50th Anniversary Perspective: Solid-State Multistimuli, Multiresponsive Polymeric Materials

Foldamer-Based Mechanoresponsive Materials: Molecular Nanoarchitectonics to Advanced Functions

Artificial molecules that respond to external stimuli such as light, heat, chemical signals, and mechanical force have garnered significant interest due to their tunable functions, variable optical properties, and mechanical responses. Particularly, mechanoresponsive materials featuring molecules that respond to mechanical stress or show force-induced optical changes have been intriguing due to their extraordinary functions. Despite the promising potential of many such materials reported in the past, practical applications have remained limited, primarily because their functions often depend on irreversible covalent bond rupture. Foldamers, oligomers that fold into well-defined secondary structures, offer an alternative class of mechanoactive motifs. These molecules can reversibly sustain mechanical stress and efficiently dissipate energy by transitioning between folded and unfolded states. This review focuses on the emerging properties of foldamer-based mechanoresponsive materials. We begin by highlighting the mechanical responses of foldamers in their molecular form, which have been primarily investigated using single-molecule force spectroscopy and other analytical methods. Following this, we provide a detailed survey of the current trends in foldamer-appended polymers, emphasizing their emerging mechanical and mechanochromic properties. Subsequently, we present an overview of the state-of-the-art advancements in foldamer-appended polymers, showcasing significant reports in this field. This review covers some of the most recent advances in this direction and draws a perspective for further development.

Theory and simulation of ligand functionalized nanoparticles - a pedagogical overview

Synthesizing reconfigurable nanoscale synthons with predictive control over shape, size, and interparticle interactions is a holy grail of bottom-up self-assembly. Grand challenges in their rational design, however, lie in both the large space of experimental synthetic parameters and proper understanding of the molecular mechanisms governing their formation. As such, computational and theoretical tools for predicting and modeling building block interactions have grown to become integral in modern day self-assembly research. In this review, we provide an in-depth discussion of the current state-of-the-art strategies available for modeling ligand functionalized nanoparticles. We focus on the critical role of how ligand interactions and surface distributions impact the emergent, pre-programmed behaviors between neighboring particles. To help build insights into the underlying physics, we first define an "ideal" limit - the short ligand, "hard" sphere approximation - and discuss all experimental handles through the lens of perturbations about this reference point. Finally, we identify theories that are capable of bridging interparticle interactions to nanoscale self-assembly and conclude by discussing exciting new directions for this field.

Trainable bioinspired magnetic sensitivity adaptation using ferromagnetic colloidal assemblies

Nature has already suggested bioinspired functions. Beyond them, adaptive and trainable functions could be the inspiration for novel responsive soft matter beyond the state-of-the-art classic static bioinspired, stimulus-responsive, and shape-memory materials. Here, we describe magnetic assembly/disassembly of electrically conducting soft ferromagnetic nickel colloidal particles into surface topographical pillars for bistable electrical trainable memories. They allow magnetic sensing with adaptable and rescalable sensitivity ranges, enabled by bistable memories and kinetic concepts inspired by biological sensory adaptations. Based on the soft ferromagnetism of the nanogranular composition and the resulting rough particle surfaces prepared via a solvothermal synthesis, triggerable structural memory is achieved by the magnetic field-driven particle assembly and disassembly, promoted by interparticle jamming. Electrical conversion from current to frequency for electrical spikes facilitates rescalable and trainable frequency-based sensitivity on magnetic fields. This work suggests an avenue for designing trainable and adaptable life-inspired materials, for example, for soft robotics and interactive autonomous devices.

Highly efficient grafting of hetero-complementary amidinium and carboxylate hydrogen-bonding/ionic pairs onto polymer surfaces

We describe a grafting methodology, based on thiol-fluoroarene chemistry, to efficiently incorporate complementary hydrogen-bonding carboxylate and amidinium groups into polymer backbones. The process was optimized both in solution and on the surface of processed films, with the aim to produce materials showing hetero-complementary adhesion.

Connecting Molecular Exchange Dynamics to Stress Relaxation in Phase-Separated Dynamic Covalent Networks

A suite of phase separated dynamic covalent networks based on highly tunable dynamic benzalcyanoacetate (BCA) thia-Michael acceptors are investigated. In situ kinetic studies on small molecule model systems are used in conjunction with macroscopic characterization of phase stability and stress relaxation to understand how the molecular dynamics relate to relaxation modes. Electronic modification of the BCA unit strongly impacts the exchange dynamics (particularly the rate of dissociation) and the overall equilibrium constant (Keq) of the system, with electron-withdrawing groups leading to decreased dissociation rate and increased Keq. Critically, below a chemistry-defined temperature cutoff (related to the stability of the hard phase domains), the stress relaxation behavior of these phase separated materials is dominated by the molecular exchange dynamics, allowing for networks with a tailored thermomechanical response.

