Rudolf Zentel

Liquid Crystalline Elastomers as Actuators and Sensors

This review collects recent developments in the field of liquid crystalline elastomers (LCEs) with an emphasis on their use for actuator and sensor applications. Several synthetic pathways leading to crosslinked liquid crystalline polymers are discussed and how these materials can be oriented into liquid crystalline monodomains are described. By comparing the actuating properties of different systems, general structure-property relationships for LCEs are obtained. In the final section, how these materials can be turned into usable devices using different interdisciplinary techniques are described.

Giant lateral electrostriction in ferroelectric liquid-crystalline elastomers

Mechanisms for converting electrical energy into mechanical energy are essential for the design of nanoscale transducers, sensors, actuators, motors, pumps, artificial muscles, and medical microrobots. Nanometre-scale actuation has to date been mainly achieved by using the (linear) piezoelectric effect in certain classes of crystals (for example, quartz), and ‘smart’ ceramics such as lead zirconate titanate. But the strains achievable in these materials are small—less than 0.1 per cent—so several alternative materials and approaches have been considered. These include grafted polyglutamates (which have a performance comparable to quartz), silicone elastomers (passive material—the constriction results from the Coulomb attraction of the capacitor electrodes between which the material is sandwiched) and carbon nanotubes (which are slow). High and fast strains of up to 4 per cent within an electric field of 150 MV m-1 have been achieved by electrostriction (this means that the strain is proportional to the square of the applied electric field) in an electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer. Here we report a material that shows a further increase in electrostriction by two orders of magnitude: ultrathin (less than 100 nanometres) ferroelectric liquid-crystalline elastomer films that exhibit 4 per cent strain at only 1.5 MV m-1. This giant electrostriction was obtained by combining the properties of ferroelectric liquid crystals with those of a polymer network. We expect that these results, which can be completely understood on a molecular level, will open new perspectives for applications.

Liquid Crystalline Ordering and Charge Transport in Semiconducting Materials

Organic semiconducting materials offer the advantage of solution processability into flexible films. In most cases, their drawback is based on their low charge carrier mobility, which is directly related to the packing of the molecules both on local (amorphous versus crystalline) and on macroscopic (grain boundaries) length scales. Liquid crystalline ordering offers the possibility of circumventing this problem. An advanced concept comprises: i) the application of materials with different liquid crystalline phases, ii) the orientation of a low viscosity high temperature phase, and, iii) the transfer of the macroscopic orientation during cooling to a highly ordered (at best, crystalline-like) phase at room temperature. At the same time, the desired orientation for the application (OLED or field-effect transistor) can be obtained. This review presents the use of molecules with discotic, calamitic and sanidic phases and discusses the sensitivity of the phases with regard to defects depending on the dimensionality of the ordered structure (columns: 1D, smectic layers and sanidic phases: 2D). It presents ways to systematically improve charge carrier mobility by proper variation of the electronic and steric (packing) structure of the constituting molecules and to reach charge carrier mobilities that are close to and comparable to amorphous silicon, with values of 0.1 to 0.7 cm(2)  · V(-1)  · s(-1) . In this context, the significance of cross-linking to stabilize the orientation and liquid crystalline behavior of inorganic/organic hybrids is also discussed.

Overcoming the PEG-addiction: well-defined alternatives to PEG, from structure–property relationships to better defined therapeutics

Synthetic methods in polymer chemistry have evolved tremendously during the last decade. Nowadays more and more attention is devoted to the application of those tools in the development of the next generation of nanomedicines. Nevertheless, poly(ethylene glycol) (PEG) remains the most frequently used polymer for biomedical applications. In this review, we try to summarize recent efforts and developments in controlled polymerisation techniques that may allow alternatives to PEG based systems and can be used to improve the properties of future polymer therapeutics.

Liquid-crystalline ordering as a concept in materials science: from semiconductors to stimuli-responsive devices.

While the unique optical properties of liquid crystals (LCs) are already well exploited for flat-panel displays, their intrinsic ability to self-organize into ordered mesophases, which are intermediate states between crystal and liquid, gives rise to a broad variety of additional applications. The high degree of molecular order, the possibility for large scale orientation, and the structural motif of the aromatic subunits recommend liquid-crystalline materials as organic semiconductors, which are solvent-processable and can easily be deposited on a substrate. The anisotropy of liquid crystals can further cause a stimuli-responsive macroscopic shape change of cross-linked polymer networks, which act as reversibly contracting artificial muscles. After illustrating the concept of liquid-crystalline order in this Review, emphasis will be placed on synthetic strategies for novel classes of LC materials, and the design and fabrication of active devices.

