Preparation and tribological studies of C60 thin film chemisorbed on a functional polymer surface

A novel self-assembled C60 film was prepared by chemical adsorption of C60 molecules onto an amino-group-containing polyethyleneimine-coated silicon substrate surface. The contact angle of distilled water on the C60 film was measured, the thickness was determined by means of ellipsometric analysis, and the morphology was observed with an atomic force microscope. The tribological properties of the films were investigated as well. It was found that the C60 thin film had a contact angle of about 72 degrees and thickness of 1.8 nm and exhibited a surface domain microstructure composed of fullerene clusters. Due to the hydrophobicity and low surface energy, the C60 film possessed good adhesive resistance and had an adhesive force of about 7.1 nN, which was about an order of magnitude lower than that of the silicon substrate surface. Moreover, the C60 film showed good friction reduction, load-carrying capacity, and antiwear ability, which were attributed to the higher mechanical stiffness and elastic modulus of C60 molecules. Besides, the friction coefficient decreased with increasing sliding velocity and normal loads, due to the rolling effect of the physisorbed C60 molecules.     

Vibrational spectra of C60.C8H8 and C70.C8H8 in the rotor-stator and polymer phases

C(60).C(8)H(8) and C(70).C(8)H(8) are prototypes of rotor-stator cocrystals. We present infrared and Raman spectra of these materials and show how the rotor-stator nature is reflected in their vibrational properties. We measured the vibrational spectra of the polymer phases poly(C(60)C(8)H(8)) and poly(C(70)C(8)H(8)) resulting from a solid-state reaction occurring on heating. On the basis of the spectra, we propose a connection pattern for the fullerene in poly(C(60)C(8)H(8)), where the symmetry of the C(60) molecule is D(2h). On illuminating the C(60).C(8)H(8) cocrystal with green or blue light, a photochemical reaction was observed leading to a product similar to that of the thermal polymerization.     

In-situ infrared spectroscopy of a photoirradiated potassium-doped C60 film

Photodimerization between C₆₀ molecules in a potassium-doped C₆₀ film (a mixture of α-C₆₀ and K₃C₆₀ phases) has been investigated using in-situ Fourier-transform (FT) infrared spectroscopy in combination with tight-binding molecular dynamics calculations. A comparison of the experimental spectrum of C₆₀ dimers, which was found to be a main product in the photoirradiated K-doped film using an FT mass spectrometer, with the theoretical spectra for several isomers of C₁₂₀ suggests that C₁₂₀ species with structures similar to a bucky peanut were formed in the photoirradiated film.     

Polymer decoration: The orientation of polymer folds as revealed by the crystallization of polymer vapors

A new technique of decoration of the fold surface of polymer crystals is described. It is similar in methodology to gold decoration, but makes use of vapors of crystallizable polymers, principally polyethylene, as decorating material. Upon condensation and crystallization, the highly anisometric decorating molecules become oriented parallel to the fold direction. They build up small crystalline lamellae which, seen edge-on, appear as elongated rods and can be easily observed by conventional transmission electron microscopy. This decoration technique reveals the sectorization of polymer single crystals grown from solution. It can be used to determine the local fold orientation (with a resolution of ca. 10 nm) of polymers crystallized under a wide range of conditions, including crystallization from the bulk. The technique reveals that under all crystallization conditions so far investigated the outermost part of the polymer fold surface is relatively ordered, and the folds and/or loops are, as a rule, oriented nearly parallel to the macroscopic growth front.     

Thermal degradation of the polymer obtained by oxidation of C60 fullerene by oleum

Degradation of (C₆₀) _n polymer and the C₁₂₀ and C₁₂₀O dimers at different temperatures was studied by IR and ESR spectroscopy. The formation of C₆₀ was revealed. The polymer stability was found to be higher than the stability of the dimers. Peculiar features of the IR spectra of three-dimensional hyper-crosslinked polymers obtained by liquid-phase polymerization of C₆₀ are discussed.     

The structure of water in polymer systems as revealed by Raman spectroscopy

A review on the structure of water in aqueous polymer systems as revealed by Raman spectroscopy is presented. Various interpretations and analysis procedures for Raman bands of liquid water which have been proposed are introduced. The structure and hydrogen-bonding properties of water which exist in aqueous polymer solutions and gels are described. Effects of chemical properties of polymer chains and size of water domains surrounded by polymer chains on the structure of water are discussed.     

