Soft Building Blocks Research Articles

Self-assembled soft alloy with Frank-Kasper phases beyond metals.

Soft building blocks, such as micelles, cells or soap bubbles, tend to adopt near-spherical geometry when densely packed together. As a result, their packing structures do not extend beyond those discovered in metallic glasses, quasicrystals and crystals. Here we report the emergence of two Frank-Kasper phases from the self-assembly of five-fold symmetric molecular pentagons. The μ phase, an important intermediate in superalloys, is indexed in soft matter, whereas the ϕ phase exhibits a structure distinct from known Frank-Kasper phases in metallic systems. We find a broad size and shape distribution of self-assembled mesoatoms formed by molecular pentagons while approaching equilibrium that contribute to the unique packing structures. This work provides insight into the manipulation of soft building blocks that deviate from the typical spherical geometry and opens avenues for the fabrication of 'soft alloy' structures that were previously unattainable in metal alloys.

Size Control of On-Surface Self-Assembled Nanochains Using Soft Building Blocks.

Owing to their conformational flexibility, soft molecules with side chains play a crucial role in molecular self-assembly or self-organization processes toward bottom-up building of supramolecular nanostructures. However, the influence of the rotating side chains in the confined space and subsequent surface-confined supramolecular self-assembly remains rarely explored. Herein, using the spatial confinement effect between soft building blocks, we realized size control on surface-confined supramolecular coordination self-assembly through the synergy between the repulsive steric hindrance and the attractive chemical interactions. Combining scanning tunneling microscopy with density functional theory calculations and Monte Carlo simulations, we elucidated the effective repulsive force generated by the thermal wiggling motions of the soft building blocks, allowing length tuning of the self-assembled chain structures. Through a delicate balance between the repulsive interaction induced by the spatial confinement effect and the coordinate chemical interaction, we provide a new strategy for controlling the geometry of the on-surface supramolecular nanostructures.

Reconfigurable Soft Robots by Building Blocks.

Soft robots are of increasing interest as they can cope with challenges that are poorly addressed by conventional rigid-body robots (e.g., limited flexibility). However, due to their flexible nature, the soft robots can be particularly prone to exploit modular designs for enhancing their reconfigurability, that is, a concept which, to date, has not been explored. Therefore, this paper presents a design of soft building blocks that can be disassembled and reconfigured to build different modular configurations of soft robots such as robotic fingers and continuum robots. First, a numerical model is developed for the constitutive building block allowing to understand their behavior versus design parameters, then a shape optimization algorithm is developed to permit the construction of different types of soft robots based on these soft building blocks. To validate the approach, 2D and 3D case studies of bio-inspired designs are demonstrated: first, soft fingers are introduced as a case study for grasping complex and delicate objects. Second, an elephant trunk is used for grasping a flower. Third, a walking legged robot. These case studies prove that the proposed modular building approach makes it easier to build and reconfigure different types of soft robots with multiple complexshapes.

Stretch‐Safe: Magnetic Connectors for Modular Stretchable Electronics

Wearable healthcare devices monitor the condition of patients outside the hospital and hence increase treatment capacity and resources. At the same time, patients benefit from wireless data transmission and conformable device designs, speeding up the rehabilitation process and integration into normal daily activities. Yet, the successful implementation of personalized treatment creates an extensive engineering challenge as devices need to adapt to a vast number of different patients and treatment protocols. Herein, soft building blocks of mobile health (mHealth) devices that reversibly assemble through a magnetic click‐on mechanism are introduced. Fabrication of reliable magnetic connectors with an inherent safeguard mechanism allows the realization of personalized wearable mHealth devices, independent of the (desired) measurement technique. Stretchable elastomer‐based units combined with imperceptible electrodes are protected from overstretching by controlling the opening force of the magnetic connections. The stretchable devices retain both electrical and mechanical functionality for more than 10 000 opening cycles, on par with the standard for universal serial bus type C (USB C) connectors used in consumer electronics. A fully functional and autonomous pulse sensor wristband, assembled from the reliably connecting circuits, demonstrates the feasible implementation for mHealth devices. The plug‐and‐play modularity ensures the applicability independent of a patient's needs, without sacrificing functionality and durability.

