Preparation And Performance Of Polymer/Melanin Composites

Melanin,a well-known macromolecule that are widely distributed in many parts of nature.Polydopamine is also a major pigment of naturally occurring melanin（eumelanin）.As a kind of macromolecule containing constitutional unit with catechol,amine,and imine.Melanin can scavenge radical,absorb ultraviolet rays and exhibits excellent photo-protective,adhesive properties,and photothermal conversion ability.Various substrates could be successfully functionalized with melanin.The functional groups on melanin can serve as both the starting points for covalent modification with desired molecules,so it’s easy to modify surface of diverse materials.There are two methods to prepare melanin models:natural melanin model and synthetic melanin-like model.The natural melanin model has been obtained from a biological source.The synthetic melanin-like model is obtained by chemical oxidation of dopamine or enzymatic oxidation of precursor molecules.Both natural melanin and melanin-like pigment show similar physical and chemical properties.Polymer composites with UV-shielding properties were prepared based on photo-protective properties of melanin.A significant UV shielding and high transparency polymer composite was easily obtained via tuning the structure of melanin.What’s more,polymer/melanin composites,superior nacre-like layered material,and smart thermo-driven actuator with high performance have been fabricated due to the strong interaction between melanin and various functional groups.Self-healable polymer nanocomposites were fabricated based on the photothermal conversion nature of melanin and“mechanical annealing”process.The results will provide the fundamental knowledge for synthesis of melanin,modification of polymer,and technical support on fabrication of novel melanin composites with excelent properties.The main cotents and results are summarizeds as follows:1.Sepia eumelanin（SE）,a biomacromolecule,was developed to prepare the excellent UV-shielding poly（vinyl alcohol）（PVA）composites with better photostability.PVA/SE film blocks most ultraviolet light below 300 nm even with a low concentration of SE（0.5 wt%）,which still keeps its high transparency in the visible spectrum.What’s more,PVA/SE films show better photostability without side-effect,i.e.,photocatalytic activity of inorganic blockers.The mechanism for outstanding UV-shielding properties and photostability of PVA/SE film was illuminated,based on the photothermal conversion and the well-known radical scavenging capabilities of SE.SE dissipates UV radiation via converting the absorbed photon energy into heat to keep harmful UV light out.As free radical scavenger,SE also reacts with radical species,as well as reduced oxygen and generated active oxygen species.2.Bioinspired melanin-like“polydopamine”solid nanoparticles（Dpa-s NPs）and hollow nanoparticles（Dpa-h NPs）as UV-absorbers were introduced to enhance the UVshielding performance of polymer.UV irradiation protocols were set up,allowing selective study of the extra-shielding effects of Dpa-s versus Dpa-h NPs.In contrast to PVA/Dpa-s films,PVA/Dpa-h films exhibit stronger UV-shielding capabilities and can almost block the complete UV region（200⁴00 nm）.The excellent UV-shielding performance of the PVA/Dpa-h films mainly arises from multiple absorption because of the hollow structure and large specific area of Dpa-h NPs.Moreover,the wall thickness of Dpa-h NPs can be simply controlled from 28 to 8 nm,depending on the ratio between PS and dopamine.The resulting films with Dpa-h NPs（wall thickness～8 nm）maintains relatively high transparency to visible light because of the thinner wall thickness.3.Melanin-like NPs with different sizes were synthesized by oxidation of dopamine.Based on the study for the influence of melanin-like NPs on UV-shielding properties,photostability and transparency of material.A significant short-wavelength shielding and high visible light transparency polymer nanocomposite is easily obtained via tuning the NPs size.In particular,the nanocomposite film with melanin-like NPs（diameter¹5 nm）loading even as low as 1 wt%blocks most ultraviolet light below 340 nm and still keeps high visible light transparency（83%）in the visible spectrum.The highly dispersed