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Papers by Keith M. Azzopardi

physica status solidi (b), 2015

ABSTRACT

physica status solidi (RRL) - Rapid Research Letters, 2015

ABSTRACT

physica status solidi (RRL) - Rapid Research Letters, 2015

Dalton Trans., 2015

The geometric perturbation of the cyclopropyl ring in [LAu(S)](n+) (S = cyclopropyl(methoxy)carbe... more The geometric perturbation of the cyclopropyl ring in [LAu(S)](n+) (S = cyclopropyl(methoxy)carbene) complexes has been recently proposed as an indirect experimental probe of the [LAu](n+) electron-donating power, but experimental data are available only for a phosphine ligand [Brooner et al., Chem. Commun., 2014, 50, 2420, L = P(t-Bu)2(o-biphenyl)]. We broaden the study through DFT geometry optimization of a large number of systems, including anionic, neutral and cationic ligands. We combine these results with the accurate calculation, through charge displacement analysis, of the Dewar-Chatt-Duncanson components of the Au-carbene bond. The results demonstrate a linear correlation between the distortion of the cyclopropyl ring (Δd) and the Au → C π back-donation, which enables us to confidently estimate back-donation from a simple geometry optimization or, when available, from experimental data such as X-ray crystal structures. Consequently, Δd can be reliably used to quantitatively determine the position of each system in the continuum between the carbocationic and carbene extremes and the percentage of back-donation that S is able to accept (Pback). In particular, Pback results to be vanishing with cationic ligands, between 18 and 27% with neutral phosphines and carbenes and around 50% with anionic ligands. Finally, we study the effect of the heteroatom on the substrate, showing that the absolute value of the back-donation is enhanced by around 25% when the methoxy is substituted by a methyl group. Despite this, since the absence of the heteroatom also enhances the maximum capacity of the carbene to accept back-donation, the position of the systems in the continuum moves only slightly toward the carbene end.

Acta Biomaterialia, 2015

Tendons are visco-elastic structures that connect bones to muscles and perform the basic function... more Tendons are visco-elastic structures that connect bones to muscles and perform the basic function of force transfer to and from the skeleton. They are essential for positioning as well as energy storing when involved in more abrupt movements such as jumping. Unfortunately, they are also prone to damage, and when injuries occur, they may have dilapidating consequences. For instance, there is consensus that injuries of tendons such as Achilles tendinopathies, which are common in athletes, are difficult to treat. Here we show, through in vivo and ex vivo tests, that healthy tendons are highly anisotropic and behave in a very unconventional manner when stretched, and exhibit a negative Poisson's ratio (auxeticity) in some planes when stretched up to 2% along their length, i.e. within their normal range of motion. Furthermore, since the Poisson's ratio is highly dependent on the material's microstructure, which may be lost if tendons are damaged or diseased, this property may provide a suitable diagnostic tool to assess tendon health. We report that human tendons including the Achilles tendons exhibits the very unusual mechanical property of a negative Poisson's ratio (auxetic) meaning that they get fatter rather than thinner when stretched. This report is backed by in vivo and ex vivo experiments we performed which clearly confirm auxeticity in this living material for strains which correspond to those experienced during most normal everyday activities. We also show that this property is not limited to the human Achilles tendon, as it was also found in tendons taken from sheep and pigs. This new information about tendons can form the scientific basis for a test for tendon health as well as enable the design of better tendon prosthesis which could replace damaged tendons.

physica status solidi (b), 2014

Scientific Reports, 2015

Auxetic mechanical metamaterials are engineered systems that exhibit the unusual macroscopic prop... more Auxetic mechanical metamaterials are engineered systems that exhibit the unusual macroscopic property of a negative Poisson's ratio due to sub-unit structure rather than chemical composition. Although their unique behaviour makes them superior to conventional materials in many practical applications, they are limited in availability. Here, we propose a new class of hierarchical auxetics based on the rotating rigid units mechanism. These systems retain the enhanced properties from having a negative Poisson's ratio with the added benefits of being a hierarchical system. Using simulations on typical hierarchical multi-level rotating squares, we show that, through design, one can control the extent of auxeticity, degree of aperture and size of the different pores in the system. This makes the system more versatile than similar non-hierarchical ones, making them promising candidates for industrial and biomedical applications, such as stents and skin grafts.