Structure and Properties of Metallosupramolecular Polymers with a Nitrogen-Based Bidentate Ligand

The solid-state properties of supramolecular polymers that feature metal-ligand (ML) complexes are, in addition to the general nature of the monomer, significantly affected by the choice of ligand and metal salt. Indeed, the variation of these components can be used to alter the structural, thermal, mechanical, and viscoelastic properties over a wide ranges. Moreover, the dynamic nature of certain ML complexes can render the resulting metallosupramolecular polymers (MSPs) stimuli-responsive, enabling functions such as healing, reversible adhesion, and mechanotransduction. We here report MSPs based on the bidentate ligand 6-(1'-methylbenzimidazolyl) pyridine (MBP), which is easily accessible and forms threefold coordination complexes with various transition metal ions. Thus, a poly(ethylene-co-butylene) telechelic was end-functionalized with two MBP ligands and the resulting macromonomer was assembled with the triflate salts of either Zn2+, Fe2+, or Ni2+. All three MSPs microphase separate and adopt, depending on the metal ion and thermal history, lamellar or hexagonal morphologies with crystalline domains formed by the ML complexes. The melting transitions are well below 200 °C, and this permits facile (re)processing. Furthermore, defects can be readily and fully healed upon exposure to UV-light. While the three MSPs display similar moduli in the rubbery regime, their extensibility and tensile strength depend on the nature of the ML complex, which similarly affects the long-range order and dynamic behavior.

Recent advances in permeable polymersomes: fabrication, responsiveness, and applications

Polymersomes are vesicular nanostructures enclosed by a bilayer-membrane self-assembled from amphiphilic block copolymers, which exhibit higher stability compared with their biological analogues (e.g. liposomes). Due to their versatility, polymersomes have found various applications in different research fields such as drug delivery, nanomedicine, biological nanoreactors, and artificial cells. However, polymersomes prepared with high molecular weight components typically display low permeability to molecules and ions. It hence remains a major challenge to balance the opposing features of robustness and permeability of polymersomes. In this review, we focus on the design and strategies for fabricating permeable polymersomes, including polymersomes with intrinsic permeability, the formation of nanopores in the membrane bilayers by protein insertion, and the construction of stimuli-responsive polymersomes. Then, we highlight the applications of permeable polymersomes in the fields of biomimetic nanoreactors, artificial cells and organelles, and nanomedicine, to underline the challenges in the development of polymersomes as soft matter with biomedical utilities.

High-contrast reversible multiple color-tunable solid luminescent ionic polymers for dynamic multilevel anti-counterfeiting

Dynamic color-tunable luminescent materials, which possess huge potential applications in advanced multilevel luminescence anti-counterfeiting, are of considerable interest. However, it remains challenging to develop simple high-contrast reversible multiple (triple or more than triple) color-tunable high-efficiency solid luminescent materials with low cost, facile synthesis, and good processability. Herein, by simply grafting charged multi-color AIEgen-based chromophores into polymers, a series of high-efficiency multiple color-tunable luminescent single ionic polymers are constructed through tuning feed ratios, counter anions and reaction solvents. Remarkably, some ionic polymers can not only achieve rare high-contrast reversible multiple color-tunable emission in solid states in response to different solvent stimuli, but also could realize excitation-dependent color-tunable emission. To the best of our knowledge, such charming multiple (triple or more than triple) color-tunable solid polymers responding to multiple external stimuli are still rare. Based on comparative studies of emission spectra, excitation spectra and fluorescence lifetimes before and after swelling, it could be inferred that solvent stimuli could induce microstructure changes of these ionic polymers and then change the aggregated-states of their corresponding AIE-active emission centers. Moreover, the different solvent stimuli could induce to produce different degrees of microstructure changes, resulting in their unique multiple color-tunable emission. More significantly, these smart color-tunable ionic polymers show great promise for applications in dynamic multilevel (three-level or even more than three-level) anti-counterfeiting.

