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The method of converting insulating polymers into conducting 3D porous graphene structures, so-called laser-induced graphene (LIG) with a commercially available CO2 laser engraving system in an ambient atmosphere, resulted in several applications in sensing, actuation, and energy. In this paper, we demonstrate a combination of LIG and a smart hydrogel (poly(N-vinylcaprolactam)—pNVCL) for multiresponsive actuation in a humid environment. Initiated chemical vapor deposition (iCVD) was used to deposit a thin layer of the smart hydrogel onto a matrix of poly(dimethylsiloxane) (PDMS) and embedded LIG tracks. An intriguing property of smart hydrogels, such as pNVCL, is that the change of an external stimulus (temperature, pH, magnetic/electric fields) induces a reversible phase transition from a swollen to a collapsed state. While the active smart hydrogel layer had a thickness of only 300 nm (compared to the 500 times thicker actuator matrix), it was possible to induce a reversible bending of over 30° in the humid environment triggered by Joule heating. The properties of each material were investigated by means of scanning electron microscopy (SEM), Raman spectroscopy, tensile testing, and ellipsometry. The actuation performances of single-responsive versions were investigated by creating a thermoresponsive PDMS/LIG actuator and a humidity-responsive PDMS/pNVCL actuator. These results were used to tune the properties of the multiresponsive PDMS/LIG/pNVCL actuator. Furthermore, its self-sensing capabilities were investigated. By getting a feedback from the piezoresistive change of the PMDS/LIG composite, the bending angle could be tracked by measuring the change in resistance. To highlight the possibilities of the processing techniques and the combination of materials, a demonstrator in the shape of an octopus with four independently controllable arms was developed.

Dissertation, RWTH Aachen University, 2020; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen, Diagramme (2021). = Dissertation, RWTH Aachen University, 2020, This thesis deals with the synthesis and properties of new photochromic polymers, low-molecular compounds and their supramolecular complexes. The first chapter describes the goal and motivation of this work. The second chapter presents an overview of isomerization of photochromic groups, e.g. spiropyrans and azobenzenes, and their application in the design and synthesis of photochromic materials. The third chapter describes the preparation and properties of microgel-surfactant complexes, in which the surfactant molecules contain a photoisomerizable azo-group. For this purpose, microgels based on copolymers of vinylcaprolactam and vinylimidazole were neutralized with sulfonic acid containing surfactant molecules. In the structure of the wedge-shaped surfactant molecules three long-chain alkyl groups are connected with a sulfonated azobenzene unit via an ester group. The alkyl chain contains 8 or 12 carbon atoms. The ionic binding of the surfactants to the microgels has a mutual influence. On the one hand, the micelle formation of the microgel-bound surfactants is altered compared to freely dissolved surfactants. On the other hand, the solubility behavior of the surfactants changes. These association and aggregate formation processes can in turn be influenced by the switchability of the surfactants under irradiation. The results show that the increase of the degree of neutralization of the vinylimidazole groups leads to the increase of the activation energy of the cis-trans-transition of the azobenzene groups. The activation energy also depends on the chain length. The longer the chain, the higher the activation energy. These effects result from 1) aggregation of wedge-shaped groups 2) low degree of neutralization of the vinylimidazole groups (no possibility to form microdomains). It has also been shown that the modification with the surfactant molecules has an influence on volume phase transitions of the microgels. While the thermal shrinkage behavior at low degrees of neutralization (25%) was only slightly less pronounced than that of the unmodified microgels, an increased degree of neutralization significantly affected the temperature dependence and led to the reduction in the hydrodynamic radius. The samples with the degree of neutralization of 100% further showed the decrease of the hydrodynamic radius: with the surfactant with dodecyl groups, the microgels largely lose their thermal sensitivity, while the increase of the transition temperature was observed for the surfactants with shorter alkyl chains. Chapter 4 deals with the domain formation of pure surfactant molecules. Under dry conditions and with low water content, the surfactant molecules form inverse micelles, which can also adopt a cylindrical structure. In this chapter the dependence of the structure on the water content was systematically investigated. With increasing relative air humidity (RH), the amount of absorbed water increases significantly for the surfactant with dodecyl chains, unlike in comparison with one with octyl groups. In the previous case, water sorption is accompanied by a significant increase in ion conductivity and a phase transition. In particular, an increase in RH leads to a transition from a lamellar to a columnar phase; this results in the formation of one-dimensional water channels along the axis of the supramolecular columns. In the case of the molecules with shorter alkyl chains, the lamellar phase exists over the entire RH range with prominent swelling at high RH values, which then also lead to a two-dimensional water channel structure. NMR diffusometry was used to address the molecular mobility in the lyotropic mesophases of the wedge-shaped amphiphilic sulfonate molecules. Chapter 5 focuses on the design, synthesis, and investigation of core-shell nanoparticles that contain spiropyran groups. The particles were synthesized using a two-stage polymerization procedure: 1) preparation of a spiropyran-containing polystyrene-based core via emulsion polymerization and 2) seed polymerization of N-isopropylacrylamide to build up a thermosensitive shell. This synthetic approach provides a simple method for the preparation of monodisperse particles with up to 4 mol.-% of spiropyran groups. The incorporation of such groups renders the polystyrene core pH-sensitive, that is expressed in the blue shift of the absorbance by adding acid, and UV-responsive: irradiation with light of 365 nm wavelength leads to isomerization from the spiropyran isomer to the merocyanine form, which is stable for up to 12 hours at room temperature. In addition, the formation of the merocyanine group is accompanied by both a significantly increasing absorbance in the range of 450-510 nm and an increase of the hydrodynamic radius of the particles through the formation of a solvation envelope. The presence of the poly-N-isopropylacrylamide shell makes the nanoparticles thermosensitive: their temperature of the volume-phase transition is 33° C. This chapter clearly shows the new approach to the synthesis of nanoparticles with multisensory properties. Overall, the work presented here describes a couple of interesting examples, where the sensitivity of different types to stimuli of different origins (temperature, pH and light) can be combined in multifunctional materials, therefore, there is great potential for their use in nanotechnology, Published by RWTH Aachen University, Aachen

