Study On Shape Memory Poly(d,l-lactide-co-trimethylene Carbonate) Based Composites And Their Direct-inkwrite 4D Printing

Since the concept of 4D printing was first proposed in 2013,it has attracted extensive attention from the academic community.4D printing is defined as the shape,properties,and functionalities of 3D printed structures change over time under external stimuli,which introduces time as the fourth dimension.One important method to realize 4D printing is to 3D print smart materials with intrinsic stimuli responsiveness.Shape memory polymers are a kind of important smart materials that could recover to their original shapes from programmed temporary shapes under external stimuli.They possess the advantages of flexible designability,light weight,easy processability,etc.The combination of shape memory polymers with 3D printing can not only broaden the types of 4D printed materials in terms of material design,but also realize the active 4D shape transformation of 3D printed structures over time,bringing new applications in the fields of electronics,biomedicine,and soft robotics.The construction method of 4D printing is the same as 3D printing.Among them,the direct ink writing（DIW）method shows great advantages due to a wide variety of materials,open source apparatus,and convenience of fabricating composites and multi-material structures.At present,the types of shape memory polymers that can be used for direct ink writing-based 4D printing are relatively limited.The actuation method mainly relies on a single stimulus,and the shape transformation form is relatively simple.Therefore,exploring novel types of shape memory polymers,actuation methods and applications for direct ink writing based 4D printing has become the focus of current research.In this thesis,we chose to study poly（d,l-lactide-co-trimethylene carbonate）（PLMC）,a type of thermo-responsive shape memory polymer.The printing ink is prepared by dissolving it into a volatile solvent,dichloromethane.We regulate the ink to possess shear-thinning behavior and fast solvent evaporation rate to meet the requirement for direct ink writing.We systematically investigate the influence of ink concentration and printing parameters on the properties of 3D printed structures and construct complex structures of PLMC from one-dimensional（1D）to three-dimensional（3D）,and microscopic to macroscopic scale.By comparing the properties between solvent-cast and as-printed PLMC,we find the direct ink writing method has almost no influence on materials properties.The direct-ink-write 4D printed PLMC has a relatively low glass transition temperature（T_g）around 50℃and excellent thermo-responsive shape memory properties,as the shape fixity ratio（R_f）and shape recovery ratio（R_r）are both above 99.7%.This attributes from the physical crosslinking interaction in the PLMC chains.Taking the direct-ink-write 1D filament,2D nonwoven fabric,and 3D scaffold as an example,we demonstrate and analyze the macroscopic shape transformation of these 4D printed structures under heat stimulus slightly above the body temperature.The 4D printed PLMC structures show advantages of low actuation temperature（45℃）,fast response speed（0.5 s）,and good mechanical properties.The electrical conductivity of nanocomposites is beneficial to extend the actuation method of direct-ink-write 4D printed structures to electrical actuation that avoids the limitation of direct heat stimulus.Based on DIW 4D printed PLMC,we study the performance of 4D printed electro-responsive shape memory composites.By introducing CNT into PLMC,we systematically investigate the thermal,mechanical and thermomechanical properties.With the increase of CNT content,the shape retention ability of printed structures also increases.When the CNT content is10 wt%,the composite exhibits a volume conductivity of 65 S m^-1.The R_f and R_r of PLMC/CNT nanocomposites are also close to 100%after four continuous shape memory cycles,exhibiting excellent and repeatable shape memory properties.The DIW 4D printed structure recovers to its original shape within 16 s under a voltage of 25 V.We further introduce Norrish typeⅡphotoinitiators to modify PLMC into cross-linking polymer（c-PLMC）by the hydrogen abstraction reaction.Compared to PLMC,the T_g of c-PLMC drops by 10℃while the mechanical property increases and the thermal stability remains.The c-PLMC/CNT composites exhibit good mechanical properties and conductive properties with a volume conductivity of 68 S m^-1.The 4D printed liquid sensors from c-PLMC/CNT composites could be used to monitor different solvent environments.We prove the liquid sensing capability in various solvents by both theoretical calculations and experiments.We demonstrate the good environmental adaptability and flexibly tunable detection range of the shape-changing liquid sensors based on their excellent electro-responsive shape memory effect.Concerning the actuation method of DIW 4D printed structures is relied on one stimulus,the number of deformation shapes is low and the deformation mode is relatively simple,we further introduce PTMC with similar chemical structures into the dual shape memory PLMC to prepare PLMC/PTMC material by physical blending.By regulating the composition of PLMC/PTMC,we find the weight ratio at 50:50 has the broadest glass transition region（around 40℃）,which creates the possibility for triple shape memory effect.The DIW 4D printed structures realize sequential active deformation under heat stimulus at 45℃and 70℃.Then we introduce Fe₃O₄ to prepare magnetic nanocomposites and the DIW 4D printed PLMC/PTMC/Fe₃O₄ structure realizes sequential active deformation under an alternating magnetic field of 30 kHz.Finally,by compositional and structural design,we endow DIW 4D printed multi-material structure with sequential magnetic and thermo stimuli-responsiveness.The multiple-stimuli actuation method allows the structure to change shape between five complex configurations,with global and local shape programming controllability.