Magnetic field-driven particle assembly and jamming for bistable memory and response plasticity

Unlike classic synthetic stimulus-responsive and shape-memory materials, which remain limited to fixed responses, the responses of living systems dynamically adapt based on the repetition, intensity, and history of stimuli. Such plasticity is ubiquitous in biology, which is profoundly linked to memory and learning. Concepts thereof are searched for rudimentary forms of "intelligent materials." Here, we show plasticity of electroconductivity in soft ferromagnetic nickel colloidal supraparticles with spiny surfaces, assembling/disassembling to granular conducting micropillars between two electrodes driven by magnetic field B. Colloidal jamming leads to conduction hysteresis and bistable memory upon increasing and subsequently decreasing B. Abrupt B changes induce larger conduction changes than gradual B-changes. Periodic B pulsing drives to frequency-dependent facilitation or suppression of conductivity compared to exposing the same constant field. The concepts allow remotely controlled switching plasticity, illustrated by a rudimentary device. More generally, we foresee adaptive functional materials inspired by response plasticity and learning.

Plasticization of a Semicrystalline Metallosupramolecular Polymer Network

The assembly of ligand-functionalized (macro)monomers with suitable metal ions affords metallosupramolecular polymers (MSPs). On account of the reversible and dynamic nature of the metal-ligand complexes, these materials can be temporarily (dis-)assembled upon exposure to a suitable stimulus, and this effect can be exploited to heal damaged samples, to facilitate processing and recycling, or to enable reversible adhesion. We here report on the plasticization of a semicrystalline, stimuli-responsive MSP network that was assembled by combining a low-molecular-weight building block carrying three 2,6-bis(1'-methylbenzimidazolyl) pyridine (Mebip) ligands and zinc bis(trifluoromethylsulfonyl)imide (Zn(NTf₂)₂). The pristine material exhibits high melting (T _m = 230 °C) and glass transition (T _g ≈ 157 °C) temperatures and offers robust mechanical properties between these temperatures. We show that this regime can be substantially extended through plasticization. To achieve this, the MSP network was blended with diisodecyl phthalate. The weight fraction of this plasticizer was systematically varied, and the thermal and mechanical properties of the resulting materials were investigated. We show that the T _g can be lowered by more than 60 °C and the toughness above the T _g is considerably increased.

Healing Strategy Based on Space Adjustment for Cross-Linked Polymer Networks

Healable materials are notable for their ability to recover from mechanical damage. Most methods for preparing cross-linked healable materials require the introduction of healing agents or supramolecular interactions in solvent environments. Hence, a strategy without the addition of functional component remains a key challenge. Herein, a healing strategy based on space adjustment is proposed with cross-linked poly(octadecyl acrylate) as a model, and this strategy demonstrates that the predesigned holes in cross-linked networks can supply the possibility for polymer coils to move and decrease the space density of the networks during the annealing process. As a result, the motilities of coils are enhanced, which allows them to easily penetrate and entangle in fracture sites. In contrast with the untreated cross-linked poly(octadecyl acrylate), which cannot heal, the space-adjusted poly(octadecyl acrylate) readily heals, and the highest healing efficiency is 96%. The ways in which the extent of space adjustment and the content of the cross-linking agent affect the healing efficiency are discussed, and the mechanism of the space adjustment strategy is studied through rheology research. This strategy concentrates on adjusting the spatial density of the network without the need for any functional design, which may be applied in various polymer systems.

Smart E-Textiles: Overview of Components and Outlook

Smart textiles have gained great interest from academia and industries alike, spanning interdisciplinary efforts from materials science, electrical engineering, art, design, and computer science. While recent innovation has been promising, unmet needs between the commercial and academic sectors are pronounced in this field, especially for electronic-based textiles, or e-textiles. In this review, we aim to address the gap by (i) holistically investigating e-textiles' constituents and their evolution, (ii) identifying the needs and roles of each discipline and sector, and (iii) addressing the gaps between them. The components of e-textiles-base fabrics, interconnects, sensors, actuators, computers, and power storage/generation-can be made at multiscale levels of textile, e.g., fiber, yarn, fabric, coatings, and embellishments. The applications, current state, and sustainable future directions for e-textile fields are discussed, which encompasses health monitoring, soft robotics, education, and fashion applications.

