Chain dynamics in foamed polysiloxane materials: Influence of the tin catalyst phase on material properties

We report on our underpinning science activities investigating the through life performance properties of foamed polysiloxanes. These materials are manufactured through tin octanoate based condensation cure chemistry. The tin catalyst plays an important role, not only in determining crosslink density and microstructure, but also potentially influencing properties (compression set and load retention) important to through life performance. In this study, Mossbauer Spectroscopy and X-ray fluorescence has been used to assess the nature and distribution of the tin species and show evidence for highly localized catalyst regions, including a mobile tin phase. In addition, testing was carried out using a NMR MOUSE (Mobile Unit for Surface Exploration) together with a CPMG (Carr Purcell Meiboom Gill) spin echo sequence to probe chain dynamics through the measurement of spin-spin or transverse relaxation (T2) responses. We have used model polysiloxanes to demonstrate that the NMR MOUSE capability is relatively sensitive to crosslink density. In general, the more resticted the environment around a given chain structure, the more rapidly it is likely to relax, and potential chain confinement effects of the tin phase may be assessed through the measurement of T2 parameters. Our work shows that the tin catalyst has a binding action and affects the dynamics of the relatively more rigid environments (such as crosslinks) as well as the mobility of chain ends and mid chain components (long T2). Load retention properties are reduced with increased tin catalyst loading, and this ties in with the influence of the tin particles on chain dynamics. The observations reported here support our initiative to develop predictive models representative of the through life performance of foamed polysiloxanes.