Tough Polymer Research Articles

Modeling of toughening effect in rigid particulate-filled polymer composites by artificial intelligence: a review

Low fracture toughness is the Achilles heel of many polymers for practical application, particularly for long-term and special applications such as medical implants and critical part design. The toughening of polymers is a complicated process, and the toughening mechanisms in many composite systems have not yet been fully understood. Machine learning (ML) has emerged as a promising tool for modeling various complex systems in the field of materials science. In this respect, ML appears to be an effective, affordable, and accurate technique in the modeling of the toughening process. This review article provides an overview of the recent ML-based approach for analyzing toughening effects in rigid particulate-filled polymer composites. The results summarized here would contribute to further possibilities for researchers to explore emerging trends for designing toughing and high-performance polymers.

Assessing the effect of the experimental parameters in the evaluation of the essential work of fracture in high-strength thin sheets

The essential work of fracture methodology (EWF) has been successfully adopted to evaluate the fracture toughness of various metals and polymers. However, some aspects of the methodology are still far less understood, such as the influence of the experimental parameters on EWF measurement in thin metal sheets. In the present paper, the ligament range criterion of the EWF approach was revised for several advanced high-strength steels (AHSS). The validity of the upper and lower ligament length limits given by the ESIS protocol is redefined and rationalized according to the necking capability and the plasticity behaviour of the different AHSS grades. The work provides a new criterion to define the minimum ligament length to be tested, based on the minimum distance required by the crack to fully develop the necking capability of the material. The width constraint is too restrictive and has no effect on the deviation from linearity in the upper range. On the other hand, the maximum ligament length is proven to be controlled by the size of the plastic zone as proposed by the ESIS protocol.

Tough and redox-mediated alkaline gel polymer electrolyte membrane for flexible supercapacitor with high energy density and low temperature resistance

To meet the practical application requirements of the flexible supercapacitors, the improvements of the toughness and energy density attract more and more attention. Herein, a physically cross-linked double network alkaline gel polymer electrolyte (AGPE) membrane based on PVA and kappa-carrageenan is developed. In this AGPE membrane, KOH plays a dual role including carrier donator and ion cross-linker, which makes the AGPE membrane show the unique improvement of ionic conductivity (0.31 S/cm) and mechanical properties (tensile strength of 1.62 MPa and tensile elongation of 670%) at the same time. In order to improve energy density, the redox-active mediator p-phenylene diamine has been introduced in the AGPE membrane, endowing the fabricated supercapacitor with high electrode specific capacitance (741 F/g) and energy density (18.3 Wh/kg) with a maximum power density of 800 W/kg. Meanwhile, the supercapacitor can exhibit great flexibility and toughness, and maintain stable capacitance performance under bending and stretching state. Moreover, the supercapacitor shows an excellent low temperature resistance and it is able to maintain 82% of its room-temperature capacitance even at -40 °C. This investigation offers a strategy to prepare gel polymer electrolyte membrane with high adaptive ability of deformation and low temperature, which has potential application in various flexible, portable and wearable electronics. • Dual role KOH can improve conductivity and mechanical properties simultaneously. • PPD can signally increase the capacitance and energy density of EDLC supercapacitor. • Supercapacitor shows ideal electrochemical properties under deformation and coldness.

Synergistic toughening of polypropylene by thermoplastic starch acetate andSEBS‐MAH

AbstractThe plasticized starch exhibits flexible properties, similar to that of elastomer. In this paper, thermoplastic starch acetate (TPAS) and maleic anhydride grafted poly[styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene] (SEBS‐MAH) were used to toughen polypropylene (PP) through melt blending. It is found TPAS and SEBS‐MAH have a synergistic toughening effect on PP, especially the notched impact strength of PP/TPAS/SEBS‐MAH (70/15/15) blend is as high as 83.4 kJ/m2, but only 7.2 kJ/m2of that for PP/TPAS/SEBS (70/15/15) blend. The structure‐property relationship of the PP/TPAS/SEBS‐MAH blends was investigated. Compared with PP/TPAS/SEBS (70/15/15) blend, the TPAS domains are dispersed homogeneously in the PP/TPAS/SEBS‐MAH (70/15/15) blend with the domain size of TPAS less than 0.2 μm. The incorporation of SEBS‐MAH not only improves the compatibility between PP and TPAS but also facilitates the formation of a core‐shell structure with TPAS as the core and SEBS‐MAH as the shell. Thus formed core‐shell particles play an important role in toughening PP. The spreading coefficient of the multiphase blend reveals that only when the SEBS‐MAH content is higher than TPAS, the core‐shell structure can be formed. Otherwise, TPAS and SEBS‐MAH will disperse separately and the toughness will not be improved. This work provides a new insight into polymer toughening with TPAS.

