Heino Finkelmann

University of Freiburg, Freiburg, Baden-Württemberg, Germany

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Publications (216)529.14 Total impact

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    ABSTRACT: 4-Alkoxy-4'-cyanoazobenzenes are organic chromophores with great applicability within present nanotechnology. However, the use of such azo dyes for obtaining light-triggered artificial muscle-like actuators remains still unexplored. Achieving further knowledge about the thermal back reaction and isomerisation mechanism for these types of azoderivatives is essential to get photo-actuators with the desired abilities. Despite the push-pull nature of the 4-alkoxy-4'-cyanoazobenzene chromophore, it has been experimentally demonstrated that it isomerises via an inversion mechanism. The opto-mechanics of the prepared elastomeric material has also been investigated. For this system, a maximum opto-mechanical response of ca. 2.5 kPa has been registered, which is independent of the working temperature of the photoactuator.
    Physical Chemistry Chemical Physics 03/2014; · 3.83 Impact Factor
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    ABSTRACT: Nematic networks with three different concentrations of polar and nonpolar mesogens and the same concentration of a novel cross-linking agent give rise to unusual liquid single crystal elastomers (LSCEs) that are transparent monodomain nematic networks with smectic clusters. The largest spontaneous length change is observed in the sample with 70 mol% of the polar mesogen which also has the highest glass transition temperature and smectic clusters with a slowly increasing but nearly constant layer spacing on cooling from 90°C to 25°C. X-ray scattering intensity from smectic clusters with layer spacings that monotonically increase on cooling first increases to a maximum at C corresponding to clusters of about 30 layers. Below , the scattering intensity decreases as the number of layers in a cluster decreases. To account for this surprising nonlinear behavior that correlates with nonlinear features of the networks’ macroscopic spontaneous shape change and birefringence, a model is proposed where dislocations form in the layers at . Below , more dislocations form to break down the layer structure. The possibility of dislocation formation at independent of mesogenic concentrations is attributed to a conformational change in the crosslinker which is present at the same concentration in the three LSCEs.
    Advances in Condensed Matter Physics 09/2013; 2013. · 1.18 Impact Factor
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    Antoni Sánchez-Ferrer, Heino Finkelmann
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    ABSTRACT: Monodomains of nematic main-chain liquid-crystalline elastomers containing azoderivatives in the polymer backbone have been studied by means of opto-mechanical experiments at different temperatures for the evaluation of the mechanical response, kinetic rates, activation energies and the isomerization mechanism. The increase in the crosslinking density of the network enhances the light-induced stress in the sample, leading to a better performance and more tunability of the systems when compared to nematic side-chain liquid-crystalline elastomers.
    Soft Matter 02/2013; 9:4621-4627. · 4.15 Impact Factor
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    ABSTRACT: We report the synthesis of a thin nematic elastomer film where the major director of the mesogenic moieties (i.e. the average orientation of the main molecular axes) is permanently oriented parallel to the film's normal. This allows us to directly observe the orientation of the minor director (average orientation of the minor molecular axes) using optical techniques. We find that the elastomer exhibits spontaneous biaxiality at room temperature and the minor director is oriented in large domains. When a mechanical field is applied perpendicular to the major director, the domains of the minor director reorient into the direction of strain above a characteristic threshold, forming a monodomain with 3D orientational long-range order. This reorientation process is accompanied by a change in the slope of the stress–strain curve and X-ray scattering experiments reveal that the reorientation of the major director only sets in after the reorientation of the minor director is complete.
    Soft Matter 02/2013; 9(9):2646-2651. · 4.15 Impact Factor
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    ABSTRACT: Ever since the first theoretical phase diagram that was published by Straley in 1974, extensive theoretical efforts have been made on predicting the molecular shape and properties of mesogens that can form a biaxial-nematic phase. The theories suggest that mesogens, whose form is board-like rather than rod-like, are more likely to form biaxial phases. In this study we show that the onset of phase-biaxiality for nematic main-chain polymers can indeed, without significantly changing the glass transition temperature, be shifted to considerably higher temperatures by laterally broadening the mesogenic moieties. The investigations were performed by means of 2H-NMR spectroscopy on directly deuterated polymers and yield results that differ considerably from what could hitherto be observed in spin-probe measurements on side-chain polymers with similarly broadened mesogens.
    Soft Matter 01/2013; 9(5):1674-1677. · 4.15 Impact Factor
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    ABSTRACT: We present studies on bulk smectic-A copolymer networks with end-on attached homeotropically oriented mesogens that show spontaneous optical biaxiality at room temperature. Orthoscopic and conoscopic investigations under uniaxial extension in the layer planes give first evidence of the orientability of the minor director in mechanical fields yielding biaxial monodomains with 3-d orientational long-range order of all three principle axes. This is an important step towards the synthesis of permanently oriented biaxial monodomain elastomers for which highly interesting mechanical and optical properties are expected.
    Macromolecular Rapid Communications 10/2012; · 4.93 Impact Factor
