| Hydrogen bond is a kind of important non-covalent bond,and can endow the materials with stimuli and adaptable properties over interaction strength from several kJ to tens kJ per mole.Hydrogen bonding relates to significant life process,like protein folding and base pairing in nucleic acid,and affects various properties of synthetic polymer materials,such as crystalline structures,and miscibility of polymer blends.Owing to its fundamental importance in many branches of science,a growing interest has occurred in the study of physical and chemical properties of hydrogen bonds both experimentally and theoretically in physical,chemical,and biological field.The use of hydrogen bond to trigger interpolymeric complexes?IPCs?has emerged as an easy method to obtain new materials with specific properties,such as self-healing,thermal stability,stimulus response and so on.The stability of hydrogen bond complexes is sensitive to the environment.Under the stimulation of the external environment,the hydrogen bond complexes will show significant physical or chemical changes,such as phase,shape,electric field,surface energy,reaction rate,permeation rate and recognition performance.Due to its unique property,hydrogen bond complexes have been widely studied as an intelligent material.Until now,IPCs has been successfully implemented in producing polymer complex thin films,microcapsules,gels.However,as our best to know,no one have utilized hydrogen bond to manufacture textile fibers.Other polymer complex has been prepared into fibers.Stereocomplex complex of poly?D-lactic acid??PDLA?and poly?L-lactic??PLLA?has been prepared into fiber with wet,dry spinning,and melt spinning techniques.Polyelectrolyte complex cannot melt and is hard to dissolve,so it cannot be produced into fiber with traditional spinning technique.Yamamoto et al.demonstrated the fabrication of polyelectrolyte complex fiber by continuously drawing from the complexation interface of oppositely charged polymers.Polyelectrolyte complex fibers,such as chitosan/gellan,chitosan/alginate,poly?L-lysine?/poly?L-glutamic acid?and protein/DNA,were successfully prepared,and were used for tissue engineering scaffold,gene delivery,and controlled release.IPCs can change significantly in terms of the polymer solubility through controlling IPCs formation and deformation by some ways,such as adjusting solution pH,changing solvent composition,controlling ionic strength.Through controlling IPCs formation and deformation,it can release soluble and insoluble transition of polymer blend in the solvent,which make it possible to manufacture fibers by wet spinning.Here,we first put forward a simple and economical method based on hydrogen bond complex formation to successfully prepare polymer complex fiber.First,a spinnable fluid is obtained by restricting hydrogen bonds,and then it is extruded through a spinneret into a coagulation bath where hydrogen bonds are built to induce fiber formation.In this paper,high elastic PAA/PEO fibers and conductive rGO/PEO fibers were prepared by linear and two-dimensional molecular.After PDA treatment,?PAA/PEO?@PDA fibers show a hierarchical structure.Moreover,adsorption of a compact layer of CNT in the swollen state combines highly elasticity with conductivity.the main results are summarized as follows:?1?PAA/PEO fibers are prepared by wet spinning.First,a spinnable fluid is obtained by restricting hydrogen bonds and then it is extruded through a spinneret into a coagulation bath where hydrogen bonds are built and the fibers are formed.NaOH is deliberately added into the solution to break hydrogen bonds between PAA and PEO.Due to the restriction of hydrogen bonds,the solution is homogenous,and the viscosity is fit for the spinning process.The solution is extruded into a 0.1 M HCl solution.In an acidic environment,PAA is protonated and hydrogen bonds between PAA and PEO are formed,which leads to fiber formation.SEM images show that the hydrogen-bonded PAA/PEO fiber is smooth and dense.The fiber exhibits large elongation.It can be drawn to more than 12×its original length without breaking.The elastic behavior shows dependence on the molar ratio of PAA and PEO,environment humidity,and molecular structure.PAA/PEO fibers prepared with different molar ratios have been characterized by differential scanning calorimetry?DSC?.The fibers show only one glass transition without melting peak.The glass transition temperatures?Tg?of fibers F3-1,F2-1,F1-1,F1-2,and F1-3 are 80,73,33,20,and 14?,respectively.PEO is a semicrystalline polymer and its melting peak is 62?.The lack of any endothermic peak indicates that the crystallization of PEO is totally suppressed.XRD pattern further demonstrates that the PEO crystallization in PAA/PEO fibers is prevented.Also,observation of only one glass transition demonstrates that PAA and PEO are intimately mixed.As the PEO molar ratio increases,PAA/PEO fibers will change from ductile plastic to rubber.the mechanical properties of the PAA/PEO fibers are very sensitive to the environmental humidity.Hydrogen-bonded PAA/PEO fibers?F1-2?were incubated in different humidity environments.As the humidity increases,the fiber has a higher elongation but lower strength.Specifically,after incubation in RH 30%,the initial modulus is 100 MPa,the ultimate stress is 10.2 MPa and the ultimate strain is 500%.After incubation in RH 90%,the initial modulus decreases to 1.5 MPa and the stress at break is only 2.2 MPa,while the elongation at break is as high as 1200%.The results show that water acts as a plasticizer and enhances the mobility of polymer chains.In order to investigate the effect of molecular structure,Two other hydrogen-bonded polymer complexes,namely,poly?methacrylic acid?