt,phase separation after cooling and solidification.In addition,the interaction of the blend components and the molecular chain mobility have an important effect on the crystallization and phase separation behavior of the blend.Introducing nanoparticles into such crystalline/amorphous miscible blends also influences the phase separation and/or crystallization processes because of their possible nucleation effect and the selective adsorption with one component.Carbon nanotubes(CNTs)are typical inorganic nanometer-fillers with large length-diameter ratio and specific surface area,so they can show a heterogeneous nucleation effect on the crystallization behavior of polymers.Therefore,it can be expected that adding CNTs into the PVDF/PMMA binary blend can promote the crystallization and phase separation of this blend.The effect of CNTs on the crystallization and phase separation of polymer blends mainly depends on CNTs selectively adsorbing PMMA components.The selective adsorption can be affected by the mobility of PMMA chains and the surface structure of CNTs,which will more or less influence the CNTs' role in the phase behavior of binary blend system.Based on this,this thesis systematically studies how PMMA with different molecular weights and CNTs influence the crystallization and phase separation of PVDF/PMMA blend,and investigates the inductive effect of CNTs.Furthermore,the relationship between the microstructure of blend composites and the dielectric properties were studied.The main conclusions are listed as follows:(1)PVDF/PMMA binary blends and PVDF/PMMA/CNT nanocomposites were prepared through melt-blending process.The effects of PMMA of different molecular weights and CNTs on the crystallization and phase separation of blends and their composites were investigated.CNTs are evenly distributed in composites and show no significant influence on the molecular chain's mobility.The molecular weight of PMMA(PMMA-L)shows a positive effect on the storage modulus,viscosity,and glass transition temperature of composites.As the molecular weight of PMMA increases,the movement of the molecular chain of the system is hindered.Besides,the crystallization study shows that as molecular weight of PMMA increases,the crystallinity of blends and their composites gradually declines,while CNTs play a more apparent role in promoting the crystallization of PVDF in PVDF/PMMA-S blends and their composites.Moreover,the concentration fluctuations are more likely to occur in PVDF/PMMA-S blends,but the presence of CNTs under shear conditions inhibits the movement of molecular chains in composites and weakens the fluctuation of composites.Thus,it can be concluded that CNTs induce the compatibility of PVDF/PMMA blends.Moreover,based on observation of the crystallization after melt annealing,the concentration fluctuation of the composite materials in the molten state has a significant effect on the subsequent cooling crystallization process,and strong fluctuations in concentration facilitate the crystallization of PVDF.(2)To study the effects of the CNT surface on the crystallization and phase separation of composites,CNTs were grafted with maleic anhydride polar functional groups(CNTs-f).Microstructure characterization shows that CNTs and CNTs-f are evenly dispersed in the composites.The modified CNTs-f have stronger interfacial interactions with the blended system.The crystallization study further reveals that the unmodified CNTs exhibit anisotropic nucleation to promote PVDF crystallization,while the modified CNTs-f,which selectively absorb a large amount of PMMA on the surface,exhibit an inhibited nucleation effect.The concentration fluctuation results show that unmodified CNTs increase the density of molecular chain entanglement and inhibit the concentration fluctuation of the system,while modified CNTs-f decrease the internal entanglement density and enhance the molecular chain mobility,thus promoting the concentration fluctuations of composites in molten state.(3)To investigate how CNTs and different PMMA contents influence the dielectric properties of composites,a small amount of PMMA was introduced into PVDF/CNTs composites.The addition of PMMA greatly improves the dispersion of CNTs in composites.According to crystallization study,the heterogeneous nucleation of CNTs promotes the crystallization of PVDF,while PMMA hinders the crystallization of PVDF,and two crystalline structures of different platelet thicknesses are formed in the system.The crystal form of PVDF is not changed by CNTs or PMMA.The study of dielectric properties shows that the improvement of CNTs' dispersibility contributes to the interfacial polarization of composites.Meanwhile,the PMMA adsorbed on the surface of CNTs avoids the leakage of the CNTs.The synergistic effect contributes to the increment of the dielectric constant and the decline of the dielectric loss.