MQ NMR and SPME Analysis of Nonlinearity in the Degradation of a Filled Silicone Elastomer

Abstract

Radiation-induced degradation of polymeric materials occurs through numerous, simultaneous, competing chemical reactions. Although degradation is typically found to be linear in adsorbed dose, some silicone materials exhibit nonlinear 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 in electronic encapsulation and in biomedical applications), using traditional mechanical testing, NMR spectroscopy, and sample headspace analysis using solid-phase microextraction (SPME) followed by gas chromatography/mass spectrometry (GC/MS). The mechanical data and 1H spin−echo NMR spectra indicated that radiation exposure leads to predominantly cross-linking over the cumulative dose range studied (0−250 kGy) with a rate roughly linear with dose. 1H multiple-quantum NMR spectroscopy detected a bimodal distribution in the network structure, as expected from the proposed structure of Sylgard 184. The MQ NMR spectra further indicated that the radiation-induced structural changes were not linear in adsorbed dose and that competing chain scission mechanisms made a greater contribution to the overall degradation process in the range of 50−100 kGy (although cross-linking still dominated). 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.

Thermal Degradation Behavior and Product Speciation in Model Poly(dimethylsiloxane) Networks

“…Although it is well known that the thermal stability of polysiloxanes is sensitive to the presence of Lewis acids/bases [6] and organo-metallic catalysts [9], the overall degradation pathways of polysiloxanes have been shown to be remarkably resistant to change in the face of additives, filler materials, catalyst residues or other contaminants [10]. A similar finding was recently reported by Chinn et al [11] for a commercial silicone formulation exposed to increasing amounts of ionizing radiation. Broadly speaking, a polysiloxane will always thermally degrade above its ceiling temperature to yield 30-80% of the D 3 cyclic and a tailing distribution of higher cyclics from D 4 to D [20][21][22][23][24][25][26][27][28][29][30] .…”

Molecular weight distributions of irradiated siloxane-based elastomers: A complementary study by statistical modeling and multiple quantum nuclear magnetic resonance

“…As discussed in detail previously, distribu tions of RDCs can be extracted for polymeric materials by probing multiple quantum coherences among protons along the chain [22][23][24][25][26][27]. In these works, a "bu ild-up" of thes e multiple qu antum (MQ) coherences is measured through a suitable NMR experim ent, and the RDC distrib ution is typically ex tracted from this data in a model-free way using T ikhonov regularization software, FTIKREG [20].…”

“…In these works, a "bu ild-up" of thes e multiple qu antum (MQ) coherences is measured through a suitable NMR experim ent, and the RDC distrib ution is typically ex tracted from this data in a model-free way using T ikhonov regularization software, FTIKREG [20]. This program is able to derive a series of possible normalized RDC distributions that would m anifest the observed NMR response; and through a detail ed statistical treatment of many such distributions, a "m ost likely" candidate can be chosen [27].…”

“…(1) and Eqn. (2), significant insights into the relative changes of polymer material properties due to chem ical degradation have been previously reported [23,[25][26][27].…”

“…Multiple quantum nuclear m agnetic resonance (MQ-NMR) has been used with much success over the past decade to reveal detailed information on polymer dynamics, microstructure, and degradation [20][21][22][23][24][25][26][27]. In m any of these studi es, relative changes in polym eric properties are tracked in c onsiderable detail, a llowing for the extraction and the id entification of otherwise subtle or invisible chem ical and physical pheno mena.…”