Polymeric multimaterials by photochemical patterning of crystallinity

Quantitative Mechanochemistry: A Chemical Tool to Bridge Polymer Physics and Mechanics of Soft Polymer Networks

In recent years, mechanochemistry has imposed itself as a novel promising chemical tool to bridge the gap between polymer physics and continuum mechanics in soft materials. The suitable incorporation of force-sensitive molecules (mechanophores) in load-bearing positions in soft (entropic) polymer networks and in linear chains has provided a tool to detect stresses and bond scission in 2D and 3D through the intensity of an optical signal. We review recent results linking the optical signal detected upon mechanophore activation with the applied mechanical load. Recent investigations have addressed critical questions, such as detecting and quantifying stress fields and measuring quantitative damage by bond scission in diverse cases, including failure in uniaxial tension, crack propagation in continuous loading, cyclic fatigue, or crack initiation in uniaxial and triaxial tension. We also discuss the requirements to go from simple imaging to quantitative detection, enabling comparisons between different materials and the calibration of continuum mechanics models. In ideal cases, the optical signal provides highly sensitive information on the size and intensity of damage zones in front of cracks-regions that would otherwise be undetectable.

Shape Memory Networks With Tunable Self-Stiffening Kinetics Enabled by Polymer Melting-Recrystallization

Shape memory polymers (SMPs) are deformable materials capable of recovering from a programmed temporary shape to a permanent shape under specific stimuli. However, shape recovery of SMPs is often accompanied by the evolution of materials from a stiff to soft state, leading to a significant decrease in strength/modulus and thereby impacting their practical applications. Although some attempts are made to pursue the SMPs with self-stiffening capability after shape recovery, the modulus increase ratio is much limited. Inspired by the recrystallization process of CaCO3 during crab molting, a novel and universal strategy is developed to construct water-free self-stiffening SMPs by using a single thermal stimulus through harnessing the polymer melting-recrystallization. The shape recovery is achieved through the melting of polymer primary crystals, followed by the self-stiffening via polymer recrystallization at the same recovery temperature, in which the modulus increase rate and ratio can be programmed in a wide range. Additionally, conceptual applications of these self-stiffening SMPs as artificial stents with self-enhancing supporting function are successfully demonstrated. This work is believed to provide new insights for developing the advanced shape memory devices.

Photo-Tunable Elastomers Enabling Reversible, Broad-Range Modulation of Mechanical Properties Via Dynamic Covalent Crosslinkers

Modulating the mechanical properties of soft materials with light is essential for achieving customizable functionalities. However, existing photo-responsive materials suffer from limited mechanical performance and a restricted tunable range. Here, a photo-tunable elastomer is developed by incorporating a urethane acrylate network with selenosulfide-based dynamic covalent crosslinkers, achieving high tensile strength exceeding 1.2 MPa in their stiff state and variable Young's modulus within a 0.8 MPa range. These crosslinkers undergo selenosulfide photo-metathesis, gradually breaking under ultraviolet light and reforming under visible light, enabling fine control over the modulus, strength, and stretchability of the elastomer. In terms of controllability, the design supports multiple tunable states, which allow for the use of intermediate mechanical properties. Moreover, by modeling the crosslinking density changes with reaction kinetics, modulus variation is predicted as a function of light exposure time. The light-induced modulation facilitates localized mechanical property adjustments, generating transformable multi-material structures and enhancing fracture resistance. Integrating these crosslinkers into different polymer networks provides a strategy for creating various photo-tunable elastomers and gels.

Synthesis innovations for crystallizing covalent organic framework thin films on biological and non-biological substrates

Thin film technology has emerged as a pivotal field with numerous industrial applications. Depending on their properties-such as magnetic characteristics, conductivity, architectural structure, stability, and functional backbones-thin films are widely utilized in optoelectronics, thin-film coatings, solar cells, energy storage devices, semiconductors, and separation applications. However, for all these applications, thin films must be securely attached to specific substrates, and substrate compatibility with both the thin film and the film-growth process is crucial for optimal performance. In this review, we emphasize the significance of growing thin films, particularly covalent organic framework (COF) thin films, on suitable substrates tailored for various applications. For separation technologies, polymer thin films are commonly fabricated on porous polymeric or metal-based membranes. In contrast, thin films of metals and metal oxides are typically deposited on conducting substrates, serving as current collectors for energy storage devices. Semiconductor thin films, on the other hand, are often grown on silicon or glass substrates for transistor applications. Emerging COF thin films, with their tunable properties, well-defined pore channels, and versatile functional backbones, have demonstrated exceptional potential in separation, energy storage, and electronic and optoelectronic applications. However, the interplay between COF thin films and the substrates, as well as the compatibility of growth conditions, remains underexplored. Studies investigating COF thin film growth on substrates such as metals, metal oxides, glass, silicon, polymers, ITO, and FTO have provided insights into substrate properties that promote superior film growth. The quality of the film formed on these substrates significantly influences performance in applications. Additionally, we discuss the stabilization of biological substrates, like peptide-based biomimetic catalysts and enzymes, which often suffer from instability in non-aqueous environments, limiting their industrial use. Growing COF membranes on these biological substrates can enhance their stability under harsh conditions. We also highlight techniques for growing COF membranes on biological substrates, ensuring the preservation of their structural integrity and functional properties.