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    ABSTRACT: New mesogens presenting smectic A (SmA) phases and capable of hosting lithium salts are designed. The mesogens comprise a vinyl-functionalized spacer to allow further reaction to the polymer backbone, an aromatic core and ethylene oxide chains, able to coordinate lithium ions. Copolymerizing these monomers with a suitable crosslinker yields the first lithium containing liquid crystalline elastomers (LCEs). The SmA structure where the ethylene oxide chains are microphase separated in layers is fixed by the crosslinking and permanent macroscopic orientation is obtained. Diffusion and conductivity measurements of the monomer sample show a large anisotropy of the ion mobility (100 for the cation and 400 for the anion). In the elastomer the anisotropy of the lithium mobility is comparable to that in the monomers.
    Macromolecular Rapid Communications 03/2012; 33(5):386-91. · 4.93 Impact Factor
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    ABSTRACT: A high-resolution calorimetric study has been performed in the vicinity of the isotropic to smectic-A phase transition of smectic liquid crystalline elastomers. The transition is weakly first order for both investigated samples. Nevertheless, different features have been observed when the crosslinking density is changed, such as substantial broadening of the transition and decrease of the involved latent heat. The latter is sluggishly released in a range of few K.
    Molecular Crystals and Liquid Crystals 01/2012; 553(SI):193. · 0.53 Impact Factor
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    ABSTRACT: The shear mechanical properties of nematic side-chain liquid-crystal elastomers swollen with a nematic solvent are studied. The results show that the elasticity of the network is Gaussian for elastomers oriented before crosslinking and non-Gaussian for the usual twice-crosslinked elastomers oriented by stretching the network formed after the first crosslinking step. For systems oriented before crosslinking, a crossover between Gaussian and non- Gaussian behavior is observed at a particular weight concentration of solvent. In contrast to the current belief, the neoclassical model based on Gaussian rubber elasticity cannot provide a correct description of the twice-crosslinked systems.
    Macromolecular Chemistry and Physics 12/2011; 212(24). · 2.39 Impact Factor
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    ABSTRACT: Microactuators are an essential component in microsystems or microdevices, and in applications that include artificial muscles, pumps, valves, or switchers. Liquid-crystalline elastomers are a new class of actuator material in the field of microsystem technologies, which can be used in standard processes. This newly developed actuator provides new possibilities in microfluidics because of its dimensional changes activated by the increase in temperature.
    Advanced Materials 09/2011; 23:4526-30. · 14.83 Impact Factor
  • Dominic Kramer, Heino Finkelmann
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    ABSTRACT: Smectic-A elastomers combine one-dimensional translational order of rod-like segments with the rubber elasticity of a polymer network. In recent years, detailed investigations were carried out on elastomers showing a global prolate chain conformation. In this communication, the first experiments on fluorinated S(A) elastomers exhibiting a global oblate chain conformation are presented, where the polymer chains are on average compressed along the layer normal of the lamellar phase structure. The mechanical anisotropy is studied by means of thermoelastic experiments and stress-strain measurements. For the first time, the layer compression modulus B of smectic elastomers is directly measured. B is significantly larger as compared to conventional low molar mass liquid crystals and decreases significantly with increasing local disorder introduced by the isotropic crosslinker.
    Macromolecular Rapid Communications 07/2011; · 4.93 Impact Factor
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    ABSTRACT: Azoderivatives show faster thermal cis-to-trans isomerisation kinetics when they are under the influence of the nematic mean field than dissolved in isotropic solution. The local order parameter induced in the azo-dye by the host mesogen determines the rate of its thermal cis-to-trans isomerisation process. In this way, AZO6-LC mixtures with the same order parameter exhibit identical isomerisation rates independently of the mesogen chemical nature. The temperature dependence of the local order parameter of the chromophore is related with the thermal activation parameters of the isomerisation process and they are characteristic for each AZO6-LC nematic mixture.
    Physical Chemistry Chemical Physics 06/2011; 13(23):11233-8. · 3.83 Impact Factor
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    ABSTRACT: A review of the mechanisms that influence the critical behaviour of the nematic monodomain main-chain and side-chain liquid-crystalline elastomers is presented. The crosslinking density, the applied mechanical load and the temperature at which the second crosslinking step takes place are examples of such mechanisms. This knowledge is a powerful tool when the tuning of the critical behaviour and, thus, of the thermo-mechanical response of the elastomers is needed.
    Molecular Crystals and Liquid Crystals 06/2011; 547(1):91/[1781]-96/[1786]. · 0.53 Impact Factor
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    ABSTRACT: It is well known that the proper substitution of the azobenzene core allows tuning the thermal cis-to-trans isomerisation kinetics of azo dyes. The thermal isomerisation process of nitro-substituted azobenzenes is accelerated up to 13 times with respect to that in the common isotropic solvents when they are doped in nematic low molar mass liquid crystals. This kinetic acceleration is even stronger when these azo dyes are covalently linked to a nematic siloxane polymer. In this environment, the isomerisation process is accelerated more than 10(3) times. This effect is presented herein for the first time. The possible application of the networks obtained as possible photo-actuators has been also considered.
    Chemistry 05/2011; 17(23):6518-23. · 5.93 Impact Factor
