Synthesis, characterization and optical properties of fluorinated poly(aryl ether)s containing phthalazinone moieties

A series of novel fluorinated poly(aryl ether)s containing phthalazinone moieties (FPPEs) have been prepared by a modified synthetic procedure for optical waveguide applications. The obtained random copolymers exhibited excellent solubility in polar organic solvents, high glass transition temperatures (T_gs: 185-269 °C), good thermal stabilities (the temperatures of 1% weight loss: 487-510 °C) and good optical properties. By adjusting the feed ratio of the reactants, the refractive indices of TE and TM modes (at 1550 nm) could be well controlled in the range of 1.575-1.498 and 1.552-1.484, respectively. The optical losses of the FPPEs exhibited relatively low values (less than 0.27 dB/cm at 1310 nm). Additionally, the thermo-optic coefficient (dn/dT) values of the FPPEs at 1310 nm and 1550 nm (TE mode) ranged from −0.97 × 10⁻⁴ °C to −1.33 × 10⁻⁴ °C and from −0.96 × 10⁻⁴ °C to −1.29 × 10⁻⁴ °C, respectively.     

Thermally stable NLO poly(amide-imide)s via sequential self-repetitive reaction

A series of thermally stable side-chain second-order nonlinear optical (NLO) poly(amide-imide)s via sequential self-repetitive reaction (SSRR) have been developed. This SSRR is based on carbodiimide (CDI) chemistry. Three difunctional azo chromophores (DR19, NDPD and DNDA) were, respectively, reacted with excessive amount of 4,4′-methylene-diphenylisocyanate (MDI) to form poly-CDI, and subsequently trimellitic anhydride (TMA) was added to obtain an intermediate, poly(N-acylurea). Poly(N-acylurea) exhibits excellent organosolubility, which enables the fabrication of high quality optical thin films. Moreover, its moderate glass transition temperature (T_g) characteristic allows the NLO-active polymer to exhibit high poling efficiency. After in situ poling and curing process, N-acylurea moieties were converted to amide-imide structures via SSRR, and the T_gs of the polymers were elevated significantly up to 70 °C higher than that of the poly-CDI sample. Electro-optical coefficients, r₃₃ of about 5.2-25.2 pm/V at 830 nm were obtained. Good temporal stability (80 °C) and waveguide optical losses (3.8-6.6 dB/cm at 830 nm) were also obtained for these polymers.     

Synthesis and characterization of partly fluorinated poly(phthalazinone ether)s crosslinked by allyl group for passive optical waveguides

It is necessary to introduce cross-linkable groups onto polymer chains as the processability and thermal stability of the polymers for passive waveguide device applications are very dependent on their cross-linking capabilities. Herein a series of novel cross-linkable allyl-containing fluorinated poly(phthalazinone ether)s (Allyl-FPPEs) have been prepared by a modified polycondensation of 4-(4-hydroxylphenyl)(2H)-phthalazin-1-one (DHPZ), decafluorobiphenyl (DFBP), 4,4′-(hexafluoroisopropylidene)diphenol (6F-BPA), and 3,3′-diallyl-4,4′-dihydroxybiphenyl (DA-DHBP) for optical waveguide applications. The obtained random polymers were characterized by FT-IR, NMR and GPC. The resulting polymers having good solubility in polar organic solvents at room temperature, can be easily spin-coated into thin films with attracting optical quality, good thermal stabilities (the temperatures of 1% mass-loss after curing: 455-503 °C), and high glass transition temperatures (T_gs: 167-251 °C) which could further increase by about 20 °C after thermal cross-linking. The crosslinked polymer films exhibit good optical properties. By adjusting the feed ratio of the reactants, the refractive indices of TE and TM modes (at 1550 nm) could be well controlled in the range of 1.4998-1.5618 and 1.4954-1.5520, respectively. The optical losses of the crosslinked polymers possess rather low values, less than 0.3 dB/cm at 1550 nm.     

Physical aging of thin glassy polymer films monitored by gas permeability

The physical aging at 35 °C of three glassy polymers, polysulfone, a polyimide and poly(2,6-dimethyl-1,4-phenylene oxide), has been tracked by measurement of the permeation of three gases, O₂, N₂, and CH₄, for over 200 days. Several techniques were used to accurately determine the thickness of films (∼400 nm-62 μm) in order to obtain absolute permeability coefficients and to study the effects of film thickness on the rate of physical aging. Each film was heated above the polymer T_g to set the aging clock to time zero; ellipsometry revealed that this procedure leads to isotropic films having initial characteristics independent of film thickness. A substantial pronounced aging response, attributed to a decrease in polymer free volume, was observed at temperatures more than 150 °C below T_g for thin films of each polymer compared to what is observed for the bulk polymers. The films with thicknesses of approximately 400 nm of the three polymers exhibit an oxygen permeability decrease by as much as two-fold or more and about 14-15% increase in O₂/N₂ selectivity at an aging time of 1000 h. The results obtained in this study were compared with prior work on thickness dependent aging. The effects of crystallinity on physical aging were examined briefly.     