A Continuous Flow Synthesis of Micrometer‐Sized Actuators from Liquid Crystalline Elastomers

Adv. Mater. 2009, 21, 4859–4862 2009 WILEY-VCH Verlag G T IO N Liquid crystalline elastomers (LCEs) are weakly crosslinked polymers that contain shape-anisotropic moieties (mesogens) that self organize into liquid crystalline phases. It was proposed by de Gennes in 1975 that these materials would undergo a shape change during the phase transition from the liquid crystalline to the isotropic state, if all mesogens were ordered into the same direction, forming a monodomain. The ability to change shape on application of a certain external stimulus led to the fabrication of actuators based on these ‘‘intelligent’’ materials. These are mainly macroscopic actuators, having sizes of millimeters or centimeters. However, in recent years there has been a growing interest in the preparation of micrometer sized actuators, as these are interesting for novel fields of science such as micromechanics and robotics. Here we present the use of a microfluidic setup to prepare monodisperse, monodomainic, and micrometer-sized liquid-crystalline elastomer beads that show a strong and rapid shape change of about 70% in length during the phase transition. We show that the particle size as well as the quality of the monodomain can be controlled by the operating parameters of the microfluidic setup. The key step in preparing LCE-based actuators is the overall orientation of the liquid crystalline director (ideally the formation of a monodomain) before the material is crosslinked. Methods used in the previous cited works are mainly the stretching of pre-crosslinked films, the drawing of fibers from a polymer melt and the use of electric or magnetic fields. All these methods have in common that they are complex multistep processes that are difficult to automate and not suitable for the continuous preparation of a large number of actuators. In addition, they are problematic for preparing samples in the micrometer size region. Using microfluidics on the other hand allows the continuous preparation of a large number of monodisperse micro-particles with a minimum of time and effort. In addition we argue, that the polymerization of the droplets while they are flowing through a tube increases the tendency of the mesogens to adopt a monodomainic director field configuration, thus giving the particles actuation properties. In our approach a liquid crystallinemonomer wasmixed with a crosslinker and a photoinitiator, melted into the isotropic phase and injected through a thin needle into a co-flowing stream of immiscible silicone oil. The resulting droplets were cooled into the liquid-crystalline phase and polymerized by irradiation with UV-light while flowing through a piece of thin tubing. To achieve this, we constructed a novel microfluidic setup, which is based on earlier works of Serra and Zhang. In this approach the mixing of monomer and continuous phase is carried out via a fused silica capillary in a T-junction. The main challenge for the new setup was temperature control. As all known LC-monomers are solid at room temperature, we placed the T-junction as well as the tube containing the monomer mixture in a heat bath, which was set above the monomer clearing temperature. Two syringe pumps were used, one providing the flow of the continuous phase, the other one for pushing a low viscous oil into the monomer tube, while also providing flow. An uplight microscope was used to observe droplet formation at the end of the needle. The tube containing the monomer droplets continued out of the heat bath and was rolled onto a hot plate with its temperature set to that of the liquid crystalline phase of the monomer. UV-light was shone on the tube, initiating radical polymerization and crosslinking inside the droplets in the LC-phase. Several aspects were considered for choosing an appropriate LC material for this project. Polymeric materials were excluded because their viscosities are too high to be pumped through thin capillaries while in melt. In order to induce an orientation of the mesogens, the material has to be processed (crosslinked) in the liquid crystalline phase, thus making the use of solvents to reduce viscosity impossible. Therefore we needed a monomer that was already liquid crystalline and could easily and rapidly be polymerized and crosslinked in the flow setup. In addition, a strong coupling between the mesogen and the resulting polymer is important, as this is a prerequisite for a strong shape change of the actuator. Finally, the transition temperature for the material must occur in a temperature range to which the whole reactor setup can be heated. The choice fell on a three-core mesogen with a polymerizable acrylate group attached laterally over a flexible spacer (see Fig. 1 for chemical structure), which was described earlier by Patrick Keller. It has a nematic phase between 72 and 98 8C and has been used for actuator applications before. For the preparation of crosslinked polymer particles this LC-monomer was mixed with 10mol % of hexanedioldiacrylate (chemical structure also in Fig. 1) and 2wt % of the photoinitiator Lucirin TPO. The mixture was melted and injected to the monomer tube of the setup. In a microfluidic setup the particle size is controlled mainly by two parameters: the viscosity of

Quantum Dot−Block Copolymer Hybrids with Improved Properties and Their Application to Quantum Dot Light-Emitting Devices

To combine the optical properties of CdSe@ZnS quantum dots (QDs) with the electrical properties of semiconducting polymers, we prepared QD/polymer hybrids by grafting a block copolymer (BCP) containing thiol-anchoring moieties (poly(para-methyl triphenylamine-b-cysteamine acrylamide)) onto the surfaces of QDs through the ligand exchange procedure. The prepared QD/polymer hybrids possess improved processability such as enhanced solubility in various organic solvents as well as the film formation properties along with the improved colloidal stability derived from the grafted polymer shells. We also demonstrated light-emitting diodes based on QD/polymer hybrids, exhibiting the improved device performance (i.e., 3-fold increase in the external quantum efficiency) compared with the devices prepared by pristine (unmodified) QDs.