An unprecedented MXene-based actuator was developed, which mimicked the sophisticated leaf structure and its functionality., Natural leaves, with elaborate architectures and functional components, harvest and convert solar energy into chemical fuels that can be converted into energy based on photosynthesis. The energy produced leads to work done that inspired many autonomous systems such as light-triggered motion. On the basis of this nature-inspired phenomenon, we report an unprecedented bilayer-structured actuator based on MXene (Ti3C2Tx)–cellulose composites (MXCC) and polycarbonate membrane, which mimic not only the sophisticated leaf structure but also the energy-harvesting and conversion capabilities. The bilayer actuator features multiresponsiveness, low-power actuation, fast actuation speed, large-shape deformation, programmable adaptability, robust stability, and low-cost facile fabrication, which are highly desirable for modern soft actuator systems. We believe that these adaptive soft systems are attractive in a wide range of revolutionary technologies such as soft robots, smart switch, information encryption, infrared dynamic display, camouflage, and temperature regulation, as well as human-machine interface such as haptics.

Increasing interest for the application of polypeptide-based smart systems in the biomedical field has developed due to the advantages given by the peptidic sequence. This is due to characteristics of these systems, which include: biocompatibility, potential control of degradation, capability to provide a rich repertoire of biologically specific interactions, feasibility to self-assemble, possibility to combine different functionalities, and capability to give an environmentally responsive behavior. Recently, applications concerning the development of these systems are receiving greater attention since a targeted and programmable release of drugs (e.g. anti-cancer agents) can be achieved. Block copolymers are discussed due to their capability to render differently assembled architectures. Hybrid systems based on silica nanoparticles are also discussed. In both cases, the selected systems must be able to undergo fast changes in properties like solubility, shape, and dissociation or swelling capabilities. This review is structured in different chapters which explain the most recent advances on smart systems depending on the stimuli to which they are sensitive. Amphiphilic block copolymers based on polyanionic or polycationic peptides are, for example, typically employed for obtaining pH-responsive systems. Elastin-like polypeptides are usually used as thermoresponsive polymers, but performance can be increased by using techniques which utilize layer-by-layer electrostatic self-assembly. This approach offers a great potential to create multilayered systems, including nanocapsules, with different functionality. Recent strategies developed to get redox-, magnetic-, ultrasound-, enzyme-, light- and electric-responsive systems are extensively discussed. Finally, some indications concerning the possibilities of multi-responsive systems are discussed.

Fabrication strategies for programmed hydrogels that provide precise spatial control with predetermined responses to external stimuli are highly desirable. In this study a partially reversible light-driven assembly (PRLDA) method is introduced to construct multi-responsive hydrogels utilizing microgel (MG) particle building blocks (swollen diameter of 107 nm). No other material is required to prepare the gels beyond the MGs themselves. We demonstrate facile preparation of multi-responsive hydrogels that are reversibly responsive to light, pH and temperature using phototriggered covalent interlinking of coumarin-based MGs. The gels have photo-tuneable moduli and swelling ratios and show light-assisted healing and re-shaping. Remarkably, the intrinsic fluorescence of the gels undergoes a reversible light-triggered wavelength-shift. The emission peak blue-shifted from 420 nm to 390 nm upon irradiation with 365 nm light. The PRLDA gels can be constructed using either positive or negative photo-patterning. We show that the gels can be exploited for multi-responsive cytocompatible actuators, grippers and ON/OFF circuit components as well as anti-counterfeit gels. The PRLDA method provides new insight into programmed gel property control and has excellent potential for biomaterial and optoelectronic applications.

The 2-fold thermoresponsive diblock copolymer PSPP498-b-PNIPMAM144, which consists of a zwitterionic polysulfobetaine (PSPP) block and a nonionic poly(Nisopropylmethacrylamide) (PNIPMAM) block, is prepared by consecutive RAFT polymerizations. It combines the upper and lower critical solution temperature (UCST and LCST) behaviors, respectively, of the constitutive homopolymers in aqueous solution. We investigate the temperature-dependent phase behavior and the self-assembled structures of the block copolymer in D2O by turbidimetry and by small-angle neutron scattering (SANS) in salt-free solution and in the presence of small amounts of NaCl and NaBr. For comparison, solutions of PNIPMAM homopolymer in D2O are studied as well. Turbidimetry indicates thermally induced schizophrenic aggregation behavior for PSPP498-b-PNIPMAM144. SANS reveals that conventional star-like core−shell micellar structures are formed above the LCST transition, whereas below the UCST-transition, structure formation is much less pronounced. This is attributed to the different types of interactions, namely hydrophobic and ionic ones, dominating in the different regimes. Despite the increased polarity contrast between the zwitterionic and the nonionic blocks, and the much wider separation of the UCST- and LCST-based cloud points, CPUCST and CPLCST, the structural features of the new PSPP498-b-PNIPMAM144 resemble the ones found previously for the also 2-fold thermoresponsive analogue PSPP432-b-PNIPAM200, for which both phase transition temperatures nearly coincide. Remarkably, the addition of small amounts of NaBr or NaCl to the solution of PSPP498-b-PNIPMAM144 causes a significant increase of CPUCST, as well as minor but notable changes in the self−assembled structures, but no gross alterations of the phase behavior.