Rheology of nitrile rubber with hybrid crosslinked network composed of covalent bonding and hydrogen bonding

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Abstract
A hybrid crosslinked network composed of covalent bonding and non-covalent bonding was constructed in nitrile rubber (NBR) by using the compound crosslinking agents dicumyl peroxide (DCP) and N,N-methylenebis acrylamide (MBA). DCP not only acted as a chemical crosslinking agent for NBR but also initiated MBA association onto the rubber chains. In addition to the crosslinked network of covalent bonds, the acylamino groups of MBA could associate reversibly into junctions between rubber chains to form additional hydrogen bonds. It was found that the total crosslinking density of NBR increased with the increasing MBA amount. Both dynamic mechanical analysis and dynamic rheological measurement results indicated that samples with more hydrogen bonding were more sensitive to deformation and temperature. For the vulcanized NBR samples with MBA, their glass transition temperature was higher than those of vulcanized samples only with DCP and slightly increased with increasing MBA amount. The existence of hydrogen bonding led to a shorter linear viscoelastic region in vulcanized NBR samples with MBA. In addition, compared with the samples vulcanized with only DCP, the vulcanized samples containing MBA presented a higher storage modulus in the low frequency region but a lower one in the high frequency region. These results indicated that the hydrogen bonding induced by MBA decreased both the motion ability of the chain segments between the crosslinking points and the energy dissipation generated by internal friction of the polymer chain.

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