Isodimorphic aliphatic copolyester as midblock of poly(l-lactide)-based triblock copolymers towards largely enhanced impact toughness

Biobased dihydroxy-terminated aliphatic copolyesters were synthesized from melt-polycondensation of 1,6-adipic acid with 1,6-hexanediol and 1,10-decanediol. The isodimorphic behavior of the copolyesters was explored to identify the pseudo-eutectic copolyester, which was selected as macro-initiator for ring opening polymerization of l-lactide to prepare poly(l-lactide)-b-poly(hexamethylene-co-decamethylene adipate)-b-poly(l-lactide) (PLLA-b-PHDA-b-PLLA) triblock copolymers varying PLLA block length. The blocky structure was confirmed by GPC and NMR, and the influence of PLLA block length on physical and mechanical properties was investigated. Impact testing showed that the impressive impact strength was achieved, that is, the values of PLLA10-PHDA-PLLA10 and PLLA20-PHDA-PLLA20 were largely enhanced to 357.2 J/m and 259.6 J/m, respectively, about 9–12 times greater than that of PLLA homopolymer (28.6 J/m). SEM micrographs of impact fractured surface showed that the triblock copolymers underwent a transition from brittle fracture of neat PLLA to ductile fracture, which is attributed to the introduction of flexible aliphatic polyester as middle soft building block of triblcok copolymers. This work demonstrated isodimorphic aliphatic copolyesters as midblock of PLLA-based triblock fully biobased biodegradable copolymers towards highly toughened PLLA material.

The influence of silica nanoparticles on thermal degradation and mechanical properties of nanocomposites based on aliphatic polyurethanes

Nanocomposites based on aliphatic polyurethanes have recently attracted a lot of attention regarding economical and ecological aspects, due to their improved thermal and mechanical properties. The aim of this paper was to investigate the influence of silica nanoparticles, differing in size and specific surface, on thermal stability and degradation, lifetime and mechanical characteristics of the obtained nanocomposites. Two series of nanocomposites based on aliphatic polyurethanes were obtained by using a single-step procedure and by addition of silica nanoparticles of types A380 or N999 at different loadings (0,0 0.15, 0.5, 1,0 and 3.5 wt.%). It was found that the increase in heating rate caused shifting of the onset temperature to higher values (from 283 to 312 ?C). According to the shape of DTG curves, it was observed that the degradation mechanism of prepared nanocomposites consists of two overlapping processes, related to the scission of hard and soft building blocks. Based on DTG results, the addition of larger N999 silica nanoparticles induced lower thermal degradation, shifting the maximum rate temperatures of the first and second degradation stages to lower values, and caused the change in the degradation mechanism. Addition of smaller silica nanoparticles (A380) did not significantly affect the mechanism of the degradation reaction, indicating homogeneity of the obtained nanocomposites. The presence of A380 nanoparticles improved thermal stability of nanocomposites, by increasing the onset temperature from 286 ?C for the pristine elastomer to 303 ?C for the sample containing 3.5 wt.% of silica. Existence of interactions of A380 silica nanoparticles with hard and soft phases was observed, based on the increase in the maximum rates of the first and second degradation steps. The activation energy of thermal degradation of polyurethanes modified with A380 silica nanoparticles was obtained by using the Flyn-Wall and Toop models. Dependence of the activation energy and the lifetime of nanocomposites based on aliphatic polyurethanes on the silica content were estimated. The highest Ea values (determined for 1 and 5 % weight loss) were found for nanocomposites containing 0.5 and 0.15 wt. % of A380 silica nanoparticles (121 and 161.2 kJ/mol). A negative effect of the silica addition on mechanical properties of nanocomposites was observed. The polyurethanes containing smaller SiO2 particles (A380) had a higher tensile strength, elongation at break and hardness as compared to the elastomers filled with larger silica nanoparticles (N999).

