H.-W. Schmidt

University of Bayreuth , Bayreuth, Bavaria, Germany

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Publications (28)83.61 Total impact

  • M. Kersch, L. Pischke, H.-W. Schmidt, V. Altstädt
    Polymer. 01/2014;
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    ABSTRACT: The influence of physical aging on the electret properties before corona charging of three amorphous polymers, polyetherimide (PEI), poly(phenylene ether) (PPE), and polystyrene (PS), as well as with blends of PPE and PS, was investigated. The degree of aging was monitored by determining the enthalpy relaxation h using differential scanning calorimetry (DSC). The electret performance was evaluated by isothermal potential decay (ITPD) at elevated temperatures and by thermal stimulated discharge (TSD) measurements. It was demonstrated that physical aging below the glass transition temperature substantially improves the electret performance of amorphous polymers by reducing the free volume and thus hindering charge motion. As an example, the performance of nonaged PEI was improved by physical aging at 200 °C for 4 days from 18 to 95% retained charge after 24 h at 120 °C. A similar beneficial influence of physical aging on the charge storage capability was achieved using blends of PPE with PS.
    Journal of Polymer Science Part B Polymer Physics 03/2010; 48:990-997. · 2.22 Impact Factor
  • D.P. Erhard, R. Giesa, V. Altstädt, H.-W. Schmidt
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    ABSTRACT: The performance of a polymeric electret material depends on many parameters, and besides chemical structure, charging conditions, and application temperature, other factors, such as grade, manufacturer, processing history, and additive package, are critical. Commercial polyetherimide (PEI) Ultem®1000 films exhibited satisfying electret properties as revealed by an isothermal potential decay (ITPD) to 75% of the initial surface charge after 24 h at 90°C. It was found that after purification by reprecipitation this value drops to 34% and that the same PEI synthesized by two different methods revealed to be a very poor electret with charge retention of almost zero. Assuming that an additive in the commercial material might be responsible for this behavior, we identified an organophosphonite which is commonly used as antioxidant in high-temperature polymers. We incorporated this additive by melt compounding into purified PEI and found a dramatic increase in charge retention to 79% of the initial charge at an additive load level of 0.5 wt %. By immersing Ultem®1000 films in water, the electret behavior was further improved and almost 100% charge retention was achieved.
    J. Appl. Polym. Sci. 03/2010; 115:1247-1255.
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    ABSTRACT: Composition comprises at least one polycarbonate and at least one isoindole 1,3-dione derivative (I). Composition comprises at least one polycarbonate and at least one isoindole 1,3-dione derivative of formula (I). R 1-R 6, R 1 3-R 1 6H, (pseudo)halo, OH, NH 2, NO 2, 1-20C alkyl, 1-20C cycloalkyl, 2-20C alkenyl, 2-20C alkynyl, 1-20C aryl, 1-20C alkoxy, 1-20C acyl, 1-20C aldehyde, 1-20C carboxylic acid, 1-20C carboxylic acid ester, amide or acyl halo; m, n : 0-10; and T 1, T 2organic group. Independent claims are included for: (1) the isoindole 1,3-dione derivative (I), where m and n are 1-10 and T 2 is phenoxy phenyl moiety of formula (II); (2) a polymer element comprising the composition; (3) preparing the polymer element from the composition by pressing, scraping, spreading, centrifuging, spraying, extrusion, blow molding and/or compacting; (4) an electret element comprising the polymer element that is charged; (5) preparing the electret element, comprising producing the polymer element and charging it; and (6) electromechanical converter comprising the polymer element or the electret element. R 6 1-R 7 4H, (pseudo)halo, OH, NH 2, NO 2, 1-20C alkyl, 1-20C cycloalkyl, 2-20C alkenyl, 2-20C alkynyl, 1-20C aryl, 1-20C acyl, 1-20C aldehyde, 1-20C carboxylic acid, 1-20C carboxylic acid ester, amide or acyl halo.
    Ref. No: EP2159222 (A1), Year: 03/2010
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    ABSTRACT: An electret is a dielectric material exhibiting a quasi-permanent electric charge. The aim of this work is to give access to polycarbonate (PC) electret materials with a distinctly improved electret performance. In this context, the charge storage capabilities of corona charged films of several PC grades were investigated by isothermal potential decay measurements at 90°C. The most promising PC grade was further improved by a new class of bisimide additives. The best additive at the most efficient concentration in combination with a physical aging step yielded excellent PC electrets maintaining almost their complete applied charge even at 90°C.