melanin-like NPs have a small size and high accessible surface areas in the polymer matrix and made it easy for shortwavelength（UV light）to be absorbed and long-wavelength（visible light）to pass.4.Polyurethane（PU）/melanin composites with enhanced tensile strength and toughness were successfully fabricated via in situ polymerization.It is found that the tensile strength,elongation-at-break,and toughness are improved from 5.6 MPa,770%,and 33 MJ/m³ for PU to 51.5 MPa,1880%,and 413 MJ/m³ for PU/melanin（2 wt%）nanocomposite,respectively.Micromorphology indicates that individualized natural melanin are specifically linked to the hard domains of PU chains and fine dispersed in matrix.FTIR,DSC,and AFM results suggest melanin induce s an improvement in degree of phase separation,which result in remarkable enhancements in mechanical properties of PU.However,with further increasing content of melanin,a relatively large-scale phase separation is formed and lead to a decrease in mechanical properties of PU.5.Graphene oxide（GO）was reduced by dopamine with simultaneous coating by melanin-like pigment“polydopamine”in aqueous solution to yield polydopamine-capped reduce GO（PDG）.The artificial nacre nanocomposite materials of poly（vinyl alcohol）（PVA）and PDG presenting layered structure had been successfully constructed via a vacuum-assisted assembly process,in which PDG and PVA served as“bricks”and“mortar”,respectively.A combination of hydrogen bonding,strong adhesion and friction between PDG nanosheets and PVA chains result in enhancements for mechanical properties.The tensile strength,elongation at break,and toughness of PDG-PVA nanocomposite reaches to 327±19.3MPa,8±0.2%,and 13.0±0.7 MJ·m^-3,which is simultaneously 2.4,8,and 7 times higher than that of nature nacre.More interestingly,the obtained nanocomposites demonstrate a high anisotropy of thermal conductivity（k_∥/k_⊥³80）.Combined with superior mechanical properties and high anisotropy of thermal conductivity make these biomimetic materials promising candidates in aerospace,tissue engineering,and thermal management applications.6.A class of GO based artificial nacre material with quadruple hydrogen-bonding interactions was fabricated by functionalization of PDG with 2-ureido-4[1H]-pyrimidinone（UPy）self-complementary quadruple hydrogenbonding units followed by supramolecular assembly process.The artificial nacre displays a strict“brick-and-mortar”structure,with PDG nanosheets as the brick and UPy units as the mortar.The resultant nanocomposite shows an excellent balance of strength and toughness.Because of the strong strengthening via quadruple hydrogen bonding,the tensile strength and toughness can reach 325.6±17.8 MPa and 11.1±1.3 MJ·m^-3,respectively,thus exceeding natural nacre,and reaching 3.6 and 10times than that of a pure GO artificial nacre.Furthermore,after further H₂O treatment,the resulting H₂O-treated PDG-UPy actuator displays significant bending actuations when driven by heat.This work provides a pathway for the development of artificial nacre for their potential applications in energy conversion,temperature sensor,and smart thermo-driven actuator.7.A high mechanical performance and light-induced self-healable poly（ε-caprolactone）（PCL）nanocomposites were fabricated using PDG as a nanofiller with the aid of the fold-thermal compression（so-called“mechanical annealing”）cycles effect.To improve the dispersion and interfacial interactions between PDG nanosheets and PCL matrix,PCL chains were firstly grafted onto PDG nanosheets by ring-opening polymerization ofε-caprolactone.Most notably,superior mechanical performances were successfully achieved by tailoring the periodic“mechanical annealing”process.The tensile strength of PCL nanocomposite reaches up to 41.6 MPa,and the yield strength is as high as 22.7 MPa after 10 cycles periodic of“mechanical annealing”,which is simultaneously 2.4 and 2.6 times higher than that of pure PCL.Moreover,the photothermal conversion nature of melanin-like pigment has endowed the nanocomposite self-healable functionality.The temperature of nanocomposites can rapidly surpass the melting temperature of polymer upon exposure to near-infrared（NIR）light and thus allow fast NIR light-induced self-healing and recover mechanical properties of the nanocomposites.