Shape morphing of hydrogels by harnessing enzyme enabled mechanoresponse

Hydrogels have been designed to react to many different stimuli which find broad applications in tissue engineering and soft robotics. However, polymer networks bearing mechano-responsiveness, especially those displaying on-demand self-stiffening and self-softening behavior, are rarely reported. Here, we design a mechano-controlled biocatalytic system at the molecular level that is incorporated into hydrogels to regulate their mechanical properties at the material scale. The biocatalytic system consists of the protease thrombin and its inhibitor, hirudin, which are genetically engineered and covalently coupled to the hydrogel networks. The catalytic activity of thrombin is reversibly switched on by stretching of the hydrogels, which disrupts the noncovalent inhibitory interaction between both entities. Under cyclic tensile-loading, hydrogels exhibit self-stiffening or self-softening properties when substrates are present that can self-assemble to form new networks after being activated by thrombin or when cleavable peptide crosslinkers are constitutional components of the original network, respectively. Additionally, we demonstrate the programming of bilayer hydrogels to exhibit tailored shape-morphing behavior under mechanical stimulation. Our developed system provides proof of concept for mechanically controlled reversible biocatalytic processes, showcasing their potential for regulating hydrogels and proposing a biomacromolecular strategy for mechano-regulated soft functional materials.

Polymer Mechanochemistry in Microbubbles

Polymer mechanochemistry is a promising technology to convert mechanical energy into chemical functionality by breaking covalent and supramolecular bonds site-selectively. Yet, the mechanochemical reaction rates of covalent bonds in typically used ultrasonication setups lead to reasonable conversions only after comparably long sonication times. This can be accelerated by either increasing the reactivity of the mechanoresponsive moiety or by modifying the encompassing polymer topology. Here, a microbubble system with a tailored polymer shell consisting of an N2 gas core and a mechanoresponsive disulfide-containing polymer network is presented. It is found that the mechanochemical activation of the disulfides is greatly accelerated using these microbubbles compared to commensurate solid core particles or capsules filled with liquid. Aided by computational simulations, it is found that low shell thickness, low shell stiffness and crosslink density, and a size-dependent eigenfrequency close to the used ultrasound frequency maximize the mechanochemical yield over the course of the sonication process.

Thermally and Magnetically Programmable Hydrogel Microactuators

The rapid development in micro-machinery enabled the investigation of smart materials that can embody fast response, programmable actuation, and flexibility to perform mechanical work. Soft magnetic actuators represent an interesting platform toward combining those properties. This study focuses on the synthesis of micro-actuators that respond to thermal and magnetic stimuli using micro-molding with a soft template as a fabrication technique. These microsystems consist of a hydrogel matrix loaded with anisotropic magnetic nanospindles. When a homogeneous magnetic field is applied, the nanospindles initially dispersed in monomer solution, align and assemble into dipolar chains. The ensuing UV-polymerization creates a network and conveniently arrests these nanostructures. Consequently, the magnetic dipole moment is coplanar with the microgel. Varying the shape, volume, and composition of the micro-actuators during synthesis provides a temperature-dependent control over the magnetic response and the polarizability. Beyond isotropic swelling, shaping the hydrogel as long thin ribbons with a passive layer on one side allows for differential swelling leading to bending and twisting deformations, for example, 2D- or 3D-spiral. These deformations involve a reversible amplification of the magnetic response and orientation of the hydrogels under magnetic field. Temperature control herewith determines the conformation and simultaneously the magnetic response of the micro-actuators.

Tuning the Elasticity of Nanogels Improves Their Circulation Time by Evading Immune Cells

Peptide receptor radionuclide therapy is used to treat solid tumors by locally delivering radiation. However, due to nephro‐ and hepato‐toxicity, it is limited by its dosage. To amplify radiation damage to tumor cells, radiolabeled nanogels can be used. We show that by tuning the mechanical properties of nanogels significant enhancement in circulation half‐life of the gel could be achieved. We demonstrate why and how small changes in the mechanical properties of the nanogels influence its cellular fate. Nanogels with a storage modulus of 37 kPa were minimally phagocytosed by monocytes and macrophages compared to nanogels with 93 kPa modulus. Using PET/CT a significant difference in the blood circulation time of the nanogels was shown. Computer simulations affirmed the results and predicted the mechanism of cellular uptake of the nanogels. Altogether, this work emphasizes the important role of elasticity even for particles that are inherently soft such as nano‐ or microgels.