Tough polymer with a gradual spatial change in the hydrogen bond density controlled by simple one-pot copolymerization

We synthesized multiphase copolymers with a gradual change in hydrogen bond (H-bond) density at the phase boundary via simple one-pot copolymerization of two monomers. The copolymers had a gradient distribution of H-bonding groups along the chain due to the reaction rate difference between the two monomers. The gradient copolymers formed a microphase-separated structure consisting of hard phases surrounded by a soft phase, with a gradual change in monomer composition between the two phases. The mechanical properties of the copolymers depended on the total number of H-bonding units introduced in a chain; that is, more H-bonding groups led to higher rigidity and toughness, while fewer H-bonding groups contributed to better fatigue recoverability. The toughest polymer obtained in this study exhibited excellent stress at break (∼23 MPa) and toughness (∼38 MJ m−3) thanks to the efficient energy dissipation by H-bonds in the soft phase. We demonstrated that the simple one-pot copolymerization method could reproduce the gradual compositional change and multiphase structure, which provided a new design principle for tough polymers.

Small multiamine molecule enabled fire-retardant polymeric materials with enhanced strength, toughness, and self-healing properties

The combination of high fire retardancy, high strength, and great toughness as well as good healability is essential for successful real-world applications of polymeric materials in the fields of packaging, electronics & electrics, and optical devices. To date, there have been few successes in achieving such performance portfolios in polymers due to their different and even mutually exclusive governing mechanisms. Inspired by the nanoconfinement effect that governs the unique mechanical properties of spider silk, we, herein, rationally design a multifunctional small molecule, HCPA, that can serve as a fire retardant and hydrogen-bond crosslinker for poly(vinyl alcohol) (PVA). Benefiting from the dual-phase fire-retardancy effect and the dynamic cross-linking effect, the addition of 5.0 wt% of HCPA enables PVA to achieve a desired self-extinguishment in combination with a high tensile strength of 133 MPa and a toughness of 112 MJ/m3. In addition, the as-prepared polymer material exhibits a high healing efficiency of over 90% (based on strength) if triggered by water. This proof-of-concept opens numerous opportunities for the creation of self-extinguishing, strong, tough, and self-healing polymers for many high-end applications in the above-mentioned industries.

Trapping probabilities of multiple rings in end-linked gels

To create a tough polymer network material with rings as a movable crosslink, it is important to prepare materials in which multiple linear chains penetrate a ring or a ring complex. The ring size is an important factor in realizing novel mechanical properties. Establishment of close correspondence between simulations and experiments is effective for selecting a target ring polymer for synthesis. The penetration of a ring by a linear chain in a mixed system containing rings and linear chains can be evaluated using the trapping probability of rings in the end-linked gel. By performing coarse-grained molecular dynamics (CGMD) simulations of the Kremer–Grest model corresponding to poly(dimethylsiloxane) (PDMS) chains, we directly clarified the relationship between the penetration and trapping probability of rings in the end-linked gel. In addition, we introduced the concept of the ring–bridge probability, which is the probability that multiple chains penetrate a ring or a ring complex. The dependence of the trapping probability and ring–bridge probability on the ring size were investigated. To establish a correspondence between ring complexes, such as bonded rings and spiro-multicyclic polymers, a mixed system consisting of complexes of bonded three-rings and linear chains was evaluated through the CGMD simulations and confirmed via PDMS experiments. A close correspondence between the experimental and simulation results enables the prediction of the trapping and bridge probabilities of spiro-multicyclic polymers. The findings illustrate that simulation-based prediction can substantially contribute to the development of polymer network materials with ring–linear blends.

