Elastomeric Networks Research Articles

Extrudable Covalently Cross‐Linked Thio‐Urethane Liquid Crystalline Elastomers

AbstractLiquid crystalline elastomer networks cross‐linked by dynamic covalent bonds (xLCEs) possess the remarkable capability of being (re)processed during plastic flow at high temperatures, while also allowing for the (re)programming of their local alignment. However, the current generation of xLCEs cannot be extruded directly into filaments or injection‐molded into shapes capable of actuation. Here, a novel poly(thio‐urethane) (PTU) LCE thermoset is presented. The inclusion of dibutyltin dilaurate as a catalyst enables the manipulation of the elastic–plastic transition through two distinct pathways. The dissociative equilibrium exchange between the thio‐urethane linkage to form separate thiol and isocyanate competes with the associative exchange of thio‐urethane and a free isocyanate or thiol groups. The temperature of fast plastic flow onset is ≈160 ˚C, and the associated bond‐exchange activation energy is ≈54 kJ mol−1. The hydrogen bonding below the elastic‐plastic transition makes the elastomer robust and extremely ductile with a failure strain exceeding 600% and a Shore hardness A of 72, preventing any creep during cyclic thermal actuation. The versatility of this dynamically cross‐linked elastomer is successfully demonstrated by extruding it using a commercial twin‐screw extruder, injection molding it into required shapes, and hot‐pressing it into flat films or shapes.

Impact of amine functionalization on poly(furfuryl alcohol) properties

Polyfurfuryl alcohol (PFA) is a biobased thermoset with high stiffness. As carbonyls are formed via furan ring opening during polymerization, their inevitable presence was exploited to tune the PFA properties. Various diamines were employed to react with these carbonyls and consequence on thermal and mechanical properties were evaluated. Short and rigid diamines and long and flexible diamines were respectively selected for this functionalization. The more rigid amines lead to an increase of the Tg of the material compared to the pristine PFA but the ultimate properties were not particularly improved. Flexible polyetheramines such as Jeffamine D-2000 (J2000) combined with PFA allows the formation of a flexible elastomeric network (Tg = −44 °C) presenting high thermal stability due to the presence of the thermally stable furanic chains. For PFA/J2000 samples, a rigid crust is formed in oxidative conditions at T > 175 °C when the PFA content is relatively important (>50 % w/w). This phenomenon is massively impacting the ultimate mechanical properties and the thermal stability.

Introducing "MEW2" Software: A Tool to Analyze MQ-NMR Experiments for Elastomers.

Low-field time-domain proton Nuclear Magnetic Resonance (NMR) spectroscopy is an attractive and powerful tool for studying the structure and dynamics of elastomers. The existence of crosslinks and other topological constraints in rubber matrices (entanglements and filler-rubber interactions, among others) renders the fast segmental fluctuations of the polymeric chains non-isotropic, obtaining nonzero residual dipolar couplings, which is the main observable of MQ-NMR experiments. A new software, Multiple quantum nuclear magnetic resonance analyzer for Elastomeric Networks v2 (MEW2), provides a new tool to facilitate the study of the molecular structure of elastomeric materials. This program quantitatively analyzes two different sets of experimental data obtained in the same experiment, which are dominated by multiple-quantum coherence and polymer dynamics. The proper quantification of non-coupled network defects (dangling chain ends, loops, etc.) allows the analyzer to normalize the multiple quantum intensity, obtaining a build-up curve that contains the structural information without any influence from the rubber dynamics. Finally, it provides the spatial distribution of crosslinks using a fast Tikhonov regularization process based on a statistical criterion. As a general trend, this study provides an automatic solution to a tedious procedure of analysis, demonstrating a new tool that accelerates the calculations of network structure using 1H MQ-NMR low-field time-domain experiments for elastomeric compounds.

