Architectured materials: Straining to expand entanglements.

Abstract

Ordinary materials decrease in density when elastically stretched and increase in density when compressed, in any direction. A few rare types of crystal and some porous solids, called stretch-densified materials, have directions in which this behaviour is reversed — stretch increases density while compression decreases it1,2. Now, writing in Nature Materials, David Rodney et al. have demonstrated materials that increase their volume when either stretched or compressed, becoming less dense3. In so doing, they remove the asymmetry found for both ordinary materials and rare stretchdensified materials, wherein volume changes occur in opposite directions for stretch and compression. The authors demonstrate this unusual behaviour for reversibly deformable structures that are easily made from a coiled nylon fibre or NiTi alloy wire. They self-entangled a coiled fibre to make a low-density, disordered fibre ball and then compressed this ball into a cylinder at a temperature sufficient to set the structure, without causing inter-fibre welding. Deformation of cylinder length reversibly increased cylinder volume by 29.7% for 32.3% cylinder stretch and by 25.9% for 20.1% cylinder compression for a self-entangled coiled structure (SECS) made from superelastic NiTi wire. To expand volume during stretch and during compression, an axially symmetric material must have an axial Poisson ratio of below and above 0.5, respectively, and the SECSs yield this by providing a Poisson ratio that goes from near zero or slightly negative4 for large stretch to about 1.0 for large compression3. Useful insight into the reported behaviour can be obtained by considering a simple coil-based structural model that provides both stretch dilation and compression dilation. An increase in the force constant of the coiled fibres on going from tensile elongation to compression is needed for achieving these properties in the model, and the existence of such a force constant transition is supported by the observation3 that the Young modulus for the SECSs dramatically increased as a tensile strain changed to a compressive strain. An abrupt 25-fold decrease in the fibre force constant has been observed with increasing mechanical load for high-strength nylon fibres that were tightly coiled by twist insertion without using a mandrel5. The explanation for this transition is that the coils in the non-stretched fibre are pulled out of contact by stretching, reducing the fibre force constant. The present model has the lateral strut structure found in an ordinary wine rack, except that the rigid struts of the wine rack are replaced by coiled polymer fibres (or coiled metal wires), which provide high stiffness in compression and low stiffness in elongation. The only deformation modes allowed in the model are strut elongation during stretch and hinge deformation during compression. Figure 1 schematically depicts structural changes for compressive and tensile strains. The force constant for strut compression is much larger than for changing the wine rack angle, θ, that volume increases with increasing tensile compression by an increase of θ. The force constant for strut stretch dramatically decreases when coils are not contacting, which enables strut elongation to dominate over wine rack hinging, resulting in a volume increase during stretch. While this simple model cannot capture the complex deformations and structural features of the SECSs, it does illustrate how the switching of a force constant can result in a material that increases volume when either compressed or stretched. Note that pure ARCHITECTURED MATERIALS

DOI: 10.1038/nmat4436

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Cite this paper

@article{Baughman2016ArchitecturedMS, title={Architectured materials: Straining to expand entanglements.}, author={Ray H Baughman and Alexandre F. Fonseca}, journal={Nature materials}, year={2016}, volume={15 1}, pages={7-8} }