INVESTIGATION OF THE CHAIN-LENGTH DEPENDENCE OF SELF-DIFFUSION OF POLY(DIMETHYLSILOXANE) AND POLY(ETHYLENE OXIDE) IN THE MELT WITH PULSED-FIELD GRADIENT NMR

The chain length dependence of the self-diffusion coefficients of PDMS and PEO has been measured with pulsed field gradient NMR (PFG NMR). With PDMS the maximum molar mass region attainable with PFG NMR could be investigated. For molar masses larger than about 10M(e) the proportionality D approximately M-2 according to the strict reptation picture was observed; at small M Rouse-like diffusion is observed separated from the reptation part by a D(M) dependence characterized by a strong slowing down of the self-diffusion coefficient. The monomeric friction coefficients for PDMS derived from diffusion with the Doi-Edwards theory agree satisfactorily with those obtained from viscosity and also neutron scattering. Quantitatively, a discrepancy remains between the high molar mass part of D(M) where the monomeric friction coefficient is by a factor of about 0.5-0.6 smaller than that determined from the Rouse part of D(M) at low molar masses. Within experimental accuracy, for PDMS the activation energy of diffusion is equal to the activation energy of viscosity. For PEO the activation energy decreases from 24 to 18 kJ/mol at a molar mass of about 20 000 g mol-1.