Microgel that swims to the beat of light

Complementary to the quickly advancing understanding of the swimming of microorganisms, we demonstrate rather simple design principles for systems that can mimic swimming by body shape deformation. For this purpose, we developed a microswimmer that could be actuated and controlled by fast temperature changes through pulsed infrared light irradiation. The construction of the microswimmer has the following features: (i) it is a bilayer ribbon with a length of 80 or 120 [Formula: see text]m, consisting of a thermo-responsive hydrogel of poly-N-isopropylamide coated with a 2-nm layer of gold and equipped with homogeneously dispersed gold nanorods; (ii) the width of the ribbon is linearly tapered with a wider end of 5 [Formula: see text]m and a tip of 0.5 [Formula: see text]m; (iii) a thickness of only 1 and 2 [Formula: see text]m that ensures a maximum variation of the cross section of the ribbon along its length from square to rectangular. These wedge-shaped ribbons form conical helices when the hydrogel is swollen in cold water and extend to a filament-like object when the temperature is raised above the volume phase transition of the hydrogel at [Formula: see text]. The two ends of these ribbons undergo different but coupled modes of motion upon fast temperature cycling through plasmonic heating of the gel-objects from inside. Proper choice of the IR-light pulse sequence caused the ribbons to move at a rate of 6 body length/s (500 [Formula: see text]m/s) with the wider end ahead. Within the confinement of rectangular container of 30 [Formula: see text]m height and 300 [Formula: see text]m width, the different modes can be actuated in a way that the movement is directed by the energy input between spinning on the spot and fast forward locomotion.

Stiffness Tomography of Ultra-Soft Nanogels by Atomic Force Microscopy

The softness of nanohydrogels results in unique properties and recently attracted tremendous interest due to the multi-functionalization of interfaces. Herein, we study extremely soft temperature-sensitive ultra-low cross-linked (ULC) nanogels adsorbed to the solid/water interface by atomic force microscopy (AFM). The ultra-soft nanogels seem to disappear in classical imaging modes since a sharp tip fully penetrates these porous networks with very low forces in the range of steric interactions (ca. 100 pN). However, the detailed evaluation of Force Volume mode measurements allows us to resolve their overall shape and at the same time their internal structure in all three dimensions. The nanogels exhibit an extraordinary disk-like and entirely homogeneous but extremely soft structure-even softer than polymer brushes. Moreover, the temperature-sensitive nanogels can be switched on demand between the ultra-soft and a very stiff state.

Metal Coordination Induces Phase Segregation in Amphipolar Arborescent Copolymers with a Core–Shell–Corona Architecture

Arborescent copolymers with a core–shell–corona (CSC) architecture were synthesized and the topology of the molecules was challenged (constrained) through intramolecular interactions, resulting in phase separation breaking the symmetry of radial density. The inner poly(2-vinylpyridine) shell of these arborescent polystyrene-g-[poly(2-vinylpyridine)-b-polystyrene] molecules can self-assemble by binding metallic salts and acids in apolar and intermediate-polarity solvents. Upon loading with HAuCl₄, the characteristics of the polymer templates govern the “loading sites” of the metal within the molecules. Unique morphologies were observed for the metal-loaded G0–G4 arborescent copolymers investigated, namely, spherical, toroidal, raspberry-like, spherical nanocage, and a new worm-in-sphere morphology. The reason for the emergence of such morphologies is the interplay among intramolecular interactions of unlike polymer segments, solvent selectivity, the entropic elasticity of the arborescent substrate, and phase segregation induced by coordination with the charged metallic species. Meanwhile, the stability of the arborescent molecules against aggregation provides intramolecular phase segregation with imposed “confined” geometry and thus leads to nonconventional morphologies. Furthermore, the size of the arborescent molecules is much smaller than that of other known particles (droplets) serving as confined geometries. Computer simulations were used to model the mesostructure of the arborescent copolymers, to demonstrate the influence of solvent selectivity, together with HAuCl₄ loading, on the evolution of the morphology of the macromolecules.

