Influence of magic angle spinning on T1H of SBR studied by solid state 1H NMR

Abstract

Listen

Translate article

We have investigated the influence of the high centrifugal pressure caused by fast magic-angle spinning (MAS) on the molecular motion of styrene–butadiene rubbers (SBR) filled with SiO2 (SBR/Si composite) though solid-state magic-angle spinning nuclear magnetic Resonance (1H MAS NMR) measurements. Because the 1H–1H dipolar interaction of elastomers is weak compared with that of glassy polymers, a narrower 1H linewidth is observed under fast MAS. The temperature dependence of the 1H spin-lattice relaxation time (T1H) revealed that the T1H minimum increases with the MAS rate. Furthermore, we observed a difference in the temperature dependence of T1H between end-chain-modified SBR and normal (unmodified) SBR in the SBR/Si composites. The temperature dependence of T1H is described by the Bloembergen–Purcell–Pound theory, with the assumption that the correlation time obeys the Williams–Landel–Ferry empirical theory. The fitting showed that the molecular motion does not change significantly until a MAS rate of 20 kHz, with the motional mode changing considerably at a MAS rate of 25 kHz. The motion of SBR in the unmodified SBR/Si composite was greatly affected by the fast MAS rates. Furthermore, the plot of the estimated centrifugal pressure versus the T1H minimum resembled the stress–strain curve. This result enables the detection of macroscopic physical deformation by the microscopic parameter T1H. Temperature-dependent curves of T1H for the SBR with and without end-chain modification in the SBR/Si composites showed the increment of the T1H values as the MAS rate increases and the considerable change at the fast MAS rate of 25 kHz. The increase of T1H values is represented by the factor f that decides the observed T1H minimum value. The plot of centrifugal pressure caused by MAS as a function of 1/f exhibited the similar trend to that of the stress–strain curve. The differences in the T1H between the end-chain-modified and normal SBR under fast MAS was attributed to the Payne effect.