The structure of a cyanobiphenyl side chain liquid crystalline poly(silylenemethylene)

The structure of a side chain liquid crystalline poly(silylenemethylene) (-(SiCH₃R-CH₂)-: R=O(CH₂)₁₁O-Ph-Ph-CN, Ph=phenyl) (CN-11) has been studied by X-ray diffraction and differential scanning calorimetry (DSC). The DSC results showed that CN-11 has transitions at ∼92 °C (T₂) and ∼147 °C (T₁) during both cooling and immediate heating. A third transition occurred at ∼50 °C (T₃) during heating after annealing at room temperature. The X-ray fiber pattern of the CN-11 annealed at room temperature showed several wide and small angle reflections which were indexed by a monoclinic unit cell with parameters a=16.8 Å, b=7.42 Å, c=43.6 Å and β=102.1° (b: fiber direction), representing a crystal structure with layer thickness of ∼43 Å. Upon heating at T₃, the crystal structure became less ordered (but somewhat more ordered than smectic A (S_A) and smectic C (S_C)). This was followed by S_A (or S_C) phase at T₂, and ultimately an isotropic state (I) at T₁. The observed layer thickness (∼43 Å) is about ∼1.5 times the most extended side chain length, indicating a double-layer structure with tilted or interdigitated side chains. The X-ray fiber pattern had a four-point pattern at d=4.52 Å, suggesting that the side chains in the crystal are likely to be tilted by 56° from the polymer fiber axis.     

Dependence of shrinking kinetics of poly(N-isopropylacrylamide) gels on preparation temperature

Preparation temperature dependence of equilibrium swelling degree and shrinking kinetics of poly(N-isopropylacrylamide) gel has been investigated by optical microscopic measurements. The degree of swelling, d/d₀, at 20 °C was found to be strongly dependent on the preparation temperature, T_prep, where d and d₀ are the diameter of gel during observation and preparation, respectively. The value of d/d₀ was about 1.2 for T_prep=20 °C, but steeply increased by approaching the phase separation temperature ≈32.0 °C. Above 32.0 °C, d/d₀ decreases stepwise to 1.46. This upturn in d/d₀ was correlated with spatial inhomogeneities in gels. That is, the gel became opaque by increasing T_prep. Though the shrinking half-time, t_1/2, of gel was on the order of 500 min for T_prep≤20 °C, t_1/2 decreased to 2 min for T_prep≥26 °C. Hence, a rapid shrinking was attained by simply increasing T_prep. The physical implication of this rapid shrinking in gels was discussed in conjunction with the gel inhomogeneities and a thermodynamic theory of swelling equilibrium.     

Castor oil-based thermosets with varied crosslink densities prepared by ring-opening metathesis polymerization (ROMP)

Two castor oil-based monomers, (1) norbornenyl-functionalized castor oil (NCO), which has ∼0.8 norbornene rings per fatty acid chain and (2) norbornenyl-functionalized castor oil alcohol (NCA), which has ∼1.8 norbornene rings per fatty acid chain, have been prepared. Ring-opening metathesis polymerization (ROMP) of different ratios of NCO/NCA using the 2nd generation Grubbs catalyst results in rubbery to rigid biorenewable-based plastics with crosslink densities ranging from 318 to 6028 mol/m³. Increased crosslink densities result in shorter gelation times, better incorporation of the monomers into the polymer network, and much less soluble materials in the bulk materials. The increased crosslink densities obtained by adding NCA enhance the thermal properties, including the glass transition temperature (T_g) and room temperature storage modulus, which increase from −17.1 °C to 65.4 °C and from 2.4 MPa to 831.9 MPa, respectively. The TGA results, where T₁₀ increased from 285 °C to 385 °C, illustrate that improved thermal stabilities can be obtained for thermosets with higher crosslink densities. Young’s modulus (11-407 MPa), tensile strength (1.6-18 MPa) and toughness (0.14-1.6 MPa) are also improved dramatically with higher crosslink densities.     

Characterisation of activated nanoporous carbon for supercapacitor electrode materials

Commercial nanoporous carbon RP-20 was activated with water vapor in the temperature range from 950 °C to 1150 °C. The XRD analysis was carried out on nanoporous carbon powder samples to investigate the structural changes (graphitisation) in modified carbon that occurred at activation temperatures T ? 1150 °C. The first-order Raman spectra showed the absorption peak at 1582 cm⁻¹ and the disorder (D) peak at 1350 cm⁻¹. The low-temperature N₂ adsorption experiments were performed at −196 °C and a specific surface area up to 2240 m²g⁻¹ for carbon activated at T = 1050 °C was measured. The cell capacitance for two electrode activated nanoporous carbon system advanced up to 60 F g⁻¹ giving the specific capacitance ∼240 F g⁻¹ to one electrode nanoporous carbon ∣1.2 M (C₂H₅)₃CH₃NBF₄ + acetonitrile solution interface. A very wide region of ideal polarisability for two electrode system (∼3.2 V) was achieved. The low frequency limiting specific capacitance very weakly increases with the rise of specific area explained by the mass transfer limitations in the nanoporous carbon electrodes. The electrochemical characteristics obtained show that some of these materials under discussion can be used for compilation of high energy density and power density non-aqueous electrolyte supercapacitors with higher power densities than aqueous supercapacitors.     