A crosslinked waterborne poly(vinyl acetate) for greenhouse gas fixation with improved elastomeric properties, shape-memory ability, and recyclability

The development of raw materials from methane and carbon dioxide holds significant potential to cut greenhouse gas (GHG) emissions and reduce the global carbon footprint. As an important monomer, vinyl acetate (VAc) can be produced by the consumption of acetic acid, which will potentially contribute to the reduction of GHGs emission. However, polymers based on VAc are usually prepared from emulsion polymerization in water which brings about the challenges of using modification processes to improve their properties. Here, a newly developed crosslink was able to establish covalently adaptable networks directly in water, enabling the formation of a poly(vinyl acetate) (PVAc) composite. As a result, PVAc emulsions using a monomer of 100% VAc could be transformed from a soft and weak adhesive polymer into an elastomeric polymer composite network with an elongation (775%) at break and flexibility. With the enhanced rubbery modulus, the polymer possesses characteristics for shape-memory material applications with 93% shape fixity and 99% shape recovery after repeated deformation and shape fixity for three times. The recyclability of the PVAc thermosets was demonstrated after granulating and remolding three times, representing an opportunity for designing a greenhouse gas fixing material.

Stretchable, Strong, Recyclable Helicide Elastomer Based on Dynamic Covalent Interactions.

Current methods for making and disposing synthetic polymers have been widely pursued and are largely unsustainable. As a part of the solution, the reversible nature of dynamic covalent bonds emerges as an extraordinarily diverse and valuable feature in the development of exotic molecules and extended structures. With these bonds, it should be possible to construct recyclable and mechanically interlocked molecular structures using relatively simple precursors with preorganized geometries. A new helicide-based elastomer network is developed here with self-healing, recycling, and degradation features using a similar concept. The best self-healing performance (100%) was noted over 10-20 min, with various H2O, HCl, and NaOH solutions that delivered mechanical properties in the 1-1.4 MPa range. For hydrolytic degradation, the parameters are defined based on the type of binding, the pH of the solutions, and the copolymer network, which endowed a degradation time of approximately 4-11 h for each prepared sample. However, due to the reversible nature of the dynamic bonds, the material showed good recyclable mechanical properties compared to the pristine samples after five consecutive cycles, which meet the requirements of recyclable materials and recyclable packaging.

Correlating Nanomechanical Mapping and Single-Molecule Localization Microscopy for Local Properties in Interpenetrating Network Elastomers

Correlating Nanomechanical Mapping and Single-Molecule Localization Microscopy for Local Properties in Interpenetrating Network Elastomers

Mechanochromic Response of Spiropyran‐Incorporated Liquid Crystalline Elastomer Network and the Mechanochromic Enhancement by Nano Zinc Oxide

AbstractSpiropyran (SP) based mechanochromic systems are intensively interesting in the research area of mechanochromic materials. Herein, an SP mechanophore is chemically incorporated into a liquid crystalline elastomer (LCE) network through Michael addition. The rigidity of mesogens and the orientation of polymer main chains, in which the aligned mesogens and SP moieties are covalently attached, facilitate the transduction of applied force to SP mechanophore to trigger mechanochromic reaction. The LCE network provides enough elasticity for mechanochromic activation. The obtained SP‐LCEs demonstrate strong and reversible mechanochromic response upon mechanical compression. The mechanochromic efficiency and sensitivity are substantially enhanced by doping the SP‐LCE matrix with nano zinc oxide such that it is easy to write on the material surface by using a thin iron wire. The authors hope such SP‐LCE materials will be further developed for real‐time stress detection applications.

Programming Liquid Crystalline Elastomer Networks with Dynamic Covalent Bonds

AbstractA liquid crystalline elastomer (LCE) network consisting of dynamic covalent bonds (DCBs) is referred as a LCE vitrimer. The mesogen alignment and the network topology can be reprogrammed locally in the LCE vitrimer by activating the bond exchange reactions using an external stimulus. After removal of the external stress, a new network is formed and the reprogrammed shape can be fixed, leading to a different set of the physical properties of the LCE vitrimers. Herein, this type of emerging materials is reviewed by a brief introduction of the fundamentals of LCEs, followed by discussions of various DCBs and the design principles for LCE vitrimers. After a presentation of different strategies to improve the stability and reprogrammability of the registered mesogen alignment, approaches to prepare LCE vitrimers with complex shapes and their actuations are discussed. Potential applications such as self‐healing and recycling, mechanochromic effects, and post‐functionalization of nanopores are also reviewed, followed by the conclusion of the remaining challenges and opportunities.

