J. L. Valentín, P. Posadas, A. Fernández-Torres, M. A. Malmierca,
L. Gonzáles, W. Chassé, K. Saalwächter.
and Chain Dynamics in Diene Rubbers Vulcanized with Different Cure Systems.
Macromolecules 43, 4210-4222 (2010). DOI 10.1021/ma1003437.
In this study, we focus on qualitative differences in the network structure and
dynamics of natural as well as poly(butadiene) rubber in dependence of the cure
system (sulfur/accelerator or organic peroxide) used in the vulcanization
process. The spatial homogeneity of the distribution of chemical and physical
cross-links in the network is assessed via the quantitative measurement of
proton-proton residual dipolar couplings as measured by static multiple-quantum
(MQ) NMR spectroscopy at low field. The experiment also provides information on
the apparent correlation time of fast segmental fluctuations that dominate chain
relaxation processes at lower temperature, for which we also find characteristic
differences. Vulcanization via a radical mechanism (using organic peroxides)
leads to networks with a high content of nonelastic defects (loops or dangling
chains), a rather inhomogeneous distribution of cross-links, and modified
(slower) local dynamics, as compared to networks obtained by sulfur
vulcanization. These microstructural factors can be related with the well-known
differences in the macroscopic properties of diene rubbers vulcanized with
different cure systems.