Preparation And Properties Of Multi-stimuli Responsive Shape Memory Polyurethane Composites

Shape memory polymers（SMP）are smart-responsive materials that can change their initial shape under external force and fix their temporary shape.It could recover from the temporary shape to its initial shape by various stimuli-responsive modes,such as heat,electricity,light,solvent,p H,and magnetic field,and realize the transformation between different shapes.The strong perceptual adaptability in response to the external environment makes it reveals a wide prospect of application in many fields,such as smart textiles,biomedicine,aerospace,and advanced manufacturing.It is of great significance to conduct in-depth theoretical and experimental research on the special shape memory properties of SMP.Currently,most SMP have the disadvantages of a single deformation,a single stimulus response factor,a single response mode to external stimuli and the difficulty of remote control,making it hard to meet people’s increasingly smart application requirements for SMP materials.How to realize programs,multi-responsive shape memory,and simultaneous changes of other important properties with deformation have become the core problems to be solved urgently in the field of structural design and preparation for SMP.In recent years,the emergence of multi-stimuli responsive shape memory polymers（MRSMP）provided new methods for the development and design of SMP,promoting the further development of smart materials.In this project,thermoplastic polyurethanes（TPU）and polylactic acid（PLA）were used as the matrix,and different methods such as blending modification,copolymerization synthesis,and 3D printing technology were used to design and prepare multi-responsive shape memory polymers.Thus,it provides new methods for solving the technical problems of single stimulus-response mode,difficult remote control,and single deformation response of SMP,which play an important and positive role in promoting the development of SMP.The main works are as follow:（1）In this work,a fast water-responsive shape memory hybrid polymer based on thermoplastic polyurethane（TPU）has been prepared by crosslinking with hydroxyethyl nano cellulose（CNC）and multi-walled carbon nanotubes（CNT）.The effect of CNT content on the electrical conductivity of TPU/CNC/CNT nanocomposite was investigated for the feasibility of being strain sensors.The mechanical and water-responsive shape memory effects were studied comprehensively to explore their durability.Results indicated good mechanical properties and sensing performance for TPU matrix fully crosslinking with CNC and CNT.The water-induced shape fixity ratio（R_f）and shape recovery ratio（R_r）were achieved at 49.65%and 76.64%,respectively,indicating that the deformed composite was able to recover its original shape under a stimulus.The TPU/CNC/CNT samples under their fixed and recovered shapes were tested to investigate their sensing properties,such as periodicity,frequency,and repeatability of the sensor spline under different loadings.Results indicated that the hybrid composite could sense large strains accurately for more than 10³ times,and water-induced shape recovery can,to some extent,maintain the sensing accuracy after the material fatigue.With such good properties,we envisage that this kind of composite may play a significant role in developing new generations of water-responsive sensors or actuators.（2）The 3D printing filament of PLA/TPU/TMC was successfully prepared by melting extrusion,the PLA and TPU powders with a mass ratio of 7:3 as the shape memory polymer matrix and TMC as the feedback factor of temperature change.Furthermore,the continuous conductive fiber of the PLA/TPU/TMC@SMA was prepared by combining of the PLA/TPU/TMC filament and SMA fiber.A conductive braid was prepared to investigate the thermal uniformity and stability.Moreover,the PLA/TPU/TMC@SMA strip with excellent SMA fiber distribution was designed and fabricated via 3D printing technology.FT-IR results showed that PLA,TPU,and TMC were crosslinked by hydrogen bonds.Based on the DSC result of the PLA/TPU/TMC filament and the shape memory response temperature test result of the SMA,the deformation（45.8°C）and color change（43.0°C）of the PLA/TPU/TMC@SMA shape memory composite system could meet the requirements of the rapid synchronous responses of shape and color when the temperature was set at 60°C.From the results of the tensile test and bending test,when the content of TMC is 6 wt%,the mechanical properties of the PLA/TPU/TMC@SMA strip are prominent,its elongation at break is 3.29%,the tensile strength is 19.05 MPa,and the color depth of the strip is appropriate.Therefore,the PLA/TPU/TMC-6 was selected as the 3D printing filament to