Dispersion Strategies And Role Of Interfacial Phenomena In Dielectric Polymer Nanocomposites

Open Access
Author:
Khodaparast, Payam
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 18, 2014
Committee Members:
  • Zoubeida Ounaies, Dissertation Advisor
  • Michael T Lanagan, Committee Member
  • Evangelos Manias, Committee Member
  • James Hansell Adair, Committee Member
Keywords:
  • polymer nanocomposite
  • dispersion
  • dielectric
Abstract:
Owing to unique characteristics of nanoparticles such as high surface to volume ratio, it is postulated that nanoparticle-modified polymers exhibit properties beyond those predicted by effective media theories. In the case of dielectric nanoparticles in a polymer, it is expected that dielectric properties of the nanocomposite are dominated by the expansive interface rather than anticipated by the inherent properties of individual components. An in-depth review of dielectric polymer nanocomposites shows conflicting trends where addition of nano-sized particles resulted in increase or decrease in dielectric properties. This contradictory behavior could mainly stem from 1- the state of dispersion of nanoparticles and 2-The unique nature of interface based on the particle-polymer system. The hypothesis of the proposed research is that the role of the interfacial region is not only influenced by its expansive nature but is also governed by their interaction at nanoscale regime. In order to achieve a high internal surface area, the first important challenge to address is controlling the state of dispersion and disaggregation of nanoparticles. Therefore the first goal of this research is studying the effectiveness of different processing methods in achieving uniform nanoscale dispersion in dielectric polymer nanocomposites. Silane functionalization of titania nanoparticles is investigated as one possible solution of better dispersion of titania in PVDF polymer where two coupling agents namely, aminopropyltriethoxy silane called as APS, and Nonafluorohexyltriethoxysilane called as FHES, are studied. FHES is shown to be more effective in reducing the average aggregate size of titania nanoparticles in PVDF matrix to below 100nm, whereas the average aggregate size in untreated and APS-functionalized TiO2/PVDF nanocomposite was approximately one to two orders of magnitude higher than that. Dielectric permittivity of FHES-functionalized TiO2/PVDF nanocomposite, showed improvements over untreated and APS-functionalized TiO2/PVDF nanocomposite at weight fractions up to 10wt% (5vol%), as a result of higher interfacial area and resulting polarization at interface. However, DC dielectric breakdown and maximum achievable stored energy density did not show any dependence on average aggregate size or the type of silane surface treatments; both DC dielectric breakdown and maximum achievable stored energy density showed a similar decreasing trend as the titania weight fraction increased for all three cases. Besides silane functionalization of titania, in-situ and ex-situ sol-gel synthesis of titania were also studied as another effective method to achieve nanoscale dispersion in titania/PVDF nanocomposites. In- situ sol-gel technique resulted in uniform nanodispersion of titania in PVDF and 30% higher dielectric permittivity at 1kHz compared to commercial titania/PVDF composite with the same wt% of titania phase. However, high dielectric loss and lower DC dielectric breakdown were also observed in the in-situ sample, due to ionic impurities and trapped residues of the sol-gel process, which is a disadvantage of in-situ technique. On the other hand, ex-situ technique showed uniform dispersion of titania aggregates in the range of 100nm homogeneously dispersed in PVDF matrix. Dielectric permittivity and DC dielectric breakdown both improved in case of ex-situ; leading to 20% improvement in storage energy density of ex-situ titania/PVDF compared to pure PVDF sample; therefore this processing technique is a promising one. As to the second goal of this research, understanding the role of interfacial phenomena on final dielectric behavior of polymer nanocomposites, other metal oxides, namely, alumina, silica and magnesia are considered. In addition to varying contrast in dielectric permittivity with PVDF, these metal oxide particles also bring dissimilar surface chemistries in terms of type and concentration of physisorbed and chemisorbed water on their surfaces. Alumina nanoparticles in particular showed relatively higher amount of physisorbed and chemisorbed water on the surface; it also exhibited nanoscale dispersion in PVDF. Dielectric permittivity of alumina/PVDF nanocomposites, despite similar dielectric constant values for both phases, showed higher improvements compared to the other particles. Dehydrated alumina/PVDF nanocomposites also showed similar increase in dielectric permittivity. Therefore, a second important conclusion of this work is that improvements in dielectric permittivity in alumina/PVDF is mostly a result of dipolar interactions of chemisorbed water in form of hydroxyl group on the surface of alumina and C-F dipoles in PVDF chain at the interface. The combination of nanoscale dispersion in alumina/PVDF and dipolar interaction with PVDF verifies that interfacial phenomena could be significant enough in nanocomposites to lead to improvements in final dielectric permittivity of nanocomposite systems and a step forward to resolving the role of interfacial phenomena in dielectric behavior of polymer nanocomposites based on type and surface chemistry of nanoparticles.