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od 2020-09-20

Title: Influence of the cross-link density and the filler content on segment dynamics in dry and swollen natural rubber studied by the NMR dipolar-correlation effect

Source: Journal of Chemical Physics

Year : 1999

Abstract:

Chain dynamics in a series of natural rubbers with different cross-link densities was studied using the dipolar correlation effect (DCE) on the stimulated echo. In dry samples, amplitudes of the stimulated echo were shown to be affected by attenuation mechanisms due to the DCE and spin exchange which occurs between the protons of CH and CH3 (and/or CH2) groups of polyisoprene chains. The estimated spin exchange time was 0.04 s. The DCE free of undulations due to spin exchange was examined using a special modification of the radio frequency pulse sequence with additional pi-pulses inserted in the free-evolution intervals. Modulations of stimulated echo amplitudes could completely be suppressed. A strong dependence of the DCE on the cross-link density was observed. Attenuation curves of the dipolar-correlation quotient, that is, the quotient of the stimulated and the primary echo amplitudes, were fitted using analytical expressions. The mean-squared fluctuation of the dipolar coupling constant, , and correlation times were determined. In dry samples, values of were shown to scale with the number of segments between cross-links (N) as proportional to N-1.3 ± 0.2. The attenuation of echo amplitudes due to the DCE was insensitive to the increase of a carbon black content by approximately a factor of two. In swollen samples, attenuations of the stimulated echo were free of modulations by spin exchange thus permitting measurements of the DCE with a standard three-90 degrees pulse sequence. Contrary to dry samples, the attenuation curves contained slowly decaying components with relative intensities less than or equal to 10%. These components were subtracted from the total signal before the dipolar-correlation quotient was evaluated. scales with N with nearly the same exponent as in dry rubber. The DCE of dry and swollen rubber is suggested as a method for the determination of cross-link densities.
(C) 1999 American Institute of Physics.

DOI: 10.1063/1.480478   (Pobrane:  2020-10-25)