Spectral,thermal, and electrical properties of poly(o- and m-toluidine)-polystyrene blends prepared by emulsion pathway

Conducting poly(o-toluidine) (POT) and poly(m-toluidine) (PMT) blends containing 10, 30, 50, 70, and 90 % wt/wt of polystyrene (PSt) were prepared by employing a two-step emulsion pathway. The bands characteristic of both polystyrene and POT/PMT are present in the IR spectra of POT–PSt and PMT–PSt blends. The UV-visible spectra of POT–PSt and PMT–PSt blends exhibit two bands around 313 and 610 nm, confirming that some amount of POT/PMT base is present in the blends. The EPR parameters such as line width and spin concentration reveal the presence of POT/PMT salt in the respective blends. The TGA, DTA, and DSC results suggest a higher thermal stability for the POT and PMT blends than that for the respective salts. The conductivity values of POT(70)–PSt(30) and POT(90)–PSt(10) blends are almost the same (1.1 × 10⁻² and 1.3 × 10⁻² S cm⁻¹, respectively) and these values are very close to that of pure POT salt, suggesting that POT can be blended with up to 30% wt/wt of PSt to improve its mechanical properties without a significant drop in its conductivity. The conductivity values of PMT–PSt blends are lower than those of the corresponding POT–PSt blends by two to three orders of magnitude, indicating that POT is a better system than PMT to prepare blends by this method. The dielectric constant and tan δ values of the blends increase with the amount POT/PMT and are greater than that of polystyrene. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2291–2299, 1998     

Adsorption of anionic methyl orange dye by polyaniline,poly(o-methylaniline) and poly(o-methoxyaniline)

This study reports the effect of substituents in the ortho position of polyaniline on the adsorption capacity to remove the anionic dye methyl orange (MO) from an aqueous solution. The aim of this study is the synthesis of polyaniline (PANI) and its derivatives, poly-o-methylaniline (poly-o-toluidine, POT) and poly-o-methoxyaniline (poly-o-anisidine, POA) for the adsorption removal of MO dye. All polymers were obtained by oxidative polymerization of the corresponding monomers and characterized by scanning electron microscopy (SEM) and infrared spectroscopy (IR). The average particle size of the polymer was about 200 nm. The effect of various parameters such as pH, temperature, adsorption time and initial concentration was analyzed. It was found that the adsorption capacity for dye removal increases from 50.68 to 222.56 mg g⁻¹ for PANI, from 16.89 to 66.57 mg g⁻¹ for POT, and from 97.26 to 532.54 mg g⁻¹ for POA with an increase in the initial dye concentration from 5 up to 50 mg L⁻¹. For all polymers, the equilibrium state of MO adsorption was reached in 50 min. The results showed that MO adsorption on PANI, POT, and POA is well described by a pseudo second order kinetic model. Isothermal studies have shown that adsorption is in good agreement with the Langmuir isotherm model, as evidenced by higher values of correlation coefficients. Based on the data of thermodynamic studies, it was concluded that MO adsorption on PANI, POT, and POA is spontaneous and endothermic.     

Highly conductive soluble terpolymers of aniline,toluidine, and o‐aminobenzoic acid/m‐aminobenzenesulfonic acid

Two series of terpolymers, one of o‐/m‐toluidine and aniline with o‐aminobenzoic acid and the other of o‐/m‐toluidine and aniline with m‐aminobenzenesulfonic acid, have been synthesized by oxidative polymerization via an emulsion method with ammonium persulfate as the oxidant and HCl as the external dopant. The terpolymers exhibit excellent solubility and retain the high conductivity (∼1 S cm⁻¹) characteristic of the unsubstituted homopolymer, polyaniline. The terpolymers also possess higher thermal stability than polyaniline. This can be attributed to the presence of internal doping groups and substituents, which introduce flexibility to the otherwise rigid polyaniline backbone. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3040–3048, 2005     

Graft copolymers having hydrophobic backbone and hydrophilic branches. XXIII. Particle size control of poly(ethylene glycol)-coated polystyrene nanoparticles prepared by macromonomer method