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    ABSTRACT: In recent years, a pronounced interest in materials that are perspective in applications for everyday life has start to grow. Our interest has been focused on different conversions of energies in such advanced materials. We have studied a giant conversion of thermal to mechanical energy, i.e., thermomechanical effect in nematic liquid crystal elastomers, a giant conversion of electrical to thermal energy, i.e., electrocaloric effect in PLZT thin films, and an electric control of magnetization, i.e.,magnetoelectric effect, in soft multiferroics such as mixtures of SCE9 ferroelectric liquid crystal and magnetic nanoparticles.
    3rd Jožef Stefan International Postgraduate School Students Conference, JSI,; 05/2011
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    ABSTRACT: Para-substituted azophenols exhibit a fast thermal cis-to-trans isomerization rate in ethanol, which can be transferred to the solid state by obtaining liquid-crystalline elastomeric systems. The absence of protic solvent is compensated by the establishment of hydrogen bonding between azophenol monomers that are close to each other. Opto-mechanical experiments reveal that azophenol-containing liquid single-crystal elastomers are valuable materials for light-controlled actuators exhibiting relaxation times of 1 s at room temperature.
    Organic Letters 05/2011; 13(9):2282-5. · 6.14 Impact Factor
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    ABSTRACT: The mechanical behaviour of monodomain nematic side-chain liquid-crystalline elastomers containing azoderivatives as pendant groups or crosslinkers has been studied under UV irradiation and in the darkness at different temperatures. From the evaluation of the opto-mechanical experiments, the mechanical efficiency, kinetic rates, activation energies and the isomerization mechanism of the azocompounds in the liquid-crystalline matrix could be determined, as well as the effect of the chemical constitution of the azobenzene derivatives and their role in the elastomeric network.
    Macromolecular Rapid Communications 04/2011; 32:671-678. · 4.93 Impact Factor
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    ABSTRACT: We report effects of disorder due to random cross-linking on the nematic to smectic-A phase transition in smectic elastomers. Thermoelastic data, stress-strain relations and high-resolution x-ray scattering profiles have been analyzed for two related compounds with a small and a larger nematic range, respectively, each for 5% as well as 10% cross-links. At 5% cross-link density the algebraic decay of the positional correlations of the smectic layers survives in finite-size domains, providing a sharp smectic-nematic transition. At an increased cross-link concentration of 10% the smectic order disappears and gives way to extended short-range layer correlations. In this situation neither a smectic-nematic nor a nematic-isotropic transition is observed anymore. The occurrence of disorder at a relatively large cross-link concentration only, indicates that smectic elastomers are rather resistant to a random field. The temperature dependence of the correlation lengths and thermoelastic behavior suggest a shift to a "parasmectic" regime of a first-order smectic-isotropic transition.
    Physical Review E 04/2011; 83(4 Pt 1):041703. · 2.31 Impact Factor
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    ABSTRACT: To gain insight into the origins of phase biaxiality in nematic polymers and elastomers the relation of molecular dynamics and biaxiality is studied. 2H-NMR investigations of various nematic polymers differing in the attachment geometry of the mesogens to the polymer (side-chain side-on, end-on and main-chain attachment) are presented. The mesogens are 2H-labelled at specific positions of their spacers and in the aromatic core. The NMR experiments are supplemented by dielectric relaxation measurements of polymers labelled with dipole groups. Only minor differences are found for the molecular dynamics of polymers and elastomers and comparing the various labelling positions suggests that the mesogenic moiety rotates as a whole. We find broad biaxial nematic phases for all investigated systems and a slowing down of the rotational diffusion around the mesogens′ long axes upon approaching the glass transition. It seems that in nematic polymers and elastomers phase biaxiality sets in when the rotational diffusion around the long axis gets hindered enough to allow biaxial nematic order. Comparing different attachment geometries we find that the side-on attachment of the mesogens seems to hinder the rotation of the molecules around their long axes considerably more than the other geometries.
    Soft Matter 03/2011; 7(6):2387-2401. · 4.15 Impact Factor
  • Isabel Kundler, Etsushi Nishikawa, Heino Finkelmann
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    ABSTRACT: The influence of uniaxial mechanical stress applied parallel or perpendicular to the director axis (optical axis) of nematic and smectic-A elastomers having a permanent macroscopically ordered monodomain structure is investigated. Due to the different phase structures applying an external mechanical stress leads to completely different responses with respect to the young moduli and reorientation processes of the director. Nematic elastomers exhibit a weak anisotropy of the moduli parallel (μ11 and perpendicular (μ) to the optical axis with μ11 / μ = 1.5. The moduli are mainly determined by the rubber elasticity of the poly(siloxane) network. Applying the mechanical stress perpendicular to the initial director axis causes a rotation of the director axis that is connected with a periodic pattern formation and a ‘soft elastic’ response. For smectic-A elastomers μ11/μ is in the order of 102. While μ reflects a rubber elastic response similar to that of the nematic system, μ11 indicates an enthalpy-elastic behaviour of the one-dimensional long range order of the smectic layers. In this direction reorientation of the phase structure with a deformation of the smectic layers occurs above a threshold elongation. In contrast to the nematic networks, a deformation perpendicular to the optical axis causes no director reorientation, displaying the liquid-like properties within the smectic layers.
    Macromolecular Symposia 03/2011; 117(1):11 - 19.

