Inhomogeneities and Chain Dynamics in Diene Rubbers Vulcanized with Different Cure Systems

ROLE OF COAGENTS IN PEROXIDE VULCANIZATION OF NATURAL RUBBER

The drawbacks of peroxide vulcanization can largely be overcome by introducing suitable co-curing agents (coagents) in the formulation. The role of various coagents, such as zinc diacrylate (ZDA), trimethylolpropane trimethacrylate (TMPTMA), and triallyl cyanurate (TAC) in the peroxide vulcanization of natural rubber (NR) was studied by Fourier transform infrared spectroscopy. Cross-link density was measured by the equilibrium–swelling technique. Cross-linking mechanism of peroxide in NR was interpreted by comparing the spectra of cured and uncured vulcanizates. The predominance of hydrogen abstraction over the radical addition was established (at 160 °C). Coagent ZDA produces ionic as well as covalent cross-links in the vulcanizate. Ionic cross-links have the ability to slip along the hydrocarbon chains and thus resemble polysulfidic cross-links. Hence, ZDA can be chosen for applications where good mechanical properties are required. Coagent TMPTMA produces covalent cross-links between polymer chains and is suitable for high-modulus applications. TAC, although it bridges through covalent cross-links, is not a suitable coagent for highly unsaturated rubbers like NR.

MICROSTRUCTURE AND MOLECULAR DYNAMICS OF ELASTOMERS AS STUDIED BY ADVANCED LOW-RESOLUTION NUCLEAR MAGNETIC RESONANCE METHODS

Nuclear magnetic resonance (NMR) certainly belongs to the most powerful spectroscopic tools in rubber science. Yet the often high level of experimental and in particular instrumental sophistication represents a barrier to its widespread use. Recent advances in low-resolution, often low-field, proton NMR characterization methods of elastomeric materials are reviewed. Chemical detail, as normally provided by chemical shifts in high-resolution NMR spectra, is often not needed when just the (average) molecular motions of the rubber components are of interest. Knowledge of the molecular-level dynamics enables the quantification and investigation of coexisting rigid and soft regions, as often found in filled elastomers, and is further the basis of a detailed analysis of the local density of cross-links and the content of nonelastic material, all of which sensitively affect the rheological behavior. In fact, specific static proton NMR spectroscopy techniques can be thought of as molecular rheology, and they open new avenues toward the investigation of inhomogeneities in elastomers, the knowledge of which is key to improving our theoretical understanding and creating new rational-design principles of novel elastomeric materials. The methodological advances related to the possibility of studying not only the cross-link density on a molecular scale but also its distribution and the option to quantitatively detect the fractions of polymer in different states of molecular mobility and estimate the size and arrangement of such regions are illustrated with different examples from the rubber field. This concerns, among others, the influence of the vulcanization system and the amount and type of filler particles on the spatial (in)homogeneity of the cross-link density, the amount of nonelastic network defects, and the relevance of glassy regions in filled elastomers.

Recoupled separated-local-field experiments and applications to study intermediate-regime molecular motions

A specific separated-local-field NMR experiment, dubbed Dipolar-Chemical-Shift Correlation (DIPSHIFT) is frequently used to study molecular motions by probing reorientations through the changes in XH dipolar coupling and T₂. In systems where the coupling is weak or the reorientation angle is small, a recoupled variant of the DIPSHIFT experiment is applied, where the effective dipolar coupling is amplified by a REDOR-like π-pulse train. However, a previously described constant-time variant of this experiment is not sensitive to the motion-induced T₂ effect, which precludes the observation of motions over a large range of rates ranging from hundreds of Hz to around a MHz. We present a DIPSHIFT implementation which amplifies the dipolar couplings and is still sensitive to T₂ effects. Spin dynamics simulations, analytical calculations and experiments demonstrate the sensitivity of the technique to molecular motions, and suggest the best experimental conditions to avoid imperfections. Furthermore, an in-depth theoretical analysis of the interplay of REDOR-like recoupling and proton decoupling based on Average-Hamiltonian Theory was performed, which allowed explaining the origin of many artifacts found in literature data.