    Macromol. Chem. Phys. 01/2010;
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    ABSTRACT: The charge decay in isotactic polypropylene (i-PP) films of 50 µm thickness, containing three kinds of additives, namely a trisamide, a bisamide and a fluorinated compound, with concentrations in the range 0.004–1 wt% was studied. Compression molding was used to produce the films. The samples were either surface-charged by a corona method or volume-charged by mono-energetic electron beams of different energies, having penetration depths up to 6 µm. In all cases, surface potentials of about 200 V were chosen. After charging the films, the decay of the surface potential was studied either by an isothermal discharge method at 90 °C or by thermally stimulated discharge measurements. The results show a dependence of the decay rate on the kind of additive used, on additive concentration and on the energy of the injected charges. In particular, for samples with fluorinated additives, the stability of the surface potential decreases markedly with increasing electron energy, while such a dependence is very weak for samples containing the bisamide additive and does not exist at all for samples with the trisamide additive. These observations are tentatively explained by the radiation-induced generation of relatively mobile negative ions originating from the bisamide and fluorinated additives.
    Journal of Physics D Applied Physics 02/2009; 42:065410 (8pp). · 2.53 Impact Factor
  • D.P. Erhard, R. Giesa, H.-W. Schmidt
    8th International Technical Symposium on Polyimides & High Performance Functional Polymers (STEPI 8); 06/2008
  • D.P. Erhard, R. Giesa, V. Altstadt, H.W. Schmidt
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    ABSTRACT: The aim of the present work was to synthesize PEIs with different degrees of fluoro substitution in order to further improve the electret performance of the promising Ultemreg1000 and to better understand the correlation of the polymer chemical structure with the electret properties.
    Electrets, 2008. ISE-13. 13th International Symposium on; 01/2008
  • D. Lovera, V. Altstadt, H.-W. Schmidt
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    ABSTRACT: In this study, multiphasic blends were prepared by the addition of different styrene-based-copolymers to a blend PPE/PS of 75/25 composition. The aim is to give insights into the influence of the polymer chemical structure on the charge storage of polymer blends.
    01/2008;
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    ABSTRACT: Cellular polymer films have an enhanced charge storage capability, since the charge drift across the film is hindered by the presence of voids. Furthermore, modulating the morphology of those cellular polymer structures, very high piezoelectric activities can be obtained. In this work, polypropylene (PP) and polyetherimide (PEI) cellular films were produced using novel methods and their morphology, charge storage properties and piezoelectricity were investigated.
    01/2008;
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    ABSTRACT: In the present paper, the effect of several additives on charge transport in specially prepared polypropylene PP samples is investigated.
    01/2008;
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    ABSTRACT: A comparative study on the influence of chemical structure and solubility of a series of low-molecular-weight 1,4-phenylene-bisamides in isotactic polypropylene (i-PP) was conducted to explore their performance as electret additives. The effect of the symmetry of the molecules, of the type of substitution and of the length of the alkyl chain on the solubility in the i-PP melt and on the charge storage properties was investigated.
    01/2008;
  • D. Lovera, V. Altstadt, H.-W. Schmidt
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    ABSTRACT: The charge storage behavior is a fundamental property of polymers used in electromechanical-transducer applications. Polymer blends can potentially show a higher charge storage capability than the neat constituents, since the phase boundaries between the individual components can act as trapping sites. In this work, the charge storage behavior of polyphenylene ether (PPE) is enhanced by the addition of selected amounts of polystyrene (PS) and poly(styrene-isobutylene-styrene) (SIBS). While the binary blends PPE/PS are miscible and are expected to form one homogeneous phase, the ternary blends PPE/PS/SIBS are multiphasic. The blend charge storage behavior was related to the macro- and micro- morphology as well as to the charging method.
    01/2008;
