Study of crosslinking density in polydimethylsiloxane networks by DSC

A calorimetric study for estimating crosslinking densities of polydimethylsiloxane (PDMS) networks is presented. It involves the experimental determination of the heat capacities of the polymer with (C p i ) and without (C p 0 ) crosslinking. The theoretical basis of the method is described as well as the procedure used for the determination of the C p s. The crosslinking densities of PDMS networks, determined by absorbency (swelling) measurements, using the Flory-Rehner equation, and by the method described here, are compared. The PDMS, obtained by the anionic synthesis of octamethylcyclotetrasiloxane, was analyzed by gel permeation chromatography and infrared spectroscopy. Separate samples of the polymer were further crosslinked at different network densities. The C p s and the thermal stabilities were determined by DSC and TGA, respectively. The results indicate that the crosslinking density ratios of the polymer networks calculated by the relation ΔC p i /C p 0 are in reasonable agreement with those obtained from absorbency measurements. The crosslinking density can also be obtained from heat capacity measurements if the density of the network is known at the temperature that ΔC p is obtained