Advanced Engineering Materials, 2014

ABSTRACT Chiral systems may exhibit auxetic behavior, i.e. they may have a negative Poisson’s rat... more ABSTRACT Chiral systems may exhibit auxetic behavior, i.e. they may have a negative Poisson’s ratio. This particular property has led to their being studied extensively by several authors. A Finite Elements Study is presented here, investigating the mode of connection between the nodes and ligaments in the anti-tetrachiral structure. The results show that the amount of gluing material used to attach the ligaments to the node will not affect the Poisson’s ratio, but may have a large influence on the observed stiffness of the structure (Young’s modulus). It is also shown that the stiffness of the glue will have a large effect on the mode of deformation of the chiral system. This change in mechanism was found to effect the stiffness of the structure but not its Poisson’s ratios.

RSC Adv., 2014

ABSTRACT Auxetics are materials which have a negative Poisson’s ratio, that is, upon uniaxial ten... more ABSTRACT Auxetics are materials which have a negative Poisson’s ratio, that is, upon uniaxial tensile loading, they also expand in a direction perpendicular to the applied force. Here, we analyze the elastic constants of stishovite, a high pressure silica polymorph which is known to be a significant constituent of the earth’s mantle, and show that it exhibits negative Poisson’s ratio when stressed in a range of directions in the (100), (010) and (001) planes at specific ambient pressure ranges. We explain this behaviour through mechanisms involving rotations and distortions of the constituting octahedra. These findings have important practical implications since stishovite is one of the hardest known oxides, and has proven to be important to various fields ranging from seismology to materials science.

physica status solidi (b), 2013

Chiral systems are a class of structures, which may exhibit the anomalous property of a negative ... more Chiral systems are a class of structures, which may exhibit the anomalous property of a negative Poisson's ratio. Proposed by Wojciechowski and implemented later by Lakes, these structures have aroused interest due to their remarkable mechanical properties and numerous potential applications. In view of this, this paper investigates the on-axis mechanical properties of the general forms of the flexing anti-tetrachiral system through analytical and finite element models. The results suggest that these are highly dependent on the geometry (the ratio of ligament lengths, thicknesses, and radius of nodes) and material properties of the constituent materials. We also show that the rigidity of an anti-tetrachiral system can be changed without altering the Poisson's ratio.

physica status solidi (b), 2015

ABSTRACT

physica status solidi (b), 2015

ABSTRACT

physica status solidi (RRL) - Rapid Research Letters, 2015

ABSTRACT

physica status solidi (RRL) - Rapid Research Letters, 2015

Dalton Trans., 2015

The geometric perturbation of the cyclopropyl ring in [LAu(S)](n+) (S = cyclopropyl(methoxy)carbe... more The geometric perturbation of the cyclopropyl ring in [LAu(S)](n+) (S = cyclopropyl(methoxy)carbene) complexes has been recently proposed as an indirect experimental probe of the [LAu](n+) electron-donating power, but experimental data are available only for a phosphine ligand [Brooner et al., Chem. Commun., 2014, 50, 2420, L = P(t-Bu)2(o-biphenyl)]. We broaden the study through DFT geometry optimization of a large number of systems, including anionic, neutral and cationic ligands. We combine these results with the accurate calculation, through charge displacement analysis, of the Dewar-Chatt-Duncanson components of the Au-carbene bond. The results demonstrate a linear correlation between the distortion of the cyclopropyl ring (Δd) and the Au → C π back-donation, which enables us to confidently estimate back-donation from a simple geometry optimization or, when available, from experimental data such as X-ray crystal structures. Consequently, Δd can be reliably used to quantitatively determine the position of each system in the continuum between the carbocationic and carbene extremes and the percentage of back-donation that S is able to accept (Pback). In particular, Pback results to be vanishing with cationic ligands, between 18 and 27% with neutral phosphines and carbenes and around 50% with anionic ligands. Finally, we study the effect of the heteroatom on the substrate, showing that the absolute value of the back-donation is enhanced by around 25% when the methoxy is substituted by a methyl group. Despite this, since the absence of the heteroatom also enhances the maximum capacity of the carbene to accept back-donation, the position of the systems in the continuum moves only slightly toward the carbene end.