Photoreversible Bond-Based Shape Memory Polyurethanes with Light-Induced Self-Healing, Recyclability, and 3D Fluorescence Encryption

Developing a shape memory polyurethane with high mechanical properties, excellent self-healing has become a huge challenge for the development of smart materials. Herein, we report the design and fabrication of a shape memory polyurethane network terminated with coumarin units (HEOMC-PU) to address this conundrum. The synthesized HEOMC-PU exhibits exceptional mechanical performance with a breaking elongation of 746% and toughness of 55.5 MJ·m-3. By utilizing the dynamically reversible behavior of coumarin units to repair the damaged network, the efficient self-healing performance (99.2%) of HEOMC-PU is obtained. In addition, the prepared network and light-induced dynamic reversibility endow the HEOMC-PU with both liquid-state remoldability and solid-state plasticity, respectively, enabling polyurethane to be recycled and processed multiple times. Furthermore, based on the fluorescence responsive characteristic of coumarin, HEOMC-PU with a fluorescent pattern can be deformed into specific three-dimensional configurations by combining photolithography, self-healing, and the shape memory effect. Such a multilevel and multidimensional anti-counterfeiting platform with rewritable fluorescent patterns and reconfigurable shapes can open up a new encryption approach for future intelligent anti-counterfeiting.

Light-Responsive Hexagonal Assemblies of Triangular Azo Dyes

The rational design of small building block molecules and understanding their molecular assemblies are of fundamental importance in creating new stimuli-responsive organic architectures with desired shapes and functions. Based on the experimental results of light-induced conformational changes of four types of triangular azo dyes with different terminal functional groups, as well as absorption and fluorescence characteristics associated with their molecular assemblies, we report that aggregation-active emission enhancement (AIEE)-active compound (1) substituted with sterically crowded tert-butyl (t-Bu) groups showed approximately 35% light-induced molecular switching and had a strong tendency to assemble into highly stable hexagonal structures with AIEE characteristics. Their sizes were regulated from nanometer-scale hexagonal rods to micrometer-scale sticks depending on the concentration. This is in contrast to other triangular compounds with bromo (Br) and triphenylamine (TPA) substituents, which exhibited no photoisomerization and tended to form flexible fibrous structures. Moreover, non-contact exposure of the fluorescent hexagonal nanorods to ultraviolet (UV) light led to a dramatic hexagonal-to-amorphous structure transition. The resulting remarkable variations, such as in the contrast of microscopic images and fluorescence characteristics, were confirmed by various microscopic and spectroscopic measurements.

Temperature Responsive PBT Bicomponent Fibers for Dynamic Thermal Insulation

Thermoresponsive self-crimping polybutylene terephtlate (PBT)-based bicomponent fibers were fabricated by melt-spinning to serve as primary constituents for textiles, such as nonwoven battings, for an adaptive single insulting layer. Due to the intrinsically mismatching modulus and coefficient of thermal expansion (CTE), the fibers curl or straighten with temperature, similar to the concept of Timoshenko's bimetallic strip. Maximizing the curvature is driven by an optimum of fiber diameter, overall CTE, and fiber moduli, which are all affected by drawing ratio and, consequently, fiber's microstructure. A draw ratio of 2.33 yielded the best combination of mechanical and thermal properties; it was observed that increasing the draw ratio does not necessarily increase the self-crimping behavior. Tests performed on non-woven battings of these fibers exhibited comparable thermoreponsive behaviors to polypropylene-based thermoresponsive fibers from previous studies in the -20 °C to 20 °C temperature range, which has potential for wearable insulations for both commercial and defense sectors alike.

Ca2+-Chelation-Induced Fabrication of Multistimuli-Responsive Charged Nanogels from Phospholipid-Polymer Conjugates and Use for Drug/Protein Loading

Thermoresponsive phospholipid-poly(N-isopropylacrylamide) (PL-PNIPAM) conjugates were synthesized via reversible addition fragmentation chain transfer polymerization mediated by a phospholipid-modified trithiocarbonate. Temperature triggered the micellization of the PL-PNIPAM conjugate to form phosphate group-decorated micelles in the aqueous solution. Driven by the chelation of phospholipids and Ca²⁺, the PL-PNIPAM conjugate and Ca²⁺ ions formed size-tunable nanoclusters at a temperature beyond the lower critical solution temperature. To fabricate cross-linked nanogels, NIPAM was copolymerized with N-succinimidyl acrylate (NSA) to obtain the PL-P(NIPAM-co-NSA) conjugate bearing pendent cross-linkable functionalities. Subsequently, the size-controllable nanogels containing disulfide linkages were generated at 37 °C by cross-linking the PL-P(NIPAM-co-NSA)/Ca²⁺ nanoclusters with cystamine through modulation of Ca²⁺ concentrations. These negatively charged nanogels demonstrate temperature/pH/reduction triple responsiveness. The nanogels can be efficiently loaded with doxorubicin (DOX) and proteins with various isoelectric points. The DOX-loaded nanogels exhibited a temperature/pH/reduction triple-responsive release profile. The immobilized RNase A, BSA, and GOx retained the protein bioactivity. The release of RNase A-loaded nanogels possesses a temperature-responsive profile. The immobilization of Lys and cytochrome C in nanogels inhibited protein bioactivity. However, the addition of NaCl triggered the recovery of bioactivity. These multistimuli-responsive nanogels can provide a versatile platform applicable in biotechnology and drug/protein delivery.