4D Printing of a Light-Driven Soft Actuator with Programmed Printing Density

There is a growing interest in the concept of four-dimensional (4D) printing that combines a three-dimensional (3D) manufacturing process with dynamic modulation for bioinspired soft materials exhibiting more complex functionality. However, conventional approaches have drawbacks of low resolution, control of internal micro/nanostructure, and creation of fast, complex actuation due to a lack of high-resolution fabrication technology and suitable photoresist for soft materials. Here, we report an approach of 4D printing that develops a bioinspired soft actuator with a defined 3D geometry and programmed printing density. Multiphoton lithography (MPL) allows for controlling printing density in gels at pixel-by-pixel with a resolution of a few hundreds of nanometers, which tune swelling behaviors of gels in response to external stimuli. We printed a 3D soft actuator composed of thermoresponsive poly(N-isopropylacrylamide) (PNIPAm) and gold nanorods (AuNRs). To improve the resolution of printing, we synthesized a functional, thermoresponsive macrocrosslinker. Through plasmonic heating by AuNRs, nanocomposite-based soft actuators undergo nonequilibrium, programmed, and fast actuation. Light-mediated manufacture and manipulation (MPL and photothermal effect) offer the feasibility of 4D printing toward adaptive bioinspired soft materials.

Tuning the Structure and Properties of Ultra-Low Cross-Linked Temperature-Sensitive Microgels at Interfaces via the Adsorption Pathway

The structure of poly(N-isopropylacrylamide) (PNIPAM) microgels adsorbed onto a solid substrate is investigated in the dry and hydrated states by means of atomic force microscopy (AFM). We compare two different systems: a regularly cross-linked microgel containing 5 mol % cross-linker and ultra-low cross-linked microgels (ULC) prepared without a dedicated cross-linker. Furthermore, we compare three different adsorption processes: (i) in situ adsorption from solution, (ii) spin-coating, and (iii) Langmuir-Blodgett deposition from an oil-water interface. The results demonstrate that the morphology and the temperature-induced collapse of microgels adsorbed onto a solid substrate are very different for ultra-low cross-linked microgels as compared to regularly cross-linked microgels, despite the fact that their general behavior in solution is very similar. Furthermore, the morphology of ULC microgels can be controlled by the adsorption pathway onto the substrate. Absorbed ULC microgels are strongly deformed when being prepared either by spin-coating or by Langmuir-Blodgett deposition from an oil-water interface. After rehydration, the ULC microgels cannot collapse as entire objects, instead small globules are formed. Such a strong deformation can be avoided by in situ adsorption onto the substrate. Then, the ULC microgels exhibit half-ellipsoidal shapes with a smooth surface in the collapsed state similar to the more cross-linked microgels. As ULC microgels can be selectively trapped either in a more particle-like or in a more polymer-like behavior, coatings with strongly different topographies and properties can be prepared by one and the same ultra-low cross-linked microgel. This provides new opportunities for the development of smart polymeric coatings.

A Light-Driven Microgel Rotor

The current understanding of motility through body shape deformation of micro-organisms and the knowledge of fluid flows at the microscale provides ample examples for mimicry and design of soft microrobots. In this work, a 2D spiral is presented that is capable of rotating by non-reciprocal curling deformations. The body of the microswimmer is a ribbon consisting of a thermoresponsive hydrogel bilayer with embedded plasmonic gold nanorods. Such a system allows fast local photothermal heating and nonreciprocal bending deformation of the hydrogel bilayer under nonequilibrium conditions. It is shown that the spiral acts as a spring capable of large deformations thanks to its low stiffness, which is tunable by the swelling degree of the hydrogel and the temperature. Tethering the ribbon to a freely rotating microsphere enables rotational motion of the spiral by stroboscopic irradiation. The efficiency of the rotor is estimated using resistive force theory for Stokes flow. This research demonstrates microscopic locomotion by the shape change of a spiral and may find applications in the field of microfluidics, or soft microrobotics.

Cellular responses to beating hydrogels to investigate mechanotransduction

Cells feel the forces exerted on them by the surrounding extracellular matrix (ECM) environment and respond to them. While many cell fate processes are dictated by these forces, which are highly synchronized in space and time, abnormal force transduction is implicated in the progression of many diseases (muscular dystrophy, cancer). However, material platforms that enable transient, cyclic forces in vitro to recreate an in vivo-like scenario remain a challenge. Here, we report a hydrogel system that rapidly beats (actuates) with spatio-temporal control using a near infra-red light trigger. Small, user-defined mechanical forces (~nN) are exerted on cells growing on the hydrogel surface at frequencies up to 10 Hz, revealing insights into the effect of actuation on cell migration and the kinetics of reversible nuclear translocation of the mechanosensor protein myocardin related transcription factor A, depending on the actuation amplitude, duration and frequency.

