Characterization of polydimethylsiloxane elastomer degradation via cross-linker hydrolysis

Degradation kinetics based on rheological studies are reported for endlinked silane terminated polydimethylsiloxane (PDMS) networks cross-linked with 2,4,6-triallyloxy-1,3,5-triazine as a function of cross-link density and the relative humidity to which the network is exposed. These unique PDMS networks cure in the absence of water but degrade under humid conditions due to cross-linker hydrolysis. Rheological measurements show an exponential decay of the storage modulus (G′) with time under conditions of high humidity (greater than 45%) and a linear decay under conditions of low humidity (lower than 22%). Optical studies show that at high humidity two domains exist wherein a domain of degraded polymer fluid surrounds a central region of a soft elastomer. However, a uniform degradation with absence of any interface is observed under low humidity conditions. Thus a crossover from 'surface' degradation from the edge to 'bulk' degradation is observed between 45 and 22% relative humidity. The time to degrade to half the original modulus (t1/2) ranges from a few hours at high humidity to days at low humidity. At high humidity t1/2 scales with cross-link density, but it is independent of cross-link density at low humidity. A result that is consistent with 'surface' degradation and 'bulk' degradation, respectively.