Rapid Stress Relaxation, Multistimuli-Responsive Elastomer Based on Dual-Dynamic Covalent Bonds and Aniline Trimer

Covalent adaptable networks (CANs) are an emerging kind of smart materials in which cross-links are reversible upon some stimuli and then provide malleability and a stimuli-responsive ability to the materials. There is a trend to endow CANs with multistimuli-responsive capabilities and rapid stress relaxation to pursue more advanced applications. To integrate these two features into one material, here, dual-dynamic covalent bonds (imines and boronic esters) and aniline trimer (ACAT) were incorporated into the styrene butadiene elastomer as dynamic cross-links. The obtained CANs were demonstrated with rapid stress relaxation and a relatively low activation energy of 36 ± 1 kJ mol^-1, resulting from the synergistic effect of dual-dynamic covalent bonds to rearrange the network at a faster rate than for either imines or boronic esters. Because of the dynamic nature of imines or boronic esters, the elastomer can be recycled upon heat. Moreover, the appearance and configuration of the elastomer could also be manipulated by pH and light because of the inclusion of ACAT. All in all, the coupled multistimuli-responsive behavior and rapid stress relaxation in one single elastomer would potentially be applicable for sensors and actuators with good recyclability.

Mechanically robust supramolecular polymer co-assemblies

Supramolecular polymers are formed through non-covalent, directional interactions between monomeric building blocks. The assembly of these materials is reversible, which enables functions such as healing, repair, or recycling. However, supramolecular polymers generally fail to match the mechanical properties of conventional commodity plastics. Here we demonstrate how strong, stiff, tough, and healable materials can be accessed through the combination of two metallosupramolecular polymers with complementary mechanical properties that feature the same metal-ligand complex as binding motif. Co-assembly yields materials with micro-phase separated hard and soft domains and the mechanical properties can be tailored by simply varying the ratio of the two constituents. On account of toughening and physical cross-linking effects, this approach affords materials that display higher strength, toughness, or failure strain than either metallosupramolecular polymer alone. The possibility to combine supramolecular building blocks in any ratio further permits access to compositionally graded objects with a spatially modulated mechanical behavior.

Solvent- and Light-Sensitive AIEE-Active Azo Dye: From Spherical to 1D and 2D Assemblies

Fluorescent molecular assembly systems provide an exciting platform for creating stimuli-responsive nano- and microstructured materials with optical, electronic, and sensing functions. To understand the relationship between (i) the plausible molecular structures preferentially adopted depending on the solvent polarity (such as N,N-dimethylformamide [DMF], tetrahydrofuran [THF], and toluene), (ii) the resulting spectroscopic features, and (iii) self-assembled nano-, micro-, and macrostructures, we chose a sterically crowded triangular azo dye (3Bu) composed of a polar molecular core and three peripheral biphenyl wings. The chromophore changed the solution color from yellow to pink-red depending on the solvent polarity. In a yellow DMF solution, a considerable amount of the twisted azo form could be kept stable with the help of favorable intermolecular interactions with the solvent molecules. By varying the concentration of the DMF solution, the morphology of self-assembled structures was transformed from nanoparticles to micrometer-sized one-dimensional (1D) structures such as sticks and fibers. In a pink-red toluene solution, the periphery of the central ring became more planar. The resulting significant amount of the keto-hydrazone tautomer grew into micro- and millimeter-sized 1D structures. Interestingly, when THF-H₂O (1:1) mixtures were stored at a low temperature, elongated fibers were stacked sideways and eventually developed into anisotropic two-dimensional (2D) sheets. Notably, subsequent exposure of visible-light-irradiated sphere samples to solvent vapor resulted in reversible fluorescence off↔on switching accompanied by morphological restoration. These findings suggest that rational selection of organic dyes, solvents, and light is important for developing reusable fluorescent materials.