Supramolecular polymerization: challenges and advantages of various methods in assessing the aggregation mechanism

Oligothiophenes with branched alkyl end groups show distinct aggregation in organic solvents. The process of supramolecular polymerization is assessed by three different methods (UV-vis absorption and fluorescence emission spectroscopy and dynamic light scattering) to exclude artifacts. An apparent dependence of the degree of aggregation on the concentration of the oligomers is observed. Above the upper limit of concentration (a lower micromolar range for the present class of compounds), experimental data delivered conflicting results and the concentration should not therefore be exceeded. Scanning force microscopy and molecular dynamics simulations confirm the formation of one-dimensional aggregates with presumably helical arrangement of the achiral monomers.

The swimming of a deforming helix

Many microorganisms and artificial microswimmers use helical appendages in order to generate locomotion. Though often rotated so as to produce thrust, some species of bacteria such Spiroplasma, Rhodobacter sphaeroides and Spirochetes induce movement by deforming a helical-shaped body. Recently, artificial devices have been created which also generate motion by deforming their helical body in a non-reciprocal way (A. Mourran et al. Adv. Mater. 29, 1604825, 2017). Inspired by these systems, we investigate the transport of a deforming helix within a viscous fluid. Specifically, we consider a swimmer that maintains a helical centreline and a single handedness while changing its helix radius, pitch and wavelength uniformly across the body. We first discuss how a deforming helix can create a non-reciprocal translational and rotational swimming stroke and identify its principle direction of motion. We then determine the leading-order physics for helices with small helix radius before considering the general behaviour for different configuration parameters and how these swimmers can be optimised. Finally, we explore how the presence of walls, gravity, and defects in the centreline allow the helical device to break symmetries, increase its speed, and generate transport in directions not available to helices in bulk fluids.

Diffusion of Gold Nanorods Functionalized with Thermoresponsive Polymer Brushes

Understanding the diffusion of gold nanorods (AuNRs) and their composites in dispersion is important at fundamental level and in fields as diverse as material science, nanobiotechnology to drug delivery. The translational and rotational diffusion of AuNRs decorated with thermoresponsive poly( N-isopropylacrylamide) brushes having hydrophilic and hydrophobic end groups was investigated in the dilute regime by dynamic light scattering. The same series of functionalized AuNRs were studied in the isotropic concentrated dispersions by high-resolution NMR diffusometry. The dependence of translational and rotational diffusivity upon molecular weight and polymer end group were measured as a function of temperature in the region of the brush phase transition. The effective hydrodynamic radius of AuNR composites proved to be the most sensitive quantity to the temperature-induced phase transition of brushes, allowing the evaluation of the brush thickness in the swollen and collapsed states.

Probing the Internal Heterogeneity of Responsive Microgels Adsorbed to an Interface by a Sharp SFM Tip: Comparing Core-Shell and Hollow Microgels

Microgels composed of thermoresponsive polymer poly( N-isopropylacrylamide) (PNIPAM) are interfacial active. Their adsorption leads to deformation, causing conformational changes that have profound effects on the macroscopic properties of these films. Yet, methods to quantitatively probe the local density are lacking. We introduced scanning force microscopy (SFM) to quantitatively probe the internal structure of microgels physically adsorbed on a solid (SiO₂)/water interface. Using a sharp SFM tip, we investigated the two types of microgels: (i) core-shell microgels featuring a hard silica core and a PNIPAM shell and (ii) hollow microgels obtained by dissolution of the silica core. Thus, both systems have the same polymer network as the peripheral structure but a distinctly different internal structure, that is, a rigid core versus a void. By evaluating the force-distance curves, the force profile during insertion of the tip into the polymer network enables to determine a depth-dependent contact resistance, which closely correlates with the density profiles determined in solution by small-angle neutron scattering. We found that the cavity of the swollen hollow microgels is still present when adsorbed to the solid substrate. Remarkably, while currently used techniques such as colloidal probe or reflectometry only provide an average of the z-profile, the methodology introduced herein actually probes the real three-dimensional density profile, which is ultimately important to understand the macroscopic behavior of microgel films. This will bridge the gap between the colloidal probe experiments that deform the microgel globally and the insertion in which the disturbance is located near the tip.

