The PhD meeting of this month will take place on January 29th. Akshay will give a talk about:
Studies on the effect of thermo-oxidative ageing in unfilled NR, SBR and their blends
time: 4 pm
place: SR 1.03 Von-Seckendorff-Platz 1
Elastomer blends are of extreme technological and economic importance as they offer greater possibilities in realizing the desired product properties compared to their constituent polymers. On a microscopic level, a difference in reactivity of the constituent rubbers in the blends may cause diffusion of curatives between the blend phases, leading to different extents of curing of the phases thereby affecting the properties of the blends . As a result, measurement of the phase-specific crosslink density (CLD) becomes important. Widely popular techniques such as equilibrium swelling of the vulcanizate based on Flory-Rehner equation , and stress-strain modeling (rheometric)  lack phase-resolved determination of CLDs. NMR has so far been successfully used for phase-resolved determination [3, 4], albeit qualitatively. In this work, the CLDs in sulfur-vulcanized natural rubber (NR), styrene butadiene rubber (SBR) single vulcanizates and their blend phases were measured by low-field time-domain  as well as high-field magic-angle spinning  multiple quantum 1H-NMR spectroscopy. The residual dipolar coupling constants (RDCCs), which are a measure of the CLDs, were obtained by the fitting parameters of the double-quantum build-up curves. A correlation of microstructural information for a range of blend compositions before and after thermo-oxidative ageing is thus made. For the unaged case, the distributions of the RDCCs suggest similar values for NR and SBR vulcanizates, indicating that both NR and SBR vulcanize to similar extents, which suggests that the vulcanization system distributes itself homogeneously over the blend phases. For the thermo-oxidatively aged specimens, a wider distribution of RDCCs was observed indicating modifications in the sulfur bonds with an added molecular rigidity for NR-rich recipes.
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