Dendronized Gelatins Showing Both LCST and UCST-type Thermoresponsive Behavior

Modification of natural polymers with stimuli-responsive synthetic moieties witnesses convergences of superior properties from natural polymers and stimuli-responsiveness to generate new intelligent materials. This was usally performed through synthetic polymers...

Pyridine-containing block copolymeric nano-assemblies obtained through complementary hydrogen-bonding directed polymerization-induced self-assembly in water

A diversity of pyridine-containing polymeric nanomaterials with controllable structures and multiple responses were developed through complementary hydrogen-bonding directed polymerization-induced self-assembly in aqueous solution.

Stimuli-responsive temporary adhesives: enabling debonding on demand through strategic molecular design

Stimuli-responsive temporary adhesives constitute a rapidly developing class of materials defined by the modulation of adhesion upon exposure to an external stimulus or stimuli. Engineering these materials to shift between two characteristic properties, strong adhesion and facile debonding, can be achieved through design strategies that target molecular functionalities. This perspective reviews the recent design and development of these materials, with a focus on the different stimuli that may initiate debonding. These stimuli include UV light, thermal energy, chemical triggers, and other potential triggers, such as mechanical force, sublimation, electromagnetism. The conclusion discusses the fundamental value of systematic investigations of the structure-property relationships within these materials and opportunities for unlocking novel functionalities in future versions of adhesives.

Self-Immolative System for Disclosure of Reactive Electrophilic Alkylating Agents: Understanding the Role of the Reporter Group

The development of stable, efficient chemoselective self-immolative systems, for use in applications such as sensors, requires the optimization of the reactivity and degradation characteristics of the self-immolative unit. In this paper, we describe the effect that the structure of the reporter group has upon the self-immolative efficacy of a prototype system designed for the disclosure of electrophilic alkylating agents. The amine of the reporter group (a nitroaniline unit) was a constituent part of a carbamate that functioned as the self-immolative unit. The number and position of substituents on the nitroaniline unit were found to play a key role in the rate of self-immolative degradation and release of the reporter group. The position of the nitro substituent (meta- vs para-) and the methyl groups in the ortho-position relative to the carbamate exhibited an influence on the rate of elimination and stability of the self-immolative system. The ortho-methyl substituents imparted a twist on the N–C (aromatic) bond leading to increased resonance of the amine nitrogen’s lone pair into the carbonyl moiety and a decrease of the leaving character of the carbamate group; concomitantly, this may also make it a less electron-withdrawing group and lead to less acidification of the eliminated β-hydrogen.

Prevent or Cure-The Unprecedented Need for Self-Reporting Materials

Self-reporting smart materials are highly relevant in modern soft matter materials science, as they allow for the autonomous detection of changes in synthetic polymers, materials, and composites. Despite critical advantages of such materials, for example, prolonged lifetime or prevention of disastrous material failures, they have gained much less attention than self-healing materials. However, as diagnosis is critical for any therapy, it is of the utmost importance to report the existence of system changes and their exact location to prevent them from spreading. Thus, we herein critically review the chemistry of self-reporting soft matter materials systems and highlight how current challenges and limitations may be overcome by successfully transferring self-reporting research concepts from the laboratory to the real world. Especially in the space of diagnostic self-reporting systems, the recent SARS-CoV-2 (COVID-19) pandemic indicates an urgent need for such concepts that may be able to detect the presence of viruses or bacteria on and within materials in a self-reporting fashion.

Charge-balanced terpolymer poly(diethylaminoethyl methacrylate-hydroxyethyl methacrylate-2-acrylamido-2-methyl-propanesulfonic acid) hydrogels and cryogels: scaling parameters and correlation with composition

