Auxetic polypropylene fibres:Part 1 - Manufacture and characterisation

Abstract

The term 'auxetic' is applied to materials that possess a negative Poisson's ratio ν. The use of auxetic polymers has been limited because of problems with deploying them in their fabricated forms, i.e. as 10 mm diameter cylinders. This paper reports the successful development of a processing route to produce a more useful and usable form of auxetic polymeric material, namely fibres. A conventional polymer processing technique (melt spinning) is the basis of this technique, with novel modifications. Video extensometry was used to measure the Poisson's ratio and a value of ν = -0.60±0.05 was obtained.     

Unusual Elastic and Mechanical Behaviors of Copper Phosphate Glasses with Different Copper Valence States

Elastic and mechanical properties such as Young's modulus E, Poisson's ratio ν, Debye temperature θ_D, Vickers hardness H_v, fracture toughness K_c, and fracture surface energies γ_f of yCuO_x·(100−y)P₂O₅ glasses (y= 45, 50, 55) with different copper valence states, i.e., R(Cu⁺) = Cu⁺/(Cu⁺+ Cu²⁺), at room temperature (humidity 64%) have been examined. The following features have been found: (1) the glass transition temperature (218–434°C), H_v (2.7–4.4 GPa), E (50.6–78.2 GPa), and θ_D (358–434 K) decrease largely with increasing R(Cu⁺); (2) the mean atomic volume, K_c (0.56–1.14 MPa·m^1/2), and γ_f (1.9–11.2 J·m⁻²) tend to increase with increasing R(Cu⁺); (3) 50CuO_x·50P₂O₅ glasses with R(Cu⁺) = 0.42 and 0.55 have a high resistance against crack formation in Vickers indentation tests and no crack is observed in the 45CuO_x·55P₂O₅ glass with R(Cu⁺) = 0.57 under an applied load of about 98 N. The results demonstrate that elastic and mechanical properties of yCuO_x·(100−y)P₂O₅ glasses depend strongly on the copper valence state and the CuO_x/P₂O₅ ratio. The unusal mechanical and elastic properties of copper phosphate glasses are well explained qualitatively by considering unique oxygen coordination and bonding states of Cu⁺ ions, i.e., lower coordination number and more covalent bonding compared with Cu²⁺ ions.     

Mechanical characterization of a polysiloxane-derived SiOC glass

Silicon oxycarbide glass with the composition Si_1.0O_1.6C_0.8 was synthesized from a commercial polysiloxane by polymer pyrolysis. Dense SiOC samples were obtained by cross linking of the polysiloxane followed by warm pressing to form cylindrical samples and subsequent pyrolysis of the shaped polymer at 1100 °C in Ar. Hardness (H), Young's modulus (E) and Poisson's ratio (ν) of the as-prepared SiOC glass were evaluated from indentation studies and from acoustic microscopy. Indentation studies showed that E depends on the applied load and amounts to 90 GPa for low load and to 180 GPa for high load. Average values of 6.4 and 101 GPa were obtained for H and E, respectively, by the Vickers indentation method. Acoustic microscopy analysis yielded values of 96 GPa and 0.11 for E and ν, respectively. Compared to vitreous silica, the Young's modulus of the SiOC glass is about 1.3–1.5 times higher. To the knowledge of the present authors, the measured Poisson's ratio (ν = 0.11) is the lowest reported so far for glasses and polycrystalline ceramics.     

