Thermal and dynamic mechanical properties of vitamin E infused and blended ultra‐high molecular weight polyethylenes

Abstract

Vitamin E (or a-tocopherol) is an alternative via to thermal treatments to achieve oxidative stability of gamma or electron beam irradiated ultra-high molecular weight polyethylenes (UHMWPE) used in total joint replacements. Our aim was to study the effects of vitamin E on the molecular dynamics and microstructural properties of UHMWPE. We hypothesized that the antioxidant would plasticize UHMWPE. Vitamin E was incorporated into UHMWPE at different concentrations by diffusion and blending and detected by ultraviolet and infrared spectroscopies from 500 ppm and 4000 ppm, respectively. Dynamic mechanical thermal analysis was used to characterize the influence of this antioxidant in the relaxations of the raw material. Differential scanning calorimetry and transmission electron microscopy served to characterize thermal and microstructure properties, respectively. Vitamin E concentrations above 3% by weight significantly reduced the degree of crystallinity and increased the melting transition temperature of raw UHMWPE. The presence of increasing concentrations of a-tocopherol introduced and/or strengthened the beta relaxation, which was also shifted toward gradually lower temperatures and had rising activation energies up to 188 kJ/mol. In addition, the gamma relaxation remained unaltered on vitamin E addition. Therefore, no plasticizing effects of vitamin E on the molecular dynamics of UHMWPE could be confirmed from mechanical spectroscopy data. However, the a relaxation was modified in intensity and location due to the changes in the degree of crystallinity introduced by the incorporation of vitamin E.

Dielectric behavior induced by vitamin E and electron beam irradiation in ultra high molecular weight polyethylene

“…Therefore, higher crystalline content and the prevalent structural linearity of Nitta and Tanaka may result in the absence of this relaxation. However, in a recent work by Puértolas et al, this β relaxation was weakly detected in GUR1050 by means of dynamic mechanical thermal analysis.…”

Sliding wear behaviour of oriented ultrahigh molecular weight polyethylene

“…This relaxation was only noticed at higher frequencies for UHMWPE having residual strains of 0, 0.07 and 0.2, and appeared at temperatures ranging between −10 °C and 10 °C. According to the literature, the β‐relaxation process in UHMWPE may involve motions of loop and loosely tie molecules. The orientation process by tension leads to a stretching mechanism of the tie molecules, which may explain the disappearance of this relaxation process at residual strains above 0.2.…”

“…At 22 °C, the loss factor increased from 0.065 to 0.11 when the strain level was increased from 0 to 0.85 indicating that the orientation increased the energy dissipation ability of UHMWPE through molecular rearrangements. The α‐relaxation process observed from 30 °C is attributed to motions of the crystalline phase . This relaxation generally exhibits two successive tan δ peaks, called α 1 and α 2 , due to intralamellar grain boundary motions, like the rotation of the crystal grains in the direction of external excitation, and due to rotational oscillations of polymer chains within the crystal grains, respectively .…”

Microstructure, thermooxidation and mechanical behavior of a novel highly linear, vitamin E stabilized, UHMWPE

“…In the case of vitamin E infused UHMWPE, the thermal processes used to incorporate vitamin E are performed at temperatures over 120°C, and this implies that an annealing process takes place simultaneously to the diffusion of the antioxidant. Thermal annealing is known to produce a small increase in the crystallinity of UHMWPE associated to the gradual thickening of lamellar crystals [24], and this also occurs to vitamin E infused UHMWPEs as long as the antioxidant concentration does not excess 0.4% by weight [25].…”