Comparative TMA and NMR investigation of a structure of carbon-black-free cured butadiene rubber

Abstract

Butadiene rubbers (BRs) differing in isomeric content and cured with different amounts of dicumyl peroxide (DCP) have, according to TMA investigations, regions with two glass transition temperatures (low and high) and different thermal expansion properties. BR network has chemical and cluster type (physical localised) branching junctions. For rubbers of the same isomeric content, shares of such regions depend on concentration of DCP. This fact suggests cis–trans isomerization at curing, which increases the content of a high-temperature region with predominantly chemical branching junctions, because cluster type branching junctions start to relax at T g″. It is supposed that an essential change in T g, if DCP concentration in the rubber compound is varied, is determined by the different molecular mobility of the chains included in associated structures of cluster junctions of a network of cured rubber. Such mobility also increases if 1,2 (vinyl) isomer is present in the BR chain. The latter is probably distributed uniformly between the chains of both regions. In a low-temperature region it could act as an antiplasticizer, whereas in a high-temperature region as a plasticizer. The molecular weight distribution of the chains between the junctions of a low-temperature region is virtually bimodal for low DCP concentration in rubber compounds. This could result from some zones with different crosslinking degree within the network. It is believed that this region has branching junctions, which are of a low crosslink density of chemical types and a high crosslinking density of cluster type junctions. A wide spectrum of isomeric content in neat and cured BR when a medium content of DCP was used meant that a low-temperature region had unimodal and symmetric MWD between the junctions. In a high-temperature region the exact opposite phenomenon may be observed — separation of chain segment length giving a bimodal MWD. A possible reason for this is non-uniform distribution of 1,2 (vinyl) isomer within the chains of a high-temperature region. The shift in MWD between the junctions from bimodal to unimodal as DCP content is raised, which was identified using TMA, confirms our 2H NMR findings. The two splits in the 2H NMR spectra observed at lower DCP contents could result from regions with different crosslink density, as we have argued in a recently published paper. All these conclusions confirm the applicability of the adopted TMA for testing a complex structure of rubbers.