Degradable thermosets via orthogonal polymerizations of a single monomer

Crosslinked thermosets are highly durable materials, but overcoming their petrochemical origins and inability to be recycled poses a grand challenge1-3. Many strategies to access crosslinked polymers that are bioderived or degradable-by-design have been proposed, but they require several resource-intensive synthesis and purification steps and are not yet feasible alternatives to conventional consumer materials4-8. Here we present a modular, one-pot synthesis of degradable thermosets from the commercially available, biosourced monomer 2,3-dihydrofuran (DHF)9. In the presence of a ruthenium catalyst and photoacid generator, DHF undergoes slow ring-opening metathesis polymerization to give a soft polymer; then, exposure to light triggers strong acid generation and promotes the cationic polymerization of the same DHF monomer to spatially crosslink and strengthen the material10-12. By manipulating catalyst loading and light exposure, we can access materials with physical properties spanning orders of magnitude and achieve spatially resolved material domains. Importantly, the DHF-based thermosets undergo stimuli-selective degradation and can be recycled to the monomer under mild heating. The use of two distinct polymerization mechanisms on a single functional group allows the synthesis of degradable and recyclable thermoset materials with precisely controlled properties.

Coordinatively stiffen and toughen polymeric gels via the synergy of crystal-domain cross-linking and chelation cross-linking

Polymer gels have been widely used in flexible electronics, soft machines and impact protection materials. Conventional gels usually suffer from the inherent conflict between stiffness and toughness, severely hampering their applications. This work proposes a facile yet versatile strategy to break through this trade-off via the synergistic effect of crystal-domain cross-linking and chelation cross-linking, without the need for specific structure design or adding other reinforcements. Both effects are proven to boost the mechanical performance of the originally weak gel, and result in a stiff and tough conductive gel, achieving significant enhancements in elastic modulus and toughness by up to 366-, and 104-folds, respectively. The resultant gel achieves coordinatively enhanced stiffness (110.26 MPa) and toughness (219.93 MJ m-3), reconciling the challenging trade-off between them. In addition, the presented strategy is found generalizable to a variety of metal ions and polymers, offering a promising way to expand the applicability of gels.

Multimaterial Thermoset Synthesis: Switching Polymerization Mechanism with Light Dosage

The synthesis of polymeric thermoset materials with spatially controlled physical properties using readily available resins is a grand challenge. To address this challenge, we developed a photoinitiated polymerization method that enables the spatial switching of radical and cationic polymerizations by controlling the dosage of monochromatic light. This method, which we call Switching Polymerizations by Light Titration (SPLiT), leverages the use of substoichiometric amounts of a photobuffer in combination with traditional photoacid generators. Upon exposure to a low dose of light, the photobuffer inhibits the cationic polymerization, while radical polymerization is initiated. With an increased light dosage, the buffer system saturates, leading to the formation of a strong acid that initiates a cationic polymerization of the dormant monomer. Applying this strategy, patterning is achieved by spatially varying light dosage via irradiation time or intensity allowing for simple construction of multimaterial thermosets. Importantly, by the addition of an inexpensive photobuffer, such as tetrabutylammonium chloride, commercially available resins can be implemented in grayscale vat photopolymerization 3D printing to prepare sophisticated multimodulus constructs.