Dispersion of surface acoustic waves in polycrystalline diamond plates

Five free-standing polycrystalline diamond plates were grown by electron-assisted hot filament CVD on molybdenum using different deposition parameters. The homogeneity of the elastic properties was studied for each side of the samples by surface acoustic waves (SAWs) using pulsed laser excitation and piezoelectric detection. From the velocities of the longitudinal acoustic bulk waves (LABWs) and SAWs, values of the Poisson ratio of approximately ν=0.14±0.06 were obtained. The Young's modulus varied between E=928±40 GPa and E=1098±43 GPa. Anomalous dispersion of the SAWs on the nucleation side was observed in some samples. In addition the mean velocity of the SAWs was generally higher on the nucleation side than on the growth side. This indicates that the elastic properties changed considerably during the growth process. Measurements of the grain size by scanning electron microscopy (SEM), of the phase purity by Raman spectroscopy, and the texture by X-ray diffraction (XRD) were performed in order to estimate the contribution of different effects to the dispersion and the side dependence of the SAW velocities. In addition Fourier transform infrared (FTIR) spectroscopy was applied to study the influence of the hydrogen content.     

Elastic behaviour of zirconium titanate bulk material at room and high temperature

Zirconium titanate (ZrTiO₄) is a well known compound in the field of electroceramics, however, its potential for structural applications has never been analysed. Moreover, it is compatible with zirconia, thus, zirconium titanate–zirconia composites might have potential for structural applications in oxidizing atmospheres. Nevertheless, there are currently no data about elastic properties of zirconium titanate materials in the literature. In view of the importance of these properties for the structural integrity of components subjected to high temperature and mechanical strains, an attempt was done in this work to determine the elastic properties of ZrTiO₄, both at room and high temperature. Young's modulus (161 ± 4 GPa), shear modulus (61 ± 1 GPa) and Poisson's ratio (0.32 ± 0.01) values at room temperature have been estimated for a fully dense single phase ZrTiO₄ material from experimental data of sintered single phase ZrTiO₄ materials with different porosities (6–19%). Values for room temperature Young's modulus are in agreement with those obtained by nanoindentation. Young's modulus up to 1400 °C shows an unusual dependence on temperature with no significant variation up to 500 °C an extremely low decrease from 500 to 1000 °C (≈0.02–0.03% every 100 °C) followed by a larger decrease that can be attributed to grain boundary sliding up to 1400 °C.     

Unidirectional fiber–polymer composites: Swelling and modulus anisotropy

The solvent swelling of unidirectional rubber–fiber composites was studied. The amount of matrix swelling was constrained to the extent that would be predicted from the thermodynamic theories of elasticity and polymer–solvent interaction. The geometry of swelling was found to be orthotropic in nature. A simple trigonometric function was derived to relate linear deformation due to swelling to the angle which the direction of its measurement makes with the fiber direction. The validity of the derivation was demonstrated experimentally. Considering swelling to be the imposition of tensile forces of equal magnitude in all directions, and considering a swelling-induced linear deformation to be analogous to a tensile compliance, a simple set of relationships between elastic parameters and their direction of measurement was derived: where E_θ, G_θ, v_θ, and η_θ are Young's modulus, shear modulus, Poisson's ratio, and the shear coupling ratio measured in a longitudinal transverse plane at an angle with the fiber direction, respectively, and E_L, G_LT, and θ_LT are the longitudinal Young's modulus, the longitudinal transverse shear modulus, and the longitudinal transverse Poisson ratio, respectively. Further simplifying the case of combined transverse isotropy and special orthotropy was the conclusion that 1/G_LT = 1/E_T + (1 + 2v_LT)/E_L. The relationships for G and E were experimentally demonstrated.     

Novel variations in the microstructure of the auxetic microporous ultra‐high molecular weight polyethylene. Part 1: Processing and microstructure

Recent publications have detailed how auxetic (negative Poisson's ratio) polymers have been fabricated by a novel thermal processing route consisting of three welldefined stages—compaction, sintering and extrusion. In this paper, the compaction stage of the processing route is omitted and the effects of this on the structural integrity, microstructure and Poisson's ratios of the extrudates examined. The effects of varying the processing parameters of the sintering and extrusion stages are studied so that a set of conditions that produce a highly fibrillar auxetic material with sufficient structural integrity to allow mechanical properties to be evaluated can be defined. Poisson's ratios as low as ?4 have been obtained.     