Monodisperse polymeric nanospheres, which consist of polystyrene cores and poly(ethylene glycol) (PEG) branches on their surfaces, were prepared by the dispersion copolymerization of styrene (St) with PEG macromonomers that had a methacryloyl (MMA-PEG) or p-vinylbenzyl (St-PEG) end group in various organic solvent/water media. Electron spectroscopy for chemical analysis (ESCA) of the nanosphere surfaces indicated that PEG macromonomer chains were favorably located on their surfaces. The morphologies of the nanospheres were observed via a scanning electron micrograph (SEM), and particle sizes were estimated by a submicron particle analyzer. When both the concentration of macromonomers and molecular weight were higher, small nanospheres in diameter were obtained. Larger nanospheres in diameter were obtained using macromonomers with low molecular weight at lower concentration. The functions that correlate the diameter (D_n) on different concentration units were D_n = K[St]^0.64[MMA-PEG]^{−0.53±0.01}[I]^−0.49 and D_n = K[St]^0.80[St-PEG]^{−0.69±0.01}[I]^−0.22, where [I], [St], [MMA-PEG], and [St-PEG] are initiator, styrene, MMA-PEG, and St-PEG macromonomer concentration in feed, respectively. When the reaction parameters such as the molecular weight of the macromonomers were properly chosen, the particle size could be controlled in a range from ca. 80 to 3100 nm. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2155–2166, 1999     

Swelling of polyacrylamide gels in polyacrylamide solutions

Swelling behavior of polyacrylamide (PAAm) and polyacrylamide-co-polyacrylic acid (PAAm-co-PAAc) gels was investigated in aqueous solutions of monodisperse PAAms with molecular weights (M_w) ranging from 1.5 × 10³ to 5 × 10⁶ g/mol. The volume of the gels decreases as the PAAm concentration in the external solution increases. This decrease becomes more pronounced as the molecular weight of PAAm increases. The classical Flory–Huggins (FH) theory correctly predicts the swelling behavior of nonionic PAAm gels in PAAm solutions. The polymer–polymer interaction parameter χ₂₃ was found to decrease as the molecular weight of PAAm increases. The swelling behavior of PAAm-co-PAAc gels in PAAm solutions deviates from the predictions of the FH theory. This is probably due to the change of the ionization degree of AAc units depending on the polymer concentration in the external solution. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1313–1320, 1998     

Investigation of some physicochemical properties of camphor sulfonic acid (CSA)–doped poly(o-anisidine) (PoAN) and CSA-doped PoAN/ABS composites

Camphor sulfonic acid (CSA)–doped poly(o-anisidine) (PoAN) has been found to possess the electrical conductivity of 10 S/cm. This value is rather low compared to that of (≈200 S/cm) CSA-doped polyaniline, which may arise from a limited transformation of coil-like conformation to an expanded one. Viscosity data and optical absorption spectra provided strong evidence for the existence of coil-like CSA-doped PoAN chains in m-cresol. The shift of IR bands of benzenoid and quinoid rings toward a lower wave number and the appearance of the bands at ≈1600 and 1173 cm⁻¹ (the IR is inactive but becomes active on doping) suggested the protonation of CSA-doped PoAN. The thermogravimetric profile of CSA-doped PoAN showed a five-step decomposition pattern with the thermal stability up to 134 °C. The mass spectra taken simultaneously with the thermogram revealed that dopant CSA is completely eliminated around 360 °C, but the breakdown of polymer chains does not occur very briskly at this temperature. Composites of CSA-doped PoAN with insulating acrylonitrile–butadiene–styrene copolymer (ABS) were fabricated, and it was found that the composites showed a percolation behavior in which the electrical conductivity rose sharply at about 4 weight % of CSA-doped PoAN in the ABS composite. The STM data demonstrated the formation of a continuous path at the percolation threshold. The Poole–Frenkel effect was observed for the conduction scheme in the CSA-doped PoAN/ABS composite. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4596–4604, 1999     

