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Liquid crystalline polymers and elastomers
Current Opinion in Solid State and Materials Science 6 (2002) 545–551
Liquid crystalline polymers and elastomers Sabine Mayer, Rudolf Zentel* Institute of Organic Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10 – 14, 55099 Mainz, Germany Received 29 November 2002; received in revised form 24 February 2003; accepted 24 February 2003
Abstract The results of experiments and theory correlate increasingly well in the investigation of liquid crystalline polymers. Current work focuses on rheology and the behavior of blends of thermotropic LC polymers, due to reemerging economic interest in these materials. Research on liquid crystalline elastomers concentrates on potential applications such as artificial muscles and lasing. 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Liquid crystalline (LC) polymers generally possess all the properties that are typical for low molar mass liquid crystals in addition to the properties of polymers [1–3]. As a result, new property combinations arise, which cannot be found in low molar mass liquid crystals. The polymer nature of LC polymers allows, for example, a ‘locking in’ of the LC orientation either below T g or by the formation of a densely crosslinked network. Alternatively, slightly crosslinked LC polymers (LC elastomers) offer the possibility of combining the rubber elasticity of polymer networks with LC phases [4]. LC polymers can be prepared by incorporation of formanisotropic units (so-called mesogenic groups) either into the polymer chain or by linking them as side groups to a polymer chain. In the first case LC main chain polymers result; in the second case LC side group polymers (Fig. 1). LC main chain polymers rely on the fact that every polymer chain is form-anisotropic (thickness greater than length). Therefore, if the polymer backbone is rigid enough, the polymer chain itself acts as a mesogenic element. All polymers with stiff chains can consequently be expected to preferably form nematic phases. However, often these phases can not be observed directly, because the expected melting temperatures of rigid polymers are far above their temperatures of decomposition (above 300 8C).
*Corresponding author. Tel.: 149-6131-392-0361; fax: 149-6131392-4778. E-mail address: [email protected] (R. Zentel).
In these cases the addition of a suitable solvent can lower the melting temperature and allow observation of the LC phase (so-called lyotropic LC main chain polymers). Alternatively, flexible polymer chains can be functionalized with form-anisotropic mesogens known from low molar mass liquid crystals. If a decoupling element, the so-called flexible spacer, is used to link the mesogens as side groups to the polymer chain, then LC phases can be observed (so-called LC side chain polymers). In these cases the orientation of polymer chains and mesogenic side groups are largely decoupled from each other, and both subsystems are able to follow their inherent orientational tendencies. The properties of LC main chain and side chain polymers are rather different. LC main chain polymers are interesting, from an economic point of view, as materials for high strength fibers and self-reinforcing plastics, while LC side group polymers are of interest for optical components (see the contribution by D.J. Broer in this issue). In main chain polymers, the polymer chain itself is the mesogenic element. Therefore, the role of the polymer chain in the occurrence of an LC phase is evident. Side chain polymers, however, are composed of two, more or less independent subsystems (the mesogens and the polymer chain). While the ordering of the mesogens and their phase sequence is very similar to that of low molar mass liquid crystals, the exact role of the polymer chain is not yet clear. LC elastomers combine the properties of LC polymers with those of polymer networks (Fig. 1). In these systems a macroscopic deformation of the crosslinked sample leads to a corresponding equilibrium deformation of the polymer
1359-0286 / 03 / $ – see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016 / S1359-0286(03)00011-1
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chains. Hence LC elastomers seem to be ideal systems to investigate the interaction of the orientation of the polymer chains (induced by mechanical fields) and that of mesogenic groups (induced by electric fields). IUPAC has recently set standards for the naming and description of LC polymers, which are published in Refs. [5,6]. This review discusses some current trends in research into liquid crystalline polymers and elastomers. We concentrated on articles published in 2001 and 2002. It was not our intention to cover all activities. There are three important topics: I. Based on a reestablished economic interest in thermotropic LC polymers [7], there is an increasing amount of work being carried out on the rheology and behavior of blends of these materials [7–13]. II. Crosslinked LC elastomers are of great interest in the academic area with basic questions concerning the interaction of the LC director and the polymer network [14–20]. These elastomers are also promising for applications in areas such as actuators (artificial muscles) [21–23], damping [24], and optics (tunable lasers) [25,26]. LC ionomers lie between LC networks and polymer blends. Work on these materials extends from crosslinking by ionic clusters and incorporation of transition metals up to nano composites from different LC materials [27–29]. III.Optical applications of LC polymers and elastomers are a major area of activity, with lasers being the most prominent topic [25,26,30,31], directly followed by organic light-emitting diodes (OLEDs) with polarized emission [32,33]. Another field of inquiry covers azocontaining LC polymers, in which the azo group allows either a shift of the phase transition temperatures [23] or the orientation of the LC phase [34,35].
