Auxetic Polymers Research Articles

The shear performance of uniaxially thermoformed auxetic polymer foams

We study the shear behavior of pristine and uniaxially thermoformed auxetic foams using three different shear testing techniques and finite element simulations. A novel simple shear test rig with sliding lateral supports was developed and compared to fixed simple shear and diagonal pure shear rigs. The three rigs yield similar results at shear strains under 8 %, but the fixed simple shear rig shows a significant rise in shear modulus and stiffness at higher strains. In contrast, the sliding simple shear and diagonal pure shear rigs exhibit similar linear results up to 20 % shear strain. The thermoformed auxetic foam displays strong orthotropy, with a shear modulus of 35 kPa along the thermoforming direction and 0.1 MPa in the transverse directions. The pristine foam shows good isotropy under shear, with a similar 50 kPa shear modulus in all directions. The auxetic foam expands slightly along the thermoforming direction at high strains, while the pristine foam and auxetic foam in other directions show noticeable shrinkage. Finite element models based on 3D μ-CT scans, using a new boundary coupling element to apply periodic boundary conditions, closely matched the experimental results. The simulations revealed the deformation mechanisms within the foam cell structures that contributed to the observed orthotropic shear behavior.

Auxetic polymer networks: The role of crosslinking, density, and disorder

Low-crosslinked polymer networks have recently been found to behave auxetically when subjected to small tensions, that is, their Poisson’s ratio ν becomes negative. In addition, for specific state points, numerical simulations revealed that diamond-like networks reach the limit of mechanical stability, exhibiting values of ν = −1, a condition that we define as hyper-auxeticity. This behavior is interesting per se for its consequences in materials science but is also appealing for fundamental physics because the mechanical instability is accompanied by evidence of criticality. In this work, we deepen our understanding of this phenomenon by performing a large set of equilibrium and stress–strain simulations in combination with phenomenological elasticity theory. The two approaches are found to be in good agreement, confirming the above results. We also extend our investigations to disordered polymer networks and find that the hyper-auxetic behavior also holds in this case, still manifesting a similar critical-like behavior as in the diamond one. Finally, we highlight the role of the number density, which is found to be a relevant control parameter determining the elastic properties of the system. The validity of the results under disordered conditions paves the way for an experimental investigation of this phenomenon in real systems, such as hydrogels.

Molecular Auxetic Polymer of Intrinsic Microporosity via Conformational Switching of a Cavitand Crosslinker

AbstractAuxetics are materials characterized by a negative Poisson's ratio (NPR), an uncommon mechanical behavior corresponding to a transversal deformation tendency opposite to the traditional materials. Here, the first example of a synthetic molecular auxetic polymer obtained by embedding a conformationally expandable cavitand as a crosslinker into a rigid polymer of intrinsic microporosity (PIM) is presented. The rigidity and microporosity of the polymeric matrix are pivotal to maximizing the expansion effect of the cavitand that, under mechanical stress, can assume two different conformations: a compact vase one and an extended kite form. The auxetic behavior and the corresponding NPR of the proposed material is predicted by a specific micromechanical model that considers the cavitand volume expansion ratio, the fraction of the cavitand crosslinker in the polymer, and the mechanical characteristics of the polymer backbone. The reversible auxetic behavior of the material is experimentally verified via the digital image correlation technique performed during the mechanical tests on films obtained by blending the auxetic crosslinked polymer with pristine PIM. Two specific control experiments prove that the mechanically driven conformational expansion of the cavitand crosslinker is the sole responsible for the observed NPR of the polymer.

Multifunctional Nanogenerator-Integrated Metamaterial Concrete Systems for Smart Civil Infrastructure.

