CROSS-LINK FLUCTUATIONS AND THE NMR PROPERTIES OF STRAINED POLY(DIMETHYLSILOXANE) NETWORKS

The doublet structure observed in the H-2 NMR spectra of deformed poly(dimethylsiloxane) (PDMS) networks is interpreted in this paper in terms of strain-dependent cross-link fluctuations. Previously this feature has been used as evidence for an orientation-induced weak nematic network ordering. From the experimental results the magnitude of the junction fluctuations in the undeformed state is estimated to be approximately 1/3 of the phantom network result. Using a theory developed by Flory, these restricted junction fluctuations can be related to the elastic properties of the network. In particular, they were related to the deviations of the elastic modulus from the phantom chain result. When expressed in terms of the Mooney-Rivlin constants C1 and C2, the Flory theory, together with the deduced value of the junction fluctuations obtained from the NMR results, gives C2/C1 = 0.18. Using literature data, this value is in close agreement with the value C2/C1 = 0.21 obtained from mechanical measurements on almost identical networks.