Elastomer Nanocomposites: Effect of Filler-Matrix and Filler-Filler Interactions.

The reinforcement of elastomers is essential in the rubber industry in order to obtain the properties required for commercial applications. The addition of active fillers in an elastomer usually leads to an improvement in the mechanical properties such as the elastic modulus and the rupture properties. Filled rubbers are also characterized by two specific behaviors related to energy dissipation known as the Payne and the Mullins effects. The Payne effect is related to non-linear viscoelastic behavior of the storage modulus while the Mullins or stress-softening effect is characterized by a lowering in the stress when the vulcanizate is extended a second time. Both effects are shown to strongly depend on the interfacial adhesion and filler dispersion. The basic mechanisms of reinforcement are first discussed in the case of conventional rubber composites filled with carbon black or silica usually present in the host matrix in the form of aggregates and agglomerates. The use of nanoscale fillers with isotropic or anisotropic morphologies is expected to yield much more improvement than that imparted by micron-scale fillers owing to the very large polymer-filler interface. This work reports some results obtained with three types of nanoparticles that can reinforce rubbery matrices: spherical, rod-shaped and layered fillers. Each type of particle is shown to impart to the host medium a specific reinforcement on account of its own structure and geometry. The novelty of this work is to emphasize the particular mechanical behavior of some systems filled with nanospherical particles such as in situ silica-filled poly(dimethylsiloxane) networks that display a strong polymer-filler interface and whose mechanical response is typical of double network elastomers. Additionally, the potential of carbon dots as a reinforcing filler for elastomeric materials is highlighted. Different results are reported on the reinforcement imparted by carbon nanotubes and graphenic materials that is far below their expected capability despite the development of various techniques intended to reduce particle aggregation and improve interfacial bonding with the host matrix.

Peroxide‐based crosslinking of solid silicone rubber, part II: The counter‐intuitive influence of dicumylperoxide concentration on crosslink effectiveness and related network structure

AbstractApplication of elastomers in general demands the conversion of their soluble networks into crosslinked structures. This abrupt change causes several modifications, both in the atomic/molecular level and at the macro‐scale. In this study, solid silicone rubber (high molecular weight poly(dimethylsiloxane)), was crosslinked with dicumylperoxide (DCP), a widely used crosslinking agent by the rubber industry. The changes caused by different DCP concentrations were investigated, aiming to bring attention to the molecular transformations, usually neglected when processing‐oriented studies are conducted. DCP concentration showed a limited contribution to the network's molecular dynamics, which was found to be mainly dominated by entanglements. The dominance of entanglements over other molecular constraints, like crosslink points, justifies the threshold and counter‐intuitive behavior of tensile and hardness properties. However, differences were found in the crystallization ability after crosslinking, when the more crosslink points were introduced, the lower the crystallinity was and the less stable the PDMS crystallites were. In addition to providing a deeper understanding of an industrially applied rubber system n terms of the effective concentration of DCP, and the reasoning behind such concentration, the findings of this study add to the state‐of‐the‐art comprehension of elastomeric networks, and how they behave on a molecular level.

Finite element implementation of a gradient-damage theory for fracture in elastomeric materials

We present a finite element implementation procedure for a phase-field framework for fracture in elastomeric materials based on the gradient-damage theory. Governing equations of macroscopic and microscopic force balances, and constitutive theories for large elastic deformation and damage are summarized, and the computational implementation is described in significant detail. To facilitate the computational implementation of the gradient-damage theory for elastomeric materials in a widely available finite element program, the source codes are provided as online Supplemental Materials to this paper. Furthermore, we provide a comparative study of the gradient-damage models with two distinct driving forces for damage: (1) entropy-driven and (2) internal energy-driven. We then show that the internal energy-driven damage model presents more realistic descriptions of the failure that accompanies extreme stretching and scission in elastomeric networks.