Tailoring smart hydrogels through manipulation of heterogeneous subdomains

The mechanical interactions among integrated cellular structures in soft tissues dictate the mechanical behaviors and morphogenetic deformations observed in living organisms. However, replicating these multifaceted attributes in synthetic soft materials remains a challenge. In this work, we develop a smart hydrogel system featuring engineered stiff cellular patterns that induce strain-driven heterogeneous subdomains within the hydrogel film. These subdomains arise from the distinct mechanical responses of the pattern and film domains under applied mechanical forces. Unlike previous studies that incorporate reinforced inclusions into soft matrices to tailor material properties, our method manipulates the localization, integration, and interaction of these subdomain building blocks within the soft film. This enables extensive tuning of both local and global behaviors. Notably, we introduce a subdomain-interface mechanism that allows for the concurrent customization and decoupling of mechanical properties and shape transformations within a single material system-an achievement rarely accomplished with current synthetic soft materials. Additionally, our use of in-situ imaging characterizations, including full-field strain mapping via digital imaging correlation and reciprocal-space patterns through fast Fourier transform analysis of real-space pattern domains, provides rapid real-time monitoring tools to uncover the underlying principles governing tailored multiscale heterogeneities and intricate behaviors.

Controlled patterning of crystalline domains by frontal polymerization

Materials with hierarchical architectures that combine soft and hard material domains with coalesced interfaces possess superior properties compared with their homogeneous counterparts1-4. These architectures in synthetic materials have been achieved through deterministic manufacturing strategies such as 3D printing, which require an a priori design and active intervention throughout the process to achieve architectures spanning multiple length scales5-9. Here we harness frontal polymerization spin mode dynamics to autonomously fabricate patterned crystalline domains in poly(cyclooctadiene) with multiscale organization. This rapid, dissipative processing method leads to the formation of amorphous and semi-crystalline domains emerging from the internal interfaces generated between the solid polymer and the propagating cure front. The size, spacing and arrangement of the domains are controlled by the interplay between the reaction kinetics, thermochemistry and boundary conditions. Small perturbations in the fabrication conditions reproducibly lead to remarkable changes in the patterned microstructure and the resulting strength, elastic modulus and toughness of the polymer. This ability to control mechanical properties and performance solely through the initial conditions and the mode of front propagation represents a marked advancement in the design and manufacturing of advanced multiscale materials.

Volumetric Additive Manufacturing of Dicyclopentadiene by Solid-State Photopolymerization

Polymerization in the solid state is generally infeasible due to restrictions on mobility. However, in this work, the solid-state photopolymerization of crystalline dicyclopentadiene is demonstrated via photoinitiated ring-opening metathesis polymerization. The source of mobility in the solid state is attributed to the plastic crystal nature of dicyclopentadiene, which yields local short-range mobility due to orientational degrees of freedom. Polymerization in the solid state enables photopatterning, volumetric additive manufacturing of free-standing structures, and fabrication with embedded components. Solid-state photopolymerization of dicyclopentadiene offers a new paradigm for advanced and freeform fabrication of high-performance thermosets.

Programming Mechanical Properties through Encoded Network Topologies

Polymer networks remain an essential class of soft materials. Despite their use in everyday materials, connecting the molecular structure of the network to its macroscopic properties remains an active area of research. Much current research is enabled by advances in modern polymer chemistry providing an unprecedented level of control over macromolecular structure. At the same time, renewed interest in self-healing, dynamic, and/or adaptable materials continues to drive substantial interest in polymer network design. As part of a special issue focused on research performed in the Polymer Science and Engineering Department at the University of Massachusetts, Amherst, this review highlights connections between macromolecular structure of networks and observed mechanical properties as investigated by the Tew research group.

New Prospects Arising from Dynamically Crosslinked Polymers: Reprogramming Their Properties

Dynamically crosslinked polymers (DCPs) have gained significant attention owing to their applications in fabricating (re)processable, recyclable, and self-healable thermosets, which hold great promise in addressing ecological issues, such as plastic pollution and resource scarcity. However, the current research predominantly focuses on redefining and/or manipulating their geometries while replicating their bulk properties. Given the inherent design flexibility of dynamic covalent networks, DCPs also exhibit a remarkable potential for various novel applications through postsynthesis reprogramming their properties. In this review, the recent advancements in strategies that enable DCPs to transform their bulk properties after synthesis are presented. The underlying mechanisms and associated material properties are overviewed mainly through three distinct strategies, namely latent catalysts, material-growth, and topology isomerizable networks. Furthermore, the mutual relationship and impact of these strategies when integrated within one material system are also discussed. Finally, the application prospects and relevant issues necessitating further investigation, along with the potential solutions are analyzed.

