Aspects of the polycondensation of hydroxyfunctional polydimethylsiloxanes.
This work is a contribution to the study of the curing of
silicone prepolymers (hydroxyfunctional polydimethyl
siloxanes) and the physical properties of the resulting
networks. The kinetics of curing of one of the prepolymers
has been studied with a simple new inexpensive piece of
equipment i.e the Vibrating Needle Curemet~r (VNC). The order of reaction with respect to crosslinker and catalyst concentration has been determined to be unity and 0.24 respectively while an attempt has been made to determine the order of reaction with respect to silanol group concentration in the prepolymer. The effect of. crosslinker and catalyst concentration was observed to check the validity of the Lake and Thomas theory; namely that tear strength at a particular condition is proportional to the square root of the molecular weight between crosslinks. It has been found that varying
crosslinker concentration is a better way to test this
theory. Surprisingly the detailed physical properties such
as tear and tensile of the supposedly different prepolymer
networks showed little relationship to the molecular weight.
The effect' of blending short and long chain polymers i.e
bimodality, and that of the concentration of the crosslinker
on the physical propert; es has been investigated. It became
apparent that in addition to bimodality, the physical properties are dependent upon the amount of cross1inker used
for curing This is a factor which has not previously been
reported. It has also been shown that unfortunately, contrary to expectation bimodality has adverse effects on the physical properties of the highly filled networks. In the case of the tensile properties, the stress strain curves were analysed in light of tne phenomenological
Mooney - Rivlin (MRL) and Martin, Roath and Stiehler (MRS)
equations. Apart from successfully applying these theories
to stress-strain curves, the structure factors of networks
have been' obtained. These theories have been used in parallel to check and compare the results of one technique with the other. However, in the case of the filled HOPDMS networks it is evident that only the MRS equation can successfully describe the stress-strain data. The stress-strain data for the various HOPDMS filled networks
have been compared with one another in terms of Young's
modulus from their practical point of view.