K. L. Alderson

The strain dependent indentation resilience of auxetic microporous polyethylene

A series of tests have been conducted on (i) auxetic, (ii) compression moulded and (iii) sintered ultra high molecular weight polyethylene. The auxetic material possesses a negative Poissons ratio, ν, due to its complex porous microstructure which consists of nodules interconnected by fibrils and the sintered material has a positive ν and is microporous but does not contain fibrils. It was found that the auxetic material was both more difficult to indent than the other materials at low loads (from 10–100 N) and was the least plastic with the most rapid viscoelastic creep recovery of any residual deformation. Indeed, at low loads, where the resistance to local indentation is most elastic, the hardness increased by up to a factor of 8 on changing the Poissons ratio from ν ≈ 0 to ν ≈ −0.8. A mechanism is proposed based on local densification under the indentor of the nodules and fibrils which explains how the microstructural response of an auxetic polymer can be used to interpret the results.

The interpretation of the strain-dependent Poisson's ratio in auxetic polyethylene

Abstract The strain-dependent behaviour characteristic of auxetic (i.e. having a negative Poissons ratio) polymers has been modelled using a simple geometric model which consists of rectangular nodules intecronnected by fibrils. Careful consideration of the correct form of the model to use depending on the experimental method employed to test samples of auxetic ultra high molecular weight polyethylene (UHMWPE) has resulted in very good agreement between the experimental and theoretical Poissons ratios and total engineering strain ratios when the deformation is predominantly due to hinging of the fibrils. Auxetic UHMWPE has been processed to yield a very wide range of Poissons ratios depending on its microstructural parameters (i.e. nodule shape and size, fibril length and the angle between the fibril and nodule). These can be predicted using the model, allowing the possibility of tailoring Poissons ratio of the material.

The Design, Matching and Manufacture of Auxetic Carbon Fibre Laminates

Either an in-plane or out-of-plane negative Poisson’s ratio,, in continuous carbon fibre/epoxy resin composites can be achieved, providing the fibre volume fraction and anisotropy are high enough, by selecting suitable stacking sequences. This paper examines the use of specially designed software which allows the designer to match the mechanical properties of laminates with predicted negative to those with similar mechanical properties but positive. This has allowed auxetic and matched carbon fibre/epoxy resin laminates to be specifically designed. These laminates were then fabricated and tested, with good agreement found to theoretical predictions. This study, then, provides a route to evaluating the effect of a negative alone on properties such as fracture toughness and impact resistance.

Negative Poisson’s Ratio Polyester Fibers

Auxetic materials are referred to as those having negative Poisson’s ratio (ν). Initial work at Bolton successfully fabricated auxetic polypropylene fiber using a novel thermal melt-spinning technique. This paper reports in detail both the methods and principles involved in screening polyester powder and also the manufacturing method for successful production of auxetic polyester fibers. Videoextensometry along with micro-tensile testing were used to measure the Poisson’s ratio of the fiber. The Poisson’s ratio of the polyester fiber was found to vary between -0.65 and -0.75.

Modelling of the mechanical and mass transport properties of auxetic molecular sieves: an idealised organic (polymeric honeycomb) host-guest system.

Force field-based simulations have been employed to model the mechanical properties of a range of undeformed molecular polymeric honeycombs having conventional and re-entrant hexagon pores. The conventional and re-entrant hexagon honeycombs are predicted to display positive and negative in-plane Poissons ratios, respectively, confirming previous simulations. The structure, and mechanical and mass transport properties of a layered re-entrant honeycomb ((2,8)-reflexyne) were studied in detail for a uniaxial load applied along the x 2 direction. The mechanical properties are predicted to be stress- (strain-) dependent and the trends can be interpreted using analytical expressions from honeycomb theory. Transformation from negative to positive Poissons ratio behaviour is predicted at an applied stress of σ2 = 2 GPa. Simulations of the loading of C60 and C70 guest molecules into the deformed layered (2,8)-reflexyne host framework demonstrate the potential for tunable size selectivity within the host framework. The entrapment and release of guest molecules is attributed to changes in the size and shape of the pores in this host–guest system.

An experimental study of ultrasonic attenuation in microporous polyethylene

Abstract Auxetic foams i.e. those possessing a negative Poissons ratio, v, are known to possess improved sound absorption properties when compared with conventional foams. In this study, the ultrasonic attenuation of another class of auxetic materials, microporous polymers, and in particular ultra-high-molecular weight polyethylene (UHMWPE), was examined experimentally, in comparison with a microporous positive v form of UHMWPE and conventionally processed UHMWPE. It was found that the microporous polymers, whether auxetic or not, showed very large enhancements in attenuation coefficient, α mn , over the conventionally processed UHMWPE, with the highest measurable value being α mn = 47 dB / cm for the auxetic material, which is 1.5 times the highest value for the microporous positive v UHMWPE and more than three times that for the conventionally processed material. In addition, the samples with the largest negative Poissons ratio absorbed the signal so completely that no value of a was obtainable.

Double-Negative Mechanical Metamaterials Displaying Simultaneous Negative Stiffness and Negative Poisson’s Ratio Properties

A scalable mechanical metamaterial simultaneously displaying negative stiffness and negative Poissons ratio responses is presented. Interlocking hexagonal subunit assemblies containing 3 alternative embedded negative stiffness (NS) element types display Poissons ratio values of -1 and NS values over two orders of magnitude (-1.4 N mm-1 to -160 N mm-1 ), in good agreement with model predictions.

