Annealing effects on the solid state properties of transition-metal coordination complexes and networks based on diene polymers with palladium chloride

This study focuses on the thermal and mechanical properties of 1,2-polybutadiene and 3,4-polyisoprene with an inorganic salt, bis(acetonitrile)dichloropalladium (II). Upon mixing in THF, effective crosslinks are formed because the acetonitrile ligands of the palladium salt are displaced by olefinic pendant groups of the polymers. Using a simple nth-order irreversible kinetic rate model, the palladium-catalyzed Heck-like exothermic reaction in solid films was characterized via isothermal and nonisothermal DSC. Thermal energy and mass balances appropriate to a batch reactor are developed from first principles and applied to the isothermal DSC output curve to calculate the time dependence of reactant conversion. Relevant kinetic parameters, such as the order of the reaction, the characteristic time constant for the chemical reaction, and the activation energy, have been determined. The kinetic data suggest that the palladium-catalyzed crosslinking reactions are diffusion controlled in the solid state because the reaction order is very close to unity. Higher glass transition temperatures (T_g) are measured by dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC) when (i) palladium concentration, (ii) annealing (heat treatment) time, and (iii) annealing temperature increase. After 2 h of annealing at 80°C, which corresponds to a temperature below the first exothermic crosslinking reaction (? 115°C) during nonisothermal DSC kinetic studies, rubbery materials containing very low concentrations of PdCl₂ (i.e., 0.5 mol %) exhibit reinforced ductile stress-strain response. When annealing is performed at the peak temperature of the first exothermic event, the rubbery materials are transformed into glasses. Transition-metal compatibilization of atactic 1,2-polybutadiene and 3,4-polyisoprene via PdCl₂ is demonstrated by monitoring the glass transition obtained from dynamic mechanical tan δ profiles. The effect of annealing this ternary reactive “blend” produces a glassy material exhibiting an elevated T_g and synergistic mechanical properties. © 1996 John Wiley & Sons, Inc.