Blog

Rudolf Zentel, 
Dept. of Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, 
D-55128 Mainz, Germany
zentel@uni-mainz.de

The first paper about LC-elastomers was written by H. Finkelmann [1]. It dates to 1981, at which time he worked as junior group leader in the lab of Prof. G. Rehage at the TU-Clausthal. This work was soon after extended [2] by him. A bit later (1986), also R. Zentel [3] entered the field. Concerning his status, he was also a junior group leader at the University of Mainz associated with Prof. H. Ringsdorf (for comparison: he was 8 years younger than Finkelmann). While Finkelmann focused more on Physico-chemical aspects (he was working in an Institute for Physical Chemistry), Zentel tried to broader the chemical basis and the molecular structures (for an overview see Fig. 1 and [4]). He was working in Organic Chemistry. Over time, the field “flourished” and in about 1990 a lot of papers on LC-elastomers were available. For recent reviews see especially volume 250 of “Adv. Polymer Sci.” [5] and especially the chapters of Brömmel et al. [6] and Ohm et al. [7]
 
Fig. 1: Schematic representation of LC side-chain and LC main chain elastomers [4]
The polymer chains are marked in red and the crosslinks circled in blue; (original artwork of the author), structure similar to ref. [4]

Now, there was a second route to LC-elastomers. These were the predictions of the properties of LC-elastomers by P.G. de Gennes [8, 9 ](1969 (in French) and 1975), which include also the possibility for a shape change (nowadays most prominent for “actuators”). He was a theoretical physicist and he got the “Nobel price” later on (generally for “theory of soft matter”). However, this work was not cited in the early synthetic papers (how to make LC-elastomers) and -if yes- then not in a prominent  way (see [4] for an exception). This aspect has been discussed recently by Jan Lagerwall [10] for the first time. This raises the question of the relevance of chemistry (to really make the materials) and theoretical predictions (of what “great properties they might show) to advance a field. LC-elastomers are a fine example to show that you need both!

 
Fig. 2: Schematic picture to demonstrate the interaction of anisotropically oriented mesogens and a crosslinked polymer network (red) according to the concept of de Gennes [8, 9]. In this case (taken from [4]) it is assumed that the polymer chains try to expand parallel to the director of the mesogenic groups, which determines their anisotropic chain confirmation.

My redrawing of the ideas of the general ideas of de Gennes [8,9] are presented in Fig.  2. Independent of the type of coupling (or if there is any) mesogens and polymers will “feel each other”. In this way the polymer chains will acquire some anisotropy in the liquid crystalline phase of the mesogens. And in rubbery polymer networks (elastomers) the chain conformation determines the macroscopic size of the elastomer sample. Thus, any change of the anisotropy must lead to a shape change. So far, so good. 
But then it turned out that isotropic networks cannot be swollen with low molar mass liquid crystals in the liquid crystalline phase, see refs. [10 and 11] ( note that [11] is also ref.6 in [4]). So, networks with anisotropic liquid crystalline properties cannot be made so easily (by swelling of classical elastomers). Their preparation requires it either to find (invent) crosslinked elastomers, which are compatible (swellable) with liquid crystalline materials or to invent LC-elastomers directly. And theory cannot tell you, how to solve the challenge. But the chemical preparation [1 -5, 10] of LC-elastomers had solved the problem.

It is interesting that the early ideas of de Gennes had still an outreach, which, however, cannot be tracked today in references. At first ref. [11 cannot be accessed electronically (at list I cannot do so and I have to rely on a paper copy). Secondly, I remember having read an extended abstract from a conference in the 80´ (a paper copy of in an “old” library). It described work of some German polymer scientists. They described having prepared a block of a classical elastomer and swollen it in a nematic material (from Merck) in the isotropic phase. This worked well. But as they lowered the temperature, the system phase separated into the nematic phase of the low molar mass compound and a condensed, deswollen elastomer. And because of this fact, they wrote that they could not go on to look for further promising properties. 
But this means that prior to 1979 polymer scientist had tried to do something following the predictions of de Gennes. But it had not worked. I myself are however sure that G. Rehage (the mentor of Finkelmann) know about these experiments. And in 1982 (one year after the first paper on LC-elastomers [1]) a paper was published at the TU-Clausthal [12], which shows that the new LC-side group polymers were miscible with low molar mass liquid crystals in the nematic phase. So, the experiments predicted by de Gennes could have been done. However, now LC-elastomers could be made directly [1-7] without the need of mixing/swelling.

