When physics meets chemistry at the dynamic glass transition.

Can the laws of physics be unified? One of the most puzzling challenges is to reconcile physics and chemistry, where molecular physics meets condensed-matter physics, resulting from the dynamic fluctuation and scaling effect of glassy matter at the glass transition temperature. The pioneer of condensed-matter physics, Nobel Prize-winning physicist Philip Warren Anderson referred to this gap as the deepest and most interesting unsolved problem in condensed-matter physics in 1995. In 2005, Science, in its 125th anniversary publication, highlighted that the question of "what is the nature of glassy state?" was one of the greatest scientific conundrums for the next quarter century. However, the nature of the glassy state and its connection to the glass transition have not been fully understood owing to the interdisciplinary complexity of physics and chemistry, governed by physical laws at the condensed-matter and molecular scales, respectively. Therefore, the study of glass transition is essential to explore the working principles of the scaling effects and dynamic fluctuations in glassy matter and to further reconcile the interdisciplinary complexity of physics and chemistry. Initially, this paper proposes a thermodynamic order-to-disorder free-energy equation for microphase separation to formulate the dynamic equilibria and fluctuations, which originate from the interplay of the phase and microphase separations during glass transition. Secondly, the Adam-Gibbs (AG) domain model is employed to explore the cooperative dynamics and molecular entanglement in glassy matter. It relies on the concept of transition probability in pairing, where each domain contains e+1 segments, in which approximately 3.718 segments cooperatively relax in a domain at the glass transition temperature. This model enables the theoretical modelling and validation of a previously unverified statement, suggesting that 50 to 100 individual monomers would relax synchronously at glass transition temperature.

An entropy trap model of thermodynamic anomalies for dual-amorphous water undergoing liquid-liquid phase transition.

Water displays numerous anomalously thermodynamic behaviors. However, the working principles behind these anomalies are not well understood, and the liquid-liquid phase transition (LLPT) is often regarded as the potential reason. In this study, we developed an entropy trap model to characterize the thermodynamic LLPT in dual-amorphous water, i.e. having both low-density and high-density liquid water. From the Adam-Gibbs model and free-volume theory, thermodynamic behaviors of water have been described using the proposed model, in which the constitutive relationships among density, heat capacity, thermal expansivity and glass transition temperature have been formulated. Moreover, the glass transition and its connection to thermodynamic behaviors were also investigated for dual-amorphous water. Finally, experimental data reported in the literature were used to verify effectiveness of the proposed model. This study is expected to provide a physical insight into the anomalous thermodynamics of dual-amorphous water undergoing the LLPT.

A double-well potential model for glass transition in a glassy hydrogel undergoing bi-stable interactions with water.

The key mechanisms for achieving ultra-high mechanical properties of glassy hydrogels have not been fully understood, and it is commonly believed that their glass transitions are the crucial reasons due to the existence of significant bi-stable interactions between polymer macromolecules and water molecules. In this study, a double-well potential model is formulated to describe the mechanical properties of glassy hydrogels undergoing glass transition, by combining phase evolution theory and a rubber elasticity model. Bi-stable interactions between polymer macromolecules and water molecules (for both the trapped and free water) have been characterized using this double-well potential model, and various parameters are studied, including depth of well (for elasticity), distance between two wells (for yielding), and energy difference between two wells (for transition probability). Furthermore, constitutive stress-strain relationships are developed to explore the working principles for achieving ultra-high mechanical properties of these glassy hydrogels. Finally, the effectiveness of the proposed models is verified using finite element analysis (FEA) and also the experimental results reported in the literature, thus providing physical and mechanical insights into the ultra-high mechanical properties of glassy hydrogels.

A phase transition model in dual-amorphous water undergoing liquid-liquid transition.