/PEO?[MAA]/[EO]=1?and PAA/poly?vinylpyrrolidone??[AA]/[VPON]=1?were also spun to prepare fibers.They do not show elastic behavior,contrary to the PAA/PEO fiber.Compared with PAA,PMAA has an additional methyl group and shows strong complexation ability with PEO.30 For the PMAA/PEO system,hydrogen bonding is coupled with hydrophobic interaction between the CH3 groups of PMAA and the CH2 groups of PEG,and massive CH3 groups increase the steric hindrance which make the chain movement extraordinarily difficult compared with the PAA/PEO system.PVPON is a stronger proton-acceptor and its complexes with PAA are more stable than that formed by PEO and PAA.In both cases,one of the components?either PMAA or PVPON?is less water absorbent than the component it replaces.Thus,water-induced plastification is weaker.The underlying water plastification is insufficient for the fibers to display elastic behavior.?2?The hydrogen-bonded polymer complex fibers of PAA and PEO are prepared with the strategies that inter-chain hydrogen bonding is restricted to obtain spinnable fluid and hydrogen bonding constructing in coagulation bath made fiber formation.The hydrogen-bonded PAA/PEO fibers is modified with dopamine solution with oxidant.Owing to the oxidation,adsorption,diffusion and self-polymerization of dopamine?PDA?,the originally transparent and smooth PAA/PEO fibers appear brown and the surface becomes rough.The resultant?PAA/PEO?@PDA fibers exhibit gradient chemical and dynamic hydrogen bonded cross-linked structure.The mechanical properties of?PAA/PEO?@PDA fibers strongly depend on self-polymerization time.As the self-polymerization time elevates,the break strength increases.The break elongation and Young's modulus show little changes at the initial self-polymerization stage?3 hours and 6 hours?while the break elongation declines and Young's modulus increase after 12-hour self-polymerization.The mechanical behavior of the dopamine treated fibers appears dependent upon fiber interior characteristics.Given the preparation process,the cross-link density is expected to gradually decrease from the outer fiber surface to the center axis.Hence?PAA/PEO?@PDA fibers have a hierarchical architecture,comprised of a hydrogen bonded network and a superimposed chemically cross-linked network with a gradient distribution.The covalent network serves to bridge cracks and stabilize deformation,enabling hydrogen bonds to unzip for large deformations.Arrays of hydrogen-bonds interact to form a physically cross-linked 3D network with a well-defined architecture in the elastomers.During loading and reloading,the dynamic hydrogen bonds reversibly break,reorient,and re-form,resulting in energy dissipation.The dynamic nature of hydrogen bond has been applied to achieve self-healing property.Our hydrogen-bonding complex polymer fibers exhibit spontaneous self-healing behavior under ambient condition.The fibers are healed in relative humidity 90%at 30? for different time to precisely study their self-healing properties.Stress-strain curves show that a long healing time results in a high recovered ultimate strain.Even when contact time is as short as 1 hour,the healed fiber can be stretched up to about 350%.The fibers retain a 748±85%strain with a healing efficiency of 71%after 24-hour healing.?3?Incorporation of PDA prevents dissolution of PAA/PEO in neutral or alkaline aqueous media.While PAA/PEO fibers completely dissolve in pH 12 solution after 10-minute immersion;?PAA/PEO?@PDA fibers swell,but do not dissolve under the same conditions.The size of the fibers enlarges after swelling.For example,the swollen-state fibers at 7 pH increase dramatically in the length,diameter,and surface area,3.0-fold,4.5-fold,and 13.5-fold higher than those of original fibers,respectively.When alternately immersed in pH 1 and pH 12solutions,?PAA/PEO?@PDA fibers are observed to reversibly shrink and swell.?PAA/PEO?@PDA elasticity,coupled with reversible,pH sensitive swelling enables the incorporation of additional functionality to the PAA/PEO based fibers.Here,carbon nanotubes?CNT?are immobilized on the?PAA/PEO?