Depurination of synthetic poly(inosinic acid) analogues

A poly(inosinic acid) analogue, poly{[1′-(β-hypoxanthine-9-yl)-5′-deoxy-D -erythro-pent-4′-enofuranose]-alt-[maleic acid]} (4), was synthesized by the alternating copolymerization of nucleoside derivative 1 with maleic anhydride and subsequent hydrolysis. N-Glycosidic bonds of the polymer were spontaneously hydrolyzed to liberate hypoxanthine from the polymer backbone in a buffer solution (pH 7.4) at room temperature. The depurination rate constant of the polymer at pH 7.4 and 37°C was measured to be 1.9 × 10⁻⁶ sec⁻¹, which was 10⁵-fold higher than that (3 × 10⁻¹¹ sec⁻¹) of the depurination of DNA that occurred in the biological systems. The increase in the depurination rate was attributable to the high potential energy of the polymer caused by the crowded environment around the bases, so that the polymer was more susceptible to the hydrolysis. Since natural nucleic acids often have compact structures with the crowded environment around the bases by the intricate chain folding, the depurination may also be accelerated in a similar manner in the biological system. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3361–3365, 1999     

Synthesis and electrical–magnetic properties of polyaniline composites

Composites of polyaniline (PANI) with both conducting and ferromagnetic feature were synthesized by an improved method proposed by the authors. The electrical and ferromagnetic properties of the composites were measured as a function of the concentration of KOH solution used during polymerization. The conductivity of the composites at room temperature decreases with the increase of the concentration of KOH; the maximum conductivity of 8.0 × 10⁻¹ S/cm can be obtained when 25 wt % of concentration of KOH was used. For a high concentration of KOH, ferromagnetic properties of the composites including a high saturated magnetization (∼ 10.0 emu/g) depending on the concentration of KOH solution and a lower coercive force (H_c ≈ 0) independent of the concentration of KOH solution were observed. It has been demonstrated that magnetic particles (Fe₃O₄) with nanometer size in the composites can be attributed to the ferromagnetic properties of the composites observed. For a lower concentration of KOH solution, on the other hand, the magnetic properties of the composites can be decomposed to Curie susceptibility χ_c depending on the temperature and Pauli susceptibility χ_P independent of the temperature. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2799–2805, 1998     

Dispersion polymerization of pyrrole using ethylhydroxy-ethylcellulose as a stabilizer

Oxidative polymerization of pyrrole has been studied using FeCl₃ or (NH₄)₂S₂O₈ (APS) as oxidant, ethylhydroxyethylcellulose (EHEC) as a steric stabilizer and water or aqueous ethanol as the dispersion medium. Transmission electron micrographic images of the particles from the as-prepared dialysed dispersions in aqueous ethanol show small as well as large particles (about a decade larger) when FeCl₃ is used as the oxidant but only large particles when APS is used as the oxidant. Small particles are not found when the dispersions are prepared in water, irrespective of the oxidant used. The particle size decreases with an increase in molecular weight of the stabilizer for the same stabilizer concentration. The minimum amount of stabilizer required to support dispersion polymerization decreases upon increasing the alcohol content of the medium. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3723–3729, 1999     

Thermosensitive graphene nanocomposites formed using pyrene‐terminal polymers made by RAFT polymerization

Thermosensitive graphene‐polymer composites have been prepared by attaching poly(N‐isopropylacrylamide) (PNIPAAm) onto the basal plane of graphene sheets via π‐π stacking. Pyrene‐terminated PNIPAAm was synthesized using reversible addition fragmentation chain transfer (RAFT) polymerization via a pyrene‐functional RAFT agent. Aqueous solutions of the graphene‐polymer composites were stable and thermosensitive. The lower critical solution temperature (LCST) of pyrene‐terminated PNIPAAm was measured to be 33 °C. When the pyrene‐functional polymer was attached to graphene the resultant composites were also thermosensitive in aqueous solutions exhibiting a reversible suspension behavior at 24 °C. Atomic force microscopy (AFM) analysis revealed that the thickness of a graphene‐PNIPAAm (M_n: 10,000 and PDI: 1.1) sheet was ～5.0 nm. The surface coverage of polymer chains on the graphene basal plane was calculated to be 7.2 × 10^?11 mol cm^?2. The graphene‐PNIPAAm composite material was successfully characterized using X‐ray photoelectron spectroscopy (XPS), attenuated total reflection infrared (ATR‐IR) spectroscopy, and thermogravimetric analysis (TGA). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 425–433, 2010     