In-Gel Direct Laser Writing for 3D-Designed Hydrogel Composites That Undergo Complex Self-Shaping

Self-shaping and actuating materials inspired by biological system have enormous potential for biosensor, microrobotics, and optics. However, the control of 3D-complex microactuation is still challenging due to the difficulty in design of nonuniform internal stress of micro/nanostructures. Here, we develop in-gel direct laser writing (in-gel DLW) procedure offering a high resolution inscription whereby the two materials, resin and hydrogel, are interpenetrated on a scale smaller than the wavelength of the light. The 3D position and mechanical properties of the inscribed structures could be tailored to a resolution better than 100 nm over a wide density range. These provide an unparalleled means of inscribing a freely suspended microstructures of a second material like a skeleton into the hydrogel body and also to direct isotropic volume changes to bending and distortion motions. In the combination with a thermosensitive hydrogel rather small temperature variations could actuate large amplitude motions. This generates complex modes of motion through the rational engineering of the stresses present in the multicomponent material. More sophisticated folding design would realize a multiple, programmable actuation of soft materials. This method inspired by biological system may offer the possibility for functional soft materials capable of biomimetic actuation and photonic crystal application.

Thermodynamic Parameters of Temperature-Induced Phase Transition for Brushes onto Nanoparticles: Hydrophilic versus Hydrophobic End-Groups Functionalization

Quantification of the stimuli-responsive phase transition in polymers is topical and important for the understanding and development of novel stimuli-responsive materials. The temperature-induced phase transition of poly(N-isopropylacrylamide) (PNIPAm) with one thiol end group depends on the confinement-free polymer or polymer brush-on the molecular weight and on the nature of the second end. This paper describes the synthesis of heterotelechelic PNIPAm of different molecular weights with a thiol end group-that specifically binds to gold nanorods and a hydrophilic NIPAm end group by reversible addition-fragmentation chain-transfer polymerization. Proton high-resolution magic angle sample spinning NMR spectra are used as an indicator of the polymer chain conformations. The characteristics of phase transition given by the transition temperature, entropy, and width of transition are obtained by a two-state model. The dependence of thermodynamic parameters on molecular weight is compared for hydrophilic and hydrophobic end functional-free polymers and brushes.

Dynamic Switching of Helical Microgel Ribbons

We report on a microscopic poly(N-isopropylacrylamide) hydrogel ribbon, coated by a thin gold layer, that shows helical coiling. Confined swelling and shrinkage of the hydrogel below and above its characteristic volume phase transition leads to a temperature actuated reversal of the sense of the helix. The extent and the shape of the winding are controlled by the dimensions and mechanical properties of the bilayer ribbon. We focus on a cylindrical helix geometry and monitor the morphing under equilibrium and nonequilibrium conditions, that is, when the temperature changes faster than the volume (millisecond range). For slow temperature variations, the water release and uptake follow the equilibrium transition trajectory determined by the time needed for the diffusion of water into and out of the microscopic gel. Much faster variations of the temperature are accomplished by internal heating of embedded gold nanorods by IR-light irradiation. This causes elastic stresses that strongly affect the motions. This method enables well-reproducible deviations from the equilibrium transition path and allows us to control rather precisely the spatiotemporal transformation in a cyclic repetitive process. Actuation and response are sensitive to small variations of temperature and composition of the aqueous sol in which the gel is immersed. The principle as described may be used to detect specific analytes that bind either to the surface of the gold layer or within the gel and can modify the interaction between the water and the gel. The reported nonequilibrium morphing implies that the system dissipates energy and may also be able to perform work as required for a microscopic motor.

Cubosomes from hierarchical self-assembly of poly(ionic liquid) block copolymers

Cubosomes are micro- and nanoparticles with a bicontinuous cubic two-phase structure, reported for the self-assembly of low molecular weight surfactants, for example, lipids, but rarely formed by polymers. These objects are characterized by a maximum continuous interface and high interface to volume ratio, which makes them promising candidates for efficient adsorbents and host-guest applications. Here we demonstrate self-assembly to nanoscale cuboidal particles with a bicontinuous cubic structure by amphiphilic poly(ionic liquid) diblock copolymers, poly(acrylic acid)-block-poly(4-vinylbenzyl)-3-butyl imidazolium bis(trifluoromethylsulfonyl)imide, in a mixture of tetrahydrofuran and water under optimized conditions. Structure determining parameters include polymer composition and concentration, temperature, and the variation of the solvent mixture. The formation of the cubosomes can be explained by the hierarchical interactions of the constituent components. The lattice structure of the block copolymers can be transferred to the shape of the particle as it is common for atomic and molecular faceted crystals.