As most of the LC-elastomers are based on LC-side group polymers, and as also LC-side group polymers demonstrate the problem of coupling two subsystems (mesogens with anisotropic orientation) and polymer chains (statistical chain confirmation) it is appropriate to discuss them briefly (see Fig. 3).

 
Fig. 3: LC-side group polymers [i] and the linking of mesogens and polymer coils via a flexible spacer [13]

Polymers (above a critical molecular weight) do not mix with mesogens in the liquid crystalline phase (oligomers, however, might just act like a solvent). This happens because the isotropic polymer coil will reduce the nematic order of the mesogens slightly, whilst -at the same time- the entropy of the isotropic coil is reduced by inducing an anisotropic chain conformation (this corresponds to entropy elasticity on stretching an elastomer). So somehow, both subsystems have to be synthetically coupled to prevent their demixing. On the other side, the coupling should not be too strong, as the orientational tendencies are different. This problem was solved experimentally by introducing the concept of the flexible spacer, which made LC-side group polymers accessible (see [13] for an extended review). The fascinating property of the LC side-chain polymers is, that LC phases get stabilized [13], but if the spacer is removed (Gedankenexperiment) then they would just phase separate.

References:
[1] Liquid crystalline elastomers - A new type of liquid crystalline material H. Finkelmann, H.-J. Kock. G . Rehage. Makromol. Chem. Rapid. Chommun. 2, (1981) 317.
[2] Thermoelastic and photoelastic properties of crosslinked liquid-crystalline side chain polymers  W. Gleim. H. Finkelmann, Makromol. Chem. 188, 1489 (1987).
[3] Liquid crystalline elastomers based on liquid crystalline side group, main chain  and combined polymers
 R. Zentel, G. Reckert, Makromol. Chem. 187, 1915 (1986)
[4] Liquid crystalline elastomers,  R. Zentel, Angew. Chem. Int. Ed. Engl. Adv. Mater. 28, 1407 (1989), Adv. Mater., 321(1989)
[5] Liquid Crystal Elastomers: Materials and Applications, W. de Jeu, (eds) Adv Polym Sci 250 (2012);
[6] Preparation of Liquid Crystalline Elastomers: F. Brömmel, D. Kramer, H. Finkelmann, Adv Polym Sci 250, 1-48 (2012)
[7] Applications of Liquid Crystalline Elastomers; C. Ohm, M. Brehmer, R. Zentel  Adv Polym Sci 250, 49–94 (2012)
[8] Possibilites offertes par la reticulation de polymeres en presence d’un cristal liquide. 
 Pierre-Gilles de Gennes. Phys. Lett. A, 28 (11), 725 (1969)
[9] The Physics of Liquid Crystals, P. G . de Gennes. Clarendon Press. Oxford 1975.
[10] Liquid crystal elastomer actuators and sensors: glimpses of the past, the present and perhaps the future, 
Jan Lagerwall, Programmable Materials,0,1–42 (2023)
[11]  F. Brochard. J. Phys. (Les Ulis, Fr.) 40 (1979) 1049
[12] Phase Studies of Liquid Crystalline Side Chain Polymers Mixed with
Low Molar Mass Liquid Crystals of Similar Structure
H. Finkelmann, H.-J. Kock and G. Rehage
Mol. Cryst. Liq. Cryst.,  89, 23 (1982)
[13] Liquid Crystalline Polymers, R. Zentel  in: Liquid Crystals, H. Stegemeyer (Editor), Steinkopff Verlag Darmstadt 1994,  103 - 140

Die Deutsche Flüssigkristall-Gesellschaft trauert um Prof. Wim de Jeu, der am 3. Juni 2026 verstorben ist.

Wim de Jeu war ein inspirierender Wissenschaftler auf dem Gebiet der Physik weicher Materie. Sein besonderes Interesse galt selbstorganisierten Materialien, insbesondere flüssigkristallinen Systemen. Er wurde 1943 in Leersum in den Niederlanden geboren und starb am 3. Juni 2026 im Alter von 83 Jahren in Heerlen.