Fluorescence modulation of eosin Y in a PMMA film by diarylethene switching

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A lithium secondary battery using a thin film of polymer electrolyte as a separator

Ion-conductive polymer which shows an ionic conductivity (σ_i) of 1.4 × 10^?4S/cm at 25°C when mixed with LiClO₄ (molar ratio in Li/OE = 0.05) was used as a separator of electrodes in a lithium secondary battery. The effect of high ionic conductivity on the performance of the battery was studied. The polymer structure was and the cathode was comprised of poly(1,3,4-thiadiazole disulfide), graphite powder and the polymer electrolyte. The cell [(?)Li/polymer electrolyte/graphite–poly(disulfide) (+)] had an open circuit voltage (V_oc) of 3.25 V, a plateau voltage of 2.75 V, a discharge density (i_d) of 0.05 mA/cm² with the cathode utilization of 63%, and achieved over several tens of cycles at 25°C.     

Electric-field-induced changes in absorption and emission spectra of CdS nanoparticles doped in a polymer film

Electric-field-induced changes in absorption and emission spectra of colloidal CdS nanoparticles ranging in size from 1.0 to 5.0 nm in diameter have been measured by using electric field modulation spectroscopy. The analysis of the electroabsorption spectra indicates that the dipole moment in the first exciton state becomes larger with increasing particle size. The presence of the large dipole moment following photoexcitation into the first exciton band suggests that the CdS nanoparticles have large CT character in the first exciton state. The quantum yields both of the exciton emission and of the trap emission are markedly reduced by application of an electric field. On the basis of the direct measurements of the field-induced change in emission decay profile, it is suggested that the field-induced de-enhancement of these emission yields results from the field-induced decreases both in lifetime and in initial population of each emitting state. It is also found that the emission intensity of CdS nanoparticles increases under the UV light irradiation in air and decreases in a vacuum condition and that fluorescence lifetime in the former case is longer than that in the latter. This enhancement and de-enhancement process in emission intensity is almost reversible at least in several cycles.     

ESR spectra of C60 in concentrated sulfuric acid

On dissolution of C₆₀ in concentrated sulfuric acid, ESR spectra of paramagnetic species, which were identified as dimers (C₁₂₀ ⁺) or oligomers (nC₆₀)⁺, were recorded. The ESR spectra recorded upon the reaction of a toluene solution of C₆₀ with sulfuric acid were assigned to the radical cation C₆₀ ⁺. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2373–2376, November, 1998.     

Multiple glass-transition temperatures in thin supported films of isotactic PMMA as revealed by enhanced Raman scattering

The glass-transition temperature, Tg, of isotactic PMMA thin films has been measured for four thicknesses by enhanced Raman spectroscopy and ellipsometry. This was made possible by inserting a silica spacer layer between the film and the substrate. The use of such a spacer drastically improves the sensitivity of Raman scattering measurements. The improvement in the sensitivity allows us to study phenomena involving changes in molecular dynamics, such as the phase transition, and to probe the existence in very thin films of several thickness-dependent transition temperatures, Tg(h). This in turn is interpreted as the occurrence in the film of a layered structure. The influence of the polymer concentration on the conformation of the surface adsorbed polymer layer and therefore on Tg(h) is discussed.     

Sequence distribution of PMMA/iBuA copolymer by incremental calculation of 13C NMR spectra

An incremental method for characterizing triad and pentad distribution by ¹³C NMR spectroscopy was applied to the poly(methyl methacrylate-co-isobutyl acrylate) copolymer. Calculation of the intensity of individual lines was performed applying Bernoulli statistics, while the chemical shifts for each sequence were calculated by an incremental method. Based on these data, the carbonyl signal was simulated yielding good agreement at the triad and pentad level.     

Selective reduction of fullerene C60 by metals in neutral and alkaline media. Interaction of C60 with KOH

The reduction of fullerene C₆₀ by Zn and Mg in DMF was studied both in the presence and absence of KOH. Fullerene C₆₀ was reduced in these systems to form the C₆₀ ^n– (n = 1, 2, and 3) anions. The anions were detected by optical and ESR spectroscopies. It was found that Mg reduced C₆₀ to the monoanion, Mg/KOH and Zn reduced C₆₀ to the dianion, and Zn/KOH reduced C₆₀ to the trianion. Like KCN, potassium hydroxide adds to fullerene upon interaction with C₆₀ in DMF. The reaction of C₆₀ with KOH in benzonitrile was accompanied by the generation of the fullerene monoanion. A possible mechanism of the formation of fullerene monoanions in the presence of KOH is discussed. The degradation of the C₆₀ ^n– anions in air was studied.     