PhotoROMP: The Future Is Bright

Since the earliest investigations of olefin metathesis catalysis, light has been the choice for controlling the catalyst activity on demand. From the perspective of energy efficiency, temporal and spatial control, and selectivity, photochemistry is not only an attractive alternative to traditional thermal manufacturing techniques but also arguably a superior manifold for advanced applications like additive manufacturing (AM). In the last three decades, pioneering work in the field of ring-opening metathesis polymerization (ROMP) has broadened the scope of material properties achievable through AM, particularly using light as both an activating and deactivating stimulus. In this Perspective, we explore trends in photocontrolled ROMP systems with an emphasis on approaches to photoinduced activation and deactivation of metathesis catalysts. Recent work has yielded a myriad of commercial and synthetically accessible photosensitive catalyst systems, although comparatively little attention has been paid to achieving precise control over polymer morphology using light. Metal-free, photophysical, and living ROMP systems have also been relatively underexplored. To take fuller advantage of both the thermomechanical properties of ROMP polymers and the operational simplicity of photocontrol, clear directions for the field are to improve the reversibility of activation and deactivation strategies as well as to further develop photocontrolled approaches to tuning cross-link density and polymer tacticity.

Accessing pluripotent materials through tempering of dynamic covalent polymer networks

Pluripotency, which is defined as a system not fixed as to its developmental potentialities, is typically associated with biology and stem cells. Inspired by this concept, we report synthetic polymers that act as a single "pluripotent" feedstock and can be differentiated into a range of materials that exhibit different mechanical properties, from hard and brittle to soft and extensible. To achieve this, we have exploited dynamic covalent networks that contain labile, dynamic thia-Michael bonds, whose extent of bonding can be thermally modulated and retained through tempering, akin to the process used in metallurgy. In addition, we show that the shape memory behavior of these materials can be tailored through tempering and that these materials can be patterned to spatially control mechanical properties.

Plastics that lose their temper on demand

Multiple properties can be programmed into a single dynamic material by using heat.

Axially Encoded Mechano-Metafiber Electronics by Local Strain Engineering

Multimaterial integration, such as soft elastic and stiff components, exhibits rich deformation and functional behaviors to meet complex needs. Integrating multimaterials in the level of individual fiber is poised to maximize the functional design capacity of smart wearable electronic textiles, but remains unfulfilled. Here, this work continuously integrates stiff and soft elastic components into single fiber to fabricate encoded mechano-metafiber by programmable microfluidic sequence spinning (MSS). The sequences with programmable modulus feature the controllable localization of strain along metafiber length. The mechano-metafibers feature two essential nonlinear deformation modes, which are local strain amplification and retardation. This work extends the sequence-encoded metafiber into fiber networks to exhibit greatly enhanced strain amplification and retardation capability in cascades. Local strain engineering enables the design of highly sensitive strain sensors, stretchable fiber devices to protect brittle components and the fabrication of high-voltage supercapacitors as well as axial electroluminescent arrays. The approach allows the scalably design of multimaterial metafibers with programmable localized mechanical properties for woven metamaterials, smart textiles, and wearable electronics.

Skin-Inspired Patterned Hydrogel with Strain-Stiffening Capability for Strain Sensors

Flexible materials with ionic conductivity and stretchability are indispensable in emerging fields of flexible electronic devices as sensing and protecting layers. However, designing robust sensing materials with skin-like compliance remains challenging because of the contradiction between softness and strength. Herein, inspired by the modulus-contrast hierarchical structure of biological skin, we fabricated a biomimetic hydrogel with strain-stiffening capability by embedding the stiff array of poly(acrylic acid) (PAAc) in the soft polyacrylamide (PAAm) hydrogel. The stress distribution in both stiff and soft domains can be regulated by changing the arrangement of patterns, thus improving the mechanical properties of the patterned hydrogel. As expected, the resulting patterned hydrogel showed its nonlinear mechanical properties, which afforded a high strength of 1.20 MPa while maintaining a low initial Young's modulus of 31.0 kPa. Moreover, the array of PAAc enables the patterned hydrogel to possess protonic conductivity in the absence of additional ionic salts, thus endowing the patterned hydrogel with the ability to serve as a strain sensor for monitoring human motion.