@PDA fiber surface via pH induced swelling/shrinking.The fiber is first pretreated with the pH 7solution to initiate swelling,and subsequently immersed into the pH 7 aqueous CNT dispersion.The surface area of the swollen-state fibers is 13.5-fold higher than that of original fibers,which dramatically increases the contact of PDA coating with CNT.After adsorption of CNTs to the swollen fiber surface,the fiber is transferred to the pH 1 solution,leading to shrinkage and fastening of the CNTs into the surface structure.CNTs are deposited on the fiber surface with compact and wrinkled structure.The resistance of the CNT wrapped fibers is linearly dependent upon the applied strain.The fiber shows excellent cycling stability and repeatability for strains less than 300%.Furthermore,200 cycles of 200%strain drawing are done,and the fibers show excellent repeatability.In contrast,fibers with CNT absorbed in the non-swelling state?at pH 1?have a much high initial resistance and its resistance is unstable when the strain is higher than 60%.The conductive CNT wrapped?PAA/PEO?@PDA fiber is used to construct a wearable sensor to detect motion.The wearable sensors are attached to different positions of the human body by tape,and various physiological signals are thus obtained by measuring changes in the electrical resistance.A quick wrist bend results in a sharp pulse signal.Five independent strain sensors are also affixed to a glove to identify different human gestures.Different gestures induced differential resistance responses.For example,when fingers are bent,the resistance increases suddenly and then remains stable.Moreover,the strain sensors exhibit good response rate and cycling stability.Hence,the conductive,elastic fiber is an effective sensor for monitoring physiological movement.?4?Hydrogen-bonded complex fibers of graphene oxide and polyethylene oxide are prepared and reduced to conductive fiber.GO sheets have plenty phenol and carboxylic acid groups on surface,and hence GO and PEO should form hydrogen-bonded polymer complex.The amount of carboxylic acid is approximately 6.2 at%of all moiety.The complexation behaviors of GO and PEO at different pH values are investigated.When pH is below 2.0,PEO and GO will form the complex which precipitates out from the solution quickly.When pH is higher than 5.0,GO and PEO mixed solution will be homogenous.GO/PEO mixture solutions are observed with polarized optical microscopy?POM?.The birefringence of GO/PEO mixture shows dependence on pH value.At pH 6.0,the solution shows the strongest birefringence,and as the pH values increase from 6.0 to 12.0,the birefringence effects gradually decrease.In addition,at the same pH values,the birefringence of GO/PEO mixture is stronger than that of GO dispersions.Rheological measurement was carried out to obtain information about liquid crystal phase and hydrogen-bonded network systems.The GO/PEO solutions show shear thinning behavior,indicating the shear-induced alignment of GO platelets.The solution at pH 6.0 shows the best fluidity.With the increase in solution pH values,viscosity goes up and shows maxima at pH 10.0 before going down.The viscosity increase confirms pH dependent nematic to isotropic phase transition.At pH 6.0 the GO/PEO solution shows good fluidity and pre-orientation of GO sheets.The solution is extruded into 1.0 M HCl solution where hydrogen bonding between PEO and GO is constructed and hence the fiber forms.The fibers prepared with GO and PEO weight ratios of 90:10,70:30,and 50:50 are remarked as G90P10,G70P30,and G50P50 fiber,respectively.The as-formed wet fibers show birefringence,indicating the intrinsic orientation is inherited.The GO/PEO fibers prepared with different GO content are tested and characterized.The fiber mechanical properties are dependent on the weight ratio of GO and PEO.As the GO content elevates,the mechanical strength increases while break elongation declines.GO can be reduced to prepare graphene-based materials.The GO/PEO complex fibers are reduced with hydroiodic acid?HI?.After reduction,the fiber diameters shrink,and the fiber keeps good wounding ability and can be simply knitted by hand.The surface of the reduced fiber shows more wrinkles.The reduced fiber diameter shrinks to be 3/5 of the unreduced fiber.Compared with the unreduced fibers,the reduced fiber exhibited porous structure.Hot concentrated HI solution makes the cleavage of ether bond.The PEO chains degrade and are etched out from the fiber,which results in porous structure in the fiber.The reduced fibers show good mechanical and electrical properties.The highest tensile strength is close to 500 MPa while the conductivity is more than200 S/cm.The reduced fibers can be assembled into flexible supercapacitor,exhibiting potential for wearable electronics.The residue oxygen-containing groups can be further removed by thermal annealing of rGO.The compact stacking of highly aligned sp2 graphene sheets optimizes electron transport pathways.The electrical conductivity of rG90P10-H increase from 211 to 502 S/cm-1 after thermal annealing at 1200?. |
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