2. Rheology of LC polymers After years of doubt as to whether liquid crystalline polymers (LCPs) would be profitable enough on the market, today main chain LCPs are experiencing a boom with growth rates between 15 and 25% per year [7]. This boom does not involve those applications which were first considered for LCPs, such as high strength materials, but is related to microelectronics, in particular the mobile phone market. Due to their low melt viscosity, and their high heat resistance and form stability, main chain LCPs are the material of choice for applications in microelectronics whenever miniaturized devices are needed (surface mount technology components on PC boards) or thin channels must be filled. The call for lead-free soldering technology will increase their role in this area further, as these polymers are able to withstand the high temperatures associated with this process. This economic success is accompanied by academic work on the rheology of LCPs and their blends. An understanding of the rheology of main chain LCPs is important in two respects. First, the low melt viscosity in combination with the high strength of the final material is the basis of the success of main chain LCPs in miniaturized devices. The second aspect is their strong flow alignment (actually this is the basis for their low melt viscosity as the lowest viscosity coefficient mainly determines macroscopic flow). As a result the mechanical properties of the final material are highly anisotropic and processing has to be chosen so that the final anisotropy is satisfactory. It is therefore the ultimate goal of rheological investigations to correlate flow during processing, viscosity and flow alignment (orientation). On the basis of all previous work on this topic and the experience gained therein, scientists now combine different techniques to directly correlate rheology and shear alignment. Here we
Fig. 1. Schematic representation of LC main chain and side group polymers. The circles highlight the crosslinking points: without them soluble polymers are obtained, with them LC elastomers result.
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want to highlight work by Kornfield and Burghardt. Theory and experiment are now so refined that complex phenomena like shear stress overshoots in flow inception of semiflexible thermotropic LCPs [12,13] can be described with satisfactory correlation between theory and experiment [11]. X-ray measurements in a cone and plate shear cell allow—for the first time—the simultaneous probing of rheological properties, the degree of anisotropy and the average orientation angle relative to the flow direction [8]. The measurements show that the Larson-Doi polydomain model correctly predicts flow reversal. Alternatively, optical (conoscopic) measurements allow direct quantitative measurement of shear alignment [12]. Closer to engineering applications is work which investigates the molecular alignment in samples quenched from mixed shear-extensional flow in narrow channels [13] (a geometry similar to applications in microelectronics), as extensional flow is particularly effective for flow alignment. Blends of LCPs play a major role in technical applications. Commercial LCPs are usually copolymers with chemical heterogeneity. Therefore two-phase regions around the phase transition temperatures are of interest. In this respect blends of LCPs have been used to investigate rheology in the two phase region. Mather and colleagues observed flow-induced phase transitions in LCP blends [9]. On the other hand, LCPs are mixed with common cheap plastics thus reducing the costs of the material. A topic of great significance for applications is the concept of selfreinforcing blends of plastics (majority component) and LCPs (minority component, .10%), by the in situ preparation of LCP fibrils. For this purpose it is necessary to effectively transmit the mechanical stress from the non-LC component to the phase separated LCP. This is difficult because of the poor compatibility of LCPs and non-LC
Fig. 2. Blends with LCPs. SEM micrograph of the cryogenically fractured surfaces of a polyarylate / LCP blend (80 / 20) with 0.5 phr epoxy resin, parallel to the flow direction of the injection molded specimen. (Reproduced with permission from Ref. [10]).
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polymers. Epoxy resins as reactive compatibilizers have been shown to be suitable for this purpose [10]. With their help it is possible to prepare blends with fine LCP fibrils of high aspect ratio (Fig. 2) and with substantially improved mechanical properties.
