Designable Buckling Deformation Of Shape-memory Metamaterial And Its Impregnated Surface With Switchable Lipophilic/oleophobic Properties

In comparison with conventional shape memory alloy,shape memory polymers(SMPs)have attracted great passions due to their shape memory effect,high specific mechanical stiffness,strength,low density,easy manufacturing,stiffness tailorable and a variety of actuation approaches,resulting into them with great potential applications in aerospace,automobile,biomedicine,soft robotics and so on.However,the thermally responsive SMPs always present a low mechanical strength of 100 MPa in shape recovery process,shape memory effect driven by heating and low thermal conductivity.Therefore,3D printing technology has been employed to achieve the SMP and 3D printed structure with high mechanical strength and shape memory behavior,respectively.However,there is few studies have reported the research works on structure mechanics combined with the material mechanics.On this motivation,the first aims on the negative thermal effect in shape memory double-network hydrogel,and explore the toughening mechanism in the coolingdriven shape memory effect in hydrogel.And then,a 3D printing of auxetic shape memory metamaterial has been presented to achieve a designable buckling behavior,moreover,a constitutive relationship among material modulus,temperature and geometrical size of structure has been formulated and discussed.Consequently,a tunable hyperbolic out-of-plane deformation of 3D-printed auxetic polylactic acid(PLA)shape memory arrays has been designed.A structural optimization design has been achieved for the shape memory arrays,of which the mechanical performance and shape memory behavior both have been significantly improved,based on the principle of minimum strain-energy.Finally,a smart shape memory surface has been proposed using the SMP and shape memory hydrogel,which have been used as the substrate and soft component,respectively.A stimuli-responsive hydrogel impregnated smart surface with switchable lipophilic/oleophobic properties has been investigated by the finite element analyses and experimental measurements,resulting from the structural optimization of the size of substrate and swelling effect of hydrogel.It is expected to provide an effective approach to achieve smart surface with switchable lipophilic/oleophobic properties in the practical and potential applications.In second part,the negative thermal effect in shape memory double-network hydrogel has been explored,according to the Flory-Huggins solution theory.A heating-driven toughening mechanism has been discussed due to that the transition between hydrophilicity and hydrophobicity of molecular segment,resulting into the thermodynamic micro-phase separation.Furthermore,the Fick’s second law has been employed to formulate a thermodynamic equilibria of chemical potential,to enable a temperature-dependent multi-shape memory behavior.Finally,a comparison between the analytical results of proposed model and experimental data has been presented,and a good agreement has been found.The present thermodynamic equilibria has then been employed to experimentally achieve a two-way and three-way shape memory behavior,moreover,a toughening stress has been experimentally measured from 24.7k Pa to 184 k Pa,with an increase in temperature from 293 K to 338 K.In third part,an extended Maxwell model has been formulated to investigate the viscoelastic behavior of the PLA SMP,and a constitutive relationship among the elastic modulus,temperature and strain rate has been proposed.Then,an auxetic shape memory metamaterial,which is incorporated of cell structures,has been designed to achieve a designable buckling response using the thermomechanically coupled in-plane instability.The influence of viscoelasticity on in-plane moduli and Poisson’s ratios of shape memory auxetic metamaterial was experimentally investigated.Based on the finite element analysis,thermomechanical buckling transition behavior of the auxetic metamaterials,from negative to positive Poisson’s ratio,was further investigated as a function of geometrical morphology,temperature and strain rate.Finally,the simulation results have been verified by the experimental ones,and a well agreement between simulation and experimental results has been found.This study provides a simple and efficient way to generate auxetic structures using the designable buckling effect.In fourth part,the tunability of the bidirectionally hyperbolic deformation of thin plates under uniaxial compression,instead of the Euler buckling deformation has been investigated.Inspired by the horseshoe structure,through tuning the arc angle and radius of the central circles,bidirectionally hyperbolic out of plane deformation is achieved together with auxetic behavior using the finite element analysis.Based on the principle of minimum strain-energy and extended Maxwell model,the dependences of bidirectionally hyperbolic out of plane deformation on the temperature,arc angle,radius of the central circles and arrangement of unit cells have been studied and discussed.This study promotes a structural optimization of the 3D printing shape memory metamaterial,of which the mechanical performance and shape memory behavior both have been significantly improved.In the last but not least part,a stimuli-responsive shape memory surface with switchable lipophilic/oleophobic properties,has been proposed and studied,based on the SMP substrate and shape memory hydrogel,which is impregnated ino the substrate.Under swelling,the geometrical change of hydrogel is able to buckle the surface due to the structural confinement and create a continuous transition of surface topology.Thus,it will yield a change on surface wetting property from oleophilic to oleophobic with a contact angle of oil.We quantitatively investigate this structureproperty relationship,including surface modification,immersion time and swelling ratio,using finite element analysis and analytical modeling,and the simulation results and the modeling are in good agreement with the experimental ones.This study is expected to design a switchable lipophilic/oleophobic surface,which holds its potentials to be developed into autonomous system for future sub-sea/off-shore engineering applications to separate oil and water.