Creating multifunctional concrete materials with advanced functionalities and mechanical tunability is a critical step toward reimagining the traditional civil infrastructure systems. Here, the concept of nanogenerator-integrated mechanical metamaterial concrete is presented to design lightweight and mechanically tunable concrete systems with energy harvesting and sensing functionalities. The proposed metamaterial concrete systems are created via integrating the mechanical metamaterial and nano-energy-harvesting paradigms. These advanced materials are composed of reinforcement auxetic polymer lattices with snap-through buckling behavior fully embedded inside a conductive cement matrix.Werationally design their composite structures to induce contact-electrification between the layers under mechanical excitations/triggering. The conductive cement enhanced with graphite powder serves as the electrode in the proposed systems, while providing the desired mechanical performance. Experimental studies are conducted to investigate the mechanical and electrical properties of the designed prototypes. The metamaterial concrete systems are tuned to achieve up to 15% compressibility under cycling loading. The power output of the nanogenerator-integrated metamaterial concrete prototypes reaches 330 µW. Furthermore, the self-powered sensing functionality of the nanogenerator concrete systems for distributed health monitoring of large-scale concrete structuresisdemonstrated. The metamaterial concrete paradigm can possibly enable thedesignof smart civil infrastructure systems with a broad range of advanced functionalities.

Anisotropy in conventional and uniaxially thermoformed auxetic polymer foams

Auxetic (negative Poisson's ratio) open cell polymeric foams have been traditionally produced using thermoforming techniques coupled with the volumetric compression of conventional (pristine) off-the-shelf foams. The anisotropy of the pristine foam plays however a strong role on the performance of thermoformed auxetic foams, and this is the subject studied in this work. Micro computed tomography (μ-CT) scans indeed show that the cell structures of the pristine foam exhibit an elongated convex polyhedron shape, with more ribs oriented along the foam rising direction d 1 . The modulus along d 1 can be up to 70% larger than the one along the transverse directions, and the Poisson's ratios in different planes are also noticeable different, ranging from 0.2 to 0.85. Three types of auxetic foams uniaxially thermoformed along the three orthogonal directions of the conventional porous material are here developed, and all show different mechanical performances due to the anisotropy of the conventional baseline foam. The auxetic foams show significant auxetic behavior along the thermoforming direction only when thermoformed along the transverse directions of the pristine foam (d 2 or d 3 ) and loaded along the foam rising direction (d 1 ); in that case, the negative Poisson's ratio reach values as low as −1. In other cases, the auxetic performance is not evident, and the Poisson's ratio is close to 0. Finite Element models of the pristine and auxetic foams have been also here developed based on the 3D models from μ-CT, and the stiffness-strain curved derived from those coincide well with the experimental results. The simulated deformation mechanisms of the cell structures inside different foams help to explain the anisotropic performance of the foams and the differences between compressive and tensile properties.

Auxetic Polymer Foams: Production, Modeling and Applications

:Auxetic materials have high potential due to their exceptional properties resulting from a negative Poisson’s ratio. Recently, several auxetic polymer-based materials have been developed. In fact, several applications are looking for a lightweight material (less material consumed in production and transport) while having high mechanical performances (impact absorption, rigidity, strength, resistance, etc.). So, a balance between density and toughness/strength is highly important, especially for military, sporting, and transport applications. So auxetic materials (especially foams) can provide high impact protection while limiting the material’s weight. This article presents a review of recent advances with a focus on auxetic polymers, with particular emphasis on the auxetic polymer foams in terms of their fabrication methods and processing conditions (depending on the nature of the cellular structure), the effect of the fabrication parameters on their final properties, as well as their models and potential applications.

Effect of auxetic honeycomb angular stiffeners in cold-formed perforated and webbed steel upright section under buckling modes

This work was carried out on the buckling effects of cold-formed perforated steel columns with base auxetic polymer stiffeners. Buckling tests were carried out for three thicknesses of steel profiles (1.5–1.8 mm) with and without base stiffeners. Loading conditions were considered to be with displacement variation of 0.1 mm/s and respective axial loads and lateral displacements were noted. Results obtained states that the lateral displacement was found to be 2.2 for 1.8 mm CFS thickness and 93 kN of axial load with the use of auxetic stiffener with 14.8% of the variation in comparison without stiffener. The strain energy of absorption for auxetic stiffener is found to be high as 0.0523 at a lateral load of 80 kN for 1.8 mm CFS thickness. The maximum resistance to local, distortional, and Euler’s buckling loads was found to be high for 1.8 mm thick CFS with stiffener with 11.1%, 17.39%, and 10% in comparison without stiffener.