3. LC elastomers Liquid crystalline elastomers, which combine LC phases and their orientability by electric fields with the elasticity of polymer networks and their orientability by mechanical fields, dominate academic research. Mostly elastomers from LC side group polymers are investigated. They offer in addition potential applications in the field of mechanical actuators (artificial muscles) and optics (lasing). From the academic point of view, study of the interaction between the LC director and the polymer network is fascinating. Compared to uncrosslinked LCPs, for which flow alignment is effective, LC elastomers have the advantage that investigations can be done under quasi-equilibrium conditions. In LC elastomers a macroscopic deformation of the crosslinked sample leads to an equilibrium deformation of the polymer chains. Different degrees of orientation of the polymer chains can be achieved by varying the strain rates. The resulting orientation of the LC director can afterwards be investigated well above T g under quasiequilibrium conditions. A long-term topic of investigation has been that of LC elastomers crosslinked in a disordered state (multidomain samples) and their transformation into monodomain samples under strain. Careful stress–strain measurements [16] in nematic LC elastomers now show a very slow stress relaxation in these polydomain samples as a result of director orientation, which makes it likely that at a true mechanical equilibrium (i.e. after a very long time) the transient orientational nematic effects can relax completely. Such slow director reorientations, which complicate understanding of the mechanical properties, may, however, be useful to create materials for damping applications. Nematic LC elastomers have been found to exhibit very large loss moduli over large temperature intervals [24] with tan d values of up to 1.5. The reason for these high tan d values is not order fluctuation at the phase transitions, but the director fluctuations in the polydomain network. Dynamic mechanical measurements are the method of choice to differentiate transient and equilibrium properties. They show for various LC elastomers [17,18] that the real part of the modulus G9 (in the isotropic and nematic phase) always consists of a frequency independent contribution of the permanent network and an ‘LC contribution’ which scales with v 0.5 . As the dynamic of the network system is strongly modified by the nematic order at the glass transition temperature, it is only possible to determine the true elastic response (independent of v ) at very high
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temperatures in the isotropic phase [18]. In the smectic phase the elastic response is always dominated by a transient network with very long relaxation times [17] and it is—at the experimentally accessible time scale—not possible to reach the equilibrium value. For smectic LC elastomers, tiny balloons have been established as a model which allows mechanical measurements on well-oriented samples [14,15]. These balloons are made by crosslinking of smectic bubbles, which are curved analogs of free-standing films [14]. Stress–strain measurements parallel to the smectic layers confirm the entropy elasticity of smectic elastomers. The mechanical data can satisfactorily be described with the MooneyRivlin theory of isotropic rubbers. Measurements in tilted smectic phases (s C* ) demonstrate for the first time a coupling between mechanical stress and the LC director, and a deviation from simple rubber theory. In the case described above, the uncrosslinked polymer is oriented (free-standing film) and crosslinked thereafter. This is also the process applied for ferroelectric LC elastomers [36] and for the preparation of densely crosslinked optical components (see contribution by D.J. Broer in this issue). Alternatively monodomains of LC elastomers (liquid single crystal elastomers, LSCE) are made in a two-stage crosslinking process [19,20], in which a slightly crosslinked multidomain sample is stretched. This leads to the formation of a monodomain sample, which is stabilized in a second crosslinking step. This method was originally developed for nematic elastomers, but it can also be applied to smectics [20]. It works especially well if a narrow nematic phase is present around the clearing temperature. For the formation of cholesteric monodomains it has been modified to anisotropic de-swelling [19] where a cholesteric film is crosslinked in the presence of solvent. As it sticks to the surface of the casting vessel, it can de-swell only by a shrinkage perpendicular to the substrate. This leads to well-oriented cholesteric elastomer samples with the helix axis perpendicular to the substrate, which can be used, for example, for mirrorless lasing [25]. The application potential of LC elastomers is predominantly in the area of actuators or lasing. Two types of elastomers are under investigation as artificial muscles: (i) Generally the shape of an LSCE changes at the isotropization temperature. This is due to the coupling of director and polymer conformation. This shape change is predicted to be up to 400% [23]. The phase transition can be triggered thermally or photochemically. In order to stimulate a thermal phase transition in a free-standing sample, a nematic LC elastomer has to be heated. This can, for example, be done by coating it with carbon black and irradiating it with an IR laser [37]. The accessible strain changes are 30–45% [22]. Alternatively, the phase transition can be triggered photochemically by trans–cis isomerization of azobenzene (Fig. 3) [23]. cis-Azobenzene reduces the clear-
Fig. 3. Actuators from LC elastomers. Contraction fraction, 1 2 e T 0 (t), versus the time the sample was exposed to UV radiation for T 0 5 298 K (p), 303 K (s), 308 K (n), and 313 K (h). Inset: Recovery of the contraction of the 298 K sample after 90 min of illumination. (Reproduced with permission from Ref. [23]).