Radioactive Labeling of Defined HPMA-Based Polymeric Structures Using [18F]FETos for In Vivo Imaging by Positron Emission Tomography

During the last decades polymer-based nanomedicine has turned out to be a promising tool in modern pharmaceutics. The following article describes the synthesis of well-defined random and block copolymers by RAFT polymerization with potential medical application. The polymers have been labeled with the positron-emitting nuclide fluorine-18. The polymeric structures are based on the biocompatible N-(2-hydroxypropyl)-methacrylamide (HPMA). To achieve these structures, functional reactive ester polymers with a molecular weight within the range of 25,000-110,000 g/mol were aminolyzed by 2-hydroxypropylamine and tyramine (3%) to form (18)F-labelable HPMA-polymer precursors. The labeling procedure of the phenolic tyramine moieties via the secondary labeling synthon 2-[(18)F]fluoroethyl-1-tosylate ([(18)F]FETos) provided radiochemical fluoroalkylation yields of ∼80% for block copolymers and >50% for random polymer architectures within a synthesis time of 10 min and a reaction temperature of 120 °C. Total synthesis time including synthon synthesis, (18)F-labeling, and final purification via size exclusion chromatography took less than 90 min and yielded stable (18)F-labeled HPMA structures in isotonic buffer solution. Any decomposition could be detected within 2 h. To determine the in vivo fate of (18)F-labeled HPMA polymers, preliminary small animal positron emission tomography (PET) experiments were performed in healthy rats, demonstrating the renal clearance of low molecular weight polymers. Furthermore, low metabolism rates could be detected in urine as well as in the blood. Thus, we expect this new strategy for radioactive labeling of polymers as a promising approach for in vivo PET studies.

From Defined Reactive Diblock Copolymers to Functional HPMA-Based Self-Assembled Nanoaggregates

This paper describes the synthesis of functional amphiphilic poly( N-(2-hydroxypropyl) methacrylamide)-block-poly(lauryl methacrylate) copolymers by RAFT polymerization via the intermediate step of activated ester block copolymers (pentafluoro-phenyl methacrylate). Block copolymers with molecular weights from 12000-28000 g/mol and PDIs of about 1.2 have been obtained. The amphiphilic diblock copolymers form stable super structures (nanoaggregates) by self-organization in aqueous solution. The diameters of these particles are between 100 and 200 nm and depend directly on the molecular weight of the block copolymer. Furthermore, we investigated the impact of these nanoaggregates on cell viability and on the motility of adherent cells. Cytotoxicity was investigated by the MTS test and the fluctuation in cell shape was monitored employing ECIS (electrical cell-substrate impedance sensing). In these investigations, the formed particles are not cell toxic up to a concentration of 2 mg/mL. Thus, our polymeric particles offer potential as polymer therapeutics.

Synthesis of Hetero-Telechelic α,ω Bio-Functionalized Polymers

Reversible addition-fragmentation chain transfer (RAFT) polymerization was used to synthesize poly[diethylene glycol monomethylether methacrylate] (PDEGMA) (M(n) = 6250 g/mol, PDI = 1.14) with a pentafluorophenyl (PFP) activated ester and a dithioester end group. The hormone thyroxin (T4) was quantitatively attached to the PFP activated ester alpha end group via its amino group. The omega-terminal dithioester was not harmed by this reaction and was subsequently aminolyzed in the presence of N-biotinylaminoethyl methanethiosulfonate, yielding a polymer with a thyroxin and a biotin end group with very high heterotelechelic functionality. The polymer was characterized by (1)H, (13)C, and (19)F NMR, UV-vis, and IR spectroscopy and gel permeation chromatography. The thyroxin transport protein prealbumin with two thyroxin binding sites and streptavidin, which has four biotin binding sites, was conjugated using the biotarget labeled polymer, resulting in the formation of a protein-polymer network, confirming the heterotelechelic nature of the polymer. Polymer-protein microgel formation was observed with dynamic light scattering. To realize a directed protein assembly, prealbumin was immobilized onto a surface, exposing one of its two thyroxin binding groups and thus allowing the conjugation with the thyroxin alpha end group of the heterotelechelic polymer. The biotin omega end group of the attached polymer layer enabled the subsequent immobilization of streptavidin, yielding a defined multilayer system of two proteins connected with the synthetic polymer (efficiency of streptavidin immobilization 81% based on prealbumin). Without the polymer, no streptavidin immobilization occurred. The layer depositions were monitored by surface plasmon resonance. The synthetic approach of combining PFP activated esters with functional MTS reagents presents a powerful method for obtaining well-defined heterotelechelic (bio-) functionalized polymers.