Chromaticity of poly(o-toluidine) matrix enhanced by anion-exchange mechanism

The incorporation of 9,10-anthraquinone-1,5-disulfonate (AQS₂) into the protonated form of poly(o-toluidine) (POT), produced by oxidative polymerization of the cationic form of the monomer or by doping the basic form (POT-EB) by anion-exchange has been studied by FTIR and UV-VIS spectroscopy and mass spectrometry. The presence of sulfur and the absence of chlorine proven by elemental analysis of the polymer product confirmed the substitution of the chloride anion with AQS₂ in the matrix. Molecular mechanics (MM+) calculations suggest that the optimal geometric structure (OMG) of AQS₂-doped POT is at least three (3.92) times more stable than that of the parent chloride-doped POT (HCl-doped POT). The increase of the absorbance at about 840 nm associated with the increasing concentration of AQS₂ revealed the insertion of AQS₂ into the POT chain. This observation could be explained by the diffusion of AQS₂ in the polymer chain. Kinetic parameters of the oxidative polymerization of the cationic form of o-toluidine (o-T-HClO₄) in the presence of different amounts of AQS₂ were deduced on the basis of absorbance variations. The results of computer-oriented kinetic analysis indicate that the rate-controlling step of the o-T polymerization is governed by the Ginstling-Bronstein equation representing the three-dimensional diffusion (D4). Activation parameters of the oxidative polymerization of protonated o-T in the presence of varying amount of AQS₂ were computed and discussed.     

Highly soluble conducting poly(ethylene oxide) grafted at two sites of poly(o‐aminobenzyl alcohol)

Poly(o‐aminobenzyl alcohol) (POABA) was grafted with poly(ethylene oxide)s (PEOs) through the reaction of tosylated PEO with both the hydroxide and amine moieties of reduced POABA. Reduced POABA was prepared through the acid‐mediated polymerization of o‐aminobenzyl alcohol, followed by neutralization with an aqueous ammonium hydroxide solution and reduction with hydrazine. The grafted copolymers were very soluble in common polar solvents, such as chloroform, tetrahydrofuran, and dimethylformamide, and the copolymers with longer PEO side chains (number‐average molecular weight > 164) were even water‐soluble. The conductivities of the doped grafted copolymers decreased with increasing PEO side‐chain length because of the nonconducting PEO and its torsional effect on the POABA backbone. The conductivity of highly water‐soluble POABA‐g‐PEO‐350 was 0.689 × 10^?3 S/cm, that is, in the semiconducting range. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4756–4764, 2004     

Donor–acceptor‐based poly(toluidine‐co‐chloroaniline)s: Synthesis and characterization

A series of poly(o‐/m‐toluidine‐co‐o‐/m‐chloroaniline) copolymers of different compositions were synthesized by an emulsion method with ammonium persulfate as the oxidant. The conductivity of the copolymers was two to five orders of magnitude higher than that of the homopolymers poly(o‐toluidine) and poly(m‐chloroaniline). Among the copolymers, the copolymer of o‐toluidine and m‐chloroaniline exhibited a maximum conductivity of 0.14 S cm^?1. The conductivity of these copolymers was also higher than that of poly(aniline‐co‐chloroaniline). The properties of the copolymers were greatly influenced by the positions of the substituents and the concentrations of the individual monomers in the feed. All the copolymers were completely soluble in polar solvents such as dimethyl sulfoxide and showed higher heat stability as the chloroaniline concentration increased. These effects could be interpreted in terms of extensive hydrogen bonding and interchain linking and, therefore, higher electron delocalization in these copolymers due to the presence of electron‐rich toluidine rings adjacent to electron‐deficient chloroaniline. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1579–1587, 2005     

