NMR Studies on the Phase-Resolved Evolution of Cross-Link Densities in Thermo-Oxidatively Aged Elastomer Blends

Abstract

A knowledge of the distribution of cross-link densities (CDs) in the constituent rubber phases of an elastomer blend is essential to understand its overall service properties, but phase-resolved CD estimations have so far been limited to qualitative assessments. In a first study of its kind, we demonstrate a phase-resolved quantification of the CDs in sulfur-cured blends of natural rubber (NR) and styrene–butadiene rubber (SBR) using solid-state 1H homonuclear dipolar double-quantum (DQ) magic-angle spinning (MAS) NMR spectroscopy. The blends were also subjected to prolonged thermo-oxidative aging to monitor the chemical changes in the two phases. Analyses of the residual dipolar coupling constant (D res), arising due to spatial restrictions by cross-links to molecular motions, as observed in MAS and static 1H DQ experiments, suggest that unaged NR and SBR cross-link to similar extents. A minimum in D res after around 500 h of aging duration is observed in NR, associated with the formation of highly mobile defect fractions, as seen from free induction decay (FID) combined with transverse relaxometry, with further cross-linking up to 1000 h. SBR appears more stable, with a gradual increase in D res over the aging duration. Phase-resolved experiments reveal a somewhat less cross-linked SBR phase in an unaged 50:50 blend. The phase-specific distribution of CDs in the blend phases becomes significant upon aging, which suggests that NR ages more strongly in the blend as compared to the aged single vulcanisates, thus dictating the blend properties.

Effects of artificial weathering in NR/SBR elastomer blends

“…At the extended weathering durations, the coupling strengths of the blend are intermediate to those of NR and SBR. Interestingly, no significant variation of Dmed,1 can be observed for the three materials between 504 h and 988 h. Also, the invariance of the corresponding coupling widths suggests no significant chemical modifications in the samples beyond 500 h. In comparison to the previous work, 17 these trends highlight that sulfur-cross-linked NR is chemically unstable at higher temperatures (thermo-oxidative aging at 80 °C), whereas sulfurcross-linked SBR is more susceptible to artificial weathering.…”

“…Compounds of NR, SBR, and NR/SBR blend (50/50 ratio by parts per hundred rubber, phr) were prepared using the composition reported in a previous work 17 The three compounds were formulated based on a conventional vulcanization system consisting of 2.5 phr sulfur (Carl Roth GmbH, Germany) and 1.5 phr n-cyclohexyl-2-benzothiazole sulfenamide (CBS) accelerator (TCI Deutschland GmbH, Germany), along with 1 phr stearic acid (Carl Roth GmbH, Germany) and 3 phr zinc oxide (Carl Roth GmbH, Germany) activators.…”

“…A modified bimodal fitting function 17 can be used to separately fit these hardened surface fractions, and those that lie beneath that are comparatively less aged. The input parameters to the fitting function are a fixed coupling value Dmed,2/2π = 12 kHz, distribution value σln,2 = 0.01, and constrained fraction value obtained from the T2 analyses.…”

“…55 The same observation has been made earlier with NR exposed to thermo-oxidative aging at 80 °C. 17 Upon continued weathering the bulk of NR undergoes a reduction in cross-link density due to the decomposition of main chains. These changes are also evident in the distribution width.…”

“…Early phase-resolved studies using NMR involved the measurement of proton line-widths in solvent-swollen blend specimens. [13][14][15][16] This approach is qualitative at best, as discussed in recent work by this group, 17 in which use of a quantitative alternative was made, as explained below. Using a more direct method, proof of curative migration due to diffusion and solubility in natural rubber and polybutadiene blends has been demonstrated by Klei and Koenig by NMR imaging based upon a Carr-Purcell spin-echo pulse sequence with a gradient magnetic field.…”