P. J. Davies

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.

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.

The Effects of Processing on the Topology and Mechanical Properties of Negative Poisson’s Ratio Foams

Combined heat treatment and applied mechanical strains have been employed to modify the structure and properties of polyurethane (PU) foams. Consequently, foams with a range of pore sizes, shapes and orientations have been produced, including those possessing auxetic (negative Poisson’s ratio) behaviour. Four conversion conditions were employed: triaxial, biaxial and uniaxial compression (to linear compression ratios of 0.9, 0.8 and 0.7) and uniaxial tension to (linear extension ratios of 1.1 and 1.2). The converted foams were then observed by either SEM or optical microscopy, as appropriate, to determine the pore structure. Mechanical properties were measured using tensile testing in conjunction with videoextensometry, allowing the values of Young’s modulus and Poisson’s ratio to be ascertained. Increased anisotropy in the pore structure and elastic properties was observed. The biaxial as well as the triaxial conversion routes led to auxetic behaviour for the PU foams.Copyright