Ionic Liquid Grafted Poly(vinylidene Fluoride) And Their Investigations Of Structures And Properties

As an important functional polymer, poly(vinylidene fluoride)(PVDF) has been thrust into both academic and industrial fields. Recent years have seen a surge of unprecedented progresses made by room temperature ionic liquids(RTILs), known as functional modifying agents, for modification of common polymers. Previous studies have demonstrated that PVDF is thermodynamically miscible with IL and that physically mixed PVDF/IL blends exhibite excellent mechanical properties, high optical transparency, high conductivity and high polar phase content. When it comes to dielectric materials, however, an important problem exsits in such a miscible PVDF/IL system, that is, ion movement of IL in electric field, which leads to considerable current leakage and dielectric loss. In order to figure out this problem, limidazolium-based ILs with unsaturated double bonds on the cations were used to be grafting PVDF(forming PVDF-g-IL composite) via solid electron beam co-irradiation grafting method in this study.The structures and physical properties of PVDF-based composites were adjusted.The main contents and conclusions are as follows:(1) Study on the solid irradiation grafting of PVDF/ILIL grafted PVDF composites(i.e. PVDF-g-IL composites) have been prepared by electron beam co-irradiation grafting method, wherein cations of IL are grafting onto PVDF chains in amorphous regions while anions of IL are still free. PVDF chains were successfully grafted by IL confirmed by 1H NMR result, and the IL sequences were short, obtained by IL grafting yield result. The SEM, XRD and SAXS measurements have demonstrated that the morphologies, crystal phase and crystal long period(L) of PVDF samples are not significantly influenced by electron beam irradiation. Results of electrical and dielectric properties have proved that the immobilization of IL’s cations can decrease the conductivity, dielectric permittivity as well as dielectric loss of PVDF samples. Mechanical property results have shown that absorbed dose has effects on the elongation at break, yield strength and Yong’s modulus of PVDF samples.(2) Study on the nanostructured PVDF/IL composites with organic nanodomainsa. Nanostructured PVDF/IL composites have been fabricated by melting the PVDF-g-IL composites. The TEM, SEM and EMI measurements have demonstrated that organic nanodomains formed by PVDF-g-IL chains, with 20-30 nm in size, are homogeneously dispersed within the PVDF matrix in the nanostructured PVDF/IL(100/8) composites. A mechanism is proposed: cations of IL were locally grafting onto the PVDF amorphous chains during electron beam irradiation, forming block-like grafting copolymers of crystalline PVDF-(amorphous PVDF-g-IL)-crystalline PVDF. The PVDF-g-IL chains were immiscible with the ungrafted PVDF chains and they were microphase-separated in the melt, forming PVDF-g-IL nanodomains. The nanostructured PVDF/IL composites were thus obtained. Furthermore, it is found that PVDF-g-IL nanodomains have a nucleation effect on the PVDF crystallization studied by the DSC and POM investigations. The nanostructured PVDF/IL composites show non-polar α phase and increased crystal long period(L), compared with neat PVDF. Additionally, the nanostructured PVDF/IL composites exhibite high conductivity, high dielectric permittivity, low dielectric loss and excellent mechanical properties.b. Microphase separation of PVDF-g-IL chains has been investigated by synchrotron and DSC techniques, respectively. It is found that 1) microphase separation of PVDF-g-IL chains in the block-like grafting copolymers of crystalline PVDF-(amorphous PVDF-g-IL)-crystalline PVDF occurs when the PVDF crystals are totally melted; 2) the formation of PVDF-g-IL nanodomains undergoes a nucleation and growth process;(3) the formed PVDF-g-IL nanodomains are stable in the measured T scope.c. The structures and properties of nanostructured PVDF/IL composites have been adjusted by IL grafting yield. It is found that the PVDF-g-IL nanodomains sizes are increased and that the nanodomains are gradually connected with each other with increasing IL grafting yield. The decreased surface resistance(Rs), increased AC conductivity(σ), increased both dielectric permittivity and loss have been also attributed to the increased IL grafting yield.(3) Study on the PVDF nanocomposites with simultaneous organic conductive nanodomains and inorganic conductive nanoparticlesOn the basis of the studies above, PVDF nanocomposites with simultaneous organic conductive PVDF-g-IL nanodomaisn and inorganic conductive carbon black(CB) nanoparticles have been fabricated. The TEM and SEM measurements have shown that PVDF-g-IL nanodomains coexist with CB nanoparticles and that CB nanoparticles are adhered with PVDF-g-IL nanodomains, forming nanodomains @ CB nanoparticles structures. The DSC, SAXS and XRD results have proved that PVDF nanocomposites have higher crystallization temperature(Tc), higher melting point(Tm) and increased crystal long period(L) relative to neat PVDF, and that show dominantly non-polar α phases. Compared to physically mixed binary PVDF/IL blend and ternary PVDF/IL-CB nanocomposites, PVDF nanocomposites with double conductive nanodomains show a lowest conductivity at the same CB content. In addition, such PVDF nanocomosites exhibit high dielectric permittivity and low dielectric loss, which can be ascribed to the following reasons: 1) the use of the IL can help disperse CB nanoparticles and thus reduces the agglomeration-induced loss; 2) the IL molecules are confined within PVDF-g-IL nanodomains, thus suppressing their ion-movement-induced loss; 3) the structure of nanodomains @ CB nanoparticle provides good PVDF-CB interfaces.