Current-Driven Two-Way Shape Memory Effect Of Thermoplastic Polyamide Elastomers

Shape memory polymers(SMPs)are smart materials that can respond to external stimuli.According to the different actuation methods,SMPs can be divided into thermal-driven SMPs,current-driven SMPs,light-driven SMPs,magnetic-driven SMPs and solvent-driven SMPs.Most of the common SMPs are mainly thermal—driven SMPs which shape memory process requires the external environment to provide a persistent heat source.But the indirectly driving method enables SMPs can be applied in some special places,such as outer space,human body.According to the reversibility of the shape memory process,SMPs can be divided into one-way SMPs(1W-SMPs)and two-way SMPs(2W-SMPs).After decades of development,the relationship between 1W-SMPs structure and performance has been quite matured,but their application fields have greatly been limited due to the irreversibility of the shape memory process.The shape memory process of 2W-SMPs is reversible and can theoretically be used many times once programming.At present,most studies on 2W-SMPs focus on chemically cross-linked crystalline polymers,but the relationship between the structure and properties of physically cross-linked crystalline polymers is still unclear.Thermoplastic polyamide elastomer(TPAE)is a(AB)_n-type block copolymer formed by the condensation polymerization of polyamide hard segments and polyester or polyether soft segments.The crystallinity of the soft and hard segments of TPAE enables TPAE to have two widely different melting transition temperatures and a microphase-separated structure,which makes TPAE have the potential to realize the two-way shape memory effect.Therefore,it is of great significance to study the thermally driven two-way shape memory effect and the electric-driven two-way shape memory effect of TPAE materials.The main contents of this paper are as follows:(1)Two-way shape memory effect of thermoplastic polyamide elastomers with different ratios of soft and hard segmentsTaking four TPAE materials with different ratios of soft and hard segments as the research object which were prepared into thin film splines by vacuum hot pressing method.The TG test results show that the four TPAE materials have good thermal stability.The DSC and XRD results show that in the four untreated TPAE materials prepared by vacuum hot pressing,the hard and soft segments of the T12 material and the T22 material can be Crystallization,while T11 and T21 materials only have hard segments that can crystallize.The test results of storage modulus and mechanical properties show that all four TPAE materials have excellent mechanical properties,and theαtransition temperature of the four TPAE materials is roughly around 50℃,while theβtransition temperature is roughly around-40℃.All four TPAE materials can exhibit quasi-two-way shape memory effect and reversible two-way shape memory effect which are affected by various factors such as the ratio of soft and hard segments of the TPAE material,stretching temperature,cooling rate,tensile stress,etc.Among them,in the study of the quasi-bidirectional shape memory effect of T12 material,the response strain change can reach 16.4%under the tensile stress of 8 MPa,and the shape recovery ratio is also Close to 100%;in the study of the reversible two-way shape memory effect of T12 material,the response strain change can reach 9.7%at a tensile strain of 300%and a shape recovery temperature of 20℃,and the shape recovery ratio is also close to 100%.(2)Study on the current-driven two-way shape memory effect of thermoplastic polyamide elastomerT12/CNTs composites were prepared by ultrasonic dispersion-solvent evaporation method using multi-walled carbon nanotubes(CNTs)as conductive fillers and T12powder(0-150 um)as polymer matrix.The research shows that the conductive percolation value of T12/CNTs composites is 3 wt%,the maximum surface temperature of T12/CNTs composites increases with the increase of voltage and CNTs content,and when the content of CNTs is 5 wt%,the joule heat from energization(110-150 V)of the T12/CNTs composite is sufficient to trigger the shape recovery behavior of the T12/CNTs composite.The DMA results show that the content of CNTs has the same effect on the quasi-two-way shape memory effect and the reversible two-way shape memory effect,that is,when the content of CNTs is lower,the response degree and the change of the response strain of the T12/CNTs composite are larger than those of the pure T12 material.The response degree of T12/CNTs composites is smaller than that of pure T12 material only when the CNTs content is higher,and both decrease with the increase of CNTs content.The designed bilayer SEBS/T12/CNTs-5 composite spline produced a shape recovery immediately after energization,and its response angle varied by 3°.And the change of the response angle of the bilayer SEBS/T12/CNTs-5 composite spline can be controlled by the regulation of voltage and energization time.