An in-depth understanding of liquid-liquid phase transition (LLPT) in condensed water will gain insight into anomalous behaviors of dual-amorphous condensed water. Despite numerous experimental, molecular simulation, and theoretical studies, it is yet to achieve a widely accepted consensus with convinced evidence in the condensed matter physics for two-state liquid-liquid transition of water. In this work, a theoretical model is proposed based on the Avrami equation, commonly used to describe first-order phase transitions, to elucidate complex homogeneous and inhomogeneous condensation from high-density liquid (HDL) water to low-density liquid (LDL) water for both pure and ionic dual-amorphous condensed water. This model unifies the coupling effects of temperature and electrolyte concentration based on the new theoretical framework. The Adam-Gibbs theory is then introduced to characterize the synergistic motion and relaxation behavior of condensed water. Variations in the configurational entropy under electrostatic forces are further explored, and an analytical 2D cloud chart is developed to visualize the synergistic effect of temperature and electrolyte concentration on the configurational entropy of ionic water. The constitutive relationships among viscosity, temperature, and electrolyte concentration are derived to analyze their synergistic effects under different condensation fractions of LDL and HDL. The Stokes-Einstein relation and free volume theory are further used to analyze diffusion coefficients and densities (or apparent density) during both pure and ionic LLPT. Finally, theoretical results obtained from these models are compared with experimental results reported in literature to validate the accuracy and applicability of the proposed models, which offer significant benefits and advancements in effectively predicting physical property changes of dual-amorphous condensed water.

Machine-Learning Assisted Handwriting Recognition Using Graphene Oxide-Based Hydrogel.

Machine-learning assisted handwriting recognition is crucial for development of next-generation biometric technologies. However, most of the currently reported handwriting recognition systems are lacking in flexible sensing and machine learning capabilities, both of which are essential for implementation of intelligent systems. Herein, assisted by machine learning, we develop a new handwriting recognition system, which can be applied as both a recognizer for written texts and an encryptor for confidential information. This flexible and intelligent handwriting recognition system combines a printed circuit board with graphene oxide-based hydrogel sensors. It offers fast response and good sensitivity and allows high-precision recognition of handwritten content from a single letter to words and signatures. By analyzing 690 acquired handwritten signatures obtained from seven participants, we successfully demonstrate a fast recognition time (less than 1 s) and a high recognition rate (∼91.30%). Our developed handwriting recognition system has great potential in advanced human-machine interactions, wearable communication devices, soft robotics manipulators, and augmented virtual reality.

An extended Stokes-Einstein model for condensed ionic water structures with topological complexity.

'What is the structure of water?' This has been a perplexing question for a long time and water structure with various phases is a great topic of research interest. Topological complexity generally occurs because hydrophilic ions strongly influence the size and shape of condensed water structures owing to their kosmotropic and chaotropic transitions. In this study, an extended Stokes-Einstein model incorporating Flory-Huggins free energy equation is proposed to describe the constitutive relationship between dynamic diffusion and condensed water structure with a topological complexity. The newly developed model provides a geometrical strategy of end-to-end distance and explores the constitutive relationship between condensed ionic water structures and their dynamic diffusion behaviors. A free-energy function is then formulated to study thermodynamics in electrolyte aqueous solution, in which the condensed ionic water structures undergo topologically complex changes. Finally, effectiveness of the proposed model is verified using both molecular dynamics simulations and experimental results reported in literature.

Phase transition of supercooled water confined in cooperative two-state domain.

The question of 'what is the structure of water?' has been regarded as one of the major scientific conundrums in condensed-matter physics due to the complex phase behavior and condensed structure of supercooled water. Great effort has been made so far using both theoretical analysis based on various mathematical models and computer simulations such as molecular dynamics and first-principle. However, these theoretical and simulation studies often do not have strong evidences of condensed-matter physics to support. In this study, a cooperative domain model is formulated to describe the dynamic phase transition of supercooled water between supercooled water and amorphous ice, both of which are composed of low- and high-density liquid water. Free volume theory is initially employed to identify the working principle of dynamic phase transition and its connection to glass transition in the supercooled water. Then a cooperative two-state model is developed to characterize the dynamic anomalies of supercooled water, including density, viscosity and self-diffusion coefficient. Finally, the proposed model is verified using the experimental results reported in literature.

Untangling the mechanics of entanglements in slide-ring gels towards both super-deformability and toughness.