Amylose-iodine complex formation without KI: Evidence for absence of iodide ions within the complex

Amylose-iodine (AI) complex has been synthesized in aqueous solution without added KI. Complex formation (with solid iodine in amylose solution) is maximized at approximately 35°C and decreases beyond that temperature. Ion-selective electrode (ISE) measurements of an aqueous solution of iodine and AI complex indicate that there is no change in the I⁻ ion concentration when the complex forms. This suggests that I⁻ ions (including I, I, and others) cannot be involved in forming the AI complex. The present work also reports a new and simple method for providing both the iodine-binding capacity (IBC) of amylose and the dissociation mechanism for the AI complex. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2711–2717, 1999     

A composite of polyaniline with both conducting and ferromagnetic functions

A composite of polyaniline (PANI) with both conducting and ferromagnetic functions was synthesized by a chemical method proposed by the authors. For the electrical properties, its room-temperature conductivity was measured to be about 10⁻¹ S/cm when doped with 1.0M HCl, and it is independent of the preparation conditions, such as reaction temperature and concentration of FeSO₄ solution. Temperature dependence of the conductivity of the composites at temperature between 77 and 450 K is controlled by thermal activation and dedoping processes, which result in the decrease of conductivity with increase of temperature as T > 320 K. For their magnetic properties, unusual ferromagnetic properties with high saturated magnetization (M₂) and lower coercive force (H_c = 0) were observed. An effect of the preparation conditions on the ferromagnetic properties of composites was observed. The higher the reaction temperature and the concentration of FeSO₄ solution, the higher the saturated magnetization was observed. No hysteresis feature (i.e. H_c = 0) for any PANI composites synthesized in this paper was observed, and this is independent of the preparation conditions. This may be attributed to the nanometer size of the magnetic particles existing in composites. Thus, it suggests that the doping of PANI leads to electrical properties of composites, whereas the nanocrystalline magnetic particles (Fe₃O₄) are responsible for the observed ferromagnetic properties of PANI composites. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2129–2136, 1997     

Glucose oxidase electrodes of polyaniline,poly(o‐toluidine) and their copolymer as a biosensor: a comparative study

A simple technique is described for constructing a glucose sensor by the entrapment of glucose oxidase (GOD) in a polyaniline (PA), poly(o‐toluidine) (POT) and their copolymer poly(aniline‐co‐o‐toluidine) (PA‐co‐POT) thin films, which were electrochemically deposited on a platinum plate in phosphate and acetate buffer. The maximum current response was observed for PA, POT, and PA‐co‐POT GOD electrodes at pH 5.5 and potential 0.60 V (v. Ag/AgCl). The phosphate buffer gives fast response as compared to acetate buffer in amperometric measurements. PA GOD electrode shows the fastest response followed by PA‐co‐POT and POT GOD electrodes. Copyright © 2004 John Wiley & Sons, Ltd.     

Properties of In situ polymerized poly(3,4‐ethylenedioxythiophene)/alumina composites for energy storage applications

Composites formed by poly(3,4‐ethylenedioxythiophene) and alumina (PEDOT/Al₂O₃) have been prepared by in situ anodic polymerization. For this purpose, the stability of 1:1 and 4:1 monomer:alumina aqueous solutions has been examined as a function of the pH (2.3, 4.0, 7.0, 8.8, or 10.8). Results indicate that the monomer behaves as a dispersant that remains stable at the studied basic pHs despite they are close to the isoelectric point of alumina. Although the thermal stability of the composites is considerably affected by the pH of the reaction medium, its influence on the surface morphology is very small. Independently, of the synthetic conditions, the electrochemical properties were better for PEDOT/Al₂O₃ than for pure PEDOT, reflecting that alumina particles promote the charge mobility. The highest specific capacitance (SC; 141 F/g), which was 55% higher than that obtained for pure PEDOT, was achieved for the composite prepared at pH = 8.8 using a 4:1 monomer:alumina ratio. These conditions favor the participation of OH^– groups as secondary doping agents without degrading the polymer matrix and enhance the specific surface of the films, facilitating the ionic mobility. On the other hand, application of a multi‐step polymerization strategy has shown that interfaces originated by consecutive steps enhance the SC. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1131–1141     

