Effects of stearic acid and thermal treatment on morphology and dielectric properties of UHMWPE/POSS composites prepared by ball milling

Abstract

For the last decade, there has been lots of interest in the development of nanodielectrics as these materials may present superior dielectric properties when compared to traditional composites. However, the success of these materials relies on a proper dispersion of nanofillers within the matrix. In this work, UHMWPE/POSS composite of Ultra-High-Molecular Weight PolyEthylene (UHMWPE) with 5% wt Octaisobutyl Polyhedral Oligomeric SilSesquioxanes (POSS) was prepared by ball milling. In order to enhance the dispersion of POSS within UHMWPE, two routes were tested: 1) stearic acid in a concentration of 1% wt (versus POSS filler) was added to UHMWPE and POSS blend during ball milling; 2) a thermal treatment on UHMWPE/POSS composite at a pressure of 6.3×10 6 Pa and an average temperature of 453 K for 7200 s was carried out. The resulting composites were tested using optical microscopy, heat-flux differential scanning calorimetry, broadband dielectric spectroscopy and short-time breakdown. The results showed the first route seemed to have little effect on enhancing the dispersion of POSS within UHMWPE matrix nor the dielectric properties of UHMWPE/POSS composite. The results also showed the second route seemed to have a positive effect on enhancing the dispersion of POSS within UHMWPE matrix yet a negative effect on the dielectric properties of UHMWPE/POSS composite.

Silsesquioxane Derivatives as Functional Additives for Preparation of Polyethylene-Based Composites: A Case of Trisilanol Melt-Condensation

“…Joshi et al showed that addition of octamethylsilsesquioxane to HDPE at various loadings had little impact on crystallinity level of the polymer, however it mediated its crystallite size [ 23 ]. Guo et al also suggested that when applying octa( iso butyl)silsesquioxane for UHMWPE, the additive segregated on the spherulite interfaces, where most of the polymer amorphous phase is located [ 37 ]. The non-linear relationship between the SSQ additive loading and the crystallinity levels of PE can be explained on the basis of two levels of dispersion of the additives, where at higher loadings more agglomerates form and the less numerous, large particles of the additive provide less nucleation sites for the crystallizing polymer [ 54 , 55 ].…”

“…As organosilicon hybrid compounds, SSQs tend to improve thermal stability of polymer matrices they are introduced into, but also modify materials rheology (e.g., as plasticizers or thixotropic agents) or improve mechanical properties, such as tensile or flexural modulus, tensile strength, heat deflection temperature. Different structures have been tested, including homo-(R n [SiO 1.5 ] n ) and heterosubstituted (R n-m R’ m [SiO 1.5 ] n ) systems, bearing either alkyl [ 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 ], alkenyl [ 41 , 48 ], aryl [ 24 , 25 , 26 ], or other functional groups [ 26 , 41 , 49 , 50 , 51 , 52 ], introduced mostly by condensation, hydrosilylation, or substitution reactions. It has been proven that a choice of introduced functional groups plays a major role in miscibility of silsesquioxanes and polymer matrix and the additive tendency towards agglomeration, but also the processing method and parameters have an impact on the obtained additive dispersion in the final composite or nanocomposite [ 40 ].…”

“…Very good dispersion of the additives has been obtained when high-shearing-forces processes have been applied, such as twin-screw extrusion [ 22 , 23 , 44 ] or injection moulding [ 32 ]. Among other methods, ball milling [ 30 , 31 , 33 , 37 ], solution blending [ 34 , 35 ], or reactive melt blending [ 51 , 52 ] have been reported with moderate to good results. The dispersion level plays a huge factor on the properties of the obtained materials, and effective dispersing of the organosilicon additive is crucial to observe effects such as (nano)reinforcing action or thermal stabilization of the composition [ 11 , 12 , 13 , 14 , 15 ].…”