Preparation And Infrared Radiation Properties Of Nanocomposites Based On Helical Polyacetylene

Materials with low infrared emissivity have attracted widespread attention due to their importance in electromagnetic protection, thermal insulation and military stealthy. Organic polymers possess unique properties including low density, tractability, anti-corrosion and the most value, their adjustable composition and structure, which can provide the possibility of adjusting their emission performance efficaciously according to practical applications. Optically active polymer is beneficial to be an excellent alternative in lowering infrared emissivity because the orderly helical conformation and strong intramolecular interactions decrease the unsaturation of macromolecular chains. However, since the vibration of unsaturated chemical bonds in the organics result in unsatisfactory emissivity, pristine organic polymer is not enough to meet the requirement of practical uses. Furthermore, the undesirable instability against heat, poor mechanical strength and intractable processability of these polymers are also disadvantageous to potential technological applications so that further modification or hybridization with inorganic nanomaterials is indeed required. The hybridization process will create more interfacial synergism forces such as hydrogen bonds or electrostatic interactions between the organic and inorganic components which can alter the original vibration mode of macromolecules, atoms or pendants on interface, which may eventually have great effects on the emissivity. Thus, it has still remained a meaningful research objective to develop novel classes of optically active polymer/inorganic nanocomposites with tunable infrared radiation properties.In this paper, novel optically active amino acid-based N-propargylamide monomers were synthesized and polymerized with rhodium zwitterion catalyst to afford helical polyacetylene (HPA). The organic/inorganic nanocomposites were prepared by grafting the polymers onto the surface-modified nanoparticles. All of the polymers and nanocomposites were characterized by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), UV-Vis spectroscopy, X-ray diffraction (XRD), circular dichroism (CD) spectroscopy, thermogravimetric analysis (TGA), scanning electronic microscope (SEM), transmission electronic microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Finally, the infrared emissivity values of them were tested. Effects of microstructure and infrared emissivity value at wavelength of 8-14 μm of the polymers and nanocomposites were investigated, and the following results were obtained.1. Synthesis, characterization and infrared emissivity property study of helical amino acid-based polyacetylenesChiral amino acid-based monosubstituted acetylene monomers [N-tert-butoxy-carbonyl-L-phenylalanine-N’-propargylamide (LP), and N-tert-butoxycarbonyl-L-serine-N’-propargylamide (LS)] were (co)polymerized with rhodium zwitterion catalyst in THF to afford helical polyacetylenes (HPAs) with moderate molecular weights (4400-14800) in good yields. The optically active HPA copolymers were soluble in common organic solvents and proven to adopt predominately single-handed helical conformations according to their intense Cotton effect and large specific rotations. Various contents of amino acid units in side chains facilitated controllable helical secondary structure while the helix was primarily stabilized by hydrogen bonding and steric repulsion between the substituents. Intramolecular hydrogen bonds constructed between hydroxyl and amide groups, in particular, played a significant role in adjusting the helicity and orderliness of the helix. The infrared emissivity values of the HPAs at wavelength of 8-14 μm were measured and the correlation between helical conformations and their effect on infrared emission were systematically investigated. The results showed that more well-ordered and compact screw-sense could enhance the performance of organic polymers in lowering their infrared emissivity. Among all the as-prepared HPAs, poly(LP50-co-LS50) exhibited the lowest infrared emissivity value (ε= 0.632) and possessed excellent resistance against heat.2. Effect of monomer configuration on hydrogen bonding, secondary conformation, and infrared emissivity of the helical polyacetyleneSerine-based monosubstituted acetylene monomers were synthesized and polymerized with rhodium zwitterion catalyst in THF to afford optically active polyacetylene derivatives (LPA and DPA) and corresponding racemic polyacetylenes (RPA) with moderate molecular weights in good yields. All of the substituted polyacetylenes (HPA) were characterized by FT-IR, NMR, GPC, UV-vis, CD and TGA. LPA and DPA were soluble in common organic solvents and possessed single-handed helical conformation according to their intense Cotton effect and large specific rotations, while RPA presented random coiled polymer chain. The characterization results showed that helical structure of these polymers was created and stabilized by intra-and intermolecular hydrogen bonding between the substituents. Further, the infrared emissivity properties of the polymers