Pathways to tough yet soft materials Bosnjak, Nikola; Silberstein, Meredith N
Science (American Association for the Advancement of Science),
2021-Oct-08, 2021-10-08, 20211008, Volume:
374, Issue:
6564
Journal Article
Peer reviewed
Chemical and physical strategies allow polymer chains in hydrogels to extend or slip.
The elastic–plastic behavior of the polymer electrolyte membrane (PEM) Nafion is characterized via monotonic and cyclic uniaxial tension testing as a function of strain rate, temperature, and ...hydration. Dynamic mechanical analysis shows that, under dry (30%RH) conditions, the material begins to transition from the glassy to the rubbery state at 75
°
C, with a glass transition of 105
°
C. DMA reveals the fully hydrated state to be significantly more compliant than the dry state, with a glass transition beginning at 40
°
C. Large strain monotonic tensile tests find the rate-dependent stress–strain behavior to be highly dependent on temperature and hydration. The dry state transitions from an elastic–plastic behavior at 25
°
C to an increasingly more compliant behavior and lower yield stress as temperature is increased through the glass transition, until exhibiting a rubbery-like behavior at 100
°
C. At 25
°
C, the stress–strain behavior remains elastic-plastic for all hydrated states with the stiffness and yield stress decreasing with increasing hydration. Increasing hydration at all temperatures acts to decrease the initial elastic stiffness and yield stress. Unloading from different strains reveals the elastic-plastic nature of the behavior even for the elevated temperature and hydrated states. Cyclic loading-unloading-reloading excursions to different strains show significant nonlinear recovery at all strains past yield with a highly nonlinear reloading behavior which rejoins the initial loading path. A micromechanically motivated constitutive model consisting of an intermolecular resistance in parallel with an elastic network resistance is shown to be capable of capturing the rate, temperature, and hydration dependence of the monotonic stress-strain behavior. The intermolecular resistance captures the local intermolecular barriers to initial elastic deformation and also captures the thermally activated nature of yield; these intermolecular barriers are modeled to decrease with increasing temperature and hydration, in particular mimicking the reduction in these barriers as the material approaches and enters the glass transition regime, successfully capturing the strong temperature and hydration dependence of the stress-strain behavior. The highly nonlinear post-yield unloading and reloading suggest the development of a back stress during inelastic deformation which aids reverse plastic flow during unloading. Inclusion of a back stress which saturates after reaching a critical level provides an ability to capture the highly nonlinear cyclic loading stress response. Hence, the proposed model provides the capability to describe the complex evolution of stress and strain that occurs in PEM membranes due to the constrained hygrothermal cyclic swelling/deswelling characteristic of membranes in operating fuel cells.
The mechanical properties of polymers are highly dependent on the mobility of the underlying chains. Changes in polymer architecture can affect inter‐ and intramolecular interactions, resulting in ...different chain dynamics. Herein, an enhancement in the mechanical properties of poly(butylmethacrylate) is induced by folding the polymer chains through covalent intramolecular crosslinking (CL). Intramolecular CL causes an increase in intramolecular interactions and inhibition of intermolecular interactions. In both the glassy and rubbery states, this molecular rearrangement increases material stiffness. In the glassy state, this molecular rearrangement also leads to reduced failure strain, but surprisingly, in the rubbery state, the large strain elasticity is actually increased. An intermediate intramolecular CL degree, where there is a balance between intra‐ and intermolecular interactions, shows optimal mechanical properties. Molecular dynamics simulations are used to confirm and provide molecular mechanisms to explain the experimental results.
Balancing intra‐ and intermolecular interactions, intramolecular collapse enhances polymer stiffness and strength especially at an intermediate point where intra‐ and intermolecular interactions are balanced. In the rubbery state, highly stretchable thermoplastic plastics are obtained, reaching over 1400% strain at break. Molecular dynamics simulations support the experimental results and explain the effect of chain folding to bulk properties.