design and prepare the 3D printing structures.The thermochromic properties and reversibility of the PLA/TPU/TMC@SMA shape memory composite are excellent,which is consistent with TMC,and the thermochromic response temperature is 43.0°C.The braided specimen and 3D printing strip gradually changed from red to blue in the progress of electric heating and recovered to red during the progress of cooling,without obvious color loss.The chroma parameter of electrochromic color has good reversibility.The chroma could recover the initial chroma value after the electroheating/dry ice cooling cycle and obtain the real-time reversible electrothermal response to color change.The SMA improved the mechanical properties of the PLA/TPU/TMC@SMA strip.What’more,the shape memory properties of the PLA/TPU/TMC@SMA composites system improved significantly.Due to an efficient conductive network composed of continuous SMA,the braided specimen and 3D printing strip showed the shape memory effect of rapid electrothermal stimuli.Under 15 V,the braided specimen obtained a shape recovery rate of 83.52%in 145 s,while 98.69%for the PLA/TPU/TMC@SMA strip in 30 s.Making the PLA/TPU/TMC@SMA Shape memory composites have become an ideal choice for building high-performance electrothermal actuators.Furthermore,the electrochromic performance and electrothermal response shape memory performance of the PLA/TPU/TMC@SMA braided specimen and 3D printing strip were studied,respectively.Both showed excellent synchronous deformation and discoloration performance,while strip-printed parts show excellent deformation load capacity.（3）Herein,a series of thermoplastic polyurethane（TPU）/carbomer（CB）/nano-celluloses（CNC）multi-responsive shape-memory nanocomposite films were tailored using CB as soft segments and CNC as a crosslinker.The impact of CNC content on the mechanical properties of the TPU/CB/CNC nanocomposite films has been comprehensively studied by which the best CNC loading was reported at 5 wt.%.The chemistry and microstructure of the nanocomposite film have been precisely elucidated by FT-IR spectroscopy,¹H NMR,XRD,and DSC to verify the reaction of–CO–NH–in TPU with–COOH in CB and CNC—forming quaternary ionic and hydrogen bonds.The shape-memory properties of the TPU/CB/CNC nanocomposite film were also investigated in different conditions such as water,ethanol,acid,alkali,and heat.Moreover,the shape recovery behavior of the nanocomposite film was examined by changing p H,which affects the quaternary ammonium ionic and hydrogen bonds as switch units.Although the shape of the thin nanocomposite film has successfully recovered in the alkaline environment,the internal structure was damaged.After transfer to the acidic environment,the destroyed internal structure of the strip was revived,and its strength improved to carry a foolproof load.Eventually,the as-made nanocomposite film has a multi-responsive capability against water,ethanol,p H,and heat,representing its potential in slow-release drug and flexible robotic as well as electronic applications.（4）Herein,a multi-stimuli responsive shape memory polyurethane（MRSMPU）containing disulfide bonds（–S–S–）,bioinspired fromα-keratin hair component,was synthesized using a two-step reaction and activated via four-varied stimuli,including water,heat,redox agent,and UV-light.First,the prepolymer was prepared by the reaction between the comonomers of hexamethylene diisocyanate and polycaprolactone-2000,and once ended,1,4-butylene glycol and 3-mercapto-1,2-propanediol（C₃H₈O₂S）have been appended to the prepolymer to initiate the chain-extension reaction.The chemical structure and properties of the MRSMPU were decoded by FT-IR,XRD,DSC,and Raman spectra.The correlation between microstructure,dynamic-mechanical-analysis,and shape-memory properties of the MRSMPU was systemically discussed.As a primary clue,the opening/closing of H-bonds and disulfide bonds approved that they dictated the shape memory performance.The clear-cut results clarified that the MRSMPU endowed composition-dependent stress relaxation,and its reversible variation of storage modulus was examined by changing temperature.Owing to such molecular switches,the strip pieces were able to represent a multi-stimuli responsive shape memory effect—induced by water,heat,redox agent（Na HSO₃-H₂O₂ solutions）,and UV light.This research paves a feasible road to prepare a novel multi-stimuli responsive shape memory actuator with high potential features in flexible sensor and robotic applications.The stimuli-responsive mechanism,types and preparation methods of MRSMPU have been explored,the complexity of MRSMP molecules reduced,and developed MRSMPU with multi-stimuli responseto meet the application requirements of smart materials in complex environments.