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Erhard D.P.,Bayreuther Institute For Makromolekulforschung Bimf | Richter F.,Bayreuther Institute For Makromolekulforschung Bimf | Bartz C.B.A.,Bayreuther Institute For Makromolekulforschung Bimf | Schmidt H.-W.,Bayreuther Institute For Makromolekulforschung Bimf
Macromolecular Rapid Communications | Year: 2015

In view of the increasing significance of technology-driven devices such as microelectromechanical systems, energy-harvesting devices, and organic field effect transistors, polymer electret materials with durable electret performance at elevated temperatures become more and more important. However, typical polymer electret materials lose their performance at elevated temperatures. To provide polymer materials with improved electret performance over a broad temperature range, a series of aromatic polyimides with different degree of fluorosubstitution is presented. Isothermal surface potential decay measurements at elevated temperatures reveal that minor differences in the chemical structure have a major influence on the electret behavior. The best performance is found for the polyimide containing a hexafluoroisopropylidene moiety in both the bisanhydride- and the diamine-based unit. Excellent long-term charge storage stability at 120 °C is observed. From the initial surface charge 94% remains after 24 h. This polyimide even tolerates short-term exposure of 30 s at 300 °C with almost no loss of performance. These findings demonstrate that this particular polyimide is suitable for device applications at elevated temperatures during fabrication and use. Structure-property relations of a series of aromatic polyimides with different fluorine content reveal that hexafluoroisopropylidene moieties in both the bisanhydride and the diamine unit show the best electret performance. This polyimide features excellent long-term charge storage stability above 100 °C and even tolerates short-term exposure at 300 °C. These findings pave the way to electret device applications requiring elevated temperatures during fabrication or use. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Kersch M.,University of Bayreuth | Pischke L.,University of Bayreuth | Schmidt H.-W.,University of Bayreuth | Schmidt H.-W.,Bayreuther Institute For Makromolekulforschung Bimf | And 2 more authors.
Polymer (United Kingdom) | Year: 2014

The influence of two different trisamide-based additives on the morphological and mechanical behavior of isotactic polypropylene (PP) is investigated. Morphological investigations by wide-angle X-ray scattering (WAXS) show that only 0.04 wt.-% of one of the additives is enough to induce 80% beta modification in the crystalline part of the polymer. The materials are investigated with quasistatic (tensile testing) as well as short-time dynamic (Charpy impact strength) methods and for the first time fatigue crack growth measurements are performed on alpha- and beta-nucleated PP. In all cases a much higher toughness of the material containing the beta-nucleating agent (beta NA) can be observed. The crack growth propagation rate is one order of magnitude smaller with the use of the beta-nucleating agent compared to the alpha polymer. Scanning electron microscopy (SEM) pictures are used to support the correlation of the mechanical behavior with the morphological changes. © 2014 Elsevier Ltd. All rights reserved.

Abraham F.,Bayreuther Institute For Makromolekulforschung Bimf | Schmidt H.-W.,Bayreuther Institute For Makromolekulforschung Bimf
Polymer | Year: 2010

This paper presents 1,3,5-benzenetrisamides as colorless α-nucleating agents for poly(vinylidene fluoride). In order to screen a large variety of 1,3,5-benzenetrisamide derivatives with respect to their nucleating potential an efficient and reliable test based on polarized light microscopy was established. For selected promising compounds the concentration dependence of the PVDF crystallization temperature, the dissolution behavior of the additive in the polymer melt, and the crystallization of the additive from the polymer melt was investigated in a concentration range between 1 wt% (10,000 ppm) and 70 ppm. It was found, that only two of the investigated compounds were able to raise the crystallization temperature about 8 °C at a concentration of 140 ppm and 580 ppm, respectively. These trisamides have the advantage being soluble in the polymer melt, not featuring absorption of visible light and therefore allowing the preparation of uniform and colorless PVDF products. © 2010 Elsevier Ltd. All rights reserved.

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