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γ-irradiated PDMS gel networks cured with selected amines

Poly(dimethylsiloxane), PDMS is the most common semi-inorganic polysiloxane polymers. Diglycidyl ether terminated poly(dimethylsiloxane), PDMS-DGE can be cured with a variety of agents, like aliphatic amines [1]. Four structurally different amines were selected: 1, 3-diamino-2-propanol (DAP), diethylenetriamine (DETA), tris(2- aminoethyl)amine (TAEA) and triethylenetetramine (TETA). All amines reacted with PDMS-DGE at relatively low temperature of 50 ºC and networks with gel-like properties but without a liquid component, “gel networks” [2] were obtained. The characterization of gel networks is a challenge so numerous experimental techniques were necessary including thermal analysis, extraction, dielectric and vibrational spectroscopies. DSC thermograms revealed an endothermic transition presumably caused by reversible gel network collapse. To assess their radiation stability gel networks were irradiated to selected doses in ambient conditions at a dose rate of 7.8 Gy/s in 60Co γ-irradiation facility of LRCD, RBI. Gamma irradiation of PDMS can induce both crosslinking and degradation by chain scission. [3] Since Raman spectroscopy of unirradiated gel networks detected unreacted epoxy groups it was expected that radiation crosslinking will predominate. While the apparent firmness of gel networks irradiated up to 50 kGy was preserved, Soxhlet extraction in hexane revealed only a slight increase in gel content of most gel networks and even a slight decrease in case of TAEA cured gel network. However the changes in thermal and dielectric properties were more pronounced and the radiation response was influenced by the structure of used amine. The gel networks prepared using the amines that can either form hydrogen bonds, like DAP, or produce denser network, like TAEA with three primary amine groups, were less influenced by irradiation. The electrical conductivity of unirradiated DAP cured gel network was the lowest and increased very little on irradiation. On the other hand both the electrical conductivity and the enthalpy of endothermic transition of DETA cured gel network increased significantly on irradiation and its electrical conductivity was almost 2 orders of magnitude greater than that of corresponding unirradiated gel network. In case of irradiated TETA cured gel network a slight reduction of enthalpy was observed and conductivity change was lower than for DETA cured ones. Since extraction of irradiated TETA cured gel network showed about 10% higher gel content all this indicates less complete initial curing than in case of DETA although the diamines have very similar structure. In conclusion, although DAP, DETA, TETA and TAEA are relatively small segments of cured PDMS gel networks their structure significantly influenced not only the properties on initial curing but the outcome of irradiation even at a relatively low dose of 50 kGy.

PDMS ; aliphatic amines ; gamma-irradiation

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