Soft Microrobots Employing Nonequilibrium Actuation via Plasmonic Heating

A soft microrobot composed of a microgel and driven by the light-controlled nonequilibrium dynamics of volume changes is presented. The photothermal response of the microgel, containing plasmonic gold nanorods, enables fast heating/cooling dynamics. Mastering the nonequilibrium response provides control of the complex motion, which goes beyond what has been so far reported for hydrophilic microgels.

Probing effective slippage on superhydrophobic stripes by atomic force microscopy

While the effective slippage of water past superhydrophobic surfaces has been studied over a decade, theoretical predictions have never been properly confirmed by experiments. Here we measure a drag force on a sphere approaching a plane decorated by superhydrophobic grooves and compare the results with the predictions of semi-analytical theory developed here, which employs the gas cushion model to calculate the local slip length at the gas sectors. We demonstrate that at intermediate and large (compared to a texture period) separations the half-sum of longitudinal and transverse effective slip lengths can be deduced from the force-distance curve by using the known analytical theory of hydrodynamic interaction of a sphere with a homogeneous slipping plane. This half-sum is shown to depend on the fraction of gas sectors and its value is in excellent agreement with theoretical predictions. At small distances the half-sum of effective longitudinal and transverse slip lengths becomes separation-dependent, and is in quantitative agreement with the predictions of our semi-analytical theory.

Persulfate initiated ultra-low cross-linked poly(N-isopropylacrylamide) microgels possess an unusual inverted cross-linking structure

Cross-linking density and distribution are decisive for the mechanical and other properties of stimuli-sensitive poly(N-isopropylacrylamide) microgels. Here we investigate the structure of ultra-low cross-linked microgels by static light scattering and scanning force microscopy, and show that they have an inverted cross-linking structure with respect to conventional microgels, contrary to what has been assumed previously. The conventional microgels have the largest polymer volume fraction in the core from where the particle density decays radially outwards, whereas ultra-low cross-linked particles have the highest polymer volume fraction close to the surface. On a solid substrate these particles form buckled shapes at high surface coverage, as shown by scanning force micrographs. The special structure of ultra-low cross-linked microgels is attributed to cross-linking of the particle surface, which is exposed to hydrogen abstraction by radicals generated from persulfate initiators during and after polymerization. The particle core, which is less accessible to the diffusion of radicals, has consequently a lower polymer volume fraction in the swollen state. By systematic variation of the cross-linker concentration it is shown that the cross-linking contribution from peroxide under typical synthesis conditions is weaker than that from the use of 1 mol% N,N'-methylenebisacrylamide. Soft deformable hydrogel particles are of interest because they emulate biological tissues, and understanding the underlying synthesis principle enables tailoring the microgel structure for biomimetic applications. Deformability of microgels is usually controlled by the amount of added cross-linker; here we however highlight an alternative approach through structural softness.

Substituted Septithiophenes with End Groups of Different Size: Packing and Frustration in Bulk and Thin Films

We report on three different liquid crystalline compounds with a central septithiophene core and alkylated end groups of strongly increasing bulkiness. In principle, the thiophene cores prefer to pack parallel to optimize their π-π interactions, which becomes sterically impossible for the bulkier end groups. Using X-ray diffraction, we find that the way out of this packing dilemma is toward liquid-crystal phases of higher dimensionality in the order smectic → columnar ↔ bicontinuous cubic. For the smectic phase, packing in a monolayer is no problem; for the other ones packing considerations become more stringent in films due to the boundaries. Surface X-ray techniques and atomic force microscopy indicate an appreciable difference between monolayer and three-layer films, in which the monolayers appear to escape from packing frustration by generating superstructures. We propose a basic structure of columns parallel to the substrate that provides a compromise between preserving some π-π interactions and packing the bulky alkyl groups.