Ultra-Tough Self-Healing Hydrogel via Hierarchical Energy Associative Dissipation

Owing to high water content and homogeneous texture, conventional hydrogels hardly reach satisfactory mechanical performance. Tensile-resistant groups and structural heterogeneity are employed to fabricate tough hydrogels. However, those techniques significantly increase the complexity and cost of material synthesis, and have only limited applicability. Here, it is shown that ultra-tough hydrogels can be obtained via a unique hierarchical architecture composed of chemically coupled self-assembly units. The associative energy dissipation among them may be rationally engineered to yield libraries of tough gels with self-healing capability. Tunable tensile strength, fracture strain, and toughness of up to 19.6 MPa, 20 000%, and 135.7 MJ cm⁻3 are achieved, all of which exceed the best known records. The results demonstrate a universal strategy to prepare desired ultra-tough hydrogels in predictable and controllable manners.

Digital Light Processing 3D Printing of Soft Semicrystalline Acrylates with Localized Shape Memory and Stiffness Control

Multimaterial three-dimensional (3D) printing of objects with spatially tunable thermomechanical properties and shape-memory behavior provides an attractive approach toward programmable "smart" plastics with applications in soft robotics and electronics. To date, digital light processing 3D printing has emerged as one of the fastest manufacturing methods that maintains high precision and resolution. Despite the common utility of semicrystalline polymers in stimuli-responsive materials, few reports exist whereby such polymers have been produced via digital light processing (DLP) 3D printing. Herein, two commodity long-alkyl chain acrylates (C₁₈, stearyl and C₁₂, lauryl) and mixtures therefrom are systematically examined as neat resin components for DLP 3D printing of semicrystalline polymer networks. Tailoring the stearyl/lauryl acrylate ratio results in a wide breadth of thermomechanical properties, including tensile stiffness spanning three orders of magnitude and temperatures from below room temperature (2 °C) to above body temperature (50 °C). This breadth is attributed primarily to changes in the degree of crystallinity. Favorably, the relationship between resin composition and the degree of crystallinity is quadratic, making the thermomechanical properties reproducible and easily programmable. Furthermore, the shape-memory behavior of 3D-printed objects upon thermal cycling is characterized, showing good fatigue resistance and work output. Finally, multimaterial 3D-printed structures with vertical gradation in composition are demonstrated where concomitant localization of thermomechanical properties enables multistage shape-memory and strain-selective behavior. The present platform represents a promising route toward customizable actuators for biomedical applications.

Multimorphic Materials: Spatially Tailoring Mechanical Properties via Selective Initiation of Interpenetrating Polymer Networks

Access to multimaterial polymers with spatially localized properties and robust interfaces is anticipated to enable new capabilities in soft robotics, such as smooth actuation for advanced medical and manufacturing technologies. Here, orthogonal initiation is used to create interpenetrating polymer networks (IPNs) with spatial control over morphology and mechanical properties. Base catalyzes the formation of a stiff and strong polyurethane, while blue LEDs initiate the formation of a soft and elastic polyacrylate. IPN morphology is controlled by when the LED is turned "on", with large phase separation occurring for short time delays (≈1-2 min) and a mixed morphology for longer time delays (>5 min), which is supported by dynamic mechanical analysis, small angle X-ray scattering, and atomic force microscopy. Through tailoring morphology, tensile moduli and fracture toughness can be tuned across ≈1-2 orders of magnitude. Moreover, a simple spring model is used to explain the observed mechanical behavior. Photopatterning produces "multimorphic" materials, where morphology is spatially localized with fine precision (<100 µm), while maintaining a uniform chemical composition throughout to mitigate interfacial failure. As a final demonstration, the fabrication of hinges represents a possible use case for multimorphic materials in soft robotics.

Coordinatively Stiffen and Toughen Hydrogels with Adaptable Crystal-Domain Cross-Linking

Conventional hydrogels usually suffer from the inherent conflict between stiffness and toughness, severely hampering their applications as load-bearing materials. Herein, an adaptable crystal-domain cross-linking design is reported to overcome this inherent trade-off for hydrogels by taking full advantage of both deformation-resisting and energy-dissipating capacities of cross-linking points. Through solvent exchange to homogenize the polymer network, followed by salting out to foster crystallization, a class of sal-exogels with high number densities of uniform crystalline domains embedded in homogeneous networks is constructed. During the deformation, those adaptive crystalline domains initially survive to arrest deformation, while later gradually disentangle to efficiently dissipate energy, crucial to the realization of the desirable compatibility between stiffness and toughness. The resultant sal-exogel achieves coordinatively enhanced stiffness (52.3 ± 2.7 MPa) and toughness (120.7 ± 11.7 kJ m^-2 ), reconciling the challenging trade-off between them. This finding provides a practical and universal route to design stiff and tough hydrogels and has a profound impact on many applications requiring hydrogels with such combined mechanical properties.