ing temperature. In extreme cases, a nematic sample can be transformed into an anisotropic sample under isothermal conditions. First measurements show a shape change of 20% obtained by a variation of the order parameter. This happens while staying in the nematic phase as a result a changed reduced temperature. So far, an isothermal phase transition by photoisomerization has not been achieved. If this were to happen, much larger shape changes would have to take place. (ii) The second approach to actuators uses ferroelectric or electroclinic effects in monodomain LC elastomers with chiral smectic C* phases. The advantage of this approach is that electric signals can be used to induce the shape variation based on the electroclinic effect known for low molar mass chiral smectic C* or A* phases. An electric field applied parallel to the smectic layers induces a tilt (or a change of the tilt) of the mesogens in the smectic phase. The magnitude of this electroclinic effect is especially large in LC polysiloxanes with chiral smectic phases [38,39]. In LC elastomers this tilt leads to a thinning of the sample perpendicular to the smectic layers and an elongation parallel to the smectic layers [21]. Recently cholesteric LC elastomers have been shown to be interesting for mirrorless lasing from plastic materials [25,26]. Mirrors can be omitted here, because scattering at the cholesteric structure creates the lasing cavity. And since cholesteric elastomers are flexible materials, they represent some of the rare examples of flexible lasers. Looking at the properties in more detail, the lasing is a direct result of the cholesteric structure independent of its
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29], and incorporation of transition metals [27] to build-up of nanocomposites from LC and non-LC materials or from various LC materials (Fig. 5) [29]. LC ionomers lie between form-anisotropic and amphiphilic systems, as the ionic groups stabilize smectic phases by accumulating at the layer interface.
4. LC polymers and optics Fig. 4. Laser emission in a highly crosslinked network doped with a fluorescent dye. Integral emission as function of pump energy. (Reproduced with permission from Ref. [26]).
constituents (low molar mass cholesterics, uncrosslinked or crosslinked LC polymers). A polymer network has two benefits. First, it is possible to shift the cholesteric pitch by stretching a cholesteric elastomer [25]. In this way, the laser emission was shifted from 540 to 630 nm for one sample. Second, the polymer network stabilizes the cholesteric structure against the distortion arising from dissipated heat. As a result cholesteric networks, especially densely crosslinked structures, show higher lasing intensities and are more stable than low molecular weight cholesterics (Fig. 4) [26]. LC ionomers, in which some ionic groups are incorporated into a predominantly apolar polymer, lie between LC networks and polymer blends. Due to the incompatibility of the ionic groups with the polymer, ionic clusters are formed as nanophase separated structures. These clusters act as crosslinking points, because they contain ionic groups linked to different polymer chains. LC ionomers range from reversible crosslinking via ionic clusters [27–
Fig. 5. Nanocomposites. Schematic representation of a multilayer formation from LC ionomers and polyelectrolytes. (Reproduced with permission from Ref. [29]).
The most recent trend concerning LC polymers and optics is lasing [25,26,30,31]. This topic has already been discussed in Section 3. Another important topic is organic light-emitting diodes (OLEDs) with polarized emission [32,33]. A straightforward way to produce such OLEDs is to orient an LC hole transport material, which is also the emitter, on an orientation layer [32]. Semi-conducting polyfluorenes are the material of choice for this purpose (Fig. 6). They have good optical properties, a nematic phase (they are rigid-rod polymers), and they can freeze into a nematic glass [32]. Polyfluorenes orient well into large monodomains if a thin film obtained by spin-coating is annealed on rubbed polyimide layers. The rubbed polyimide acts simultaneously as orientation layer and hole injection layer. Starting from these results liquid crystalline electron transport layers are also under investigation [33]. Polyfluorene-based blends in which absorbed light is transferred from the host to the guest (Foerster transfer) are also promising as lasing materials [30,31]. Finally we want to briefly mention two topics which are not within the scope of this review. LC azo components for optical applications are still under investigation. They are used either to shift phase transition temperatures (see for example Ref. [23] discussed in Section 3) or for the
Fig. 6. Organic light-emitting diodes. Electroluminescence spectra of a device based on a monodomain-aligned polyfluorene, measured parallel and orthogonal to the rubbing direction of the alignment layer. (Reproduced with permission from Ref. [32]).
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photo-orientation of LC materials [34,35]. This aspect of photo-orientation is the topic of another review in this issue by Tomiki Ikeda and will not be discussed here. Lyotropic phases of block copolymers are drawing increasing interest as templates for mesoporous organic and inorganic materials [40–44]. This rapidly growing field promises applications in separation technology (chromatography, membranes) and catalysis [41,44].
