Zdeněk Starý

Steady states in extensional flow of strain hardening polymer melts and the uncertainties of their determination

The existence of a plateau, a maximum or even an overshoot of the elongational viscosity of polymer melts as a function of time or extension, respectively, is a hot topic of scientific discussions. Although not essential for the relation of elongational properties to applications, the feature is crucial for a verification of various theories. From a critical review of sophisticated comparative stressing and creep experiments on a low density polyethylene (LDPE), the existence of a pronounced stationary elongational viscosity becomes very probable. These conclusions are supported by investigations on a long-chain branched polypropylene. For an LDPE of higher molar mass with stress maxima caused by local necking within the sample, the maximum viscosities agree with the corresponding steady-state viscosities obtained from creep experiments performed by the same rheometer. The decisive role of the uniformity of the sample deformation for the accuracy of elongational experiments is pointed out. From the findin...

Behavior of Conductive Particle Networks in Polymer Melts under Deformation

The electrical properties of the composites containing conductive fillers are given by the structure of the particle network. Therefore, the behavior of these networks under deformation in the molten state is of great interest in order to obtain materials with desirable electrical properties. In this work the composites containing carbon black or carbon fibers were exposed to a shear deformation in two different experimental modes—oscillation and creep and the electrical conductivity of the materials was measured simultaneously.

Shear induced electrical behaviour of conductive polymer composites

The time-dependent electrical resistance of polymethylmethacrylate containing carbon black was measured under oscillatory shear in the molten state. The electrical signal was oscillating exactly at the doubled frequency of the oscillatory shear deformation. Moreover, the experimental results gave a hint to the development of conductive structures in polymer melts under shear deformation. It was shown that the flow induced destruction of conductive paths dominates over the flow induced build-up in the beginning of the shear deformations. However, for longer times both competitive effects reach a dynamic equilibrium and only the thermally induced build-up of pathways influences the changes in the composite resistance during the shear. Furthermore, the oscillating electrical response depends clearly on the deformation amplitude applied. A simple physical model describing the behaviour of conductive pathways under shear deformation was derived and utilized for the description of the experimental data.