Highly Stable Polymers based on Poly( m -carboranyl-siloxane) Elastomers
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Highly Stable Polymers based on Poly(m-carboranyl-siloxane) Elastomers Anthony.C.Swain, Mogon Patel and Julian.J.Murphy Materials Science and Research Division, AWE, Aldermaston, Reading, RG7 4PR ABSTRACT Poly(m-carboranyl-siloxane) elastomers containing a mixture of di-methyl- and methyl(phenyl)-silyl units were synthesised using the Ferric Chloride catalysed condensation reaction between di-chloro-di-organosilane and 1,7-bis(di-methyl(methoxy)silyl)-m-carborane. These prepared polymers were aged either by heating in air at elevated temperature or by γirradiation from a 60Co source. Multinuclear (1H, 13C and 11B) solid and solution state nuclear magnetic resonance was used to evaluate degradation. γ-irradiation doses to 1 MGy were found to induce only a small reduction in elastomer properties as evidenced by a reduction in segmental chain dynamics. Ageing at temperatures below 350oC similarly displayed a small reduction in segmental chain dynamics together and a concomitant weight loss as measured by differential scanning calorimetry. Above 350oC degradation of the elastomer was dramatic with a decreased segmental chain dynamics and oxidation of the carborane cage, vide infra. INTRODUCTION Polymeric materials commonly contain significant quantities of typically inorganic fillers, these serve to reduce feed stock costs and allow modification and improvement of materials properties. It is well known that polymer-ageing effects can often be traced to the polymer/filler interface and of its subtle modification. This tends to make the development of predictive ageing models an extremely complex undertaking. Poly(di-methylsiloxane) is a commonly used elastomeric material, which is general considered to be stable. However, the incorporation of fillers to the polymer phase under ambient conditions allows water to migrate to the filler/polymer interface. When such materials are placed in a harsh and strongly desiccating service environment certifying component, performance becomes exceedingly challenging. Poly(siloxane) materials incorporating an icosahedral C2B10H10 carborane[1], unit in their main chain[2] and have demonstrated useful properties under extreme conditions[2,3]. The carborane cage can be envisaged as a nano-scale internal modifier aiding engineering and mechanical properties whilst improving thermal stability without the problems associated with a polymer/filler/water interface. These materials maybe readily prepared by a ferric chloride catalysed reaction between di-chlorosilanes and di-substituted-methoxysilyl-terminated carboranes. Curing of these systems to network structures is achieved by the incorporation of olefinic silanes into the feed, followed by a subsequent organo-peroxide cure[4]. This network can be readily probed using solid state NMR techniques poly(m-carboranyl-siloxane)s thereby providing an insight into dynamic time scales that can reflect viscoelastic properties[5-10]. We report our NMR spectroscopy and thermal analysis of the properties of non-degraded and of thermally and rad
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