Studies of self‐diffusion of poly(dimethylsiloxane) chains in PDMS model networks by pulsed field gradient NMR

Abstract

AbstractWe have measured the diffusion of poly(dimethylsiloxane) (PDMS) chains in PDMS model networks by using a pulsed field gradient NMR technique. The model networks have been prepared by tetrafunctional endliking of linear PDMS chains having molecular weights Mn of 3,700 and 7,400 g mol−1. The diffusants have been incorporated in the networks by immersing pieces of them in PDMS linear chains with molecular weights Mn between 3,000 and 12,000 g mol−1 and molecular weight distributions Mw/Mn between 1.1 and 1.7. Although spin‐echo attenuation results were fitted to a model which takes into account polydispersity of the diffusant, these results did not exhibit any dependency upon the molecular weight distribution. The self‐diffusion coefficients of PDMS chains in the PDMS model networks were found to be smaller than in the melt, and the exponents for the diffusion coefficient dependence on Mn in the networks were found to be about‐1.3. Free diffusion will give an exponent equal to‐1, whereas free volume contributions or behavior intermediate between free and entangled diffusion will increase the magnitude of the exponent.