Dynamic and Degradable Imine‐Based Networks for 3D‐Printing of Soft Elastomeric Self‐Healable Devices

AbstractSelf‐healable degradable networks encounter a growing popularity for biomedical applications due to their ability to recover their properties after damage. Self‐healable hydrogels dominate with applications in tissue engineering and drug delivery. On the opposite and despite their potential for medical devices, self‐healable elastomers remain scarce, especially if they must be compatible with fused deposition modeling (FDM) 3D‐printing and self‐heal at physiological temperature under a hydrated state. These unmet challenges are addressed in this work with degradable elastomeric networks based on dynamic imine bonds prepared from multi(aldehyde) and multi(amine) hydrophobic PEG‐PLA star‐shaped copolymers. The star topology of these copolymers is the key feature of the strategy as it allows the design of multifunctional high molecular weights pre‐polymers that ensure an efficient dynamic chemical crosslinking while guaranteeing access to the FDM process generally restricted to thermoplastics. The proposed elastomeric networks combine high self‐healing efficiencies at 37 °C (>97%) with mechanical properties compatible with soft tissues and a linear degradation profile. Their FDM processing to produce self‐healable tubular devices is demonstrated. Finally, their cytocompatibility is assessed and confirm their potential as biodegradable elastomeric networks to be used for the design of self‐healable 3D‐printed devices for biomedical applications.

A general continuum damage model for soft composites

Soft composites, including filled rubbers, nanocomposite gels, double network hydrogels, and multiple network elastomers, all exhibit stress-softening behavior in loading–unloading cycles, also denoted as the Mullins effect. Though qualitatively this damage behavior shows similar features, the details still vary with material types. In this work, we provide a general approach towards modeling the Mullins effect for soft composites based on continuum damage mechanics. We start with formulating a one-dimensional damage model. A logarithmic damage variable is employed, while a damage potential function is adopted to obtain the evolution law for the damage variable. We then extend the one-dimensional damage model to three-dimensions. The eight-chain type micro–macro transition is used to formulate an isotropic damage model. An anisotropic damage model is constructed based on the full network model, while a new micro–macro mapping is used to determine the micro stretch from the macro deformation. With this new mapping, the full network model appears in a consistent format in that the free energy, dissipation, and stress are respectively given by the average of their directional micro counterparts. In the current approach, two constitutive functions need to be determined: the elastic free energy density and the stored energy limit. Simple forms of these two constitutive functions are provided with only a small set of model parameters. The resulting models are then successfully applied to describe the stress response in loading–unloading cycles and the anisotropic response of predeformed materials for different kinds of soft composites.

Self-Healable, Stretchable, and Super-Soft Bottlebrush Polyester Elastomers for Highly Sensitive Flexible Sensors

While much effort has been devoted to the creation of super-soft elastomers, there is a dearth of research aimed at self-healable and super-soft renewable elastomers. Herein, self-healing and super-soft renewable elastomers are developed based on bottlebrush architecture design and dynamic associative transesterification. The bottlebrush polymer precursors are generated by ring-opening metathesis polymerization (ROMP) of the norbornene-terminated macromonomer poly(ε-caprolactone-co-l-lactide) (PCLLA) (NB-PCLLA). After introducing alkyne groups onto the end of PCLLA side chains, the bottlebrush polymers are cross-linked to achieve elastomer networks via a UV-induced “click” thiol-yne reaction between the terminal alkyne groups and the dithiothreitol (DTT) additive. The resulting materials exhibit a shear modulus as low as 17 kPa, high stretchability with breaking strains as large as 315%, and a self-healing efficiency of 95.6% at an elevated temperature of 80 °C. Taking the super-softness advantage, flexible sensor devices are assembled based on the prepared elastomer, which are highly sensitive to deformation with a gauge factor of 0.347. This work demonstrates that high-performance biodegradable materials with unique properties that conventional linear polymers impossibly possess can be easily constructed by molecular architecture design using bottlebrush polymers as building blocks.

