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
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Media type:
E-Article
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
Published in:
The Journal of Chemical Physics, 111 (1999) 24, Seite 11222-11231
Language:
English
DOI:
10.1063/1.480478
ISSN:
0021-9606;
1089-7690
Origination:
Footnote:
Description:
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 π-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, 〈δΩd2〉, and correlation times were determined. In dry samples, values of 〈δΩd2〉 were shown to scale with the number of segments between cross-links (N) as ∝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° pulse sequence. Contrary to dry samples, the attenuation curves contained slowly decaying components with relative intensities ⩽ 10%. These components were subtracted from the total signal before the dipolar-correlation quotient was evaluated. 〈δΩd2〉 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.