Abstract
Elastomers filled with hard nanoparticles are of great technical importance for the rubber industry. In general, fillers improve mechanical properties of polymer materials, e.g. elastic moduli, tensile strength etc. The smaller the size of the particles, the larger is the interface where interactions between polymer molecules and fillers can generate new properties. Using temperature-modulated differential scanning calorimetry and dynamic mechanical analysis, we investigated the properties of pure styrene-butadiene rubber (SBR) and SBR/alumina nanoparticles. Beside a reinforcement effect seen in the complex elastic moduli, small amounts of nanoparticles of about 2 wt% interestingly lead to an acceleration of the relaxation modes responsible for the thermal glass transition. This leads to a minimum in the glass transition temperature as a function of nanoparticle content in the vicinity of this critical concentration. The frequency dependent elastic moduli are used to discuss the possible reduction of the entanglement of rubber molecules as one cause for this unexpected behavior.
Highlights
Nowadays, many research works are dedicated to the modification of polymer properties using nanometer sized fillers [1, 3, 13, 16, 28, 30]
We present a system (styrene-butadiene rubber (SBR) filled with untreated alumina nanoparticles) where both accelerated as well as reduced molecular dynamics can be found for the same system
The slight increase of Tg from the pure SBR system to the system filled with 20 wt% of alumina nanoparticles measures up to expectations: (i) the mere presence of the nanoparticles hinders the flow/movement of the polymer molecules, (ii) interactions between SBR molecules and the nanoparticles’ surfaces are expected to slow down the molecular dynamics of the composite system
Summary
Many research works are dedicated to the modification of polymer properties using nanometer sized fillers [1, 3, 13, 16, 28, 30]. The mobility of polymer molecules is often reduced This can be due to e.g. confinement effects by the fillers or due to interactions between polymer molecules and filler particles. Even at small filler contents, changed molecular mobility leading to changes in the glass transition behavior has been reported [8,9,10]. We present a system (styrene-butadiene rubber (SBR) filled with untreated alumina nanoparticles) where both accelerated as well as reduced molecular dynamics can be found for the same system. This leads to a minimum in the glass transition temperature at small filler concentrations. Dynamic mechanical analysis (DMA), rheometry and temperature-modulated differential scanning calorimetry (TMDSC) together with structural investigation tools have been used to shed light on this unexpected experimental finding
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