Self-Healable Elastomeric Network with Dynamic Disulfide, Imine, and Hydrogen Bonds for Flexible Strain Sensor.

Self-healable and stretchable elastomeric material is essential for the development of flexible electronics devices to ensure their stable performance. In this study, a strain sensor (PIH2 T1 -tri/CNT-3) composed of self-repairable crosslinked elastomer substrate (PIH2 T1 -tri, containing multiple reversible repairing sites such as disulfide, imine, and hydrogen bonds) and conductive layer (carbon nanotube, CNT) was prepared. The PIH2 T1 -tri elastomer had excellent self-healing ability (healing efficiency=91 %). It exhibited good mechanical integrity in terms of elongation at break (672 %), tensile strength (1.41 MPa). The Young's modulus (0.39 MPa) was close to that of human skin. The PIH2 T1 -tri/CNT-3 sensor also demonstrated an effective self-healing function for electrical conduction and sensing property. Meanwhile, it had high sensitivity (gauge factor (GF)=24.1), short response time (120 ms), and long-term durability (4000 cycles). This study offers a novel self-healable elastomer platform with carbon based conductive components to develop flexible strain sensors towards high performance soft electronics.

N-Sulfonyl Urethanes to Design Polyurethane Networks with Temperature-Controlled Dynamicity

(Re)processing of cross-linked polyurethanes (PUs) is often energy intensive and inefficient since dissociation of urethane linkages at elevated temperatures generates highly reactive isocyanate moieties that can react with a wide range of nucleophiles. In this study, we first show with a small molecule study that the introduction of N-sulfonyl urethane bonds leads to dynamic covalent exchange reactions under much milder conditions compared to regular urethane groups. Then, these exchangeable N-sulfonyl urethane motifs have been introduced, in relatively small amounts (5, 10, and 20%), in a cross-linked PU matrix in an attempt to facilitate plastic flow at lower temperatures. Rheological analysis of the elastomeric dissociative networks revealed an interesting double relaxation behavior, even for temperatures between 150 and 100 °C, which could be described by a Maxwell model with two elements, which can be related to the activated and less activated urethane bonds. Finally, the (re)processability of these sulfonyl urethanes containing PUs was demonstrated through multiple cutting and hot pressing cycles and the corresponding materials showed a good retention of thermal properties.

Vat Photopolymerization Additive Manufacturing of Tough, Fully Recyclable Thermosets.

To advance the capabilities of additive manufacturing, novel resin formulations are needed that produce high-fidelity parts with desired mechanical properties that are also amenable to recycling. In this work, a thiol-ene-based system incorporating semicrystallinity and dynamic thioester bonds within polymer networks is presented. It is shown that these materials have ultimate toughness values >16 MJ cm-3, comparable to high-performance literature precedents. Significantly, the treatment of these networks with excess thiols facilitates thiol-thioester exchange that degrades polymerized networks into functional oligomers. These oligomers are shown to be amenable to repolymerization into constructs with varying thermomechanical properties, including elastomeric networks that recover their shape fully from >100% strain. Using a commercial stereolithographic printer, these resin formulations are printed into functional objects including both stiff (E ∼ 10-100 MPa) and soft (E ∼ 1-10 MPa) lattice structures. Finally, it is shown that the incorporation of both dynamic chemistry and crystallinity further enables advancement in the properties and characteristics of printed parts, including attributes such as self-healing and shape-memory.