Novel fabrication route for auxetic polyethylene. Part 1. Processing and microstructure

One method for producing synthetic auxetic materials is starting with polymer powders and using a combination of compaction, sintering, and extrusion. This article presents a novel variation on this route, omitting the extrusion stage and generating the required microstructure by compaction followed by multiple sintering. The effects of single, double, and quadruple sintering on compacted cylinders are examined in terms of a detailed microstructural examination, study of density and dimensional variations, and measurement of Poisson's ratio. The best results were obtained by compaction followed by double sintering, resulting in a strain dependent Poisson's ratio as low as ν = ?0.32. This new technique has great potential for increasing the range of geometries that can be fabricated and is very akin to ceramic sintering techniques. POLYM. ENG. SCI., 45:568–578, 2005. © 2005 Society of Plastics Engineers     

The effects of powder morphology on the processing of auxetic polypropylene (PP of negative poisson's ratio)

With the use of a three-stage thermal processing route, similar to that used previously for the production of ultra high molecular weight polyethylene possessing a negative Poisson's ratio (i.e. displaying auxetic behavior), an auxetic form of polyethylene has been fabricated. The polypropylene is processed by the compaction, sintering, and extrusion of a powder. The importance of powder morphology on the ability of polypropylene to achieve auxetic behavior has been examined, revealing that particle shape, size, and surface roughness are critical variables for successful processing. Negative Poisson's ratios of up to −0.22 at 1.6% strain have been obtained. The data have been successfully interpreted by use of a simple geometric model based on the polymer microstructure. These suggest that, by further optimizing the processing conditions, much larger negative Poisson's ratios should be achievable.     

Fracture Behaviors and Ferroelastic Deformation in W/Cr Co‐Doped Bi4Ti3O12 Ceramics

In this paper, the influence of Cr₂O₃ additive (y = 0~0.4 wt%) on fracture behaviors, ferroelastic deformation, and mechanical strengths of Bi₄Ti_2.95W_0.05O_12.05 (BTW) Aurivillius ceramics was investigated. For these W/Cr co‐doped Bi₄Ti₃O₁₂ ceramics (BTW–yC), SEM analysis on their fractured surfaces demonstrate that the transgranular fracture is the main fracture mechanism, however, the intergranular fracture also exists in the sample at y = 0.3. Impedance analyses based electrical resonance show that their Poisson's ratio and Young's modulus vary with y in a similar trend, while the frequency constant and elastic compliance exhibit a contrary varying trend with y. On the other hand, under the Vickers indentation, the crack propagation in BTW–yC ceramics takes the form of the long‐straight extension, small‐angle deflection, and short‐distance branching at y = 0, 0.2, and 0.4, respectively. In the uniaxial compression tests, the stress–strain behavior of ceramics consist of three stages, i.e., linear elastic deformation, ferroelastic domain switching, and microcrack propagation. The ferroelastic domain switching tends to form the resistance for the microcrack propagation. Furthermore, uniaxial bending tests prove that the fracture strength is dependent on the grain size and fracture toughness, Overall, the sample at y = 0.1 gains a better mechanical strength among BTW–yC ceramics.     

Elastic properties of translucent polycrystalline cubic boron nitride as characterized by the dynamic resonance method