Publication Stats

2k Citations
529.14 Total Impact Points

Institutions

  • 1986–2013
    • University of Freiburg
      • Institute of Macromolecular Chemistry
      Freiburg, Baden-Württemberg, Germany
  • 2012
    • University of Bristol
      • School of Chemistry
      Bristol, ENG, United Kingdom
  • 2011
    • ETH Zurich
      • Institute of Food, Nutrition and Health
      Zürich, ZH, Switzerland
  • 2008–2011
    • University of Massachusetts Amherst
      • Department of Polymer Science and Engineering
      Amherst Center, MA, United States
  • 2002–2011
    • University of Barcelona
      • Departament de Química Orgànica
      Barcelona, Catalonia, Spain
  • 2010
    • Jožef Stefan Institute
      • Department of Condensed Matter Physics
      Ljubljana, Ljubljana, Slovenia
  • 2004–2008
    • Kyushu University
      • Department of Applied Quantum Physics and Nuclear Engineering
      Fukuoka-shi, Fukuoka-ken, Japan
  • 2003–2007
    • Johannes Gutenberg-Universität Mainz
      • Institute of Organic Chemistry
      Mainz, Rhineland-Palatinate, Germany
    • Tokyo Polytechnic University
      Edo, Tōkyō, Japan
  • 2005
    • University of Leipzig
      • Institut für Experimentelle Physik
      Leipzig, Saxony, Germany
  • 1983–2003
    • Technische Universität Clausthal
      Bergstadt-Clausthal-Zellerfeld, Lower Saxony, Germany