Percolating Network of Ultrathin Gold Nanowires and Silver Nanowires toward “Invisible” Wearable Sensors for Detecting Emotional Expression and Apexcardiogram

2 nm thin gold nanowires (AuNWs) have extremely high aspect ratio (≈10 000) and are nanoscale soft building blocks; this is different from conventional silver nanowires (AgNWs), which are more rigid. Here, highly sensitive, stretchable, patchable, and transparent strain sensors are fabricated based on the hybrid films of soft/hard networks. They are mechanically stretchable, optically transparent, and electrically conductive and are fabricated using a simple and cost‐effective solution process. The combination of soft and more rigid nanowires enables their use as high‐performance strain sensors with the maximum gauge factor (GF) of ≈236 at low strain (<5%), the highest stretchability of up to 70% strain, and the optical transparency is from 58.7% to 66.7% depending on the amount of the AuNW component. The sensors can detect strain as low as 0.05% and are energy efficient to operate at a voltage as low as 0.1 V. These attributes are difficult to achieve with a single component of either AuNWs or AgNWs. The outstanding sensing performance indicates their potential applications as “invisible” wearable sensors for biometric information collection, as demonstrated in applications for detecting facial expressions, respiration, and apexcardiogram.

Direct Observation of Highly Ordered Dendrimer Soft Building Blocks over a Large Area.

Developing large-area, single domain of organic soft-building blocks such as block copolymers, colloids, and supramolecular materials is one of the most important issues in the materials science and nanotechnology. Owing to their small sizes, complex molecular architectures, and high mobility, supramolecular materials are not well-suited for building large area, single domain structures. In the described study, a single domain of supramolecular columnar dendrimers was created over large area. The columnar structures in these domains have smaller (4.5 nm) diameters, higher area densities (ca. 36 Tera-dots/in(2)) and larger domains (>0.1 × 0.1 mm(2)) than those of all existing BCP and colloidal assemblies. By simply annealing dendrimer thin films between two flat solid surfaces, single domains of hexagonal columnar structures are created over large macroscopic areas. Observations made in this effort should serve as the foundation for the design of new routes for bottom-up lithography based on supramolecular building blocks.

Toward design rules of directional janus colloidal assembly.

Burgeoning interest in supracolloidal assembly has reached the point at which the field can seek so-called intelligent design rather than solely rely on evolution. Emphasizing Janus and triblock particles, this review presents a progress report on formulating design rules for the assembly of interesting structures. We discuss how to design building blocks, bearing in mind that patchy particles embody not just geometric shape but also chemical shape, that chemical shape determines particle-particle interactions, and that the assembly process can be designed to proceed in hierarchical stages. Remarks are included about the potential of kinetic and nonequilibrium control, as well as the potential for the augmented use of soft building blocks. Whereas the reverse design problem, in which arbitrarily selected structures can be designed from the bottom up, still stands as a grand challenge, the field has reached the point of understanding necessary, although not always sufficient, conditions.

Fabrication of transparent, flexible conducing graphene thin films via soft transfer printing method

As a unique type of soft building block, graphene oxide (GO) dispersions are easy to process to produce flexible transparent conductive films (TCFs). Developing methods to produce large size GO with layered structure is the key for these applications. Here, we demonstrate a method to prepare ultra-large graphene oxide (UL-GO) sheets (up to ~100μm in lateral size) in aqueous media, which are then transferred onto polyethylene terephthalate substrates through soft transfer printing method. The as produced UL-GO films have layered architectures and exhibit high electrical conductivity and transparency after the chemical reduction by using hydroiodic (HI) acid. The produced flexible TCFs deliver sheet resistance ranging 0.3–17kΩ/sq with optical transmittance ranging 72–95% depending on the thickness. The technique proposed to produce flexible TCFs is inexpensive and tunable for mass production.

Structure control of ultra-large graphene oxide sheets by the Langmuir–Blodgett method

As a unique type of soft building block, graphene oxide (GO) dispersions are easy to process to produce electronic devices. Developing methods to produce large sized GO with controllable structure is the key for these applications. Here, ultra-large graphene oxide (UL-GO) sheets (up to ∼100 μm in lateral size) were produced in aqueous media and deposited onto substrates with controllable structure, including flat, rippled, standing collapsed, folded, over folded, highly curved, and scrolled morphologies, via the Langmuir–Blodgett (L–B) technique. The MD simulations show that the GO becomes much softer than pristine graphene sheets due to the sp3-hybridization caused by oxygen containing functional groups and this is the reason why a series of distinct structures were observed. To demonstrate the fabrication of a transparent conductor, the close-packed flat UL-GO films on poly(ethylene terephthalate) (PET) were reduced with a low-temperature chemical reduction process in a hydriodic (HI) acid aqueous solution, achieving a sheet resistance as low as 8.1 kΩ/sq with 90% transparency.