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Liquid crystalline polymers and elastomers Sabine Mayer, Rudolf Zentel* Institute of Organic Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10 – 14, 55099 Mainz, Germany Received 29 November 2002; received in revised form 24 February 2003; accepted 24 February 2003
Abstract The results of experiments and theory correlate increasingly well in the investigation of liquid crystalline polymers. Current work focuses on rheology and the behavior of blends of thermotropic LC polymers, due to reemerging economic interest in these materials. Research on liquid crystalline elastomers concentrates on potential applications such as artificial muscles and lasing. 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Liquid crystalline (LC) polymers generally possess all the properties that are typical for low molar mass liquid crystals in addition to the properties of polymers [1–3]. As a result, new property combinations arise, which cannot be found in low molar mass liquid crystals. The polymer nature of LC polymers allows, for example, a ‘locking in’ of the LC orientation either below T g or by the formation of a densely crosslinked network. Alternatively, slightly crosslinked LC polymers (LC elastomers) offer the possibility of combining the rubber elasticity of polymer networks with LC phases [4]. LC polymers can be prepared by incorporation of formanisotropic units (so-called mesogenic groups) either into the polymer chain or by linking them as side groups to a polymer chain. In the first case LC main chain polymers result; in the second case LC side group polymers (Fig. 1). LC main chain polymers rely on the fact that every polymer chain is form-anisotropic (thickness greater than length). Therefore, if the polymer backbone is rigid enough, the polymer chain itself acts as a mesogenic element. All polymers with stiff chains can consequently be expected to preferably form nematic phases. However, often these phases can not be observed directly, because the expected melting temperatures of rigid polymers are far above their temperatures of decomposition (above 300 8C).
*Corresponding author. Tel.: 149-6131-392-0361; fax: 149-6131392-4778. E-mail address: [email protected] (R. Zentel).
In these cases the addition of a suitable solvent can lower the melting temperature and allow observation of the LC phase (so-called lyotropic LC main chain polymers). Alternatively, flexible polymer chains can be functionalized with form-anisotropic mesogens known from low molar mass liquid crystals. If a decoupling element, the so-called flexible spacer, is used to link the mesogens as side groups to the polymer chain, then LC phases can be observed (so-called LC side chain polymers). In these cases the orientation of polymer chains and mesogenic side groups are largely decoupled from each other, and both subsystems are able to follow their inherent orientational tendencies. The properties of LC main chain and side chain polymers are rather different. LC main chain polymers are interesting, from an economic point of view, as materials for high strength fibers and self-reinforcing plastics, while LC side group polymers are of interest for optical components (see the contribution by D.J. Broer in this issue). In main chain polymers, the polymer chain itself is the mesogenic element. Therefore, the role of the polymer chain in the occurrence of an LC phase is evident. Side chain polymers, however, are composed of two, more or less independent subsystems (the mesogens and the polymer chain). While the ordering of the mesogens and their phase sequence is very similar to that of low molar mass liquid crystals, the exact role of the polymer chain is not yet clear. LC elastomers combine the properties of LC polymers with those of polymer networks (Fig. 1). In these systems a macroscopic deformation of the crosslinked sample leads to a corresponding equilibrium deformation of the polymer
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chains. Hence LC elastomers seem to be ideal systems to investigate the interaction of the orientation of the polymer chains (induced by mechanical fields) and that of mesogenic groups (induced by electric fields). IUPAC has recently set standards for the naming and description of LC polymers, which are published in Refs. [5,6]. This review discusses some current trends in research into liquid crystalline polymers and elastomers. We concentrated on articles published in 2001 and 2002. It was not our intention to cover all activities. There are three important topics: I. Based on a reestablished economic interest in thermotropic LC polymers [7], there is an increasing amount of work being carried out on the rheology and behavior of blends of these materials [7–13]. II. Crosslinked LC elastomers are of great interest in the academic area with basic questions concerning the interaction of the LC director and the polymer network [14–20]. These elastomers are also promising for applications in areas such as actuators (artificial muscles) [21–23], damping [24], and optics (tunable lasers) [25,26]. LC ionomers lie between LC networks and polymer blends. Work on these materials extends from crosslinking by ionic clusters and incorporation of transition metals up to nano composites from different LC materials [27–29]. III.Optical applications of LC polymers and elastomers are a major area of activity, with lasers being the most prominent topic [25,26,30,31], directly followed by organic light-emitting diodes (OLEDs) with polarized emission [32,33]. Another field of inquiry covers azocontaining LC polymers, in which the azo group allows either a shift of the phase transition temperatures [23] or the orientation of the LC phase [34,35].
