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Title: Processing, structure and electrical properties of poly(vinylidene fluoride) based nanocomposites
Other Titles: Ju pian fu yi xi na mi fu he cai liao de zhi bei, jie gou he dian xing neng
聚偏氟乙烯納米複合材料的製備, 結構和電性能
Authors: Li, Yuchao (李玉超)
Department: Department of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2010
Publisher: City University of Hong Kong
Subjects: Nanostructured materials
Polymeric composites.
Notes: CityU Call Number: TA418.9.N35 L54 2010
xviii, 178 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2010.
Includes bibliographical references (leaves 150-178)
Type: thesis
Abstract: Poly(vinylidene fluoride) (PVDF) is a semi-crystalline polymer having high permittivity, excellent thermal stability and chemical resistance. Consequently, PVDF finds widespread applications in industrial and biomedical sectors for fabricating capacitors, transducers and sensors. The electrical properties of PVDF can be further improved by adding low volume fractions of nanofillers. Low loading levels of nanofillers reduce the cost and weight of resulting nanocomposites considerably. Further development of advanced PVDF-based nanocomposites with functional properties requires proper understanding of the processing, structure, electrical property, and their relationship. In general, polymer matrix with poor electrical conductivity forms the continuous phase of the composite system. A common practice for enhancing dielectric constant or electrical conductivity of composites is to add high permittivity ceramic particles or conducting carbonaceous fillers. By increasing the filler content, remarkable changes in the conductivity and dielectric constant occur near the percolation threshold. For conventional composites, high loadings of microfillers are needed to reach desired electrical properties, leading to inferior mechanical properties and poor processability of materials. In contrast, only low volume fractions of nanofillers are required for achieving such purposes. In this research work, three-dimensional ceramic nanoparticles such as barium titanate (BaTiO3) and beta silicon carbide (β-SiC), and two-dimensional graphite and silicate platelets are used to fill PVDF. Melt-compounding or solution casting process is employed to fabricate these nanocomposites. The processing, structure, thermal and electrical properties of such nanocomposites are systematically studied. For PVDF filled with BaTiO3 nanoparticles, dielectric responses of resulting nanocomposites differ greatly from their microcomposite counterparts. The dielectric constant of these nanocomposites increases with increasing filler content. However, the overall permittivity of PVDF/BaTiO3 nanocomposites is substantially lower than that of PVDF/BaTiO3 microcomposites. This is due to a deterioration of dielectric permittivity of barium titanate by reducing its grain size from micrometer down to nanometer level. Micro-grained barium titanate with a tetragonal structure is widely known to exhibit high dielectric permittivity. At nanometer regime, a transition from the tetragonal to cubic structure with low dielectric permittivity takes place. Therefore, PVDF/BaTiO3 nanocomposites requires a large nanofiller content (50 wt%) to reach a dielectric constant of ~ 23.3. Silicon carbides are widely known to exhibit several crystallographic forms or polymorphs. The most attractive polymorph for electronic applications is the beta silicon carbide (β-SiC) with a face-centered cubic structure. Beta SiC is a semiconductor with excellent dielectric permittivity and electrical conductivity. Thus SiC additions can markedly enhance the dielectric constant and electrical conductivity of PVDF. Graphite intercalation compound can be dispersed into expanded (EG) and exfoliated graphite nanoplatelets (GNPs) via rapid thermal treatment and/or sonication. The electrical properties of the polymer composites depend greatly on the aspect ratio and homogeneous dispersion of conductive fillers. GNPs with large aspect ratio, surface area and excellent electrical conductivity are ideal nanofillers for forming conducting nanocomposites. Graphite doped PVDF nanocomposites display a significant enhancement in both dielectric permittivity and conductivity at the percolation threshold compared to the PVDF/BaTiO3 and PVDF/SiC nanocomposites. In this study, PVDF/EG nanocomposites were prepared using melt-compounding while PVDF/GNP nanocomposites were fabricated by means of the solution casting process. As expected, solution cast PVDF/GNP nanocomposites exhibited much lower percolation threshold (2.4 wt%) than the melt-blended PVDF/EG system (7.8 wt%). A large dielectric constant of 173 and low dielectric loss of 0.65 were found in the PVDF/2.5 wt% GNP nanocomposite. The large permittivity was related to interfacial polarization and to the formation of many mini-capacitors in the polymer matrix. In general, hybridization of nanofillers enables the composites to attain superior electrical properties. The permittivity of PVDF/BaTiO3//GNP and PVDF/SiC//GNP hybrids is found to increase markedly by adding a small GNP content. The experimental results of nanocomposites investigated are also compared with those predicted from theoretical models. The results reveal that both logarithmic mixing rule and Maxwell-Garnett approximation can well describe the dielectric responses of PVDF composites filled with insulating ceramic nanoparticles. However, the dielectric responses of binary PVDF/GNP nanocomposites, ternary PVDF/BaTiO3//GNP and PVDF/SiC//GNP hybrids can only be explained in terms of the percolation theory.
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