Rheological modelling of the free-radical crosslinking of PDMS rubber in the presence of TEMPO nitroxide

The aim of the present work is to study the free-radical kinetics of PDMS rubber crosslinking in the presence of 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) nitroxide. For this purpose a new method based on the relationship between the kinetics of the macro-radicals coupling [Rcc(t)] was derived from a fundamental kinetic model and the viscoelastic changes of the complex shear modulus (G′(t)ω and G″(t)ω). The kinetic model takes into account the initiator (Dicumyl peroxide in the present study) decomposition and the trapped PDMS macro-radicals in the presence of a radical scavenger such as TEMPO. Activation energy Eac and collision frequency factor A0c for the bimolecular termination reaction coefficient rate kcc have been derived from the anisothermal DSC results according to the Kissinger method. Furthermore, it was observed that addition of TEMPO nitroxide can boost the initiator efficiency. The concentration variation of the active PDMS carbon-centred radicals [Rp(t)]act and the [Rcc(t)] with reaction time were predicted using this kinetic model. On the other hand, the influence of TEMPO concentration in formulation ([N]0) and effect of temperature on viscoelastic variations are studied. As a main result, the rheological modelling shows that this new method accurately predicts the time variation of complex shear modulus at any temperature and [TEMPO]/[DCP] ratio.