When Colloidal Particles Become Polymer Coils

This work concerns interfacial adsorption and attachment of swollen microgel with low- to medium-level cross-linking density. Compared to colloids that form a second, dispersed phase, the suspended swollen microgel particles are ultrahigh molecular weight molecules, which are dissolved like a linear polymer, so that solvent and solute constitute only one phase. In contrast to recent literature in which microgels are treated as particles with a distinct surface, we consider solvent-solute interaction as well as interfacial adsorption based on the chain segments that can form trains of adsorbed segments and loops protruding from the surface into the solvent. We point out experimental results that support this discrimination between particles and microgels. The time needed for swollen microgels to adsorb at the air/water interface can be 3 orders of magnitude shorter than that for dispersed particles and decreases with decreasing cross-linking density. Detailed analysis of the microgels deformation, in the dry state, at a solid surface enabled discrimination particle like microgel in which case spreading was controlled predominantly by the elasticity and molecule like adsorption characterized by a significant overstreching, ultimately leading to chain scission of microgel strands. Dissipative particle dynamics simulations confirms the experimental findings on the interfacial activity and spreading of microgel at liquid/air interface.

Waterborne physically crosslinked antimicrobial nanogels

Aqueous solutions of poly(ethylene imine) functionalized with decyl- and azetidinium groups form physically cross-linked antimicrobial nanogels.

Monolayer properties of asymmetrically substituted sexithiophene

We present a structural comparison of monolayers on a SiO2 substrate of two asymmetrically substituted sexithiophenes (6T). Molecule 1 consists of 6T with a branched alkyl chain at one end only and shows a crystalline structure. In molecule 2, the bifunctional 6T has in addition at the other end a linear alkyl chain. It displays thermotropic liquid crystalline (LC) behavior. Both compounds form readily single molecular layers from solution. Remarkably, full monolayer coverage can be achieved before multilayer growth starts. LC properties promote preordering near the interface as well as exchange of molecules between the growing domains, thus regulating the domain sizes. As a result, the LC compound 2 forms single-molecule islands with larger domain sizes compared to compound 1. Surface X-ray investigations indicate that the 6T cores are tilted relative to the layer normal. The tilt angle is as large as 54° for compound 2 compared to 28° for compound 1. For molecule 2, interfacial constraints and packing requirements because of the asymmetric substitution cause a rather loosely organized core structure.

Formation of self-assembled organosilicon-functionalized quinquethiophene monolayers by fast processing techniques

Different techniques for a relatively fast self-assembled monolayer film formation such as Langmuir-Blodgett (LB), spin-coating, and dip-coating methods have been compared using chloro[11-(5''''-ethyl-2,2':5',2″:5''',2''':5''',2''''-quinquethiophene-5-yl)undecyl]dimethylsilane as a reactive precursor. It was shown that both spin-coating and LB techniques are very promising methods for preparation of highly ordered monolayer films of organosilicon-functionalized quinquethiophene with vertical orientation of oligothiophene fragments, while dip-coating gives only partial coverage. Optimal conditions for complete filling out the substrate surface by the quinquethiophene-containing monolayer by spin-coating and LB methods have been found. Grazing incidence X-ray diffraction measurements confirmed formation of in-plane crystalline order within the monolayer film. Changes in the layer structure were established by X-ray reflectivity and grazing incidence X-ray diffraction methods.

Interplay between H-bonding and alkyl-chain ordering in self-assembly of monodendritic L-alanine derivatives

This paper reports on the synthesis and self-organizing properties of monodendrons consisting of L-alanine at the focal point and alkyl chains with different length at the periphery. The structures of thin films and monolayers are studied by temperature-resolved grazing-incidence X-ray diffraction and scanning force microscopy. The interplay between H-bonding and ordering of the alkyl chains results in a rich temperature-dependent phase behavior. The monodendrons form H-bonded stabilized clusters with the number of molecules depending on the length of the aliphatic chains and temperature. The clusters play the role of constitutive units in the subsequent self-assembly. Short alkyl chains allow the material to form thermodynamically stable crystalline phases. The molecules with longer side groups exhibit additional transitions from the crystalline phase to thermotropic columnar hexagonal or columnar rectangular liquid-crystalline phases. In monolayers deposited on highly ordered pyrolytic graphite, the materials show ordering similar to thin films. However, for the compound bearing hexadecyl chains the affinity of the alkyl groups to graphite dominates the self-assembly and thereby allows epitaxial growth of a 2D lattice with flat-on oriented molecules.

Solution-processable septithiophene monolayer transistor

Septithiophene with endgroups designed to form liquid crystalline phases and allows controlled deposition of an electrically connected monolayer. Field effect mobilies mobilities of charge carriers and spectroscopic properties of the monolayer provide evidence of sustainable transport and delocalization of the excitation through intermolecular interactions within the layer.