Amines used for low temperature curing of PDMS-based gel-networks impact γ-irradiation outcome

Four structurally different amines: 1,3-diamino-2-propanol (DAP), diethylenetriamine (DETA), tris(2-aminoethyl)amine (TAEA) and triethylenetetramine (TETA) were reacted with diglycidyl ether terminated poly (dimethylsiloxane) (PDMS-DGE) at relatively low temperature of 50 degrees C. Stable transparent networks with gel-like properties but without a liquid component, "gel-networks" were obtained. Raman spectra revealed presence of residual epoxy-group absorptions in cured gel-networks but secondary amine absorption intensities in FTIR spectra were used for monitoring of the curing reaction extent. To assess their radiation stability gel-networks were gamma-irradiated to absorbed dose of 50 kGy. Even though contribution of amine segments to PDMS-based gel-networks is relatively small, their structure significantly influenced the properties of studied PDMS-based gel-networks. The most important property of an amine was the ability to either form hydrogen bonds, or produce denser network like a triamine. However there were also significant differences in properties of gelnetworks prepared with mutually similar diamines DETA and TETA that can be considered a dimer and a trimer of ethylenediamine. In the studied gel-networks there is a complex interaction of extent of initial curing that depended on the amine used and the resulting (dis)order that strongly influenced the outcome of irradiation. An endothermic transition presumably caused by reversible gel-network collapse was observed in DSC thermograms of all the gel-networks. Its temperature depended on the amine type and shifted to higher values on irradiation. Changes in most properties like residual gel content on extraction or DC electrical conductivity and its frequency dependence were more pronounced in gel-networks prepared with diamines DETA and TETA than in those prepared with amines capable of significant secondary valent interactions (DAP and TAEA). Radiation curing and/or crosslinking seem to have occurred only in TETA-cured gel-network and it was mostly due to lower extent of initial reaction. Since swelling is a measure of gel quality and it was higher in irradiated gel-networks it may be concluded that irradiation to D = 50 kGy improved their properties.