Gerhard Eder

Shear induced crystallization, a relaxation phenomenon in polymer melts : A re-collection

Thread‐like nuclei for the crystallization of polymers which are formed at high deformation rates in a temperature range close to the equilibrium melting point appear to be practically stable at temperatures where spherulites are melting. However, the fact that the great majority of experiments on flow induced crystallization have been carried out at temperatures below the melting temperature of the spherulites leads to the conclusion that the precursors for elongated structures, as formed under those conditions, are practically stable from the moment of their creation. In other words: their relaxation times are much longer than any deformation time applied. As a consequence, deformation times as independent parameters lose their importance in these experiments. Long lasting deformations under low stresses can yield the same precursors as short term deformations under high loads. Apparently the only condition is that the applied specific work is the same.

Crystallization kinetics of two aliphatic polyketones

Abstract This is to describe the kinetics of crystallization from the quiescent melt of two aliphatic polyketones (Shell Carilon) and to document a new device called ‘Thin Slice Experiment II’ that allows to determine the number of nuclei and the growth speed of spherulites of fast crystallizing polymers in the entire temperature range from the glass point to the melting point.

Capacitance Dilatometry for the in-situ Controlled Expansion Process of Cellular Polymer-Filler Composites (Ferroelectrets)

Ferroelectrets are unusual composites, consisting of a closed-cell polymer-filler foam structure. Ferroelectrets behave like ferroelectric materials during and after internally charging the voids of the cellular structure. In order to optimize the electromechanical properties of ferroelectrets, the voids within the cellular polymer composite are expanded in a controlled manner in a high pressure gas chamber. Thereby, the gas pressure is first increased, subsequently released and finally the expanded structure is stabilized by a heat setting treatment at elevated temperatures. Capacitance dilatometry provides an easy means for the in-situ monitoring of the multi-step foaming process, and therefore allows for the control of the degree of foaming. The mechanical stabilization of the expanded cellular polymer composite structure is explained in terms of the enhanced crystallinity as a result of the heat setting process.

Modelling of polymer crystallization in temperature fields

This paper is devoted to the mathematical modelling of polymer crystallization processes in a bounded region under heat transfer conditions, i.e. in time-dependent temperature fields. A stochastic model in a general setup is developed based on the theory of marked point processes, and a hybrid model on a macroscopic scale is derived using expected values. The stochastic modelling part is supplemented by a detailed discussion of the relevant parameters and their dependence upon temperature. In the special case of one-dimensional crystallization a system of partial differential equations describing the evolution of temperature and of the degree of crystallinity is derived.

In-Situ Optical Coherence Tomography (OCT) for the Time-Resolved Investigation of Crystallization Processes in Polymers.

By application of optical coherence tomography (OCT), an interferometric noncontact imaging technique, the crystallization of a supercooled poly(propylene) melt in a slit die is monitored. Both the quiescent and the sheared melt are investigated, with a focus on experiments where solidification and flow occur simultaneously. OCT is found to be an excellent tool for that purpose since the resultant structures are strongly scattering, which is a prerequisite for application of that method. The resulting images enable for the first time to directly monitor structure development throughout the whole experiment, including final cooling to room temperature. By rendering the setup polarization-sensitive, information on the birefringence of the pertinent structures is obtained.

Simultaneous detection of optical retardation and axis orientation by polarization-sensitive full-field optical coherence microscopy for material testing

We present a polarization-sensitive full-field optical coherence microscopy modality which is capable of simultaneously delivering depth resolved information on the reflectivity, optical retardation and optical axis orientation. In this way local birefringence, inherent stress–strain fields and optical anisotropies can be visualized with high resolution, as exemplified for various technical material applications.

In‐situ Monitoring of Polymer Crystallization by Polarization‐Sensitive Optical Coherence Tomography

The use of polarization‐sensitive optical coherence tomography for monitoring the progress of a classical shear experiment in a slit die leads to new insights in the mechanisms of polymer crystallization. For the first time it is possible to get both depth and time resolved information on the development of different layers as a result of the processing conditions. By additionally measuring the transmitted light the method is at the same time fully compatible to conventional birefringence measurements. Microscopic analysis of the extruded stripes completes the description of the crystallization process.

Model for the inflation process of cellular space-charge electrets

Ferroelectrets are closed-cell polymer foams that behave like ferroelectric materials during and after internally charging the voids of the cellular structure. In order to optimize the electromechanical properties of ferroelectrets, the voids within the cellular polymer are expanded in a controlled manner in a high pressure gas chamber. Thereby, the gas pressure is first increased, subsequently released and finally the expanded structure is stabilized by a heat setting treatment at elevated temperatures. Dielectric dilatometry provides an easy means for the in-situ monitoring of the multi-step foaming process, and therefore allows for the control of the degree of foaming. The mechanical stabilization of the expanded cellular polymer structure is explained in terms of the enhanced crystallinity as a result of the heat setting process. A simple model for the gas exchange process between the gas in the voids and the gas in the high pressure gas chamber is developed. For the model a simplification of the cellular structure is used, where an air-gap is separated from the electrodes by two polymer layers. The gas transport through the polymer is modelled by Ficks law with permeabilities taken from the solution diffusion gas transport model. Good agreement between model calculations and experiment is obtained for the diffusion expansion of cellular polypropylene with CO/sub 2/ and He as blowing agents.

The role of Minkowski functionals in the thermodynamics of two-phase systems

Within this work quite old concepts from integral geometry are applied to classical equilibrium thermodynamics of two-phase systems. In addition to the area as basic interfacial quantity the full geometric characterization of the interface is used, which includes the two remaining Minkowski functionals, the mean curvature integral and the Euler Poincare characteristic. The basic energetic characteristic of the interface (i.e. the interfacial tension) is extended by two additional properties: edge force as (up to a factor 4/π) the work necessary to form a right-angled edge of unit length, and item energy as the work to form an additional item in the phase morphology. Both quantities are of increasing importance, when going to micro- and nano-scales. They are subsequently used for interfaces of arbitrary shape to derive a relationship extending the classical Young-Laplace equation. The supplementary contribution is proportional to the Gaussian curvature, with the edge force as proportionality constant. Furt...