APPLICATIONS OF FT-NMR TO CROSS-LINK DENSITY DETERMINATIONS IN NATURAL-RUBBER BLEND VULCANIZATES

Previously 90 MHz CW-NMR spectroscopy has been used to estimate the crosslink density in individual components of elastomer blends. Transfer of the technique to a 300 MHz FT instrument is not straightforward. Chemical shifts of polymer resonances in spectra of single-polymer vulcanizates are dependent on crosslink densities of the vulcanizates. Additionally, two resonances are observed for small molecules such as TMS and residual protonated solvent. The smaller resonance of each pair changes in shape and position in synchrony with the polymer signals and is considered to originate from solvent within the swollen polymer. The secondary TMS peak is used as the reference position from which to locate polymer signals in the spectrum. The position of these secondary peaks, and thus the polymer spectrum, relative to the "free' TMS is a function of crosslink density and also of carbon-black loading and type in filled vulcanizates. C-13-NMR line widths are observed to increase with crosslink density and this effect was used to study blends of NR with EPDM or a maleic acid modified EPDM. NR crosslink densities determined from C-13 line-width analyses were in good agreement with those obtained from H-1-NMR and increased crosslinking in the modified EPDM was confirmed.