Influence of Plasticity and Friction on the Contact Mechanics of Auxetic Materials

AbstractContact interactions play an important role in the tribological behavior of engineering materials. This paper develops a finite element model to investigate the contact mechanics and stress distribution of auxetic materials, i.e., materials with negative Poisson’s ratio. The model results are compared with numerical and mathematical models for isotropic auxetic polymers. The indentation of auxetic materials is analyzed for the effects of friction, plasticity and allowing separation after contact with a spherical indenter using a commercial software, abaqus. The results are discussed in terms of stress profiles, force-indentation depth curves, plasticity, friction, internal energy, compressibility, sink-in, and the pile-up of material. It is concluded that for purely elastic contact, the indentation resistance increases for auxetic materials and the inclusion of friction shifts subsurface stresses closer to the surface. However, the introduction of plasticity negates the improvement of increased indentation resistance. The pile-up of material around the indent reduces for auxetic materials which makes them more suitable for rolling/sliding contacts. The internal strain energy decreases for purely elastic contact and increases for an elastic/plastic contact.

Auxetic Polymers in Textiles - Review

Auxetic Polymers in Textiles - Review

Bioinspired design of architected cement-polymer composites

Recent advances in the design of structural composites often mimic natural microstructures. Specifically, the structure of abalone nacre with its high stiffness, tensile strength, and toughness is a source of inspiration from the process of evolution. The inspiration from nacre can lead to design of a new class of architected structural materials with superb mechanical properties. This work presents a combined experimental and computational study on a set of bioinspired architected composites created using a cement mortar cast with brick-and-mortar and auxetic polymer phases. The impact of this unit-cell polymer phase on the flexural and compressive strengths, resilience, and toughness is thoroughly studied as a function of architected geometry. All mechanical properties of the architected composite specimens are found to be greater than those of control samples due to prevention of localized deformation and failure, resulting in higher strength. The architected composites showed more layer shear sliding during fracture, whereas the control samples showed more diagonal shear failure. After initial cracking, the architected composites gradually deformed plastically due to interlocking elements and achieved high stresses and strains before failure. Results also show that composites with the architected polymer phase outperform control samples with equal volume fraction of a randomly oriented polymer fiber phase. Extensive computational studies of the proposed unit cells are also performed and the results suggest that the orientation of cells during loading is critical to achieve maximum performance of a cementitious composite. The implications of these results are immense for future development of high performing construction materials.

Fabrication and Characterization of Nonwoven Auxetic Polymer Stent

Synthetic biomaterials have better controlled physical and mechanical properties and can be used to tailor both soft and hard tissues. A tiny, expandable mesh tubes called stents keep blood vessels open and allow blood flow and treat blockage to improve quality of patient’s life. The main focus of this work is to (i) fabricate a polymeric sheet of melt electrospun polycaprolactone microfibers; (ii) tailor auxetic geometry by micromachining on polycaprolactone microfiber and polycaprolactone sheet; (iii) fabricate a cylindrical tube to make auxetic stents. Final results for mechanical characterization and performance analysis of auxetic polymer stents are discussed.

Three decades of auxetic polymers: a review

AbstractDevelopments in design and technology in the engineering and medical fields necessitate the use of smart and high-performance materials to meet higher engineering specifications. The general requirements of such materials include a combination of high stiffness and strength with significant weight savings, resistance to corrosion, chemical resistance, low maintenance, and reduced costs. Over the last three decades, it has been demonstrated that auxetic materials offer a huge potential for the fields of engineering, natural sciences, and biomedical engineering, and for many other industries, including the aerospace and defense industries, through their unique deformation mechanism and measured enhancements in mechanical properties. To meet future engineering challenges, auxetic materials are increasingly being recognized as integral components of smart and advanced materials. Although materials with a negative Poisson’s ratio have been known since the early 1900s, they did not capture researchers’ attention until the late 1980s. Since 1991, these materials have been known as auxetic materials. Since then, their benefits and applications have been expanded to all major classes of materials such as metals, ceramics, polymers, and composites, and they are also now being used in engineering applications. The goal of this review was to present the development of auxetic polymers, which were first fabricated in the form of polyurethane foam approximately three decades ago and are now used in the fabrication of non-woven nano/micropolymeric structures. This review could provide useful information for the future development of auxetic polymers.