Modelling of the mechanical and mass transport properties of auxetic molecular sieves: an idealised inorganic (zeolitic) host-guest system

Force field based simulations have been employed to model the structure, and mechanical and mass transport properties of the all-silica zeolite MFI (ZSM5—Si96O192). Undeformed and deformed MFI subject to uniaxial loading in each of the three principal directions were investigated. The mechanical properties are predicted to include negative on-axis Poissons ratios (auxetic behaviour) in the x 1–x 3 plane of the undeformed structure, and are strain-dependent. Transformation from positive-to-negative Poissons ratio behaviour, and vice versa, is predicted for most on-axis Poissons ratios at critical loading strains. Simulations of the simultaneous sorption of neopentane and benzene guest molecules onto the undeformed host MFI framework indicate a low neopentane-to-benzene loading ratio, consistent with experimental observation. The sorption of these two molecular species onto deformed MFI is Poissons ratio- and strain-dependent. Uniaxial tensile loading along a direction containing a negative on-axis Poissons ratio leads to an increase in the loading of the larger neopentane molecules with respect to benzene, strongly correlated with the increase in volume associated with auxetic behaviour.

Evidence for uniaxial drawing in the fibrillated microstructure of auxetic microporous polymers

Auxetic polymers (i.e., those with a negative Poissons ratio, i) were ®rst identi®ed in 1989 [1, 2] with the study of the microstructure and mechanical properties of a microporous form of expanded polytetra uoroethylene (PTFE). This polymer achieved a strain-dependent negative i by means of its complex microstructure, consisting of nodules interconnected by ®brils of diameter approximately 1 im, which is schematically illustrated in Fig. 1, and not by any intrinsic material property of the PTFE itself. Consequently, attempts were made to reproduce the microstructure, and hence the auxetic behavior, in other polymers; and this was successfully achieved in ultra high molecular weight polyethylene (UHMWPE) [3] and, most recently, in polypropylene (PP) [4]. The microstructure has also been reproduced in nylon, but no measurements of i have as yet been undertaken on this material. From scanning electron microscope (SEM) examination [1] and estimates of the Youngs modulus based on experimental data [5], the ®brils are believed to consist of highly ordered (i.e., crystalline) material, connected to a more amorphous disordered material forming the nodules. Between the fully developed ®bril and nodule there is expected to be a transition region. If PTFE is considered, the nodule is expected to have a Youngs modulus similar to bulk PTFE (0:3y0:7 GPa), while the ®bril has been estimated to have a modulus between 15 GPa and 75 GPa [5]. To gain a greater understanding of the mechanisms involved in the production of a strain-dependent negative i, a very simple geometric model was developed [6], which is illustrated in Fig. 1. It consists of rectangular blocks (representing the nodules), interconnected by ®brils at the corners of the rectangles. In both PTFE and UHMWPE, SEM examination has indicated that the ®bril density is typically 60 ®brils per nodule [1, 3]. Despite this approximation, along with geometric considerations, arising from modeling what are either spherical or ellipsoidal nodules as rectangles; this much-oversimpli®ed model has been found to describe, to a ®rst approximation, the behavior of auxetic PTFE [2], UHMWPE [7±9], and PP [4]. This indicated that the negative Poissons ratio effect at low strains can be primarily attributed to nodule translation due to ®bril hingeing. It does not, however, explain the general features of experimental data at higher strains for auxetic PTFE undergoing tensile loading [1]. A more sophisticated analysis has recently considered the additional stage of ®bril stretching once ®bril hingeing is complete [5]. This has allowed the general features of experimental data at higher strains to be explained for auxetic PTFE undergoing tensile loading. This letter considers theoretically and experimentally the deformation of these microstructural ®brils, with a mechanism based on ®bril hingeing and stretching being proposed and explored. The fabrication processes necessary to produce structural auxetic UHMWPE and PP have been considered at length in previous publications [3, 4, 10±12], so they will only be brie y summarized here. Basically, they consist of three distinct stages: compaction of ®nely divided powder with a rough surface, sintering, and extrusion through a conical die. More recent work has concentrated on the production of UHMWPE and nylon, with lower structural integrity but greater micro-porosity. These particular forms of auxetic polymer have been found to be highly ®brillar in both the axial and radial directions [13] and, as such, are excellent models for the study of microstructural formation. In these cases, the compaction stage of the processing route is omitted because the primary function of compaction is to impart structural integrity to the specimens [10]. Processing thus consists of sintering the polymer powder, followed by extrusion. For

Modelling of Negative Poisson's Ratio Nanomaterials: Deformation Mechanisms, Structure-Property Relationships and Applications

Analytical and Molecular Mechanics methods have been used to study the deformation mechanisms acting at the molecular level in the auxetic polymorph of crystalline silica (a-cristobalite). The analytical models indicate that a-cristobalite deforms by concurrent tetrahedral dilation and cooperative rotation when stretched along the x3 axis, and that a second phase is predicted to exist for this loading scenario, having a geometry similar to that of ‘idealised’ b-cristobalite. This is supported by preliminary Molecular Mechanics simulations, which also indicate that the cooperative rotation predicted for loading along x3 is not sufficient to describe the deformation mechanism for loading along x1. A negative hydrostatic pressure offset is observed to lead to a change in the sign of the predicted Poisson’s ratio from positive to negative, leading to improved agreement of the Molecular Mechanics model with experiment.