Double quantum NMR applied to polymer networks with low concentrations of pendant chains.

R. H. ACOSTA; G. A: MONTI; D. A. VEGA; M. A. VILLAR; E. M. VALLÉS

Mainz, Alemania

Simposio; GDCh Magnetic Resonance Division, 27th Discussion Meeting, ‘High Field Bio-NMR Symposium; 2005

GDCh Magnetic Resonance Division

Mechanical properties of a polymeric material can be directly related to its structure. In the particular case of polymer networks, chemical or physical cross-linking points give distinctive characteristics to these materials such as high elongation, better thermal stability and insolubility. Model silicone networks have been extensively studied in order to explain the influence of molecular structure on mechanical properties. Free chains and pendant chains are the more common defects in real networks; in particular, pendant chains have a strong influence on the viscoelastic properties of these networks, changing considerably the spectrum of relaxation times. However, there are few works in which the influence of network defects on the non-equilibrium properties was investigated. Recently we have sensitized model PDMS networks with controlled amounts of well characterized defects, which is an ideal system to reveal the basic mechanisms responsible for structural behaviors of more complex systems. Figure 1 is a schematic representation of a network where defects, such as free and dangling chains are shown. NMR techniques are suitable to unravel the fundamental properties of polymers. For cross linked elastomers above the glass transition temperature, Tg, NMR provides information on the structure and on the dynamics of the polymer chains in the bulk. Several 1H-NMR experiments like, transverse 1H relaxation (Hahn spin echoes), pseudo solid spin echoes, 2D-exchange experiments and multiple quantum (MQ) coherence excitation, have been used to probe different aspects of the structure and dynamics of the polymer chains at the molecular level. In particular, proton residual dipolar interactions measured by NMR provide information about the structure and molecular dynamics in soft solids like elastomers. Therefore changes in the structure and molecular dynamic can be investigated via changes induced in the residual dipolar couplings by cross-link density, the presence of fillers, the action of mechanical deformation forces, and other defects. One approach to model-free access to the residual dipolar couplings and dynamical order parameters uses MQ buildup curves recorded in the initial regime of the excitation/reconversion periods. In that work is proved rigurously that the revelant quantity for analysis of double-quantum (DQ) builds up curves in the initial regime is the second Van Vleck moment. In this work we tested the sensitivity of NMR to detect the influence of defects, at low concentrations, on the polymer network, by comparing DQ-NMR experimental data with experimental data obtained from rheological techniques. To our knowledge, no comparison between 1H NMR and dynamic elastic modulus (in the limit at the low frequency) experimental data have been made until now, as a function of the concentration of pendant chains.