Entanglement plays a critical role in determining the dynamic properties of polymer systems, e.g., resulting in slip links and pulley effects for achieving large deformation and high strength. Although it has been studied for decades, the mechanics of entanglements for stiffness-toughness conflict is not well understood. In this study, topological knot theory incorporating an extended tube model is proposed to understand the entanglements in a slide-ring (SR) gel, which slips over a long distance to achieve large deformation and high toughness via the pulley effect. Based on topological knot theory, the sliding behavior and pulley effect of entanglements among molecular chains and cross-linked rings are thoroughly investigated. Based on rubber elasticity theory, a free-energy function is formulated to describe mechanical toughening and slipping of topological knots, while the SR gel retains the same binding energy. Finally, the effectiveness of the proposed model is verified using both finite element analysis and experimental results reported in the literature.

Spatially and Reversibly Actuating Soft Gel Structure by Harnessing Multimode Elastic Instabilities.

Autonomous shape transformation is key in developing high-performance soft robotics technology; the search for pronounced actuation mechanisms is an ongoing mission. Here, we present the programmable shape morphing of a three-dimensional (3D) curved gel structure by harnessing multimode mechanical instabilities during free swelling. First of all, the coupling of buckling and creasing occurs at the dedicated region of the gel structure, which is attributed to the edge and surface instabilities resulted from structure-defined spatial nonuniformity of swelling. The subsequent developments of post-buckling morphologies and crease patterns collaboratively drive the structural transformation of the gel part from the "open" state to the "closed" state, thus realizing the function of gripping. By utilizing the multi-stimuli-responsive nature of the hydrogel, we recover the swollen gel structure to its initial state, enabling reproducible and cyclic shape evolution. The described soft gel structure capable of shape transformation brings a variety of advantages, such as easy to fabricate, large strain transformation, efficient actuation, and high strength-to-weight ratio, and is anticipated to provide guidance for future applications in soft robotics, flexible electronics, offshore engineering, and healthcare products.

Anchoring-mediated topology signature of self-assembled elastomers undergoing mechanochromic coupling/decoupling.

Soft elastomers with their ability to integrate strain-adaptive stiffening and coloration have recently received significant research attention for application in artificial muscle and active camouflage. However, there is a lack of theoretical understanding of their complex molecular dynamics and mechanochromic coupling/decoupling. In this study, a topological dynamics model is proposed to understand the anchoring-mediated topology signature of self-assembled elastomers. Based on the constrained molecular junction model, a free-energy function is firstly formulated to describe the working principles of strain-adaptive stiffening and coloration in the self-assembled elastomer. A coupled ternary "rock-paper-scissors" model is proposed to describe the topological dynamics of self-assembly, mechanochromic coupling and mechanoresponsive stiffening of the self-assembled elastomers, in which there are three fractal geometry components in the topology network. Finally, the proposed models are verified using the experimental results reported in the literature. This study provides a fundamental approach to understand the working mechanism and topological dynamics in the self-assembled elastomers, with molecularly encoded stiffening and coloration.

The nonequilibrium behaviors of covalent adaptable network polymers during the topology transition.

Vitrimers with bond exchange reactions (BERs) are a class of covalent adaptable network (CAN) polymers at the forefront of recent polymer research. They exhibit malleable and self-healable behaviors and combine the advantages of easy processability of thermoplastics and excellent mechanical properties of thermosets. For thermally sensitive vitrimers, a molecular topology melting/frozen transition is triggered when the BERs are activated to rearrange the network architecture. Notable volume expansion and stress relaxation are accompanied, which can be used to identify the BER activation temperature and rate as well as to determine the malleability and interfacial welding kinetics of vitrimers. Existing works on vitrimers reveal the rate-dependent behaviors of the nonequilibrium network during the topology transition. However, it remains unclear what the quantitative relationship with heating rate is, and how it will affect the macroscopic stress relaxation. In this paper, we study the responses of an epoxy-based vitrimer subjected to a change in temperature and mechanical loading during the topology transition. Using thermal expansion tests, the thermal strain evolution is shown to depend on the temperature-changing rate, which reveals the nonequilibrium states with rate-dependent structural relaxation. The influences of structural relaxation on the stress relaxation behaviors are examined in both uniaxial tension and compression modes. Assisted by a theoretical model, the study reveals how to tune the material and thermal-temporal conditions to promote the contribution of BERs during the reprocessing of vitrimers.