2. Rheology of LC polymers After years of doubt as to whether liquid crystalline polymers (LCPs) would be profitable enough on the market, today main chain LCPs are experiencing a boom with growth rates between 15 and 25% per year [7]. This boom does not involve those applications which were first considered for LCPs, such as high strength materials, but is related to microelectronics, in particular the mobile phone market. Due to their low melt viscosity, and their high heat resistance and form stability, main chain LCPs are the material of choice for applications in microelectronics whenever miniaturized devices are needed (surface mount technology components on PC boards) or thin channels must be filled. The call for lead-free soldering technology will increase their role in this area further, as these polymers are able to withstand the high temperatures associated with this process. This economic success is accompanied by academic work on the rheology of LCPs and their blends. An understanding of the rheology of main chain LCPs is important in two respects. First, the low melt viscosity in combination with the high strength of the final material is the basis of the success of main chain LCPs in miniaturized devices. The second aspect is their strong flow alignment (actually this is the basis for their low melt viscosity as the lowest viscosity coefficient mainly determines macroscopic flow). As a result the mechanical properties of the final material are highly anisotropic and processing has to be chosen so that the final anisotropy is satisfactory. It is therefore the ultimate goal of rheological investigations to correlate flow during processing, viscosity and flow alignment (orientation). On the basis of all previous work on this topic and the experience gained therein, scientists now combine different techniques to directly correlate rheology and shear alignment. Here we
Fig. 1. Schematic representation of LC main chain and side group polymers. The circles highlight the crosslinking points: without them soluble polymers are obtained, with them LC elastomers result.
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want to highlight work by Kornfield and Burghardt. Theory and experiment are now so refined that complex phenomena like shear stress overshoots in flow inception of semiflexible thermotropic LCPs [12,13] can be described with satisfactory correlation between theory and experiment [11]. X-ray measurements in a cone and plate shear cell allow—for the first time—the simultaneous probing of rheological properties, the degree of anisotropy and the average orientation angle relative to the flow direction [8]. The measurements show that the Larson-Doi polydomain model correctly predicts flow reversal. Alternatively, optical (conoscopic) measurements allow direct quantitative measurement of shear alignment [12]. Closer to engineering applications is work which investigates the molecular alignment in samples quenched from mixed shear-extensional flow in narrow channels [13] (a geometry similar to applications in microelectronics), as extensional flow is particularly effective for flow alignment. Blends of LCPs play a major role in technical applications. Commercial LCPs are usually copolymers with chemical heterogeneity. Therefore two-phase regions around the phase transition temperatures are of interest. In this respect blends of LCPs have been used to investigate rheology in the two phase region. Mather and colleagues observed flow-induced phase transitions in LCP blends [9]. On the other hand, LCPs are mixed with common cheap plastics thus reducing the costs of the material. A topic of great significance for applications is the concept of selfreinforcing blends of plastics (majority component) and LCPs (minority component, .10%), by the in situ preparation of LCP fibrils. For this purpose it is necessary to effectively transmit the mechanical stress from the non-LC component to the phase separated LCP. This is difficult because of the poor compatibility of LCPs and non-LC
Fig. 2. Blends with LCPs. SEM micrograph of the cryogenically fractured surfaces of a polyarylate / LCP blend (80 / 20) with 0.5 phr epoxy resin, parallel to the flow direction of the injection molded specimen. (Reproduced with permission from Ref. [10]).
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polymers. Epoxy resins as reactive compatibilizers have been shown to be suitable for this purpose [10]. With their help it is possible to prepare blends with fine LCP fibrils of high aspect ratio (Fig. 2) and with substantially improved mechanical properties.