Acta Biomaterialia, 2015

Tendons are visco-elastic structures that connect bones to muscles and perform the basic function... more Tendons are visco-elastic structures that connect bones to muscles and perform the basic function of force transfer to and from the skeleton. They are essential for positioning as well as energy storing when involved in more abrupt movements such as jumping. Unfortunately, they are also prone to damage, and when injuries occur, they may have dilapidating consequences. For instance, there is consensus that injuries of tendons such as Achilles tendinopathies, which are common in athletes, are difficult to treat. Here we show, through in vivo and ex vivo tests, that healthy tendons are highly anisotropic and behave in a very unconventional manner when stretched, and exhibit a negative Poisson's ratio (auxeticity) in some planes when stretched up to 2% along their length, i.e. within their normal range of motion. Furthermore, since the Poisson's ratio is highly dependent on the material's microstructure, which may be lost if tendons are damaged or diseased, this property may provide a suitable diagnostic tool to assess tendon health. We report that human tendons including the Achilles tendons exhibits the very unusual mechanical property of a negative Poisson's ratio (auxetic) meaning that they get fatter rather than thinner when stretched. This report is backed by in vivo and ex vivo experiments we performed which clearly confirm auxeticity in this living material for strains which correspond to those experienced during most normal everyday activities. We also show that this property is not limited to the human Achilles tendon, as it was also found in tendons taken from sheep and pigs. This new information about tendons can form the scientific basis for a test for tendon health as well as enable the design of better tendon prosthesis which could replace damaged tendons.

physica status solidi (b), 2014

Scientific Reports, 2015

Auxetic mechanical metamaterials are engineered systems that exhibit the unusual macroscopic prop... more Auxetic mechanical metamaterials are engineered systems that exhibit the unusual macroscopic property of a negative Poisson's ratio due to sub-unit structure rather than chemical composition. Although their unique behaviour makes them superior to conventional materials in many practical applications, they are limited in availability. Here, we propose a new class of hierarchical auxetics based on the rotating rigid units mechanism. These systems retain the enhanced properties from having a negative Poisson's ratio with the added benefits of being a hierarchical system. Using simulations on typical hierarchical multi-level rotating squares, we show that, through design, one can control the extent of auxeticity, degree of aperture and size of the different pores in the system. This makes the system more versatile than similar non-hierarchical ones, making them promising candidates for industrial and biomedical applications, such as stents and skin grafts.

Advanced Engineering Materials, 2014

ABSTRACT Chiral systems may exhibit auxetic behavior, i.e. they may have a negative Poisson’s rat... more ABSTRACT Chiral systems may exhibit auxetic behavior, i.e. they may have a negative Poisson’s ratio. This particular property has led to their being studied extensively by several authors. A Finite Elements Study is presented here, investigating the mode of connection between the nodes and ligaments in the anti-tetrachiral structure. The results show that the amount of gluing material used to attach the ligaments to the node will not affect the Poisson’s ratio, but may have a large influence on the observed stiffness of the structure (Young’s modulus). It is also shown that the stiffness of the glue will have a large effect on the mode of deformation of the chiral system. This change in mechanism was found to effect the stiffness of the structure but not its Poisson’s ratios.

RSC Adv., 2014

ABSTRACT Auxetics are materials which have a negative Poisson’s ratio, that is, upon uniaxial ten... more ABSTRACT Auxetics are materials which have a negative Poisson’s ratio, that is, upon uniaxial tensile loading, they also expand in a direction perpendicular to the applied force. Here, we analyze the elastic constants of stishovite, a high pressure silica polymorph which is known to be a significant constituent of the earth’s mantle, and show that it exhibits negative Poisson’s ratio when stressed in a range of directions in the (100), (010) and (001) planes at specific ambient pressure ranges. We explain this behaviour through mechanisms involving rotations and distortions of the constituting octahedra. These findings have important practical implications since stishovite is one of the hardest known oxides, and has proven to be important to various fields ranging from seismology to materials science.

physica status solidi (b), 2013

Chiral systems are a class of structures, which may exhibit the anomalous property of a negative ... more Chiral systems are a class of structures, which may exhibit the anomalous property of a negative Poisson's ratio. Proposed by Wojciechowski and implemented later by Lakes, these structures have aroused interest due to their remarkable mechanical properties and numerous potential applications. In view of this, this paper investigates the on-axis mechanical properties of the general forms of the flexing anti-tetrachiral system through analytical and finite element models. The results suggest that these are highly dependent on the geometry (the ratio of ligament lengths, thicknesses, and radius of nodes) and material properties of the constituent materials. We also show that the rigidity of an anti-tetrachiral system can be changed without altering the Poisson's ratio.

physica status solidi (b), 2015

ABSTRACT