Cubic boron nitride (cBN) is second only to diamond in a number of extreme material properties, and its performance exceeds diamond in many applications involving contact with ferrous alloys and/or high temperatures. However, its properties are less well understood. We have sintered cBN powder (2–4 μm or 8–12 μm particle size) into pure, translucent, polycrystalline compacts by pressing at a pressure of 7.7 GPa and temperatures from 2100 to 2350°C without any sintering agent. We have determined the Young's modulus E, shear modulus G, and Poisson's ratio ν of a number of translucent polycrystalline cBN compacts, in the form of free-standing disks, using the dynamic resonance method. The measured values for E, G, and ν lay in the ranges of 665–895 GPa, 295–405 GPa, and 0.11–0.15, respectively, depending on the grain size of the cBN starting material and the sintering temperature. These values may be compared with the theoretical values of E, G, and ν for pure, equiaxed, cBN of 909 GPa, 405 GPa, and 0.12, respectively. Combining the Young's modulus with previous Vickers hardness measurements, the fracture toughness K_IC of well-sintered translucent PCBN is evaluated as 6.8 MPa m^1/2. The dependence of the elastic properties on the synthesis conditions is discussed in the context of the microstructure and of related material properties.     

High‐Performance Nanocomposites of Sodium Carboxymethylcellulose and Graphene Oxide

High‐performance nanocomposites of NaCMC with GO are produced by solution casting. FESEM images reveal a good homogeneous dispersion of GO in the NaCMC matrix. The composite formation is facilitated by H‐bonding interaction between GO and NaCMC. T_g of the composites increases with increasing GO concentration. The storage modulus (G′) exhibits a maximum 174% increase over NaCMC at 1 wt% GO. The mechanical properties of the composites exhibit highest increase of tensile stress and Young's modulus of 188 ± 4% and 154 ± 11%, respectively, for 1 wt% GO. Analysis of Young's modulus (E_y) data using the Halpin‐Tsai equation suggests that the E_y data are close to the unidirectional orientation at >0.5 wt% GO, indicating more efficient load transfer at these compositions.

     

Gel‐spinning of partially saponificated poly(vinyl alcohol)

DMSO/water (80/20 volume ratio) solutions of commercial poly(vinyl alcohol)s (a‐PVA₉₉, a‐PVA₈₈) with degrees of saponification of 99.3 and 88 mol % were gel‐spun into methanol (−20 and −70°C). The dry filaments obtained were drawn at 200°C (a‐PVA₉₉) and 150–180°C (a‐PVA₈₈). The maximum draw ratio and Young's modulus were 26 and 34 GPa for a‐PVA₉₉ and 21 and 24 GPa for a‐PVA₈₈ (drawing temperature: 160°C). So, at first, the dry filaments obtained for a‐PVA₈₈ were drawn at 150–180°C until 10 times their original length. Moreover, the predrawn a‐PVA₈₈ filaments were perfectly saponificated under fixing at the both ends and then the filaments were redrawn at 200°C. The maximum draw ratio and Young's modulus for the filaments (a‐PVA_88→99) predrawn at 150°C were 28 and 39 GPa, respectively. The a‐PVA_88→99 filaments had two melting peaks (228 and 236°C). © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2872–2876, 2000     

The production and properties of poly(aryletherketone) (PEEK) rods oriented by drawing through a conical die

Oriented poly(aryletherketone) (PEEK) rods have been produced by drawing isotropic polymer through a conical die. Room temperature Young's moduli were measured by three-point bending and ranged from 5 GPa at a draw ratio of about 2 to 11 GPa at a draw ratio of about 4. Dynamic mechanical properties were explored in the range ?150 to 200°C; two loss peaks were observed, with the higher corresponding to T_g.     

Elastic behaviour of zirconium titanate-zirconia bulk composite materials at room and high temperature

Zirconium titanate-zirconia composites have potential for applications involving variations of temperature. Elastic characterization is necessary to evaluate stresses developed in materials which may be used in these kinds of applications. In this work, Young's and shear modulus and Poisson's ratio of two zirconium titanate-zirconia bulk composites (Z(Y)T70 and Z(Y)T50) have been determined at room temperature by the Impulse Excitation Technique (IET). Furthermore, Young's modulus (E) has been determined at high temperature (up to 1400 °C) for both composites. Young's modulus of Z(Y)T70 composite decreases ≈6% between room temperature and 400 °C due to the presence of zirconia. From 400 to 1400 °C, the decrease of E (≈14%) is due to the presence of zirconium titanate. Young's modulus behaviour at high temperature of Z(Y)T50 composite is determined by the degree of microcrack healing, which depends on the maximum temperature reached.     