3. LC elastomers Liquid crystalline elastomers, which combine LC phases and their orientability by electric fields with the elasticity of polymer networks and their orientability by mechanical fields, dominate academic research. Mostly elastomers from LC side group polymers are investigated. They offer in addition potential applications in the field of mechanical actuators (artificial muscles) and optics (lasing). From the academic point of view, study of the interaction between the LC director and the polymer network is fascinating. Compared to uncrosslinked LCPs, for which flow alignment is effective, LC elastomers have the advantage that investigations can be done under quasi-equilibrium conditions. In LC elastomers a macroscopic deformation of the crosslinked sample leads to an equilibrium deformation of the polymer chains. Different degrees of orientation of the polymer chains can be achieved by varying the strain rates. The resulting orientation of the LC director can afterwards be investigated well above T g under quasiequilibrium conditions. A long-term topic of investigation has been that of LC elastomers crosslinked in a disordered state (multidomain samples) and their transformation into monodomain samples under strain. Careful stress–strain measurements [16] in nematic LC elastomers now show a very slow stress relaxation in these polydomain samples as a result of director orientation, which makes it likely that at a true mechanical equilibrium (i.e. after a very long time) the transient orientational nematic effects can relax completely. Such slow director reorientations, which complicate understanding of the mechanical properties, may, however, be useful to create materials for damping applications. Nematic LC elastomers have been found to exhibit very large loss moduli over large temperature intervals [24] with tan d values of up to 1.5. The reason for these high tan d values is not order fluctuation at the phase transitions, but the director fluctuations in the polydomain network. Dynamic mechanical measurements are the method of choice to differentiate transient and equilibrium properties. They show for various LC elastomers [17,18] that the real part of the modulus G9 (in the isotropic and nematic phase) always consists of a frequency independent contribution of the permanent network and an ‘LC contribution’ which scales with v 0.5 . As the dynamic of the network system is strongly modified by the nematic order at the glass transition temperature, it is only possible to determine the true elastic response (independent of v ) at very high
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temperatures in the isotropic phase [18]. In the smectic phase the elastic response is always dominated by a transient network with very long relaxation times [17] and it is—at the experimentally accessible time scale—not possible to reach the equilibrium value. For smectic LC elastomers, tiny balloons have been established as a model which allows mechanical measurements on well-oriented samples [14,15]. These balloons are made by crosslinking of smectic bubbles, which are curved analogs of free-standing films [14]. Stress–strain measurements parallel to the smectic layers confirm the entropy elasticity of smectic elastomers. The mechanical data can satisfactorily be described with the MooneyRivlin theory of isotropic rubbers. Measurements in tilted smectic phases (s C* ) demonstrate for the first time a coupling between mechanical stress and the LC director, and a deviation from simple rubber theory. In the case described above, the uncrosslinked polymer is oriented (free-standing film) and crosslinked thereafter. This is also the process applied for ferroelectric LC elastomers [36] and for the preparation of densely crosslinked optical components (see contribution by D.J. Broer in this issue). Alternatively monodomains of LC elastomers (liquid single crystal elastomers, LSCE) are made in a two-stage crosslinking process [19,20], in which a slightly crosslinked multidomain sample is stretched. This leads to the formation of a monodomain sample, which is stabilized in a second crosslinking step. This method was originally developed for nematic elastomers, but it can also be applied to smectics [20]. It works especially well if a narrow nematic phase is present around the clearing temperature. For the formation of cholesteric monodomains it has been modified to anisotropic de-swelling [19] where a cholesteric film is crosslinked in the presence of solvent. As it sticks to the surface of the casting vessel, it can de-swell only by a shrinkage perpendicular to the substrate. This leads to well-oriented cholesteric elastomer samples with the helix axis perpendicular to the substrate, which can be used, for example, for mirrorless lasing [25]. The application potential of LC elastomers is predominantly in the area of actuators or lasing. Two types of elastomers are under investigation as artificial muscles: (i) Generally the shape of an LSCE changes at the isotropization temperature. This is due to the coupling of director and polymer conformation. This shape change is predicted to be up to 400% [23]. The phase transition can be triggered thermally or photochemically. In order to stimulate a thermal phase transition in a free-standing sample, a nematic LC elastomer has to be heated. This can, for example, be done by coating it with carbon black and irradiating it with an IR laser [37]. The accessible strain changes are 30–45% [22]. Alternatively, the phase transition can be triggered photochemically by trans–cis isomerization of azobenzene (Fig. 3) [23]. cis-Azobenzene reduces the clear-
Fig. 3. Actuators from LC elastomers. Contraction fraction, 1 2 e T 0 (t), versus the time the sample was exposed to UV radiation for T 0 5 298 K (p), 303 K (s), 308 K (n), and 313 K (h). Inset: Recovery of the contraction of the 298 K sample after 90 min of illumination. (Reproduced with permission from Ref. [23]).