Bekannt wurde Wim de Jeu vor allem durch seinen ausgeprägt physikalischen Blick auf neue flüssigkristalline Materialien und durch den Einsatz anspruchsvoller Röntgenstreumethoden zur Charakterisierung verschiedener smektischer Phasen. Darüber hinaus interessierte er sich in besonderem Maße für Struktur-Eigenschafts-Beziehungen. Damit wurde er zu einem geschätzten Partner für synthetisch arbeitende Gruppen; eine Übersicht seiner wissenschaftlichen Aktivitäten findet sich hier: (Link).

In diesem Zusammenhang leistete er wesentliche Beiträge zur Charakterisierung und Weiterentwicklung neuer polymerer flüssigkristalliner Verbindungen. Daraus entstanden enge wissenschaftliche Kontakte zu Flüssigkristall-Gruppen in Halle (Fischer), Freiburg (Finkelmann) und Mainz (Zentel). Unter anderem untersuchte er den Einfluss von Polymernetzwerken und Vernetzungspunkten in flüssigkristallinen Elastomeren [1, 2] sowie die Phasenseparation in flüssigkristallinen Blockcopolymeren [3].

Im Jahr 2018 wurde Wim de Jeu auf der Jahrestagung der Deutschen Flüssigkristall-Gesellschaft mit dem Alfred-Saupe-Preis ausgezeichnet (siehe Abb. 1). Die zu diesem Anlass gehaltene Laudatio ist hier beigefügt: (Link).

Abb. 1: Verleihung der Saupe-Medaille an Wim de Jeu. Von links: Frank Giesselmann, Wim de Jeu, Brigitte Saupe, Witwe von A. Saupe, und Rudolf Zentel.

Neben seiner wissenschaftlichen Arbeit engagierte sich Wim de Jeu in besonderem Maße in der Ausbildung von Studierenden sowie in der Förderung und Betreuung junger Wissenschaftlerinnen und Wissenschaftler. Dies zeigte sich unter anderem in seinem Beitrag zum Aufbau des Fachbereichs Naturwissenschaften an der Open University of the Netherlands, an der er von 1984 bis 1994 als Dekan der Fakultät tätig war. Auch seine drei Bücher — Physical Properties of Liquid Crystalline Materials, Liquid Crystal Elastomers: Materials and Applications und Basic X-Ray Scattering for Soft Matter — zeugen von seinem Anliegen, Wissen zugänglich zu machen und weiterzugeben. Gegen Ende seiner Laufbahn widmete er zudem viel Zeit der Ausbildung von Studierenden an der University of Massachusetts Amherst und an der RWTH Aachen.

[1] Induced long-range order in crosslinked "one-dimensional" stacks of fluid monolayers
 G.C.L. Wong, W. de Jeu, H. Shao, K.S. Liang, R. Zentel
NATURE 389, 576 - 579 (1997); https://doi.org/10.1038/39271
[2] Order and Disorder in Liquid-Crystalline Elastomers,
 WH de Jeu, BI Ostrovskii, 
 Adv. Polym. Sci., 250, 187-234 (2012); DOI 10.1007/12_2010_105

[3] Orientational Wetting in Hybrid Liquid Crystalline Block Copolymers
G.C.L. Wong, J. Commandeur, H. Fischer, WH de Jeu, Phys. Rev. Lett. 77, 5221 (1996); DOI 10.1103/PhysRevLett.77.5221

Verfasst von Rudolf Zentel

Die Flüssigkristall- und Soft-Matter-Gemeinschaft hat einen Wissenschaftler verloren, dessen Arbeiten das Fach nachhaltig geprägt haben. Wolfgang Helfrich, der im Jahr 2025 verstorben ist, hat grundlegende Beiträge sowohl zur Flüssigkristallforschung als auch zur Biophysik geleistet.

Nach seinem Physikstudium in Göttingen, München und Tübingen sowie der Promotion im Jahr 1961 an der Technischen Universität München war Helfrich in München, Ottawa und an den RCA Laboratories in Princeton tätig. Nach seiner Habilitation im Jahr 1967 kehrte er zu RCA zurück, wo er begann, theoretische Konzepte zur Struktur von Flüssigkristallen zu entwickeln.

