35 research outputs found
Peroxide-based crosslinking of solid silicone rubber, part II: The counter-intuitive influence of dicumylperoxide concentration on crosslink effectiveness and related network structure
Application of elastomers in general demands the conversion of their soluble networks into crosslinked structures. This abrupt change causes several modifications, both in the atomic/molecular level and at the macro-scale. In this study, solid silicone rubber (high molecular weight poly(dimethylsiloxane)), was crosslinked with dicumylperoxide (DCP), a widely used crosslinking agent by the rubber industry. The changes caused by different DCP concentrations were investigated, aiming to bring attention to the molecular transformations, usually neglected when processing-oriented studies are conducted. DCP concentration showed a limited contribution to the network's molecular dynamics, which was found to be mainly dominated by entanglements. The dominance of entanglements over other molecular constraints, like crosslink points, justifies the threshold and counter-intuitive behavior of tensile and hardness properties. However, differences were found in the crystallization ability after crosslinking, when the more crosslink points were introduced, the lower the crystallinity was and the less stable the PDMS crystallites were. In addition to providing a deeper understanding of an industrially applied rubber system n terms of the effective concentration of DCP, and the reasoning behind such concentration, the findings of this study add to the state-of-the-art comprehension of elastomeric networks, and how they behave on a molecular level
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MQ NMR and SPME analysis of nonlinearity in the degradation of a filled silicone elastomer
Radiation induced degradation of polymeric materials occurs via numerous, simultaneous, competing chemical reactions. Though degradation is typically found to be linear in adsorbed dose, some silicone materials exhibit non-linear dose dependence due to dose dependent dominant degradation pathways. We have characterized the effects of radiative and thermal degradation on a model filled-PDMS system, Sylgard 184 (commonly used as an electronic encapsulant and in biomedical applications), using traditional mechanical testing, NMR spectroscopy, and sample headspace analysis using Solid Phase Micro-Extraction (SPME) followed by Gas Chromatography/Mass Spectrometry (GC/MS). The mechanical data and {sup 1}H spin-echo NMR indicated that radiation exposure leads to predominantly crosslinking over the cumulative dose range studies (0 to 250 kGray) with a rate roughly linear with dose. {sup 1}H Multiple Quantum NMR detected a bimodal distribution in the network structure, as expected by the proposed structure of Sylgard 184. The MQ-NMR further indicated that the radiation induced structural changes were not linear in adsorbed dose and competing chain scission mechanisms contribute more largely to the overall degradation process in the range of 50 -100 kGray (though crosslinking still dominates). The SPME-GC/MS data were analyzed using Principal Component Analysis (PCA), which identified subtle changes in the distributions of degradation products (the cyclic siloxanes and other components of the material) as a function of age that provide insight into the dominant degradation pathways at low and high adsorbed dose