The Preparation Of Polymer Blends-based Shape Memory Materials And The Structure-Performance Relationship

Shape memory polymers(SMPs)are kinds of smart materials,which have many features,such as large deformation,response to stimulus.Hence,SMPs have a good potential application prospect in the fields of aerospace,biomedical engineering,electron device and so on.However,the most extensive research is about thermally induced SMPs,which are actuated by heating directly.Even now there are many researches about thermally induced SMPs,but the changes of microstructure in the shape memory process actuated by heating need to be explored further.Compared to the thermally induced SMPs,the SMPs actuated by new stimulus(such as light,electric field,magnetic field et al)have the advantages of non-contact and remote control.Therefore,the researchers pay more attentions to new SMPs.One is about new responsive SMPs obtained via physical method with simplicity of operation and short experimental period,and the other is about multiple-stimuli SMPs meeting special requirements.So,in this thesis,shape memory blends or composites were obtained by melt-compounding and solution blending methods.Meanwhile,the structure-property relationship of new SMPs and the molecular mechanism in shape memory process were researched further.Finally,the new SMPs actuated by infrared light/electric field/water were obtained,and their shape memory mechanisms were also stated.Firstly,in order to discuss the structure-property relationship of thermally induced SMPs,a series of poly(ethylene-co-vinyl aetate)(EVA)/poly(L-lactide)(PLLA)blends and dynamic vulcanized EVA/PLLA/blends were prepared via twin-screw extrusion.The results demonstrate that:1)For EVA/PLLA blends,the changes of phase morphology have an obvious influence on their shape memory behaviors,and the sample with co-continuous structure shows the best shape memory properties.Moreover,due to the effect of glass transition of EVA component on EVA/PLLA blend,a critical recovery temperature(53?)is observed in the shape recovery process.When the recovery temperature is below 53?,the PLLA component restrains the sample recovery.However,when the recovery temperature is above 53?,PLLA component accelerates the shape recovery.2)For dynamic vulcanized EVA/PLLA blends,due to the incorporation of PLLA crystals into PLLA component,there are two fixed phases in the dynamic vulcanized blends.One is about the chemically crosslinked networks in the EVA component,and the other is the physically croslinked networks formed by PLLA crystals.The effects of physically croslinked networks and phase morphology on shape memory properties were explored by adjusting the PLLA crystallinity.The PLLA crystallites contribute to the increase of shape recovery ratio(Rr),and lead to lower shape fixity ratio(Rf).At the same time,the blends with high EVA content shows the best shape memory properties.Furthermore,for the blends with co-continuous structure,the functions of molecular chain structures,crosslinked networks and crystalline structures in the shape memory process are exposed.Secondly,the crosslinked EVA/polycaprolactone(PCL)blends and the composites based on EVA/PCL(60/40,wt/wt)were prepared to obtain triple-SMPs and electro-active SMPs.In the composites,carbon nanotubes(CNTs)were incorporated into EVA/PCL blend.The study shows that:1)For the crosslinked EVA/PCL blends,the degree of chemically crosslinking is higher,and the crosslinked network becmmes denser in the EVA component.Moreover,the crosslinked network contributes to improve the viscosity of EVA component to lead the phase structure of blends becoming co-continuous structure.The shape memory tests demonstrate that the sample with co-continuous phase structure and complete crosslinked network can recover from the temporary shape to its original shape,and shows an obvious triple shape memory effect.2)It is proved that CNTs is selectively located in the EVA phase by theoretical prediction and experimental verification.The percolation threshold of the composites is suggested to be 0.95 wt%.When the content of CNTs is high,the conductive network formed in the composites is improved.And at applied voltage,these composites can generate enough Joule heat to increase the sample temperature up to their transition temperature.So that the blend composites containing relatively high content of CNTs exhibite excellent electrically actuated shape memory effect.Thirdly,polyving akohol(PVA)was blended with EVA via solution blending method.And to achieve water-sensitive shape memory effect,the high elasticity of EVA and the hydrophily of PVA were utilized.Due to the interaction between EVA and PVA,the phase component is hard observed from the SEM images.In consideration of the high modulus of PVA,the modulus of EVA/PVA blends increases with the increase of PVA content.But the plasticizing effect of water molecular on PVA component is obvious,so that the swelling degree and modulus of EVA/PVA blends decrease with the increase of water soaking time.Based on the modulus difference of samples in the wet/dry states,the blends containing relatively high content of PVA exhibite excellent water sensitive shape memory effect.Finally,the composites with electrically/infrared actuated shape memory effect were obtained by incorporating the graphene nanoplate(GNP)into polyurethane(PU)/PCL(50/50,wt/wt)blend and utilizing the conductivity and photo-thermal effect of GNP.From the SEM images,GNP shows a good dispersion in the composites and do not affect the miscibility between PU and PCL.With the increase of GNP content,the crystal structure of PCL has obvious change,and the GNP percolation network is formed.The percolation threshold of the composites is suggested to be 1.62 wt%.When the GNP content is high,the GNP percolation network becomes dense,and at applied voltage,the composites show excellent electrically actuated shape memory effect.Moreover,due to the photo-thermal effect of GNP,the composites show excellent photo-thermal behavior.At the same light condition,PU/PCL/GNP composites have a faster recovery speed than that of PU/PCL blend.Meanwhile,the composites with orientation structure show the complex shape memory effect and self-driven ability.