Abstract:
© 2015 American Chemical Society. Field-cycling and field-gradient <sup>1</sup>H NMR experiments were combined to reveal the segmental mean-square displacement as a function of time for polydimethylsiloxane (PDMS) and polybutadiene (PB). Together, more than 10 decades in time are covered, and all four power-law regimes of the tube-reptation (TR) model are identified with exponents rather close to the predicted ones. Characteristic polymer properties like the tube diameter a<inf>0</inf>, the Kuhn length b, the mean-square end-to-end distance <R<inf>0</inf><sup>2</sup>>, the segmental correlation time τ<inf>s</inf>(T), the entanglement time τ<inf>e</inf>(T), and the disengagement time τ<inf>d</inf>(T) are estimated from the measurements and compared to results from literature. Concerning τ<inf>d</inf>(T), fair agreement is found. In the case of τ<inf>e</inf>, agreement with rheological data is achieved when the time constant is extracted from the minimum in the shear modulus G″(ω). Concerning the TR predictions the molar mass (M) dependence of τ<inf>d</inf> is essentially reproduced. Yet, calculating τ<inf>e</inf> from τ<inf>d</inf> for PDMS yields agreement with experimental data while for PB it gets by 2 orders of magnitude too short. In no case τ<inf>e</inf> is correctly reproduced from τ<inf>s</inf>(T). Segmental and shortest Rouse times appear to coincide for PB, while in the case of PDMS the latter turns out to be longer by 1 decade.