Effect of nodule shape for modeling of auxetic microporous polymers

Previous models for describing auxetic microporous polymers adopt 2D rectangular blocks with fibril interconnections. In this paper, spherical nodes are used in order to approximate the granular nodule shape and for taking into consideration the 3D nature of the nodes. Assuming curvilinear motion of the nodes as a result of fibril rotation, a quantitative description of the Poisson’s ratio is given as a function of the nodule density packing factor. Comparison with the 3D rectangular model shows that the spherical model gives a more conservative description of the Poisson’s ratio although both models exhibit similar trend. Results reveal the significance of the assumed node shape for modeling the Poisson’s ratio of auxetic microporous polymers.

High-performance 1–3-type lead-free piezo-composites with auxetic polyethylene matrices

High piezoelectric sensitivity and large hydrostatic parameters are discussed for two 1–3-type single crystal/auxetic polymer composites. The single crystal components selected are based on lead-free (K,Na)(Nb,Ta)O3 and (Li,K,Na)(Nb,Ta)O3 solid solutions, and the auxetic polymers are characterised by Poisson’s ratios –0.83≤ ν ≤–0.29. The composites examined exhibit advantageous properties over conventional lead-based piezo-active composites and ceramics such as high longitudinal piezoelectric coefficient g33⁎~(102–103) mV·m/N, squared figure of merit (Q33⁎)2~10–11 Pa−1, hydrostatic piezoelectric coefficient gh⁎~(102–103) mV·m/N and squared figure of merit (Qh⁎)2~(10–11–10−10) Pa−1. The composites also demonstrate an infinitely large anisotropy of piezoelectric coefficients d3j⁎.

Polarisation Orientation Effects and Hydrostatic Parameters in Novel 2–2 Composites Basedon PMN–xPT Single Crystals

Polarisation orientation effects in piezo-active composites based on relaxor-ferroelectric single crystals of (1–x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 open up new possibilities to vary the effective electromechanical properties and related parameters. In the present paper the hydrostatic performance of 2–2 parallel-connected single crystal / auxetic polymer composites is studied for x = 0.28 – 0.33 and two poling directions of the single-crystal component, [001] (4mm symmetry) and [011] (mm2 symmetry) in the perovskite unit cell. The effect of the orientation of the main crystallographic axes in the single-crystal component on the hydrostatic piezoelectric coefficients d*h, eh* and squared figure of merit d*hgh* is analysed for three rotation modes in single crystals and at various anisotropy of their piezoelectric and elastic properties. The role of the auxetic polymer component is emphasised in the context of the large hydrostatic parameters and anisotropy of squared figures of merit d*3jg3j* (j = 1, 2 and 3). The largest maximum values of d*h and e*h (1990 pC/N and 42.5 C/m2, respectively) are achieved in the composite based on [011]-poled 0.71Pb(Mg1/3Nb2/3)O3–0.29PbTiO3.

Review of Mechanics and Applications of Auxetic Structures

One of the important mechanical properties of materials is Poisson’s ratio, which is positive for most of the materials. However, certain materials exhibit “auxetic” properties; that is, they have a negative Poisson’s ratio. Thus auxetic and non-auxetic materials exhibit different deformation mechanisms. A specific microscopic structure in the auxetic materials is important for maintaining a negative Poisson’s ratio. Based on their distinct nature auxetic materials execute certain unique properties in contrast to other materials, which are reviewed in this paper. Thus auxetic materials have important applications in the biomedical field which are also a part of this review article. Many auxetic materials have been discovered, fabricated, and synthesized which differ on the basis of structure, scale and deformation mechanism. The different types of auxetic materials such as auxetic cellular solids, microscopic auxetic polymers, molecular auxetic materials, and auxetic composites have been reviewed comprehensively in this paper. Modeling of auxetic structures is of considerable importance and needs appropriate stress strain configurations; thus different aspects of auxetic modeling have also been reviewed. Packing parameters and relative densities are of prime importance in this regard. This review would thus help the researchers in determining and deciding the various aspects of auxetic nature for their products.

