Network density and diene conversion in peroxide-cured gumstock EPDM rubbers. A solid-state NMR study

Peroxide-cross-linked EPM and EPDM rubbers have been investigated with magic-angle spinning 1H NMR spectroscopy and static 1H NMR relaxometry. The results yield a consistent nanoscale picture of the chemical and physical network properties in terms of chemical cross-links formed via macro-radical combination, chemical cross-links formed via addition to the double bonds, and physical cross-links resulting from chain entanglements.status: publishe

Rubber-Filler Interactions and Network Structure in Relation to Stress-Strain Behavior of Vulcanized, Carbon Black Filled EPDM

Immobilization of EPDM chains on the surface of carbon black and network structure in the rubber matrix of filled EPDM rubbers were studied by low-field proton NMR experiments. Advanced NMR experiments unambiguously show strong immobilization of EPDM chain fragments on the surface of carbon black. The thickness of the immobilized EPDM–carbon black interfacial layer is estimated to be ≥0.6 nm. The average number of monomer units per adsorption site is approximately nine, which suggests preferential chain adsorption at the crystal boundaries of carbon-black particles. The adsorbed chain fragments form physical (adsorption) junctions restricting chain mobility in the rubbery matrix outside of the interface. The cross-link density in filled EPDM is determined as a function of the filler type and its amount. The contribution of adsorption junctions to the total cross-link density is moderate as compared to the density of chemical cross-links and entanglement density. The mechanically effective network density in carbon-black-filled vulcanizates is determined by analysis of the stress–strain curves on the basis of the dynamic flocculation model. Comparison of the network density as measured by NMR and mechanical experiments shows significant differences which helps in better understanding of the reinforcement mechanism of filled rubbers. The study demonstrates that a relatively small amount of strongly adsorbed chains impacts the stress–strain properties of filled elastomers significantly.status: publishe

Mechanism for Peroxide Cross-Linking of EPDM Rubber from MAS C-13 NMR Spectroscopy

Detailed information about the chemical role of the third monomer 5-ethylidene-2-norbonene (ENB) in EPDM during peroxide cross-linking has been obtained by use of MAS 13C NMR spectroscopy and 1H NMR T2 relaxometry. Advanced 13C NMR techniques like INADEQUATE and TOCSY applied to a special EPDM grade with 13C-labeled ENB reveal a large number of new aliphatic and olefinic signals as well as indications for oxidation. The ENB unit is involved in cross-linking reactions not only (i) via addition of macroradicals to the pendent ENB unsaturation yielding aliphatic cross-link structures but also (ii) via combination of ENB-derived allyl radicals resulting in cross-link structures with intact unsaturation. The latter represents a novel pathway in the mechanism for peroxide curing of EPDM.status: publishe

Network density and diene conversion in peroxide-cured gumstock EPDM rubbers : a solid-state NMR study

Peroxide-cross-linked EPM and EPDM rubbers have been investigated with magic-angle spinning 1H NMR spectroscopy and static 1H NMR relaxometry. The results yield a consistent nanoscale picture of the chemical and physical network properties in terms of chemical cross-links formed via macro-radical combination, chemical cross-links formed via addition to the double bonds, and physical cross-links resulting from chain entanglements

Solid State Solubility of Miconazole in Poly[(ethylene glycol)-g-vinyl alcohol] Using Hot-Melt Extrusion

The use of hot-melt extrusion for preparing homogeneous API-excipient mixtures is studied for miconazole-PEG-g-PVA [poly(ethylene glycol)-poly(vinyl alcohol) graft copolymer] solid dispersions with a 5 cm(3) table-top, twin-screw corotating microcompounder (DSM Xplore). Phase behavior of PEG-g-PVA, miscibility of miconazole in PEG-g-PVA and the partitioning of miconazole between PEG and PVA amorphous phases are characterized using a combination of modulated DSC, XRPD, and solid-state (1)H and (13)C NMR methods. The (1)H NMR transverse magnetization relaxation (T(2) relaxation) method is used to analyze the phase composition and molecular mobility of the copolymer. The T(2) relaxation decay of pure PEG-g-PVA can be described by four T(2) relaxation components in the temperature range studied. PVA crystallinity is not largely affected by hot-melt extrusion and the presence of the drug. Miconazole preferably resides in the PEG amorphous phase, and its molecules are well dispersed in the PEG-g-PVA matrix using hot-melt extrusion mixing. Miconazole forms amorphous nanoclusters whose average size equals approximately 1.6 nm, indicating solid solution formation (molecular level dispersion) of the drug in the polymer.status: publishe