Microstructure evolution and mechanical behavior of a mullite fiber heat-treated from 1100 to 1300°C

In this paper, the effect of phase transformation on microstructure evolution and mechanical behaviors of mullite fibers was well investigated from 1100 to 1300°C. In such a narrow temperature range, the microstructure and mechanical properties showed great changes, which were significant to be studied. The temperature of the alumina phase transformation started at below 1100°C. The main phases in fibers were γ-Al₂O₃ and δ-Al₂O₃ with amorphous SiO₂ at 1150°C. The stable α-Al₂O₃ formed at 1200°C. Then the mullite phase reaction occurred. As the alumina phase reaction took place, the tensile strength increased with the increasing temperature. In particular, the filaments achieved the highest strength at 1150°C with 1.98 ± 0.17 GPa, and the Young's modulus was 163.08 ± 4.69 GPa, showing excellent mechanical performance. After 1200°C, the mullite phase reaction went on with the crystallization of orthorhombic mullite. The density of surface defects increased rapidly due to thermal grooving, which led to mechanical properties degrade sharply. The strength at 1200°C was 1.01 ± 0.15 GPa with a strength retention of 63.13%, and the Young's modulus was 184.14 ± 10.36 GPa. While at 1300°C, the tensile strength was 0.64 ± 0.14 GPa with a strength retention of only 40.00%.     

Characterization of jute fibers treated with soap–glycerol micelles

A tossa variety of jute fiber (Corchorus olitorious) treated with soap–glycerol micelles is characterized by infrared (IR) spectroscopy, X‐ray diffraction method, and tensilometry. The IR spectra for jute fibers treated with soap–glycerol micelles show a reduced absorption band due to O H stretching at a frequency of 3420 cm⁻¹ with almost absent OH bending frequencies, prominent CH₂ stretching and bending frequencies at 2915 and 1440 cm⁻¹ and reduced skeletal vibration at 1060 cm⁻¹. The percentage crystallinity measured by the X‐ray diffraction method increases from 45 to 53% on treated jute fibers. The tensile strength and strain percent at maximum load, Young's modulus, and work done per unit volume within an elastic limit (resilience) for treated fibers increased from 1.8 ± 0.2 to 3.43 ± 0.2 GPa, from 3.98 ± 0.1 to 4.75 ± 0.1, from 75 ± 2 to 113 ± 5 GPa, and from 26 ± 2 to 74 ± 3 MJ m⁻³, respectively. Using a stabilizing agent (2%) and a swelling agent (2% KOH), the tensile strength, strain percent, Young's modulus, and resilience increase to 4.02 ± 0.2 GPa, 4.85 ± 0.3, 154 ± 5 GPa, and 95 ± 4 MJ m⁻³, respectively. Under natural weathering at 12–30°C and 30–80% relative humidity over a prolonged period of 8 weeks, all the tensile properties for micelle‐treated fibers increase during the first 2 weeks of exposure and then decrease exponentially to the starting values. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 852–856, 2000     