Anisotropy of electromechanical properties and hydrostatic response of advanced 2–2‐type composites

AbstractThe paper reports results on the high performance of novel piezo‐active composites based on relaxor‐ferroelectric single crystals (SCs) of xPb(In0.5Nb0.5)O3–yPb(Mg1/3Nb2/3)O3–(1 − x − y)PbTiO3. We present a comparative analysis of the electromechanical properties of 2–2‐type composites based on single‐domain SCs from symmetry classes 3m and mm2. The parallel connection of the layers, the presence of an auxetic polymer component and the appropriate orientation of the main crystallographic axes of the single‐crystal layers lead to the development of large piezoelectric anisotropy and considerable hydrostatic piezoelectric response in comparison to the single‐crystal component. Effective piezoelectric coefficients $d_{3j}^{*} $ and electromechanical coupling factors $k_{3j}^{*} $ of the composites obey the conditions $d_{33}^{*} $/|$d_{31}^{*} $| ≥ 10, $d_{33}^{*} $/|$d_{32}^{*} $| ≥ 10, $k_{33}^{*} $/|$k_{31}^{*} $| ≥ 10, and $k_{33}^{*} $/|$k_{32}^{*} $| ≥ 10 at $k_{33}^{*} $ ≈ 0.6. Hydrostatic piezoelectric coefficients $d_{{\rm h}}^{*} $ ≈ 470 pC/N and $g_{{\rm h}}^{*} $ ∼ (102–103) mV m/N are one to two orders‐of‐magnitude larger than those of the single‐crystal component, which make it attractive for various piezotechnical applications. The role of the elastic properties of the components is discussed in connection with the large hydrostatic parameters.

Preface: phys. stat. sol. (b) 245/3

Preface: phys. stat. sol. (b) 245/3

Characteristics of 1–3-type ferroelectric ceramic/auxetic polymer composites

This paper presents modelling and simulation results on 1–3 piezoactive composites comprising a range of ferroelectric ceramics, which are assumed to have variable properties and an auxetic polymer (i.e. a material with a negative Poisson ratio) that improves the hydrostatic piezoelectric response of the composite. Dependences of the effective piezoelectric coefficients and related parameters of the 1–3 composites on the degree of poling, mobility of the 90° domain walls within ceramic grains, on the volume fraction of the ceramic component and on the Poisson ratio of the polymer component have been calculated and analysed. The role of the piezoelectric anisotropy and domain-orientation processes in improving and optimising the effective parameters, piezoelectric activity and sensitivity of 1–3 ferroelectric ceramic/auxetic composites is discussed.

Auxetic materials

The current status of research into auxetic (negative Poisson's ratio) materials is reviewed, with particular focus on those aspects of relevance to aerospace engineering. Developments in the modelling, design, manufacturing, testing, and potential applications of auxetic cellular solids, polymers, composites, and sensor/actuator devices are presented. Auxetic cellular solids in the forms of honeycombs and foams are reviewed in terms of their potential in a diverse range of applications, including as core materials in curved sandwich panel composite components, radome applications, directional pass band filters, adaptive and deployable structures, MEMS devices, filters and sieves, seat cushion material, energy absorption components, viscoelastic damping materials, and fastening devices. The review of auxetic polymers includes the fabrication and characterization of microporous polymer solid rods, fibres, and films, as well as progress towards the first synthetic molecular-level auxetic polymer. Potential auxetic polymer applications include self-locking reinforcing fibres in composites, controlled release media, and self-healing films. Auxetic composite laminates and composites containing auxetic constituents are reviewed and enhancements in fracture toughness, and static and low velocity impact performance are presented to demonstrate potential in energy absorber components. Finally, the potential of auxetics as strain amplifiers, piezoelectric devices, and structural health monitoring components is presented.