at wavelength of 8-14 μm were investigated at room temperature. Consequently, the LPA and DPA exhibited lower infrared emissivity values than RPA.3. Preparation, characterization, and infrared emissivity property study of SiO2/TiO2/helical polyacetylene multilayered nanospheresOptically active silica/titania/helical substituted polyacetylene (SiO2/TiO2/HPA) multilayered core-shell nanocomposites were successfully prepared by the combination of surface titania deposition on bare silica nanosphere and subsequent polymer grafting. The chiral amino acid-based HPA copolymer serving as the organic shell was optically active and adopted a predominately single-handed helical conformation. The SiO2/TiO2/HPA nanospheres were characterized by FT-IR, XRD, TEM, and SEM which record the formation of the multilayered architecture. The results clearly showed that the inorganic/organic hybrid nanoparticles exhibited hierarchical multilayered core-shell construction. The HPA outer shell experienced an enhancement in thermal stability and still maintained considerable optical activity after grafting to the SiO2/TiO2 nanosphere. The infrared emissivity value (ε= 0.548) of SiO2/TiO2/HPA nanocomposites at wavelength of 8-14μm was much lower than each of its components. The reduced infrared emissivity value proved that the strengthened interfacial interactions originating from the coating HPA had an effective synergistic effect with the semiconductive anatase TiO2 nanoparticles on silica sphere in lowering the infrared emissivity value.4. Preparation, characterization, and infrared emissivity property study of helical polyacetylene@CdSe quantum dots compositesOptically active helical polyacetylene@CdSe quantum dots (HPA@CdSe QDs) nanocomposites were fabricated by grafting HPA polymers onto the surface of semiconductor QDs through ester linkage. Optically active HPA polymer derived from chiral amino acids was copolymerized by a rhodium zwitterion catalyst, and was evidently proved to adopt a predominately single-handed helical conformation. The HPA@QDs nanocomposites were characterized by FT-IR, XPS, XRD, TEM, and TGA. The results indicated that the organic HPA matrix exhibited an enhancement in thermal stability after the hybridization with CdSe QDs, while the blended QDs maintained their original crystalline structure during grafting. The infrared emissivity properties of the HPA@QDs nanocomposites at 8-14 μm were further investigated. The data demonstrated that HPA@QDs-15 composite film possessed an infrared emissivity value of 0.501, which was much lower than pristine HPA and QDs. This might be attributed to the incorporation of optically active HPA and semiconductor QDs in a hybrid phase which had great effect in enhancing their interfacial interactions.5. Preparation, characterization and infrared emissivity property of helical polyacetylene@carbon nanotube hybridsOptically active helical polyacetylene@multiwalled carbon nanotubes (HPA@MWCNTs) nanohybrids were fabricated by wrapping HPA copolymers onto the surface of modified nanotubes through ester bonding linkage. HPA copolymer based on chiral phenylalanine and serine was pre-polymerized by a rhodium zwitterion catalyst in THF, and evidently proved to possess strong optical activity and adopt a predominately one-handed helical conformation. Various characterizations including FT-IR, XPS, XRD, and TEM results demonstrated that the HPA had been covalently grafted onto the nanotubes without destroying their original graphite structure. The wrapped HPA was found to exhibit an enhancement in thermal stability and maintained considerable optical activity after grafting. The infrared emissivity property of the nanohybrids at 8-14μm was investigated in addition. The results indicated that the HPA@MWCNTs hybrid matrix could possess a much lower infrared emissivity value (ε= 0.707) than raw MWCNTs, which might due to synergistic effect of the unique helical conformation of optically active HPA and strengthened interfacial interaction between the organic polymers and inorganic nanoparticles.6. Preparation, characterization and infrared emissivity property of helical polyacetylene@WO3 nanorod hybridsOptically active helical polyacetylene@WO3 nanorod hybrids have been successfully fabricated via a "grafting to" strategy which involves preformation of amino acid-based HPA, surface modification of WO3 nanorods and subsequent inorganic/organic grafting reaction. Various characterization data including FT-IR, CD, XRD, TEM, and TGA clearly show that the hybrids exhibit core-shell construction composing of an inorganic WO3 nanorod core and an organic HPA shell. The HPA outer shell was about 0.073 g/(g nanohybrids) and still remained considerable optical activity without any destruction to the crystalline structure of WO3. The infrared emissivity of HPA@WO3 at the wavelength of 8-14μm was further investigated, and the result demonstrated that the infrared emissivity value of HPA@WO3 hybrids was reduced to 0.527, while the bare WO3 nanorods possessed a value of 0.885. The enhancement in lowering the infrared emissivity could be attributed to the unique interfacial interactions between organics and inorganics.