Crosslink density is an important structural parameter for cured rubber. Natural rubber (NR) vulcanizates with different crosslink densities were obtained through using different sulfur and accelerator amounts and different accelerator types. The crosslink density was characterized by an 1 H-NMR technique and its influence on mechanical properties, such as Shore A hardness, 300% modulus, tensile strength, and elongation at break, of NR vulcanizates was investigated. The results showed that both the sulfur amount and the accelerator type and amount had an influence on the crosslink density of the NR networks. The relationship between total crosslink density and mechanical properties was also studied. The results, by changing either the sulfur or the accelerator amount, showed that tensile strength of NR vulcanizates reached maximum value when the total crosslink density was around 13.5 × 10−5 mol/cm3, equivalently the average molecular weight of the intercrosslink chains (Mc) was around 7000 g/mol. The max...

Influence of Crosslink Density on Mechanical Properties of Natural Rubber Vulcanizates

The effects of γ-radiation on the thermal, mechanical, and segmental dynamics of a silica filled, room temperature vulcanized polysiloxane rubber

The network structure of UV-cured acrylates was analyzed by dynamic mechanical experiments, 1H T2 NMR relaxation, and 13C NMR spectroscopy Photocured mixtures of difunctional (poly(ethylene glycol) diacrylate) and monofunctional (2-ethylhexyl acrylate) were studied The mean cross-link density and fraction of dangling chains were varied in these networks by changing the content of monofunctional acrylate Examining networks by 13C NMR spectroscopy, we observed that the conversion is high, and the contribution or the effect of side reactions that generate extra cross-links is negligible The spatial distribution of cross-links is very heterogeneous in the cured samples; ie, polyacrylate ziplike network junctions are interconnected by polyether chains An increase in the content of monofunctional monomer causes a significant decrease in the cross-link density because the monofunctional monomer acts as a chain extender NMR T2 relaxation method shows good correlation with mechanical tests with respect to 

Analysis of Network Structure of UV-Cured Acrylates by 1H NMR Relaxation, 13C NMR Spectroscopy, and Dynamic Mechanical Experiments

We have measured changes in transverse relaxation times (T2e), residual dipolar couplings (〈Ωd2〉), and the mean-squared fluctuations in the residual dipolar couplings (〈δΩd2〉), associated with cross-link density changes in a complex silica-filled polydiphenylpolysiloxane/polydimethylsiloxane (PDPS/PDMS) block copolymer composite material. The crosslinks were induced by both chemical modification of the base polymer and by radiolytic aging. We have detected 1H NMR responses from polymer chains directly interacting with the silica filler (〈δΩd2〉≫2×106 rad2 sec−2), network polymer chains not directly interacting with the silica filler (〈Ωd2〉∼2×106 rad2 sec−2 and 〈δΩd2〉∼2×106 rad2 sec−2) and non-network, low molecular weight chains and chain ends (〈δΩd2〉∼1×105 rad2 sec−2). The network domain and the non-network domain were observed to exchange spin magnetization with a correlation time of 1 sec. No evidence of spin-exchange effects on the stimulated echo were observed between the PDMS and PDPS blocks, althoug...

Residual dipolar coupling for the assessment of cross-link density changes in γ-irradiated silica-PDMS composite materials

Quantifying rubber degradation using NMR

13C-NMR MAS solid-state spectroscopy and 1H-NMR relaxation measurements have been applied for quantitative determination of crosslink density in sulfur-accelerated NR vulcanizates. It is shown that the 13C-NMR method is able to distinguish between monosulfidic crosslinks and polysulfldic structures; therefore a quantitative determination of chemical crosslinks was possible. An analysis of 13-NMR transversal relaxation measurements provided the network densities which were in good agreement with the results of the 13C-NMR measurements. Comparing stress-strain data with the spectroscopically determined crosslink density of the NR vulcanizates and calculated crosslink densities of peroxide-cured vulcanizates, it is shown that the same relation between low-strain modulus and crosslink density is valid independent of the structure of the vulcanizates. The chemical crosslink density determined by a theoretical approach based on the tube concept was in approximate agreement with the NMR results.

Structure and density of crosslinks in natural-rubber vulcanizates. A combined analysis by NMR spectroscopy, mechanical measurements, and rubber-elastic theory

Two NMR spectroscopic methods were applied to carbon black filled NR vulcanizates to determine the chemical crosslink density. Both methods give corresponding results. Addition of carbon black leads to a decrease of crosslink density with increasing filler content. The influence is more pronounced at higher sulfur content

Determination of crosslink densities of carbon black filled natural rubber sulfur vulcanizates by 13C NMR MAS spectroscopy and 1H NMR transversal relaxation

Gerald Simon

Papers

Structure and density of crosslinks in natural-rubber vulcanizates. A combined analysis by NMR spectroscopy, mechanical measurements, and rubber-elastic theory

Determination of crosslink densities of carbon black filled natural rubber sulfur vulcanizates by 13C NMR MAS spectroscopy and 1H NMR transversal relaxation