Study On Direct-Write 4D Printing Of Poly(Lactic Acid) Based Shape Memory Materials And Their Properties

Shape memory polymers?SMPs?as a kind of important smart materials are capable of returning their initial shapes after going through shape deformations under a certain extern stimulus.Active shape-changing structures based on SMPs possess various advantages including adjustable glass transition temperature?T_g?,designable actuated methods,lightweight,large deformation,etc.In recent years,this kind of smart structures has been widely investigated in the fields of aerospace,biomedicine and smart textile.Due to the limitation of traditional manufacturing methods,geometries of currently developed SMPs based active shape-changing structures are mainly in two-dimension?2D?or simple three-dimension?3D?.However,SMPs based active shape-changing structures with complex,customized and diverse geometries are urgently needed for practical application.Under this background,this thesis carries out research on direct-write 4D printing of shape memory polylactic acid materials and their properties.Based on the polymer of polylactic acid?PLA?,the thermo-,magnetic field-and electro-actuated shape memory polymer composites are designed and fabricated.Meanwhile,their direct-write 4D printing methods are established.The mechanism of materials synthesis and their shape memory effect,performance control,the principle of 4D printing and its important influencing factors,the design and mechanisms of actuated methods are systematically discussed.The results obtained herein can not only strongly support the further development of 4D printing based on shape memory polymer materials.Considering from the molecular structure,we fabricate a kind of thermal responsive shape memory polylactic acid?c-PLA?based on chemical cross-linking network by introducing UV photoinitiator of benzophenone?BP?into polylactic acid.Results show that the proposed c-PLA can present excellent thermal properties,thermal stabilities and superior shape memory effect.This is due to that the generation of chemically cross-linking network is able to effectively reduce the irreversible slippage of PLA molecules under heat.After that,we establish the direct-write 4D printing method that matches for the properties of the prepared c-PLA polymer.The influences of the printing parameters during direct-write 4D printing,including solution concentration,printing speed,printing pressure and nozzle size,on the constructed structures are studied in detail.Meanwhile,the macroscopic shape changing performances of direct-write 4D printed c-PLA structures are characterized and analyzed by taking examples of a spiral,a wave-like structure and a flower-like structure.We find that those printed structures can actively transform from 1D to 3D,2D to 3D and 3D to 3D under heat.Due to the limitation of direct heat actuation methods in practical applications,we propose study on direct-write printing of magnetically actuated shape memory composites.We construct magnetically actuated c-PLA/Fe₃O₄ shape memory nanocomposites by using c-PLA as the matrix and Fe₃O₄ nanoparticles as functional fillers.The dispersion of the nanoparticles,thermal properties,thermal stabilities and mechanical properties of the c-PLA/Fe₃O₄ composites are acquired.Characterizations of the rheological properties and solvent evaporation behavior show that the introduction of Fe₃O₄ enhances the shape maintenance of c-PLA/Fe₃O₄ and fastens the solvent evaporation behavior of the nanocomposite solution.Based on these results,c-PLA/Fe₃O₄ nanocomposite 4D structures are successfully achieved by direct-write printing technique.We characterize the magnetically actuated behavior of the direct-write 4D printed c-PLA/Fe₃O₄ nanocomposite structure.The relationship of the surface temperature with the output of the magnetic field and the shape recovery time are obtained.Based on these results,a magnetically actuated stent is design and printed.The stent can recover to its original shape in 10 s under a frequency of 30 kHz,demonstrating the achievement of fast remote actuation of 4D printed structure.A novel kind of electrically actuated shape memory nanocomposites with high conductivity is prepared by integration of core-shell silver coated carbon nanofibers?Ag@CNFs?with PLA matrix.Ag@CNFs nanofibers enable to integrate the high electrical conductivity of Ag with the high aspect ratio of CNFs.Thus,the prepared Ag@CNFs/PLA nanocomposite can present a conductivity higher than 10⁵ S/m and a percolation threshold lower than 6 vol.%.Meanwhile,the physical entanglements between Ag@CNFs and PLA enable the prepared Ag@CNFs/PLA nanocomposite with excellent shape memory behavior.The shape recovery ratio of the nanocomposite is higher than 99%.By investigating the direct-write printing process,we find that the prepared nanocomposites can be well-printed when the mass ratio of DCM/PLA is 3/7 using the nozzle of 200?m.We design and direct-write printing of a smart gripper using Ag@CNFs/PLA nanocomposite.The gripper can be electrically actuated under a voltage as low as 1 V.The research herein can provide a new method for the design of low voltage-triggered,fast-responsive electrically actuated shape memory nanocomposite and their one-step 4D printing.