Electric field effects on absorption and fluorescence spectra of trimethylsilyl- and trimethylsilylethynyl-substituted compounds of pyrene in a PMMA film

External electric field effects on absorption and fluorescence spectra of 1,3,6,8-tetrakis(trimethylsilyl)pyrene and 1,3,6,8-tetrakis(trimethylsilylethynyl)pyrene (TMSPy and TMS(E)Py, respectively) have been examined in a poly(methyl methacrylate) (PMMA) film at various concentrations at various temperatures. TMS(E)Py preferentially forms an aggregate in a PMMA film, as the concentration increases, indicating that the acetylenic groups enhance the pi-pi interactions between pyrene molecules. The change in molecular polarizability following excitation has been determined both for the monomer and for the aggregate, based on the electroabsorption spectra. The change in molecular polarizability following emission has also been determined in both compounds, based on the electrofluorescence spectra. TMSPy exhibits two excimer fluorescence emissions at high concentrations which are ascribed to the partially overlapping excimer and the sandwich-type excimer, respectively, besides the monomer fluorescence emitted from the locally excited state. The sandwich-type excimer fluorescence as well as monomer fluorescence is quenched by an electric field, whereas the fluorescence of the partially overlapping excimer is enhanced by an electric field. Excimer fluorescence of TMS(E)Py, which arises from the sandwich-type excimer, is quenched by an electric field at any temperature. Only one species of the partially overlapping excimer is confirmed in TMSPy, while no partially overlapping excimer is confirmed in TMS(E)Py.     

Switching of the helical polymer conformation in a solid polymer film

A chiral photochromic polyisocyanate was incorporated into a solid polymer matrix of poly(methyl methacrylate) (PMMA), yielding an isotropic polymer film. Isomerization of the chiral photochromic azo side groups (cis‐trans) triggers a reversible conformational change of the helical polyisocyanate backbone. Thus the chirooptical properties of the film can be switched photochemically. The isomerization of the helix is much slower than the isomerization of the azo side groups. Below T_g , the photochemically modified helix conformation is thus stable, despite thermal relaxation of the azo chromophores.     

Broken inter-C60 bonds as the cause of magnetism in polymeric C60: a density functional study using C60 dimers

Bond breaking in C60-C60 dimeric units is believed to play an important role in the onset of magnetism in 2D polymeric C60. On the basis of density-functional theory, the calculations we present here provide further insight into this mechanism through a quantitative characterization of the bond-breaking processes in the isolated dumbbell-shaped C60 dimer. In particular, the analysis of the calculated potential energy surfaces for the low-lying singlet and triplet states identifies and locates the S0-T2 crossing point, which is crucial for the transition to a magnetic state to take place under thermal conditions. These results also suggest a possible new approach to the production of magnetic polymeric C60.     

Nonvolatile polymer memory device based on bistable electrical switching in a thin film of poly(N-vinylcarbazole) with covalently bonded C60

A functional polymer (PVK-C60), containing carbazole moieties (electron donors) and fullerene moieties (electron-acceptors) in a molar ratio of about 100:1, was synthesized via covalent tethering of C60 to poly(N-vinylcarbazole) (PVK). The molecular structure and composition of PVK-C60 were characterized by FTIR, Raman, and UV-vis absorption spectroscopy, gel permeation chromatography (GPC), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CyV). The C60-modified PVK exhibited an enhanced glass-transition temperature (Tg = 226 degrees C) and good solubility in organic solvents such as toluene, tetrahydrofuran, chloroform, and N,N-dimethylformamide (DMF). It could be cast into transparent films from solutions. For a thin film of PVK-C60 sandwiched between an indium tin oxide (ITO) electrode and an Al electrode (ITO/PVK-C60/Al), the device behaved as nonvolatile flash (rewritable) memory with accessible electronic states that could be written, read, and erased. The polymer memory exhibited an ON/OFF current ratio of more than 105 and write/erase voltages around -2.8 V/+3.0 V. Both the ON and OFF states were stable under a constant voltage stress of -1 V for 12 h and survived up to 108 read cycles at -1 V under ambient conditions.     

Theoretical predictions of the structures and electronic spectra of C60NH with comparisons with the isoelectronic molecules C60O and C60CH2

Intermediate neglect of differential overlap (INDO ) calculations were used to study two structures of C₆₀NH: one of C_2ν, geometry with a bridging NH across the bond between two fused six-membered rings in C₆₀ and the other of C_s geometry with a bridging NH across the bond between a five- and a six-membered ring. We calculated the most stable isomer of C₆₀NH to be of C_2ν, symmetry. It was found that the C_2ν isomer has a protonated aziridine structure with a bridging C? C bond length of 0.1520 nm. The electronic spectra of both isomers of C₆₀NH were calculated. Comparisons were made with the isoelectronic molecules C₆₀O and C₆₀CH₂, cases in which the calculated electronic spectra for the most stable isomers C₆₀O (C_2ν) and C₆₀CH₂ (C_2ν) are in good agreement with recent experimental results. © 1995 John Wiley & Sons, Inc.     

Thermal degradation of a heteroarm starlike polymer with fullerene C60 core

The mechanism of thermal degradation of a 12-arm starlike polymer with fullerene C₆₀ core and equal number of polystyrene and poly(tert-butyl methacrylate) arms was studied by thermal desorption mass spectrometry. Thermal characteristics of the heteroarm stars were compared with those of six-arm starlike fullerene-containing polystyrenes and linear poly(tert-butyl methacrylate).