Cryo-Electron Tomography of Highly Deformable and Adherent Solid-Electrolyte Interphase Exoskeleton in Li-Metal Batteries with Ether-Based Electrolyte.

The 3D nanocomposite structure of plated lithium (LiMetal ) and solid electrolyte interphases (SEI), including a polymer-rich surficial passivation layer (SEI exoskeleton) and inorganic SEI "fossils" buried inside amorphous Li matrix, is resolved using cryogenic transmission electron microscopy. With ether-based DOLDME-LiTFSI electrolyte, LiF and Li2 O nanocrystals are formed and embedded in a thin but tough amorphous polymer in the SEI exoskeleton. The fast Li-stripping directions are along or , which produces eight exposed {111} planes at halfway charging. Full Li stripping produces completely sagging, empty SEI husks that can sustain large bending and buckling, with the smallest bending radius of curvature observed approaching tens of nanometers without apparent damage. In the 2nd round of Li plating, a thin LiBCC sheet first nucleates at the current collector, extends to the top end of the deflated SEI husk, and then expands its thickness. The apparent zero wetting angle between LiBCC and the SEI interior means that the heterogeneous nucleation energy barrier is zero. Due to its complete-wetting property and chemo-mechanical stability, the SEI largely prevents further reactions between the Li metal and the electrolyte, which explains the superior performance of Li-metal batteries with ether-based electrolytes. However, uneven refilling of the SEI husks results in dendrite protrusions and some new SEI formation during the 2nd plating. A strategy to form bigger SEI capsules during the initial cycle with higher energy density than the following cycles enables further enhanced Coulombic efficiency to above 99%.

Soluble, transparent and heat-resistant fluorinated copoly(ether imide)s containing pyridyl and biphenyl units in the main chain

A series of new fluorinated copoly(ether imide)s containing pyridyl and biphenyl moieties were prepared by thermal imidization of poly(amic acid)s derived from 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (BATB), 4-(4-trifluoromethylphenyl)-2,6-bis(4-aminophenyl)pyridine (TFMPBPP) and 3,3’,4,4’-biphenyltetracarboxylic dianhydride with various mole ratios of BATB and TFMPBPP ranging from 80/20 to 20/80. All copoly(ether imide)s were amorphous and soluble in polar solvents such as N,N-dimethylacetamide and N-methyl-2-pyrrolidone at room temperature or upon heating at 70 °C, and displayed good thermal properties with gradually increasing of a glass transition temperature ranged from 282 to 347 °C as an increase of the TFMPBPP component, the temperature at 5% weight loss of 574–586 °C, and the residue of 57–62% at 750 °C in nitrogen. Tough and flexible polymer films also exhibited outstanding mechanical properties with tensile strengths of 109.1–111.9 MPa, tensile moduli of 1.3–1.5 GPa, and elongations at break of 27.9–48.7%, low dielectric constants of 2.98–3.15 (1 MHz) and water uptake 0.36–0.55%, as well as high optical transparency with the UV-vis cutoff wavelength in the 370–382 nm range and the wavelength of 80% transparency in the range 518–550 nm.

Controlling toughness of polymer-grafted nanoparticle composites for impact mitigation.

Toughness in an entangled polymer network is typically controlled by the number of load-bearing topological constraints per unit volume. In this work, we demonstrate a new paradigm for controlling toughness at high deformation rates in a polymer-grafted nanoparticle composite system where the entanglement density increases with the molecular mass of the graft. An unexpected peak in the toughness is observed right before the system reaches full entanglement that cannot be described through the entanglement concept alone. Quasi-elastic neutron scattering reveals enhanced segmental fluctuations of the grafts on the picosecond time scale, which propagate out to nanoparticle fluctuations on the time scale 100s of seconds as evidenced by X-ray photon correlation spectroscopy. This surprising multi-scale dissipation process suggests a nanoparticle jamming-unjamming transition. The realization that segmental dynamics can be coupled with the entanglement concept for enhanced toughness at high rates of deformation is a novel insight with relevance to the design of composite materials.

