| Tear strengths have been measured for a wide variety of molecular networks under threshold conditions; i.e., at high temperatures, low rates of tearing, and with swollen samples. For all of the polymers examined, the threshold tear strength was proportional to the square root of the average molecular weight M(,c) of network strands, in agreement with theory. However, for the same M(,c), and hence for similar values of elastic modulus, different polymers showed major differences in threshold tear strength. The tear strength of polydimethylsiloxane networks was only about one-third as large as that for networks of polybutadiene and cis-polyisoprene and the values obtained for polyphosphazene networks were only about one-fifth as large, at the same M(,c). These striking differences are attributed to differences in network strand length and extensibility for the same molecular weight. The threshold tear strengths are shown to be in satisfactory quantitative agreement with theoretically-predicted values on this basis. Endlinked polydimethylsiloxane and polybutadiene networks had virtually the same threshold strength as randomly-linked networks of the same composition and M(,c).; Measurements of the strength of adhesion when one layer of endlinked PDMS is cast on a previously-cured layer have also been carried out. The two layers are only connected by molecular entanglements in this case. The threshold strength was about one-half of that for a fully-linked network.; The diffusion coefficients (D) of homologous linear polymers diffusing into endlinked PDMS networks have been measured also. They were rather large and inversely proportional to the molecular weight of the diffusing liquid.; The equilibrium swelling of endlinked PDMS networks by homologous polymers obeyed a simple version of the Flory-Huggins equation with the heat of mixing assumed to be zero. |