A statistical mechanics framework for polymer chain scission, based on the concepts of distorted bond potential and asymptotic matching

To design increasingly tough, resilient, and fatigue-resistant elastomers and hydrogels, the relationship between controllable network parameters at the molecular level (bond type, non-uniform chain length, entanglement density, etc.) to macroscopic quantities that govern damage and failure must be established. Many of the most successful constitutive models for elastomers have been rooted in statistical mechanical treatments of polymer chains. Typically, such constitutive models have used variants of the freely jointed chain model with rigid links. However, since the free energy state of a polymer chain is dominated by enthalpic bond distortion effects as the chain approaches its rupture point, bond extensibility ought to be accounted for if the model is intended to capture chain rupture. To that end, a new bond potential is supplemented to the freely jointed chain model (as derived in the uFJC framework of Buche and Silberstein (2021) and Buche et al. (2022)), which we have extended to yield a tractable, closed-form model of single chain behavior that should be amenable to continuum-level constitutive model development. Inspired by the asymptotically matched uFJC model response in both the low/intermediate chain force and high chain force regimes, a simple, quasi-polynomial bond potential energy function is derived. This bond potential exhibits harmonic behavior near the equilibrium state and anharmonic behavior for large bond stretches tending to a characteristic energy plateau (akin to the Lennard-Jones and Morse bond potentials). Using this bond potential, approximate yet highly-accurate analytical functions for bond stretch and chain force dependent upon chain stretch are established. Then, using this polymer chain model, a stochastic thermal fluctuation-driven chain rupture framework is developed. This framework is based upon a force-modified tilted bond potential that accounts for distortional bond potential energy, allowing for the derivation and subsequent calculation of the dissipated chain scission energy. The cases of rate-dependent and rate-independent scission are accounted for throughout the rupture framework. The impact of Kuhn segment number on chain rupture behavior is also investigated. The model is fit to single chain mechanical response data collected from atomic force microscopy tensile tests for validation and to glean deeper insight into the molecular physics taking place. Due to their analytical nature, this polymer chain model and the associated rupture framework can in the future be implemented in finite element models accounting for fracture and fatigue in polydisperse elastomer networks.

Influence of accelerators and curing system composition on the cure characteristics, crosslinks structure and properties of acrylonitrile‐butadiene elastomer

AbstractThe goal of this work is to study the influence of the type of vulcanization accelerator and the quantitative composition of the curing system on the cure characteristics, morphology of elastomer network, crosslink density and physico‐chemical properties of acrylonitrile‐butadiene rubber (NBR) composites. Sulfur curing systems are investigated, which consist of various types of accelerators, that is, sulfenamide, guanidine and thiuram, applied in various proportions to sulfur. The type of accelerator and the sulfur/accelerator ratio significantly affect both the cure rate and the structure and number of crosslinks resulting from the vulcanization. The highest cure rate and the lowest content of polysulfidic crosslinks are exhibited by rubber composites with efficient (EV) curing systems due to the highest content of accelerators compared to semi‐efficient (SEV) and conventional (CV) systems. The structure of crosslinks strongly affect tensile properties and hardness of the vulcanizates as well as their solvent resistance, thermal stability and resistance to thermo‐oxidative aging.

Engineering Tridimensional Hydrogel Tissue and Organ Phantoms with Tunable Springiness

AbstractBiomimicking organ phantoms with vivid biological structures and soft and slippery features are essential for in vitro biomedical applications yet remain hither to unmet challenges in their fabrication such as balancing between spatial structural complexity and matchable mechanical properties. Herein, 3D printable tissue‐mimicking elastomeric double network hydrogels with tailorable stiffness are evolved to idiosyncratically match diverse biological soft tissues by regulating the compositions of hydrogel matrix and the density of metal coordination bonds. Relying on digital light processing 3D printing, various mechanically tunable biomimetic volumetric hydrogel organ constructs with structural complexity and fidelity, including kidney, brain, heart, liver, stomach, lung, trachea, intestine, and even the intricate vascularized tissues, are fabricated faultlessly. Proof‐of‐concept 3D printed hydrogel heart and liver phantoms provide sophisticated internal channels and cavity structures and external realistic anatomical architectures that more closely mimic native organs. For the in vitro application demonstration, a 3D printed hydrogel brain phantom with tortuous cerebral arteries and slippery characters serves as an effective neurosurgical training platform for realistic simulation of endovascular interventions. This platform offers a means to construct mechanically precisely tunable hydrogel‐based biomimetic organ phantoms that are expected to be used in surgical training, medical device testing, and organs‐on‐chips.