Bio-macromolecular design roadmap towards tough bioadhesives.

Emerging sutureless wound-closure techniques have led to paradigm shifts in wound management. State-of-the-art biomaterials offer biocompatible and biodegradable platforms enabling high cohesion (toughness) and adhesion for rapid bleeding control as well as robust attachment of implantable devices. Tough bioadhesion stems from the synergistic contributions of cohesive and adhesive interactions. This Review provides a biomacromolecular design roadmap for the development of tough adhesive surgical sealants. We discuss a library of materials and methods to introduce toughness and adhesion to biomaterials. Intrinsically tough and elastic polymers are leveraged primarily by introducing strong but dynamic inter- and intramolecular interactions either through polymer chain design or using crosslink regulating additives. In addition, many efforts have been made to promote underwater adhesion via covalent/noncovalent bonds, or through micro/macro-interlock mechanisms at the tissue interfaces. The materials settings and functional additives for this purpose and the related characterization methods are reviewed. Measurements and reporting needs for fair comparisons of different materials and their properties are discussed. Finally, future directions and further research opportunities for developing tough bioadhesive surgical sealants are highlighted.

Force‐Reversible and Energetic Indole‐Mg‐Indole Cation‐π Interaction for Designing Toughened and Multifunctional High‐Performance Thermosets

AbstractNon‐covalent crosslinking has provided versatile and affordable solutions for the design of tough soft polymer materials, but it is not applied in stiff high‐performance thermosets. Here, the authors report a supramolecular approach for the design of tough high‐performance thermosets by using noncovalent sandwich‐structural indole‐Mg‐indole complexes that act as the force‐reversible and energetic crosslinking points, evading the inherent trade‐off between mechanical strength and ductility for high‐performance thermosets. Compared to traditional epoxy polymer materials, the indole‐Mg‐indole crosslinks of the network enables synchronously enhancement of mechanical strength and ductility owing to the increased interaction‐energy and efficient dissociation and reassociation behaviors given by the dynamic indole‐Mg‐indole complexes, which is quite challenging to achieve by conventional chemical methods. In addition, local manipulation of crosslinking confers the resulting thermosets with multiple fast stimuli‐responsive functions, such as recyclability, healability, and adhesion.

Synthesis and characterization of biphenyldicarboxylic acid-modified poly(1,4-cyclohexylenedimethylene terephthalate) copolyesters

A new copolyesters of poly(1,4-cyclohexylenedimethylene terephthalate-co-4,4’-biphenydicarboxylate) (PCTB) with different compositions were synthesized from terephthalic acid, 1,4-cyclohexanedimethanol and 4,4’-biphenyldicarboxylic acid (BDA) via esterification-polycondensation reaction. The synthesized copolyesters were analyzed by 1H-NMR, 13C-NMR, FT-IR, GPC, intrinsic viscosity, DSC, TGA and tensile testing, respectively. The 1H-NMR, 13C-NMR and FT-IR results verify the successful introduction of BDA into poly(1,4-cyclohexylenedimethylene terephthalate (PCT) chains. The introduction of BDA into PCT chains can effectively lower the melting point of PCT, making it easier to process. The glass transition temperature of PCTB is higher than 103 °C and its thermal stability is better than that of PCT. The rheological tests confirm the enhanced melting stability of PCT after introducing BDA into PCT chains. Furthermore, the PCTB, which owns the comparable mechanical properties to PCT, is a tough polymer with the higher Young’s modulus (above 230 MPa), the higher tensile strength (above 41 MPa) and the higher elongation at break (above 159%).