Effect of low contents of a polyaniline derivative on the crystallization and electrical properties of blends with PVDF

The effect of low levels of poly(o‐methoxyaniline) (POMA) on the crystallization, morphology, and electrical characteristics of blends with poly(vinylidene fluoride) (PVDF) were studied by infrared spectroscopy, AC electrical measurements, and optical microscopy. Undoped POMA has a strong effect in increasing the α‐phase and decreasing the β‐phase content of PVDF in blends crystallized from solution. For blends melt crystallized, doped POMA promotes much greater homogeneity than undoped POMA. Interestingly, doped POMA promotes the nucleation and growth of unringed PVDF spherulites, whereas undoped POMA hinders it. The doping state of the POMA was also extremely important in determining the electrical behavior of the blend. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1219–1224, 1999     

New aromatic benzazole polymers II: Synthesis and conductivity of benzobisthiazole-co-polymers incorporated with 4-N,N-dimethylaminotriphenylamine groups

New aromatic benzobisthiazole copolymers containing 10–70 mol % of 4-N,N-dimethylamino-triphenylamine functionality were prepared from the respective dinitrile or dicarboxylic acid monomers, terephthalic acid, and 2,5-diamino-1,4-benzene-dithiol dihydrochloride in polyphosphoric acid. At the first approximation, the copolymers containing 10 mol % or less of the triarylamino moieties in the polymer chains still preserve the capability to form anisotropic (nematic) solutions at 10 wt % polymer concentration. This is an important requirement for processing the copolymers into fibers and films with good to excellent mechanical properties. Films with good mechanical integrity were cast from the dilute methanesulfonic acid solutions of the copolymers under reduced pressure. They showed electrical conductivity values of the order of 10⁻¹¹–10⁻¹⁰ S/cm in pristine state, with four to seven orders of magnitude increase upon exposure to mild oxidizing agents such as iodine vapor. On the contrary, the parent polymer, poly(p-phenylene benzobisthiazole) is an insulator with conductivity of less than 10⁻¹² S/cm, and its conductivity does not improve at all with exposure to iodine vapor. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 713–724, 1998     

New aromatic benzazole polymers. I. Benzobisthiazole and benzobisoxazole polymers with main-chain triarylamino units

Thermally stable, nonrigid-rod poly(benzobisthiazoles), (R)TPA-PBZT , where R = H, Me, NMe₂, and OH, and poly(benzobisoxazoles), (R)TPA-PBO , where R = Me, NMe₂ containing electron-rich triarylamine groups with various para-substituents (Rs) on the pendent phenyl ring, were synthesized from either 2,5-diamino-1,4-benzenedithiol dihydrochloride or 2,4-diamino-1,5-benzenediol dihydrochloride and the respective triarylamine-based dinitrile or diacid monomer in polyphosphoric acid. Whereas (R)TPA-PBZT polymers were obtained in moderate molecular weights, analogous (R)TPA-PBO polymers were only prepared in low molecular weights. No lyotropic behaviors, characteristic of the unmodified rigid-rod benzazole polymers, as evidenced by the absence of either stir opalescence or birefringence under crosspolarizers, were observed for these homopolymers at about 10 wt % polymer concentration. Among these polymers, only (Me)TPA-PBZT and (NMe₂)TPA-PBZT formed cast films with good mechanical integrity. In their pristine state, their film conductivity values were in the range of 10⁻¹⁰–10⁻⁹ S/cm at room temperature. Upon exposure to iodine vapor, their conductivities were increased to the maximal values of 5.0 × 10⁻⁵ S/cm ( (Me)TPA-PBZT ) and 4.1 × 10⁻⁴ S/cm ( (NMe₂)TPA-PBZT ). © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 1909–1924, 1997