Mechanical and thermal properties of light weight boron-mullite Al5BO9

Al₅BO₉ is a promising thermal sealing material for hypersonic vehicles due to its low density, theoretically predicted low shear modulus, and low thermal conductivity. However, experimental investigations on the mechanical and thermal properties of bulk Al₅BO₉ have not been carried out. Herein, we report the mechanical and thermal properties of bulk Al₅BO₉ prepared by spark plasma sintering of solid-state reaction synthesized Al₅BO₉ powders. The bulk (B), shear (G), and Young's (E) moduli are 148 GPa, 85 GPa, and 214 GPa, respectively, which are close to the theoretical values. The Pugh's ratio G/B is 0.574, indicating its intrinsic damage tolerance, which is also revealed by Hertzian contact test. The Vickers hardness (H_v) is 10.8 GPa, being lower than mullite. The flexural strength, compressive strength, and fracture toughness are, respectively, 277 ± 35 MPa, 814 ± 75 MPa, and 2.4 ± 0.3 MPa·m^1/2, which are close to those of mullite. Al₅BO₉ has anisotropic coefficient of thermal expansion (CTE) in three crystallographic directions, ie α_a = (4.40 ± 0.21) × 10⁻⁶ K⁻¹, α_b = (7.11 ± 0.18) × 10⁻⁶ K⁻¹, α_c = (6.70 ± 0.29) × 10⁻⁶ K⁻¹ from Debye temperature to 1473 K, which are underpinned by its structural feature, ie lower α_a is resulted from the edge-shared AlO₆ octahedron chains along the [100] direction. The average CTE is (6.05 ± 0.06) × 10⁻⁶ K⁻¹. The thermal conductivity declines with temperature as κ = 1336.39/T + 1.97, consisting with predicted trend from Slack's model. The low thermal conductivity and low density guarantee Al₅BO₉ a promising candidate as ceramic wafer in the seal structure for hypersonic vehicles.     

Study on the Viscoelastic Poisson‘s Ratio of Solid Propellants Using Digital Image Correlation Method

Digital image correlation methods were used for further studies of the viscoelastic Poisson's ratio of solid propellants. The Poisson's ratio and the Young's relaxation modulus of solid propellants were separately determined in a single stress relaxation test. In addition, the effects of temperature, longitudinal strain, preload and storage time on the Poisson's ratio of solid propellants were discussed. The Poisson's ratio master curve and the Young's relaxation modulus master curve were constructed based on the time‐temperature equivalence principle. The obtained results showed that the Poisson's ratio of solid propellants is a monotone non‐decreasing function of time, the instantaneous Poisson's ratio increased from 0.3899 to 0.4858 and the time of the equilibrium Poisson's ratio occurred late when the temperature was varied from −30 °C to 70 °C. The Poisson's ratio increased with temperature and longitudinal strain, decreased with preload and storage time, while the amplitude Poisson's ratio increased with preload, decreases with longitudinal strain and storage time. The time of the equilibrium Poisson's ratio occurred in advance with the increase of longitudinal strain, preload and storage time.     

Strengthening the thermal Negative Poisson's ratio structures by SiC chemical vapor infiltration

Negative Poisson's ratio structures exhibit adjustable thermal expansion behavior as the thermal stress can be dispersed or offset by torsion, bending, and tension of the struts. However, the structural stability under cyclic thermal stress significantly determines the long-term durability. Strengthening the Negative Poisson's ratio structure can ensure high thermal and mechanical reliability. The work designed a heat-induced torsional Negative Poisson's ratio structures and fabricated it by 3D printing. For efficient strengthening, the preforms were further densified by chemical vapor infiltration (CVI) of SiC to enhance the reliability. Pores and gaps in the preforms were homogeneously covered and filled by the SiC, enhancing the surface finish and mechanical performance. The heat induced torsion of the structures dispersed the heat flow in one single direction, reducing the thermal stress concentration. The independent thermal expansion change of the structural unit can offset or consume the heat dissipation stress, and further improve the reliability and thermal stability through the densification process. As a result, the 120° twisted structure exhibited an average coefficient of thermal expansion (CTE) of 6. 12 × 10^?6/K from room temperature (RT) to 500 °C, and the instantaneous CTE reached the minimum value of 4.01 × 10^?6/K at 125 °C. Meanwhile, the load-bearing capacity strengthened significantly, exhibiting the optimized strength of 11.31 MPa and Young's Modulus of 36.44 GPa, revealing a significant improvement than those of preforms, promising for high load-bearing and low expansion application of structure-function integrated low expansion material.