Effects of silica and clay nanoparticles on the mechanical properties of poly(vinyl alcohol) nanocomposite hydrogels

We investigated the effects of silica(SiO 2 ) and clay nanoparticles on the tensile properties of poly(vinyl alcohol) (PVA) hydrogels containing borax. Clay/PVA/borax hydrogels possessed comparatively high tensile strength even at low concentrations of clay, but the mechanical performance drastically became worse at high concentrations. On the contrary, the tensile strength of SiO 2 /PVA/borax hydrogels increased with increasing SiO 2 concentrations under high elongation. In these hydrogels, multiple crosslinking such as physical interactions between clay and PVA, complexations between PVA (or SiO 2 ) and borate, microcrystals of PVA produced a tough polymer network. Besides, the synergistic effects of clay and SiO 2 nanoparticles on the mechanical performance of the PVA/borax hydrogels were confirmed, i.e., both extensibility and modulus were largely improved compared to clay/PVA/borax or SiO 2 /PVA/borax hydrogels. Synchrotron small-angle X-ray scattering analysis revealed that the addition of both clay and SiO 2 nanoparticles suppressed inhomogeneities in the composite gel. • Clay/PVA/borax gels possessed high tensile strength at low clay concentrations. • SiO 2 /PVA/borax gels possessed high elongation even at high SiO 2 concentrations. • The addition of SiO 2 and clay to PVA/borax gels caused high elongation and strength. • Combined use of nanoparticles and borax was effective for production of tough polymer gels.

Tough Polymer Electrolyte with an Intrinsically Stabilized Interface with Li Metal for All-Solid-State Lithium-Ion Batteries

A high-performance solid polymer electrolyte (SPE) membrane that simultaneously addresses the issues of enhanced toughness and Li-dendrite mitigation for all-solid-state Li-ion batteries (ASSLIBs) is demonstrated. The membrane has a sandwiched structure consisting of a center poly(ethylene oxide)/lithium bis(trifluoromethanesulfonyl) imide (PEO/LiTFSI) electrolyte matrix laminated on both sides with electrospun high-polarity β-phase poly(vinylidene fluoride-co-hexafluoropropylene) (β-PVDF-HFP) nanofibers. The nanofiber layers impart remarkable enhancement in both mechanical and electrochemical properties of the SPE, including a 20-fold increase in tensile strength and a 48-fold increase in toughness, along with up to 4-fold enhancement in Li-ionic conductivity. Moreover, the highly polar fluorinated nanofiber cladding layers enable a stable Li-plating/stripping interface on the Li anode to efficiently mitigate dendrite formation, while improving the electrochemical interfacial stability with the cathode. In a Li|SPE|Li symmetric cell, the use of the sandwiched SPE is demonstrated to improve the cycle stability from short-circuiting at 144 h for a pristine PEO/LiTFSI membrane to no short-circuiting even up to 3600 h (1800 Li-plating-stripping cycles). In an example of LiFePO4|SPE|Li ASSLIBs, using the sandwiched membrane enables substantial reduction irreversible capacity upon charging to the high-voltage end and more than 80% capacity retention for over 1600 h. This work presents a feasible and facile design for an SPE for high-performance ASSLIBs.

Biomimetic integration of tough polymer elastomer with conductive hydrogel for highly stretchable, flexible electronic

Flexible electronics is booming and attracts considerable interest in monitoring physiological activities and health conditions. The development of human-friend flexible electronics with both excellent human movement monitoring and flexible electrode functions at a wide working range remains highly challenging. Herein, we report a biomimetic double-layered multifunctional flexible electronic device composed of a stretchable, tough elastomer covalently coupled with a conductive, double-network hydrogel for monitoring physiological motions. The introduction of optimal physical crosslinking including hydrogen bond and metal coordination in the covalently cross-linked polyurethane elastomer effectively provides favorable flexibility and stretchability. The elastomer-hydrogel integration (EHI) shows ultra-fast responsiveness (10 ms) and resilience (246 ms) as a skin sensor and could effectively detect diverse muscle and joint movements in a highly sensitive and signal waveform-interpreting manner. Further, such a polymer integration with skin shape-adaptive hydrogel and low contact impedance enables real-time high-quality detection of electrocardiogram (ECG) by serving as flexible electrode compared to commercial Ag/AgCl electrodes, and was successfully applied to measure electrooculogram (EOG) and electromyogram (EMG). EHI-based electrodes were feasible and effective for electroencephalogram (EEG) signal detection in brain-computer interfaces (BCI) system. Altogether, this biomimetic EHI electronic device provides an advanced platform for the design of next-generation flexible healthcare diagnostic monitors and soft robots.

