Auxetic behavior of fiber networks: Paper and nonwoven fabrics

Auxetic materials/structures have been subjects of scientific curiosity due to their counterintuitive response to deformation; specifically, they exhibit a negative Poisson’s ratio. While some natural materials are auxetic, scientists and engineers have sought to produce this property in synthetic materials/structures with a goal of improving properties such as damping and producing composites with mechanically interlocking interfaces. The objective of this research was to more fully understand the out-of-plane auxetic response of engineered fiber structures, specifically paper [1] and nonwoven fabrics [2, 3]. Several paper samples and handsheets were tested to quantify their auxetic response and understand how processing and structure affected their auxeticity. The results showed that paper structures possessed a wide distribution of Poisson’s ratio values, both positive and negative. The difference in the values of Poisson’s ratio suggested a strong correlation with the fiber-network structure and the processing conditions employed during papermaking. The nonwoven fabrics studied were needle-punched nonwoven fabrics. These fabrics contained pillars of entangled fibers that were oriented roughly parallel to the thickness direction, and they were not auxetic in their as-produced state. To produce an auxetic response, a post-processing heat compression step was explored to orient more of the fibers in the plane and produce additional fiber junction points. Large, negative values for Poisson’s ratio were observed for the fabrics after heat compression. Of the available processing parameters, temperature was found to affect the auxetic response most strongly, by increasing the compression set of the fabric and, as a result, the auxetic response. Overall, these results showed some commonality between the two systems studied in that fiber network structure and fiber junctions worked together to produce an out-of-plane auxetic response. Additionally, these structural attributes may provide general design guidelines for producing these auxetic fiber structures from other materials, leading to their use in composites. References 1. Verma, P., M.L. Shofner, and A.C. Griffin, Deconstructing the auxetic behavior of paper. Physica Status Solidi (b) – Basic Solid State Physics, 2014. 251(2): 289-296. 2. Verma, P., M.L. Shofner, A. Lin, K.B. Wagner, and A.C. Griffin, Inducing out-of-plane auxetic behavior in needle-punched nonwovens. Physica Status Solidi (b) – Basic Solid State Physics, 2015. 252(7): 1455-1464. 3. Verma, P., M.L. Shofner, A. Lin, K.B. Wagner, and A.C. Griffin, Induction of auxetic response in needle-punched nonwovens: Effects of temperature, pressure and time. Physica Status Solidi (b) – Basic Solid State Physics, 2016. 253(7): 1270-1278

Viscoelastic Properties of a Liquid-Crystalline Monomer and Its Dimer

Quasielastic-light-scattering measurements are reported for a dialkoxyphenylbenzoate monomer and its dimer in their respective nematic phases. The splay and twist elastic moduli of the dimer are found to be nearly independent of molecular length. The dimer\u27s bend modulus, however, shows an anomalous increase with decreasing temperature well below the nematic-isotropic phase transition. Monomer viscosities are consistent with typical literature values, although γ1/ηsplay and ηbend seem to be larger than expected for the dimer, where γ1 is the twist viscosity. The results are discussed in terms of viscoelastic properties of semiflexible rods. The elastic properties in particular are in good agreement with a recent model by Terentjev and Petschek (unpublished)

Blocked Shape Memory Effect in Negative Poisson's Ratio Polymer Metamaterials

We describe a new class of negative Poisson’s ratio (NPR) open cell PU–PE foams produced by blocking the shape memory effect in the polymer. Contrary to classical NPR open cell thermoset and thermoplastic foams that return to their auxetic phase after reheating (and therefore limit their use in technological applications), this new class of cellular solids has a permanent negative Poisson’s ratio behavior, generated through multiple shape memory (mSM) treatments that lead to a fixity of the topology of the cell foam. The mSM-NPR foams have Poisson’s ratio values similar to the auxetic foams prior their return to the conventional phase, but compressive stress–strain curves similar to the ones of conventional foams. The results show that by manipulating the shape memory effect in polymer microstructures it is possible to obtain new classes of materials with unusual deformation mechanisms.peer-reviewe

Dissipative Field Theory with Caldeira-Leggett Method and its Application to Disoriented Chiral Condensation

The effective field theory including the dissipative effect is developed based on the Caldeira-Leggett theory at the classical level. After the integration of the small field fluctuations considered as the field radiation, the integro-differential field equation is given and shown to include the dissipative effects. In that derivation, special cares should be taken for the boundary condition of the integration. Application to the linear sigma model is given, and the decay process of the chiral condensate is calculated with it, both analytically in the linear approximation and numerically. With these results, we discuss the stability of chiral condensates within the quenched approximation.Comment: 16pages, ReV-Te

Aflatoxin in Seed Cotton During Short-Term Trailer Storage

Three experimental harvesting and storage treatments were applied to seed cotton in the Yazoo-Mississippi Delta to determine their effectiveness in preventing the development of aflatoxins during the period between harvesting and ginning. The experiments were conducted using full-scale production and ginning equipment. The experimental harvesting and storage treatments were (1) to harvest wet with dew and store without drying, (2) to harvest wet with dew and gin-dry before storage, and (3) to harvest after the dew had evaporated and store without drying. The storage period for all treatments was 7 days. Each treatment was replicated five times in each of two successive crop years. Development of aflatoxins in cottonseeds during seed-cotton storage may be minimized by ginning the picked-wet cotton by the end of the third day after harvesting and by ginning the field-dried cotton by the end of the fourth clay. Although the gin-drying treatment was successful in controlling aflatoxin production in stored cotton, it was considered impractical as a cotton-production process

Hydrogen Bonding as the Origin of Both Liquid Crystallinity and Polymer Formation in Some Supramolecular Materials

The use of lateral substitution on the aromatic rings of the diacid component of diacid/bispyridyl AABB supramolecular LC polymers is effective at retarding crystallization presumably by introducing a \u27carbuncle\u27 along the chain which interferes with the requisite efficient close packing of molecular units. We have examined both one and two chloro groups per ring and have observed a glass transition and fiber formation in the least crystalline complex

Bend elastic modulus of a bent and straight dimeric liquid crystal

Light scattering measurements in the nematic phase are reported for two dimers based upon the monomer $4,4'$-dipentyloxyphenylbenzoate. One dimer has an even number of methylene units in the spacer and, in consequence, is approximately straight in th all-trans conformation. The other dimer has one less methylene unit in the spacer and is, therefore, bent at the spacer group. We find that the ration of the twist to splay elastic modulus is nearly the same for both species, but that the ratio of the bend to splay modulus is considerably smaller in the bent dimer.Des expériences de diffusion de la lumière dans la phase nématique de deux dimères d'unité monomérique $4,4'$-dipentyloxyphenylbenzoate sont présentées. Un des dimères possède un nombre pair de groupes méthyléniques dans l'espaceur ce qui le rend approximativement linéaire dans une conformation toute trans. L'autre dimère a un groupe méthylénique de moins, ce qui impose une courbure de la molécule. Nous montrons que le rapport du module d'élasticité de torsion sur le module d'élasticité en éventail est presque le même pour les deux dimères, tandis que le rapport du module de courbure sur le module en éventail est considérablement plus petit dans le cas du dimère courbé