Im Jahr 1970 entwickelte Helfrich bei Hoffmann-La Roche in Basel gemeinsam mit Martin Schadt das erste Flüssigkristalldisplay mit verdrillter nematischer Struktur (twisted nematic LCD). Dieses Konzept bildete die Grundlage für eine Technologie, die heute in Milliarden von Geräten weltweit eingesetzt wird.

Parallel dazu leistete Helfrich wichtige Beiträge zur Physik von Membranen. In seiner Arbeit aus dem Jahr 1973 formulierte er erstmals die heute als Helfrich-Energie bekannte Beschreibung der Biegeenergie von Membranen, die auf der Krümmung basiert und bis heute eine zentrale Rolle in der Biophysik spielt. In den folgenden Jahren entwickelte er grundlegende Konzepte zu Membranfluktuationen, entropischen Wechselwirkungen und Vesikelformen, die bis heute in verschiedenen Disziplinen Anwendung finden.

Von 1973 bis zu seiner Emeritierung im Jahr 1997 war Wolfgang Helfrich Professor für Experimentalphysik an der Freien Universität Berlin. Für seine wissenschaftlichen Leistungen wurde er vielfach ausgezeichnet, unter anderem mit dem Robert-Wichard-Pohl-Preis der Deutschen Physikalischen Gesellschaft sowie im Jahr 2012 mit dem Draper Prize der US National Academy of Engineering.

Wolfgang Helfrich verband theoretische Tiefe mit einem ausgeprägten Gespür für physikalische Fragestellungen. Er bewegte sich souverän zwischen Grundlagenforschung und Anwendungen und war bekannt für seine unabhängige Denkweise sowie seine kritische Haltung gegenüber starren Strukturen in Wissenschaft und Industrie.

Er wird als Wissenschaftler in Erinnerung bleiben, der neue Perspektiven in der Erforschung von Flüssigkristallen und Membranen eröffnet hat, und als Inspiration für Kolleginnen und Kollegen sowie für seine Schülerinnen und Schüler.

Die Deutsche Flüssigkristall-Gesellschaft (GLCS / DFKG) wird ihm ein ehrendes Andenken bewahren.

by Johanna Bruckner (University of Stuttgart)

Located at the Institute of Physical Chemistry at the University of Stuttgart, Germany, the research group led by Dr. Johanna R. Bruckner deals with the fabrication and analysis of hierarchically structured materials. The group advances from lyotropic liquid crystals (LLCs) to hierarchically structured materials by taking advantage of the self-assembled structures of surfactant-, polymer-, or nanoparticle-based LLCs as precursors for well-defined solid systems. One example for this is the preparation of ordered mesoporous materials, such as silicates and metallosilicates, by direct liquid crystal templating (DLCT). In this approach, the continuous phase of an LLC is polymerized around the regularly arranged micelles, producing a precise replica of the LLC. In contrast to conventional templating methods, DLCT results in exceptionally well-defined materials (Figure 1). Due to their large surface area, uniform pore diameters, and active sites, these materials are used for catalysis, adsorption, and separation applications, as well as confinement studies. Another central research focus of the Bruckner group lies on photonic materials which selectively reflect certain electro-magnetic wavelengths through a periodic modulation of the refractive index. Such a modulation is achieved, for example, by drying chiral nematic LLCs with suitable structural and electronical properties while preserving the helical structure (Figure 2). At the heart of the group’s research is the focus on developing a detailed understanding of the initial systems and the underlying processes governing structure formation. Driven by this scientific curiosity, the group also develops new, eco-friendly LLCs and production methods to enhance sustainability and broaden their application potential.

Figure 1. A hexagonal LLC phase, as seen by polarizing optical microscopy (top), which is transferred into an ordered mesoporous material by DLCT. Transmission electron microscopy reveals the uniform cylindrical mesopores (bottom).

Figure 2. The helical modulation of the director in the chiral nematic phase gives rise to the characteristic finger print texture visible in polarizing optical microscopy (top). If the structure is retained while drying, the helical arrangement can be visualized by scanning electron microscopy (bottom left) and may lead to the selective reflection of visible light, as in the pictured xanthan film (bottom right).