Fatigue‐Resistant Interfacial Layer for Safe Lithium Metal Batteries

AbstractThe plating/stripping of Li dendrites can fracture the static solid electrolyte interphase (SEI) and cause significant dynamic volume variations in the Li anode, which give rise to poor cyclability and severe safety hazards. Herein, a tough polymer with a slide‐ring structure was designed as a self‐adaptive interfacial layer for Li anodes. The slide‐ring polymer with a dynamically crosslinked network moves freely while maintaining its toughness and fracture resistance, which allows it can to dissipate the tension induced by Li dendrites on the interphase layer. Moreover, the slide‐ring polymer is highly stretchable, elastic, and displays an ultrafast self‐healing ability, which allows even pulverized Li to remain coalesced without disintegrating upon consecutive cycling. The Li anodes demonstrate greatly improved suppression of Li dendrite formation, as evidenced by the high critical current density (6 mA cm−2) and stable cycling for the full cells with high‐areal capacity LiFePO4, high‐voltage NCM, and S cathodes.

Fatigue-Resistant Interfacial Layer for Safe Lithium Metal Batteries.

The plating/stripping of Li dendrites can fracture the static solid electrolyte interphase (SEI) and cause significant dynamic volume variations in the Li anode, which give rise to poor cyclability and severe safety hazards. Herein, a tough polymer with a slide-ring structure was designed as a self-adaptive interfacial layer for Li anodes. The slide-ring polymer with a dynamically crosslinked network moves freely while maintaining its toughness and fracture resistance, which allows it can to dissipate the tension induced by Li dendrites on the interphase layer. Moreover, the slide-ring polymer is highly stretchable, elastic, and displays an ultrafast self-healing ability, which allows even pulverized Li to remain coalesced without disintegrating upon consecutive cycling. The Li anodes demonstrate greatly improved suppression of Li dendrite formation, as evidenced by the high critical current density (6 mA cm-2 ) and stable cycling for the full cells with high-areal capacity LiFePO4 , high-voltage NCM, and S cathodes.

Control of Polymer Properties by Entanglement: A Review

AbstractChain entanglement, either cohesional or topological, distinguishes polymers from other engineering materials. It impedes the movement of molecular segments and influences the polymer rheology, morphology, and mechanical properties. Although a high level of entanglement can increase the polymer toughness, excessive entanglement should be avoided because it causes a high melt viscosity making the processing difficult. This review tended to elucidate the influence of entanglement on the polymer structure, determining the material properties and processability. A wide range of methods used to fine control the degrees of chain entanglement are summarized. The methods are applicable to polymers in solutions, melts, and condensed states with advantages and limitations discussed in detail. The authors also examined the effect of the entanglement on polymer crystallization—the mechanism remains a controversial issue. This review will provide general guidance to designing and processing polymer materials with desired properties via a rational route of controlling the chain entanglement.

An Assessment of ASTM E1922 for Measuring the Translaminar Fracture Toughness of Laminated Polymer Matrix Composite Materials.

The main objective of this work is to predict the exact value of the fracture toughness (KQ) of fiber-reinforced polymer (FRP). The drawback of the American Society for Testing Materials (ASTM) E1922 specimen is the lack of intact fibers behind the crack-tip as in the real case, i.e., through-thickness cracked (TTC) specimen. The novelty of this research is to overcome this deficiency by suggesting unprecedented cracked specimens, i.e., matrix cracked (MC) specimens. This MC exists in the matrix (epoxy) without cutting the glass fibers behind the crack-tip in the unidirectional laminated composite. Two different cracked specimen geometries according to ASTM E1922 and ASTM D3039 were tested. 3-D FEA was adopted to predict the damage failure and geometry correction factor of cracked specimens. The results of the TTC ASTM E1922 specimen showed that the crack initiated perpendicular to the fiber direction up to 1 mm. Failure then occurred due to crack propagation parallel to the fiber direction, i.e., notch insensitivity. As expected, the KQ of the MC ASTM D3039 specimen is higher than that of the TTC ASTM D3039 specimen. The KQ of the MC specimen with two layers is about 1.3 times that of the MC specimen with one layer.