Investigations On The Modification Of Poly(Vinylidene Fluoride) By Ionic Liquid And Ionic Liquid Coated Carbon Nanotubes

Poly(vinylidene fluoride)(PVDF) is a crystalline polymer having a low glass transition temperature (-35℃). It is superior in heat resistance, chemical resistance, mechanical properties, abrasion resistance, flame retardancy and weatherability. PVDF exhibits specific electrical properties because of the high polar CF2bonds in its molecular structure. Significant attentions have been paid on the modification of PVDF since1980s. In this work, ionic liquids (IL) and ionic liquid coated carbon nanotubes (IL-CNTs) have been used to modify PVDF by melt blending for the first time. The paper can be divided into two parts:(1) Structure and Properties of PVDF/IL blendsA room temperature ionic liquid,1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6], has been used to modify poly(vinylidene fluoride)(PVDF) for the first time. The miscibility, crystal structures, and physical properties of PVDF/IL blends have been investigated systematically. It was found that IL is fully miscible with PVDF with the interaction parameter (χ12) of-2.65calculated by the revised Nishi-Wang equation due to the drastic equilibrium melting temperature depression. Moreover, the incorporation of IL into the PVDF leads to drastically increased γ crystal forms of PVDF by the interaction between the>CF2and cationic ions. Dynamic mechanical analysis (DMA) and small angle X-ray scattering (SAXS) results indicate that the IL molecules are inserted into the gallery of PVDF lamellae. Small amount of IL induces the appearance of the crystal-amorphous interface relaxation of PVDF, originating from the enrichment of IL distribution near the crystal-amorphous interface. The obtained PVDF/IL blends exhibit excellent mechanical performance with significantly increased ductility and good optical transmittance. In addition, the incorporation of IL into PVDF enhances the electrical conductivity of PVDF films greatly. Therefore, novel PVDF films with high transparency, excellent antistatic property, and high polar crystal form fraction were successfully achieved.(2) Structure and Properties of PVDF/IL-CNTs nanocompositesPristine CNTs accelerate the crystallization of PVDF as an efficient nucleation agent, while the formed crystals are mainly nonpolar α crystal form with few polar β crystals. The incorporation of only ionic liquid results in the depression of PVDF melt crystallization rate due to the miscibility of IL with PVDF, but leads to higher content of polar crystals y forms than CNTs. The ionic liquid and CNTs show significant synergetic effects on both the nucleation and the formation of polar β crystals for PVDF by the melt crystallization. The addition of IL-CNTs not only improves the CNTs dispersion in PVDF matrix, but also increases the overall crystallization rate of PVDF drastically. More important, the melt-crystallized PVDF nanocomposites with IL-CNTs show100%polar β polymorphs but no α crystal forms. To our best knowledge, this is the first report on the achievements of full polar crystal form in the melt-crystallized PVDF without mechanical deformation. The IL to CNTs ratio and the IL-CNTs loading content effects on the crystallization behaviors of PVDF in the nanocomposites were also studied. It is considered that the specific interactions between>CF2with the planar cationic imidazolium ring wrapped on the CNTs surface lead to the full zigzag conformations of PVDF, thus the nucleation in polar crystals (β forms) lattice is achieved and full polar β forms crystals are obtained by the subsequent crystal growth from the nuclei. Moreover, it was found that the PVDF/IL-CNTs nanocomposites show lower modulus, but higher the elongation at break than the PVDF/CNTs composites at the same loading content.