Nanotechnology with Soft Matter: From Structures to Functions

Soft matter science is a multidisciplinary area of chemistry, physics, biology, and materials science. It essentially deals with matter such as functional polymers, amphiphiles, liquid crystals, colloids, and a number of biological molecules, in which the typical energies of interaction are comparable to thermal energies.[1] Nanotechnology involves understanding and controlling matter typically on the sub-100-nanometer scale, where unique phenomena enable novel and functional applications.[2] The endeavor in nanotechnology is to identify the chemical and physical consequences of the miniaturization and to develop nanoscale functional materials and devices, which differ in fundamental properties from the individual molecules or bulk matter.[2,3] When nanotechnology meets soft matter, the fi rst question that arises is how the soft building blocks can be tailored, manipulated, and assembled into nanostructures. This is one of the fundamental focuses in the materials research frontier. This focus has led to the generation of a broad range of soft nanoscale structures such as supramolecular architectures, gels, vesicles, polymer brushes, molecular aggregates, nanofi bers, and nanotubes. The goal of such soft nanomaterials is

Self-assembled periodic liquid crystal defects array for soft lithographic template

We have developed highly periodic patterns with sub-micrometer features over large-areas using toric focal conic domains (TFCDs) originated from smectic liquid crystal (LC) as a new self-assembling building block. TFCDs are accomplished by precisely controlling the surface and interfacial properties of smectic LC. In order to apply the smectic liquid crystal defect arrays in soft lithography, the hexagonal arrays of domain patterns are used as molds for ultraviolet (UV) curable polymers, thereby providing LC defect stamps with high spatial resolution over large areas. Our method was further utilized to transfer patterns with sub-micrometer features from the polymer stamp surface to a secondary surface by microcontact printing (μ-CP). Accordingly, we show that such LC materials can be very strong candidates for the periodic templates, compared to other soft-building blocks such as block copolymers, colloids and surfactants. We anticipate our finding be a starting point for more sophisticated lithographic applications based on liquid crystalline materials.

Tunable assembly of graphene oxide surfactant sheets: wrinkles, overlaps and impacts on thin film properties

Graphene oxide (GO) sheets are a unique type of soft building block for creating functional graphene based materials and devices. Rapid progress has been made in the chemistry and applications of GO. However, there is a pressing need for rational assembly strategies of these two-dimensional (2D) sheets, which is crucial for determining the microstructures and thus final properties of bulk GO or graphene materials. For example, wrinkles and overlaps are the two fundamental morphologies between flexible, interacting sheets that are usually convoluted in solution-processed thin films. Based on the recent discovery of the surfactant-like property of GO sheets and their pH dependent amphiphilicity, now we are able to control the tiling of such 2D sheets to obtain thin films with either a wrinkled or overlapped type of microstructure, thus allowing us to deconvolute how these two basic microstructures affect the electrical and optical properties of the final thin films. The work here provides a well-defined example of the materials science paradigm, the microstructure-properties relationship, for this new soft material.

Internal structure visualization and lithographic use of periodic toroidal holes in liquid crystals

The formation of a large-area ordered structure by organic molecular soft building blocks is one of the most exciting interdisciplinary research areas in current materials science and nanotechnology. So far, several distinct organic building blocks--including colloids, block copolymers and surfactants--have been examined as potential materials for the creation of lithographic templates. Here, we report that perfect ordered arrays of toric focal conic domains (TFCDs) covering large areas can be formed by semi-fluorinated smectic liquid crystals. Combined with controlled geometry, that is, a microchannel, our smectic liquid-crystal system exhibits a high density of TFCDs that are arranged with remarkably high regularity. Direct visualization of the internal structure of the TFCDs clearly verified that the smectic layers were aligned normal to the side walls and parallel to the top surface, and merge with the circular profile on the bottom wall surface. Moreover, we demonstrate a new concept: smectic liquid-crystal lithography. Grown in microchannels from a mixture of liquid-crystal molecules and fluorescent particles, TFCDs of the smectic liquid crystals acted as a template, trapping particles in an ordered array. Our findings pose new theoretical challenges and potentially enable lithographic applications based on smectic liquid-crystalline materials.