The Constitutive Model Of Shape Memory Polymers And The Properties Of TPI Composites

Shape Memory Polymers(SMPs)is one kind of materials which changes to its original shape from its temporary one under an appropriate stimulus,such as,temperature,moisture,pH,electric or magnetic fields.As one of the most important types of smart materials,SMPs have advantages such as low density,large recovery ability,superior process ability and low cost.During the past thirty years,SMPs have attracted a strong research interest from academia and industries.SMPs have been widely used in textile industry,medical,aerospace and microelectronics engineering.In this thesis,we focused on thermomechanical behavior of shape memory polymers,a new type of microstructure of SMPs is proposed.Base on the viscoelasticity theory,a new thermodynamics viscoelastic model of SMPs is developed.Additionally,the shape memory trans-1,4-polyisoprene(TPI),short carbon fiber/TPI composites and carbon fiber/alumina/TPI hybrid nanocomposites were developed and the shape memory and thermo-mechanical properties of shape memory TPI,short carbon fiber/TPI composites and carbon fiber/alumina/TPI hybrid nanocomposites are investigated.Firstly,a thermodynamics viscoelastic constitutive model is developed to describe the typical thermo-mechanical cycle experimentations.In this model,the changes of thermodynamic and mechanics for SMPs in cooling/heating process were described clearly and the irreversible deformations were calculated.A new type of microstructure of SMPs was proposed and new definitions of phases:active phase and frozen phase are set forth in the present study.The new thermodynamics viscoelastic model is series by two parts:mechanical parts and thermal parts.A new transform equation for the transformation of frozen phase was proposed,and the transform delay time and cooling/heating rate are considered in this transform equation.A transform equation based on the transformation of frozen phase is proposed to describe the change of thermal expansion coefficient of SMPs.Finally,the typical thermo-mechanical cycle experimentation processes are calculated using the new thermodynamics viscoelastic constitutive model.Through the comparison between the calculated results and experimental results,the rationality and accuracy of the new thermodynamics viscoelastic constitutive model is finally verified.Secondly,a new preparation technology of SMPs is developed and shape memory TPI test specimens are manufactured.The shape memory and thermo-mechanical properties of shape memory TPI are investigated by DSC test,shape memory test,static tensile tests,mechanical cyclic tests and thermo-mechanical cyclic tests.Thirdly,chopped carbon fiber reinforced TPI was developed via a proposed new manufacturing process with the aim of improving weak mechanical properties of bulk TPI bulk.Specimens of the developed shape memory polymer(SMP)composites were fabricated with carbon fiber weight fraction of 5%,7%,9%,11%and 13%,respectively.Measured are the effects of chopped carbon fiber and temperature on:(a)shape recovery ratio and rate;(b)stress-strain relationship;(c)maximum tensile stress,strain and Young's modulus;and(d)maximum stress and residual strain under a constant strain cyclic loading.In addition,SEM micrographs were also presented to illustrate the fracture surface.The present experimental results show that the SMP with 7%carbon fiber weight fraction appears to perform best in all the tests.This indicates that the 7%carbon fiber weight fraction could be the optimum value for the SMP developed using the proposed manufacturing process.Finally,carbon fiber/alumina/shape memory polymer(SMP)hybrid nanocomposites were developed with added short carbon fiber(7 wt%)and alumina nanoparticles(0.0-2.0 wt%)for improving the weak mechanical properties of TPI bulk.The shape memory properties and the effect of the weight fraction of alumina nanoparticle and chopped carbon fiber on the mechanical and thermal properties of the developed SMP hybrid nanocomposites were investigated by static tensile tests,mechanical cyclic tests and thermo-mechanical cyclic tests.The test results demonstrate that the SMP hybrid nanocomposites with chopped carbon fiber(7 wt%)and alumina nanoparticles(1 wt%)exhibit a more superior mechanical and thermal properties compared with other samples,and a good shape memory property was obtained in the developed nanocomposites.