The scaling laws relating the preparation conditions to the swelling degree, reduced modulus and effective crosslinking density of poly(diethylaminoethyl methacrylate-co-hydroxyethyl methacrylate-co-2-acrylamido-2-methyl-propanesulfonic acid), henceforth designated as PDHA, gels prepared by radical crosslinking copolymerization in a solvent mixture were reported. Charge-balanced terpolymer PDHA hydrogels and cryogels (PDHA-Hgs and Cgs) were prepared in different monomer feed compositions. The swelling dependence of the reduced modulus was described by a power law relationship G_r≈ (φ_V)^m with an exponent of m = -0.30 at low swelling degree, while in the high swelling region the scaling becomes 0.21, indicating the finite extensibility of the network chains. The scaling exponent for the swelling degree and terpolymer composition, φ_V≈ (Nν)^m, was found to be -0.13, indicating the increasing extent of the topological constraints arising from the trapped entanglements. By combining elasticity and swelling results, the scaling relationship between the apparent crosslink density and HEMA content used in the terpolymer feed was obtained as a cubic polynomial of the mol% of HEMA. In the HEMA-rich terpolymer PDHA Hgs and Cgs, the swelling degree was possibly controlled by the HEMA part of the terpolymer network, while the presence of DEAEM units in the network triggered the thermoresponsive swelling behavior. The dependence of interaction parameter χ on the volume fraction of the crosslinked terpolymer network in the swollen gel ν₂ was evaluated and the results revealed extremely strong concentration dependence of χ for all terpolymer samples. Because of their inherent properties, the resulting terpolymer gels might contribute to the improvement of the loading capacity of polymers used in anticancer drug delivery systems.

From Molecules to Polymers-Harnessing Inter- and Intramolecular Interactions to Create Mechanochromic Materials

The development of mechanophores as building blocks that serve as predefined weak linkages has enabled the creation of mechanoresponsive and mechanochromic polymer materials, which are interesting for a range of applications including the study of biological specimens or advanced security features. In typical mechanophores, covalent bonds are broken when polymers that contain these chemical motifs are exposed to mechanical forces, and changes of the optical properties upon bond scission can be harnessed as a signal that enables the detection of applied mechanical stresses and strains. Similar chromic effects upon mechanical deformation of polymers can also be achieved without relying on the scission of covalent bonds. The dissociation of motifs that feature directional noncovalent interactions, the disruption of aggregated molecules, and conformational changes in molecules or polymers constitute an attractive element for the design of mechanoresponsive and mechanochromic materials. In this article, it is reviewed how such alterations of molecules and polymers can be exploited for the development of mechanochromic materials that signal deformation without breaking covalent bonds. Recent illustrative examples are highlighted that showcase how the use of such mechanoresponsive motifs enables the visual mapping of stresses and damage in a reversible and highly sensitive manner.

Designing single trigger/dual-response release and degradation into amine-functional hyperbranched-polydendron nanoprecipitates

The synthesis of complex polymer architectures using relatively facile experimental protocols provides access to materials with the opportunity to control functionality and physical behaviour. The scope of hyperbranched-polydendron chemistries has been expanded here to include primary chains comprising amine-functional 'homopolymer', 'statistical copolymer' and amphiphilic 'block copolymer' analogues using 2-(diethyl amino)ethyl methacrylate, 2-hydroxy propyl methacrylate and t-butyl methacrylate. The different primary chain chemistry and architectures leads to a marked variation in nanoprecipitation behaviour and the response of the resulting amine-functional nanoparticles to varying pH. When acid-sensitive and acid-stable branchers, 1,4-butanediol di(methacryoyloxy)-ethyl ether and ethylene glycol dimethacrylate respectively, are utilised, nanoparticles with encapsulation properties are formed and may be triggered to either release-and-disassemble or release-disassemble-degrade to form a solution of lower molecular weight constituent primary chains.

100th Anniversary of Macromolecular Science Viewpoint: Engineering Supramolecular Materials for Responsive Applications-Design and Functionality

Supramolecular polymers allow access to dynamic materials, where noncovalent interactions can be used to offer both enhanced material toughness and stimuli-responsiveness. The versatility of self-assembly has enabled these supramolecular motifs to be incorporated into a wide array of glassy and elastomeric materials; moreover, the interaction of these noncovalent motifs with their environment has shown to be a convenient platform for controlling material properties. In this Viewpoint, supramolecular polymers are examined through their self-assembly chemistries, approaches that can be used to control their self-assembly (e.g., covalent cross-links, nanofillers, etc.), and how the strategic application of supramolecular polymers can be used as a platform for designing the next generation of smart materials. This Viewpoint provides an overview of the aspects that have garnered interest in supramolecular polymer chemistry, while also highlighting challenges faced and innovations developed by researchers in the field.