Elastomers are used in a wide range of applications because of their large strain to failure, low density, and tailorable stiffness and toughness. The mechanical behavior of elastomers derives mainly ...from the entropic elasticity of the underlying network of polymer chains. Elastomers under large deformation experience bonds breaking within the backbone chains that constitute the polymer network. This breaking of chains damages the network, can lead to material failure, and can be utilized as an energy dissipation mechanism. In the case of reversible bonds, broken chains may reform and heal the damage in the network. If the reversible bonds are dynamic, chains constantly break and reform and create a transient network. A fundamental constitutive theory is developed to model the mechanics of these polymer networks. A statistical mechanical derivation is conducted to yield a framework that takes in an arbitrary single-chain model (a Hamiltonian) and outputs the following: the single-chain mechanical response, the breaking and reforming kinetics, the equilibrium distribution of chains in the network, and the partial differential equations governing the deformation-coupled network evolution. This statistical mechanical framework is then brought into the continuum scale by using macroscopic thermodynamic constitutive theory to obtain a constitutive relation for the Cauchy stress. The potential-supplemented freely jointed chain (uFJC) model is introduced, and a parametric study of its mechanical response and breaking kinetics is provided. This single-chain model is then implemented within the constitutive framework, which we specialize and apply in two exemplary cases: the mechanical response and irreversible breakdown of a multinetwork elastomer, and the mechanical response of a dual crosslink gel. After providing a parametric study of the general constitutive model, we apply it to a hydrogel with reversible metal-coordination crosslinks. In several cases, we find that the breakdown of the network causes secondary physical mechanisms to become important and inhibit the accuracy of our model. We then discuss these mechanisms and indicate how our existing framework can be adjusted to incorporate them in the future.
Mechanochromic functionality realized through force-responsive molecules (i.e., mechanophores) has great potential for spatially localized damage warning in polymers. However, in structural plastics, ...for which damage warning is most critical, this approach has had minimal success because brittle failure typically precedes detectable color change. Herein, we report on the room-temperature mechanochromic activation of spiropyran in high T g bisphenol A polycarbonate. The mechanochromic functionality was introduced by polymerization of dihydroxyspiropyran as a comonomer while retaining the excellent thermomechanical properties of the polycarbonate. The mechanochromic behavior is thoroughly evaluated in response to changes in stress, deformation, and time, providing new insights regarding how loading history controls stress accumulation in polymer chains. In addition, a new method to incorporate mechanochromic functionality in structures without dispersing costly mechanophores in the bulk is demonstrated by using a mechanochromic laminate. The room-temperature mechanochromic activation in a structural polymer combined with the new and efficient preparation and processing methods bring us closer to the application of mechanochromic smart materials.
Non-woven materials are fiber networks consolidated by different bonding techniques. It has been found that interfiber bond fracture is a major damage mechanism in non-wovens and strongly affects ...mechanical strength and toughness. In this article, we present a micromechanically based damage model for non-wovens. The model is built upon modeling single bond breaking processes and linking local damage events to macroscopic behaviors. In this model, a nonlinear term is introduced to describe non-affine deformation of fibers at a bond. The traction load on a bonded interface is determined by considering local force balance and network constraints. A bond breaks when its traction load exceeds a critical value, and this local information is used to update the global damage state through a classical continuum damage mechanics framework. Spatial correlation of damage in the network is modeled using a non-local averaging scheme. The proposed model is applied to a commercial non-woven series. The model is able to reproduce experimentally observed behaviors including elasticity, non-linear hardening, peak load and damage localization under uniaxial tensile loading as a function of network density. Damage states predicted by the numerical simulations match well with in-situ imaging results demonstrating the predictive capability of the model. The proposed model bridges non-woven microstructure and macroscopic behaviors and thus can serve as an effective tool for future studies of the mechanics of fiber networks.
Polymer multifunctionality can be designed through the incorporation of chemical groups termed “mechanophores” that have a specific chemical transformation in response to applied force. The behavior ...of mechanophore-linked polymers depends on synthetic factors such as the choice of the mechanophore, the polymer chemistry, and the mechanophore linking architecture and on externally imposed factors such as temperature, loading mode, and loading rate. While many papers have explored changing polymer architecture, relatively few have systematically looked at these external factors, particularly temperature and loading mode. These external factors are critical for practical application of mechanophore-linked polymers, particularly for damage detection in engineering materials. Here, we use a single synthetic system to quantify the influence of these externally imposed factors. In particular, the mechanophore spiropyran (SP) is covalently bonded into lightly cross-linked poly(methyl methacrylate) (PMMA). SP is a mechanophore that has a distinct color and fluorescence change when activated through force to the merocyanine state, making it ideal for in situ studies. We monitor and analyze the full field fluorescence of SP-PMMA samples during mechanical loading under tension and compression, over 3 decades of strain rate, and over a 60 °C range in temperature. Typical SP mechanoactivation response exhibits three distinct regimes: minimal change through yield, followed by rapid intensity increase, and approach to a steady state. Stress has a strong influence on the rate of increase in SP activation, where stress increase by temperature decrease or strain rate increase substantially raises the SP activation rate. Uniaxial compression displays a qualitatively similar response to that of uniaxial tension. However, a longer flat region is observed in the case of compression as compared to tension corresponding with the larger yield strain.