A stimuli-responsive gel impregnated surface with switchable lipophilic/oleophobic properties.

In this paper, we developed a novel morphing surface technique consisting of a 3D printed miniature groove structure and injected stimuli-responsive hydrogel pattern, which is capable of switching between lipophilicity and oleophobicity under certain stimuli. Under swelling, the geometrical change of the hydrogel will buckle the surface due to the structural confinement and create a continuous transition of surface topology. Thus, it will yield a change in the surface wetting property from oleophilic to super-oleophobic with a contact angle of oil of 85° to 165°. We quantitatively investigate this structure-property relationship using finite element analysis and analytical modeling, and the simulation results and the modeling are in good agreement with the experimental ones. This morphing surface also holds potential to be developed into an autonomous system for future sub-sea/off-shore engineering applications to separate oil and water.

3D Printing of Auxetic Metamaterials with Digitally Reprogrammable Shape.

Two-dimensional lattice structures with specific geometric features have been reported to have a negative Poisson's ratio, termed as auxetic metamaterials, that is, stretching-induced expansion in the transversal direction. In this paper, we designed a novel auxetic metamaterial; by utilizing the shape memory effect of the constituent materials, the in-plane moduli and Poisson's ratios can be continuously tailored. During deformation, the curved meshes ensure the rotation of the mesh joints to achieve auxetics. The rotations of these mesh joints are governed by the mesh curvature, which continuously changes during deformation. Because of the shape memory effect, the mesh curvature after printing can be programmed, which can be used to tune the rotation of the mesh joints and the mechanical properties of auxetic metamaterial structures, including Poisson's ratios, moduli, and fracture strains. Using the finite element method, the deformation of these auxetic meshes was analyzed. Finally, we designed and fabricated gradient/digital patterns and cylindrical shells and used the auxetics and shape memory effects to reshape the printed structures.

Micro-mechanics of nanostructured carbon/shape memory polymer hybrid thin film.

This paper investigates the mechanics of hybrid shape memory polymer polystrene (PS) based nanocomposites with skeletal structures of CNFs/MWCNTs formed inside. Experimental results showed an increase of glass transition temperature (Tg) with CNF/MWCNT concentrations instead of a decrease of Tg in nanocomposites filled by spherical particles, and an increase in mechanical properties on both macro- and µm-scales. Compared with CNFs, MWCNTs showed a better mechanical enhancement for PS nanocomposites due to their uniform distribution in the nanocomposites. In nanoindentation tests using the Berkovich tips, indentation size effects and pile-up effects appeared obviously for the nanocomposites, but not for pure PS. Experimental results revealed the enhancement mechanisms of CNFs/MWCNTs related to the secondary structures formed by nanofillers, including two aspects, i.e., filler-polymer interfacial connections and geometrical factors of nanofillers. The filler-polymer interfacial connections were strongly dependent on temperature, thus leading to the opposite changing trend of loss tangent with nanofiller concentrations, respectively, at low and high temperature. The geometrical factors of nanofillers were related to testing scales, further leading to the appearance of pile-up effects for nanocomposites in the nanoindentation tests, in which the size of indents was close to the size of the nanofiller skeleton.

Nanoscale Design of Nano-Sized Particles in Shape-Memory Polymer Nanocomposites Driven by Electricity.

In the last few years, we have witnessed significant progress in developing high performance shape memory polymer (SMP) nanocomposites, in particular, for shape recovery activated by indirect heating in the presence of electricity, magnetism, light, radio frequency, microwave and radiation, etc. In this paper, we critically review recent findings in Joule heating of SMP nanocomposites incorporated with nanosized conductive electromagnetic particles by means of nanoscale control via applying an electro- and/or magnetic field. A few different nanoscale design principles to form one-/two-/three- dimensional conductive networks are discussed.