Extrusion 3D Printing of Polymeric Materials with Advanced Properties

3D printing is a rapidly growing technology that has an enormous potential to impact a wide range of industries such as engineering, art, education, medicine, and aerospace. The flexibility in design provided by this technique offers many opportunities for manufacturing sophisticated 3D devices. The most widely utilized method is an extrusion-based solid-freeform fabrication approach, which is an extremely attractive additive manufacturing technology in both academic and industrial research communities. This method is versatile, with the ability to print a range of dimensions, multimaterial, and multifunctional 3D structures. It is also a very affordable technique in prototyping. However, the lack of variety in printable polymers with advanced material properties becomes the main bottleneck in further development of this technology. Herein, a comprehensive review is provided, focusing on material design strategies to achieve or enhance the 3D printability of a range of polymers including thermoplastics, thermosets, hydrogels, and other polymers by extrusion techniques. Moreover, diverse advanced properties exhibited by such printed polymers, such as mechanical strength, conductance, self-healing, as well as other integrated properties are highlighted. Lastly, the stimuli responsiveness of the 3D printed polymeric materials including shape morphing, degradability, and color changing is also discussed.

Sustainable Water Responsive Mechanically Adaptive and Self-Healable Polymer Composites Derived from Biomass

New synthetic biobased mechanically adaptive composites, responding to water and having self-healing property, were developed. These composites were prepared by introducing plant-based cellulose nanofibrils (CNFs) at 10, 20, and 25% (v/v) concentration into a biobased rubbery poly (myrcene-co-furfuryl methacrylate) (PMF) matrix by solution mixing and subsequent compression molding technique. The reinforcement of CNFs led to an increase in the tensile storage modulus (E’) of the dry composites. Upon exposure to water, water sensitivity and a drastic fall in storage moduli (E’) were observed for the 25% (v/v) CNF composite. A modulus reduction from 1.27 (dry state) to 0.15 MPa (wet state) was observed for this composite. The water-sensitive nature of the composites was also confirmed from the force modulation study in atomic force microscopy (AFM), revealing the average modulus as 82.7 and 32.3 MPa for dry and swollen composites, respectively. Interestingly, the composites also showed thermoreversibility and excellent healing property via Diels-Alder (DA) click chemistry using bismaleimide as a crosslinker, when the scratched samples were heated at 120 °C (rDA) for 10 h and then cooled down to 60 °C (DA) followed by room temperature. The healing efficiency was obtained as about 90% from the AFM 3D height images. Thus, the composites exhibited dual stimuli-responsive behavior as mechanically adaptive water sensitive polymers with water as the stimulus and self-healing polymer using bismaleimide as an external stimulus. Therefore, this study provides guidance and new frontiers to make use of composite materials based on biopolymers for various potential smart and biomedical applications.

Terpene Based Elastomers: Synthesis, Properties, and Applications

The limited source of fossil-fuel and the predominance of petroleum-based chemistry in the manufacturing of commodity polymers has generated tremendous interest in replacing the fossil source-based polymers with renewable counterparts. The field of sustainable elastomers has grown in the past three decades, from a few examples to a plethora of reports in modern polymer science and technology. Applications of elastomers are huge and vital for everyday living. The present review aims to portray a birds-eye view of various sustainable elastomers obtained from the wide family of acyclic terpenes (renewable feedstocks from different plant oils) via various polymerization techniques and their properties, as well as plausible developments in the future applications of sustainable polymers. Not only the homopolymers, but also their copolymers with both green and commercial fossil based comonomers, are reviewed.

Viewpoint: Pavlovian Materials-Functional Biomimetics Inspired by Classical Conditioning

Herein, it is discussed whether the complex biological concepts of (associative) learning can inspire responsive artificial materials. It is argued that classical conditioning, being one of the most elementary forms of learning, inspires algorithmic realizations in synthetic materials, to allow stimuli-responsive materials that learn to respond to a new stimulus, to which they are originally insensitive. Two synthetic model systems coined as "Pavlovian materials" are described, whose stimuli-responsiveness algorithmically mimics programmable associative learning, inspired by classical conditioning. The concepts minimally need a stimulus-triggerable memory, in addition to two stimuli, i.e., the unconditioned and the originally neutral stimuli. Importantly, the concept differs conceptually from the classic stimuli-responsive and shape-memory materials, as, upon association, Pavlovian materials obtain a given response using a new stimulus (the originally neutral one); i.e., the system evolves to a new state. This also enables the functionality to be described by a logic diagram. Ample room for generalization to different stimuli and memory combinations is foreseen, and opportunities to develop future adaptive materials with ever-more intelligent functions are expected.

Stimuli-responsive luminescent supramolecular polymers based on hydrogen bonding: molecular fabrication, phase structure, and controllable-rewritable behavior

Stimuli-responsive luminescent supramolecular polymers were synthesized via hydrogen bonding, which could be further used to build controllable, rewritable paper.