MODEL NETWORKS OF END-LINKED POLYDIMETHYLSILOXANE CHAINS .3. EFFECT OF THE FUNCTIONALITY OF THE CROSS LINKS

Fractions of hydroxyl-terminated polydimethylsiloxane (PDMS) of relatively narrow molecular weight distribution were cross-linked by reacting the hydroxyl chain ends, in the undiluted state, with a trifunctional orthosilicate. Stress-strain isotherms were obtained on the resulting networks in elongation at 25° C, and were represented in terms of the equation [f*] = 2C 1 + 2C2α-1, where [f*] is the reduced stress or modulus and α is the elongation of the network. The present results on the trifunctional PDMS networks were compared with previously reported results on the corresponding tetrafunctional networks, thereby characterizing the effect of changing the network functionality φ from four to three. The observed changes were found to be in good agreement with predictions based on the most recent molecular theories of rubberlike elasticity. Specifically, experiment and theory were found to be in good agreement with regard to the factor which relates the modulus 2C1 to the structure of the network, and with regard to the ratio 2C 2/2C1 which is a measure of the extent to which the elastic deformation changes from affine to nonaffine with increasing stress. © 1979 American Institute of Physics.