A SANS study of the conformational behaviour of linear chains in compressed and uncompressed end-linked elastomers

Small-angle neutron scattering (SANS) experiments were performed to determine the conformational behaviour of unattached deuterated linear poly(dimethylsiloxane) (PDMS) chains of M_n = 51 000 g mol^-1 in uncompressed and compressed end-linked protonated PDMS networks with a minimal amount of defects. The networks were prepared from low-polydispersity precursor chains with M_n = 9900 g mol^-1, 34 800 g mol^-1, and 71 500 g mol^-1. The samples are hereafter labeled 10K, 35K, and 70K. The mixtures of the 10K network with the linear chains were prepared by cross-linking around the linear chains, while the 35K and 70K network systems were swollen with the linear chains after the network was formed. No significant variation of the radius of gyration of the linear chains was observed as a function of network mesh size, in agreement with recent Monte Carlo simulations. The results from the compressed 35K and 70K networks exhibited excess scattering at small wave vectors, indicating phase segregation, and they could not be quantitatively analyzed. The scattering data from the trapped chains in the compressed 10K networks were fitted well with a Kratky-Pored wormlike chain model, indicating segment alignment consistent with Monte Carlo simulations and deuterium NMR results.