ing temperature. In extreme cases, a nematic sample can be transformed into an anisotropic sample under isothermal conditions. First measurements show a shape change of 20% obtained by a variation of the order parameter. This happens while staying in the nematic phase as a result a changed reduced temperature. So far, an isothermal phase transition by photoisomerization has not been achieved. If this were to happen, much larger shape changes would have to take place. (ii) The second approach to actuators uses ferroelectric or electroclinic effects in monodomain LC elastomers with chiral smectic C* phases. The advantage of this approach is that electric signals can be used to induce the shape variation based on the electroclinic effect known for low molar mass chiral smectic C* or A* phases. An electric field applied parallel to the smectic layers induces a tilt (or a change of the tilt) of the mesogens in the smectic phase. The magnitude of this electroclinic effect is especially large in LC polysiloxanes with chiral smectic phases [38,39]. In LC elastomers this tilt leads to a thinning of the sample perpendicular to the smectic layers and an elongation parallel to the smectic layers [21]. Recently cholesteric LC elastomers have been shown to be interesting for mirrorless lasing from plastic materials [25,26]. Mirrors can be omitted here, because scattering at the cholesteric structure creates the lasing cavity. And since cholesteric elastomers are flexible materials, they represent some of the rare examples of flexible lasers. Looking at the properties in more detail, the lasing is a direct result of the cholesteric structure independent of its
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29], and incorporation of transition metals [27] to build-up of nanocomposites from LC and non-LC materials or from various LC materials (Fig. 5) [29]. LC ionomers lie between form-anisotropic and amphiphilic systems, as the ionic groups stabilize smectic phases by accumulating at the layer interface.
4. LC polymers and optics Fig. 4. Laser emission in a highly crosslinked network doped with a fluorescent dye. Integral emission as function of pump energy. (Reproduced with permission from Ref. [26]).
constituents (low molar mass cholesterics, uncrosslinked or crosslinked LC polymers). A polymer network has two benefits. First, it is possible to shift the cholesteric pitch by stretching a cholesteric elastomer [25]. In this way, the laser emission was shifted from 540 to 630 nm for one sample. Second, the polymer network stabilizes the cholesteric structure against the distortion arising from dissipated heat. As a result cholesteric networks, especially densely crosslinked structures, show higher lasing intensities and are more stable than low molecular weight cholesterics (Fig. 4) [26]. LC ionomers, in which some ionic groups are incorporated into a predominantly apolar polymer, lie between LC networks and polymer blends. Due to the incompatibility of the ionic groups with the polymer, ionic clusters are formed as nanophase separated structures. These clusters act as crosslinking points, because they contain ionic groups linked to different polymer chains. LC ionomers range from reversible crosslinking via ionic clusters [27–
Fig. 5. Nanocomposites. Schematic representation of a multilayer formation from LC ionomers and polyelectrolytes. (Reproduced with permission from Ref. [29]).
The most recent trend concerning LC polymers and optics is lasing [25,26,30,31]. This topic has already been discussed in Section 3. Another important topic is organic light-emitting diodes (OLEDs) with polarized emission [32,33]. A straightforward way to produce such OLEDs is to orient an LC hole transport material, which is also the emitter, on an orientation layer [32]. Semi-conducting polyfluorenes are the material of choice for this purpose (Fig. 6). They have good optical properties, a nematic phase (they are rigid-rod polymers), and they can freeze into a nematic glass [32]. Polyfluorenes orient well into large monodomains if a thin film obtained by spin-coating is annealed on rubbed polyimide layers. The rubbed polyimide acts simultaneously as orientation layer and hole injection layer. Starting from these results liquid crystalline electron transport layers are also under investigation [33]. Polyfluorene-based blends in which absorbed light is transferred from the host to the guest (Foerster transfer) are also promising as lasing materials [30,31]. Finally we want to briefly mention two topics which are not within the scope of this review. LC azo components for optical applications are still under investigation. They are used either to shift phase transition temperatures (see for example Ref. [23] discussed in Section 3) or for the
Fig. 6. Organic light-emitting diodes. Electroluminescence spectra of a device based on a monodomain-aligned polyfluorene, measured parallel and orthogonal to the rubbing direction of the alignment layer. (Reproduced with permission from Ref. [32]).
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photo-orientation of LC materials [34,35]. This aspect of photo-orientation is the topic of another review in this issue by Tomiki Ikeda and will not be discussed here. Lyotropic phases of block copolymers are drawing increasing interest as templates for mesoporous organic and inorganic materials [40–44]. This rapidly growing field promises applications in separation technology (chromatography, membranes) and catalysis [41,44].
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