Leakage and accumulation of highly stable commercial plastics has led to substantial contamination of the environment. Highly isotactic poly(propylene oxide) (iPPO) was investigated as a potential ...high-strength thermoplastic with greater susceptibility toward degradation under ambient conditions. Various stereoregular forms of iPPO including enantiopure, enantioenriched, racemic, and stereoblock were synthesized with a single catalyst architecture in the presence of chain transfer agents. These materials were found to possess the same approximate ultimate tensile strength (UTS) via uniaxial tensile elongation analysis (∼75 MPa). A serrated tensile response corresponding to stress oscillations was observed in all forms of iPPO. An investigation on strain rate dependence showed that an increase in strain rate results in the decay and disappearance of the serrated response. Further evaluation of iPPO revealed its dramatic strain hardening afforded an UTS comparable to that of nylon-6,6. Exposing iPPO to UVA light (365 nm) resulted in photolytic degradation. Following 30 days of continuous exposure at 250 μW cm–2, the M n decreased from 93 kDa to 21 kDa, while samples not exposed to UVA light remained unchanged. Through selective stabilization with antioxidant additives, we believe iPPO could be a suitable replacement for nylon-6,6 in environmentally susceptible applications.
Chemical cross-linking of polymer chains is a powerful means for tailoring the thermomechanical properties of bulk plastics. Nonetheless, upon cross-linking, processability is reduced as the plastic ...becomes thermoset. Here, molecular dynamics simulations are used to study the effects of intramolecular chemical cross-linking on chain topology and thermomechanical properties of a bulk, thermoplastic polymer. Polyethylene (PE)-like plastics are assembled purely from chains which have undergone a set level of intrachain cross-linking (to form single chain polymer nanoparticles, SCPNs). We have analyzed the chain topology at an equilibrated state in terms of chain unfolding and entanglement by radius of gyration and primitive path, respectively. The extents of both chain unfolding and chain entanglement were found to decrease with increasing intrachain cross-linking ratio. By applying simulated cooling, uniaxial tension, and uniaxial compression, we characterized the thermomechanical properties at the glassy state. The simulated mechanical testing shows that the bulk polymer becomes stiffer, stronger, and more brittle as the intrachain cross-linking ratio is increased. We observe that the failure of the SCPN-based bulk polymers is a consequence of separation between SCPNs. This study successfully elucidates the effect of intramolecular cross-linking on the thermomechanical properties at bulk, as a clear correlation is shown between the amount of covalent intrachain collapse and interchain interactions.
Mechanochemistry can lead to the degradation of the properties of covalent macromolecules. In recent years, numerous functional materials have been developed based on block copolymers (BCPs), ...however, like homopolymers, their chains could undergo mechanochemical damage during processing, which could have crucial impact on their performance. To investigate the mechanochemical response of BCPs, multiple polymers comprising different ratios of butyl acrylate and methyl methacrylate were prepared with similar degree of polymerization and stressed in solution via ultrasonication. Interestingly, all BCPs, regardless of the amount of the methacrylate monomer, presented a mechanochemistry rate constant similar to that of the methacrylate homopolymer, while a random copolymer reacted like the acrylate homopolymer. Size‐exclusion chromatography showed that, in addition to the typical main peak shift towards higher retention times, a different daughter fragment was produced indicating a secondary selective scission site, situated around the covalent connection between the two blocks. Molecular dynamics modeling using acrylate and methacrylate oligomers were carried out and indicated that dynamic phase separation occurs even in a good solvent. Such non‐random conformations can explain the faster polymer mechanochemistry. Moreover, the dynamic model for end‐to‐end chain overstretching supports bond scission which is not necessarily chain‐centered.
Unexpected mechanochemistry kinetics and selective off‐center scission behavior is shown in linear block copolymers (BCPs) stressed in solution. Molecular dynamics modeling suggests a dynamic phase separation of the blocks inducing the unusual distribution of applied forces, and promoting force focusing at the interface.
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