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Abstract
We simulate and analyse three types of two-dimensional networked polymers which have been predicted to exhibit on-axis auxetic behaviour (negative Poisson's ratio), namely (1) polyphenylacetylene networks that behave like flexing re-entrant honeycombs, commonly referred to as ‘reflexynes’, (2) polyphenylacetylene networks that mimic the behaviour of rotating triangles, commonly referred to as ‘polytriangles’ and (3) networked polymers built from calix[4]arene units. More specifically, we compute and compare their in-plane off-axis mechanical behaviour, in particular their off-axis Poisson's ratios and show that in some cases, the sign and magnitude of the Poisson's ratio are dependent on the direction of loading. We propose two functions that can provide a measure for the extent of auxeticity for such anisotropic materials and show that the polytriangles are predicted as the most auxetic when compared with the other networks with the reflexyne re-entrant networks being the least auxetic.
Acknowledgements
The financial support of the Malta Council for Science and Technology and of the Malta Government Scholarship Scheme (Grant Number ME 367/07/17 awarded to Daphne Attard) is gratefully acknowledged. The authors are also very grateful to Accelrys Inc. for the assistance they provided.
Notes
2. As shown in Table , the PCFF force-field can simulate all the systems under consideration allowing for a proper and unbiased comparison between the systems, as discussed later.
3. The SMART minimiser as implemented within MS Discover is a compound minimiser where the minimisation commences with the steepest descent algorithm followed by a conjugate gradient algorithm and terminates with a Newton Raphson algorithm.