Anca Gabriela Bejenariu

Investigation of the properties of fully reacted unstoichiometric polydimethylsiloxane networks and their extracted network fractions

We investigated the linear dynamic response of a series of fully reacted unstoichiometric polydimethylsiloxane networks and of the two corresponding network fractions namely the sol and the washed network. The sol and the washed network were separated by a simple extraction process. This way, it was possible to obtain rheological data from the washed network without interference from the sol fraction and furthermore from the sol fraction without interference from the elastic washed network. When the stoichiometry increased towards perfectly reacted networks and beyond, we observed harder networks both qualitatively and by rheology, and the properties of the two fractions became more and more different. At the gel point, the sol fraction and the washed networks have more or less identical properties which our data also show. The storage and loss moduli, G′ and G″, were analysed with the gel equation as proposed by Winter and Chambon (J Rheol 30:367–382, 1986) and Chambon and Winther (J Rheol 31:683–697, 1987). We observed that one of the investigated samples which before the swelling experiment did not show any elastic response gave an elastic washed network after swelling; this was verified by analysis with the gel equation. We also calculated the weight fraction of the sol fraction by using the theory by Villar et al. (Macromolecules 29(11):4072–4080, 1996) and compared this with experimentally found values.

Experimental evaluation of the pure configurational stress assumption in the flow dynamics of entangled polymer melts

A filament stretching rheometer was used for measuring the startup of uni-axial elongational flow followed by reversed bi-axial flow, both with a constant elongational strain rate. A narrow molecular mass distribution linear polyisoprene with a molecular weight of 483 kg/mole was subjected to the flow in the non-linear flow regime. This has allowed highly elastic measurements within the limit of pure orientational stress, as the time of the flow was considerably smaller than the Rouse time. A Doi–Edwards [J. Chem. Soc., Faraday Trans. 2 74, 1818–1832 (1978)] type of constitutive model with the assumption of pure configurational stress was accurately able to predict the startup as well as the reversed flow behavior. This confirms that this commonly used theoretical picture for the flow of polymeric liquids is a correct physical principle to apply.

High-dielectric permittivity elastomers from well-dispersed expanded graphite in low concentrations

The development of elastomer materials with a high dielectric permittivity has attracted increased interest over the last years due to their use in for example dielectric electroactive polymers. For this particular use, both the electrically insulating properties - as well as the mechanical properties of the elastomer - have to be tightly controlled in order not to destroy favorable elastic properties by the addition of particles. In the following, expanded graphite in low concentrations (up to 5 wt%) are investigated as a possible candidate as filler materials in very soft elastomers, which by the addition of traditional fillers in the necessary amounts would either lose their stability or their softness. Furthermore the influence of several mixing procedures on the electrical and mechanical properties is investigated.

Influence of micro- and nanofillers on electro-mechanical performance of silicone EAPs

The effect of different fillers on the mechanical and dielectric properties of soft elastomers has been investigated. It was found that the addition of a small amount of silica fillers would increase the Youngs modulus significantly but not simultaneously increase the tear strength sufficiently for processing as thin films. Addition of nanoclay and barium titanate nanoparticles to the soft elastomers was shown to be very favorable for the enhancement of the dielectric properties without increasing the Youngs modulus significantly and could be used for DEAP material in e.g. microprocessing where the tear strength is not crucial for processing. However, for elastomer film processing it is suggested that a combination of the nanoclay or barium titanate with either silica particles or bimodal networks would give the right tear strength together with the desired increased dielectric permittivity.