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    ABSTRACT: Enhancing the charge storage capability of thermoplastic polymers can be achieved at least by (i) the creation of obstacles such as highly elongated pores in the polymer film to substantially lengthen the pathway for charge recombination, (ii) the tailored chemical modification of the polymer backbone, and (iii) the addition of functional additives to create charge traps. This work focuses on the functional additive approach. Novel compounds to improve the electret performance of isotactic polypropylene (i-PP) were synthesized, which act as charge traps.
    01/2008;
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    ABSTRACT: In this paper we describe the use of combinatorial vapor deposition techniques for the optimization of blue organic light emitting diodes (OLEDs). In these devices the star shaped molecule 1 with a triphenylamine core and three fluorene side groups serves as hole transport and emitting layer. Compound 2 with a much lower lying HOMO and a larger bandgap is used as hole blocking layer. Using combinatorial vapor deposition 42 OLEDs with thickness gradients of both the hole transport and the hole blocking layer have been simultaneously prepared on one substrate. The physical characterization of the devices clearly shows that a hole blocking layer of the star shaped molecule 2 is necessary in order to obtain pure blue emission with CIE coordinates of x = 0.15 and y = 0.15. A thickness of only 5 nm of the blocking layer is sufficient, and with increasing layer thickness the brightness of the blue devices drops. The blue devices exhibit a brightness of 400 cd m–2 and a luminous efficiency of 2 cd A–1. The thickness variations of both the hole transport and the hole blocking layer have been made in one combinatorial evaporation experiment on a single substrate using a set of movable masks. This demonstrates how efficient combinatorial methods can be used for the development of OLEDs.
    Advanced Functional Materials 10/2007; 17(17):3456 - 3461. · 9.77 Impact Factor
  • Advanced Functional Materials 08/2007; 17(14):2336 - 2342. · 9.77 Impact Factor
  • D.P. Erhard, R. Giesa, V. Altstädt, H.-W. Schmidt
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    ABSTRACT: The aim of this work was to establish and better understand the structure-property relationship of fluorinated arom. poly(ether imide)s (PEI) with respect to their electret performance. Therefore two series of PEIs with increasing fluorine content were synthesized. All PEIs are amorphous materials with glass transition temps. above 220 Deg. Thin films with thicknesses around 100 micro m were compression molded and corona charged, and isothermal surface potential decay was investigated at 90 Deg as an accelerated test for the electret performance. In comparison to the corresponding non-fluorinated PEI, an improvement of up to thirty-fold was achieved by the incorporation of trifluoromethyl moieties. Out of the investigated fluorinated PEIs, the polymer with the best charge storage performance maintained 87% of the initial surface potential charge after annealing at 90 Deg for 24 h. It was also found that, within the investigated series, the highest fluorine content does not necessarily result in the best charge-storage properties.
    Macromolecular Chemistry and Physics 05/2007; 208:1522-1529. · 2.39 Impact Factor
  • M. Häckel, L. Kador, D. Kropp, H.-W. Schmidt
    Advanced Materials 01/2007; 19(2):227 - 231. · 14.83 Impact Factor
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    ABSTRACT: The paper presents the synthesis of azobenzene-functionalized block copolymers based on a poly(methyl methacrylate) (PMMA) segment and an azobenzene-functionalized poly(hydroxyethyl methacrylate) segment, and a basic study of blending these block copolymers with homopolymers is given. Two diblock copolymers, prepared via different routes, were synthesized by a living anionic polymerization followed by a polymer analogous reaction to attach the azobenzene side groups. Self-assembly of the block copolymers resulted in phase-separated morphologies on the nanometer scale. The photoaddressable azobenzene segments are dispersed in the PMMA matrix and locally confined. Special focus is given to the preparation and characterization of block copolymer blends with PMMA homopolymer in order to dilute the phase-separated azobenzene morphology and reduce the optical density while maintaining the confinement. The block copolymer blends were characterized with respect to their morphology and initial holographic experiments were performed.
    Macromolecules. 01/2007; 40(6):2100-2108.
  • Microscopy and Microanalysis - MICROSC MICROANAL. 01/2007; 13.

Publication Stats

88 Citations
83.61 Total Impact Points

Institutions

  • 2003–2014
    • University of Bayreuth
      • • Chair of Macromolecular Chemistry I
      • • Chair of Macromolecular Chemistry II
      • • Institute of Physics
      Bayreuth, Bavaria, Germany
  • 2005
    • Technical University Darmstadt
      • Institute of Telecommunications
      Darmstadt, Hesse, Germany