Structured Polyethylene Nanocomposites: Effects of Crystal Orientation and Nanofiller Alignment on High Field Dielectric
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Structured Polyethylene Nanocomposites: Effects of Crystal Orientation and Nanofiller Alignment on High Field Dielectric Properties Bo Li 1, C. I. Camilli 1, †, P. I. Xidas 1,2, K. S. Triantafyllidis 2, E. Manias 1,* 1
Materials Science and Engineering, Penn State University, University Park, PA 16802, U.S.A Chemistry Department, Aristotle University of Thessaloniki, GR54006 Thessaloniki, GREECE * address correspondence to [email protected]; † undergraduate student. 2
ABSTRACT In previous work we have shown that aligned high aspect-ratio (pseudo-2D) nanofillers can yield large dielectric breakdown strength (EBD) improvements for a nanocomposite with a low-crystallinity polyethylene matrix. Here, we report a systematic study which delineates the contributions of the aligned inorganic fillers and of the aligned polymer crystallites in the overall EBD improvement achieved in the nanocomposites. Specifically, extrusion blown-molded polyethylene/montmorillonite nanocomposite films were cold-stretched to various strains, to further align the nanoparticles parallel to the film surface; this filler alignment is accompanied by a commensurate alignment of the polymer crystallites, especially those heterogeneously nucleated by the fillers. A systematic series of films are studied, with increased extent of alignment of the fillers and of the crystalline lamellae (quantified through Hermans orientation order parameters from 2D X-ray diffraction studies) and the aligned structure is correlated to the electric field breakdown strength (quantified through Weibull failure studies). It is shown that aligned pseudo-2D inorganic nanofillers provide additional strong improvements in EBD, improvements that are beyond, and added in excess of, any EBD increases due to polymer-crystal orientation. INTRODUCTION Electrical breakdown strength (EBD) is an important property for dielectric materials and insulators in various applications from mobile electronic devices, stationary power systems, to hybrid electric vehicles. Manufacturing of commercial dielectric films employs biaxial stretching to improve the dielectric breakdown strength (EBD) of semicrystalline polyolefins, in particular for capacitor polypropylene and PVDF films. This methodology capitalizes on the strain-induced orientation of the crystallites in these films, and necessitates high purity and high crystallinity polymers, where EBD is to be substantially improved. Although it has long been accepted that the breakdown mechanisms are generally associated with electronic avalanche, thermal runaway, electromechanical failure, and partial discharge [1, 2], in real systems more than one of these mechanisms can be, and usually are, coupled to determine the overall breakdown behavior. In addition, the inherently stochastic nature of the electrical breakdown makes the prediction and design of high-performance polymer dielectrics even much more complicated. For polymer dielectrics, the crystallinity and crystal morphology have significant impacts on EBD. Generally, compact and space-filli
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