Composition and poling condition-induced electrical behavior of (Ba0.85Ca0.15)(Ti1−xZrx)O3 lead-free piezoelectric ceramics

Lead-free (Ba_0.85Ca_0.15)(Ti_1−xZr_x)O₃ (BCTZ) piezoelectric ceramics were fabricated by normal sintering in air atmosphere. BCTZ ceramics with x = 0.10 possess a coexistence of tetragonal and rhombohedral phases at ∼40 °C. The Curie temperature of BCTZ ceramics decreases with increasing the Zr content. Piezoelectric properties of BCTZ ceramics are dependent on the poling conditions (i.e., the poling temperature and the poling electric field), and the underlying physical mechanism is illuminated by the phase angle. The BCTZ (x = 0.10) ceramic, which locates at the existence of two phases and is poled at E ∼ 4.0 kV/mm and T_p ∼ 40 °C, exhibits an optimum electrical behavior at a room temperature of ∼20 °C: d₃₃ ∼ 423 pC/N, k_p ∼ 51.2%, 2P_r ∼ 18.86 μC/cm², 2E_c ∼ 0.47 kV/mm, ?_r ∼ 2892, and tan δ ∼ 1.53%.     

Spectroscopic investigations of the microstructure changes induced by substrate temperature in nitrogen-substituted amorphous carbon thin films

The effect of substrate temperature (T_S) on the microstructure of amorphous nitrogen-substituted carbon thin films (a-C_1−xN_x) has been studied. The a-C_1−xN_x films were deposited on Si(1 0 0) using reactive radio-frequency (RF) magnetron sputtering of a high-purity graphite target in a pure nitrogen plasma using different RF powers and different substrate temperatures T_S. Combined Fourier transform infrared (FTIR), Raman spectroscopy and residual stress measurements are used to fully characterise the films in the as-deposited state. FTIR spectra showed the existence of NCsp² and CN triple bonding, and suggested the presence of N-Csp³ bonding in the deposited films. The analysis of the spectra versus T_S reveals two rearrangement mechanisms of the microstructure. Up to 150 °C, the reversion of NCsp² to N-Csp³ and CCsp²respectively, and the increase the connectivity of the C-C network for higher T_S. The Raman features reveals that this progressive graphitisation of the material, with increasing T_S, is accompanied by a high disorder form of Csp² sites. These results are used to describe the stress variation that accompanies nitrogen evolution within the deposited films.     

DSC, dielectric and dynamic mechanical behavior of two- and three-component ordered polyurethanes

Liquid crystalline (LC) polyurethanes were made from two diisocyanates (flexible HMDI and stiff TDI) (DIs), mesogenic diol (D) and a polybutadiene-diol (B) with stoichiometric ratios of reactive hydroxy (OH) and isocyanate (NCO) groups ((NCO)_DI/((OH)_D+(OH)_B)=1/1). Two- (B/DIs, D/DIs) and three-component ((D+B)/DIs, D/B=1/1 by weight) polymers were prepared and their dielectric, dynamic mechanical and DSC behavior was investigated. For neat B, the glass transition temperature T_gB (∼−46 °C) was detected. Two-component B/HMDI and B/TDI polymers have exhibited a homogeneous structure with the glass transition temperatures T_gU∼−9 and 2 °C. On the other hand, for D/DI polymers on cooling from the melt and subsequent heating the glass transitions at T_gU∼41 °C (D/HMDI) and 58 °C (D/TDI) together with nematic and smectic mesophases were found. In three-component systems, additional glass transitions at T_gB∼−41 °C (B/D/HMDI) and −31 °C (B/D/TDI) were observed. This means that the polymers exhibit a distinct two-phase structure with soft polybutadiene (B) and hard polyurethane (D/DI) phases. In hard polyurethane phase, the glass transitions at T_gU and LC mesophases similar to those found in two-component D/DI polyurethanes were detected. Dielectric and dynamic mechanical results correlate well with DSC measurements.     

Radically crosslinked branched poly(N-allylethylenimine) with lithium triflate as a solid state polymer electrolyte

Branched poly(N-allylethylenimine) (BPAEI), a solid state polymer electrolyte host, was synthesized by allylation of branched poly(ethylenimine) (BPEI). Allylation was essentially complete with the 2 and 1° nitrogen atoms of BPEI being mono-allylated and di-allylated, respectively, and with little or no quaternization. BPAEI can be radically cross-linked with and without lithium trifluoromethanesulfonate (LiTf) present to form free-standing, homogeneous, minimally hygroscopic films. BPAEI has a glass transition temperature (T_g) of −65 °C, as measured by differential scanning calorimetry (DSC), which increases with the concentration of initiator upon cross-linking using V-50 (2,2-azobis(2-amidino-propane) dihydrochloride) to −15 °C at a 10:1 nitrogen to initiator molar ratio (N:initiator). BPAEI with 20:1 N:Li⁺ (molar ratio) LiTf has a T_g of −48 °C, which increases with the concentration of radical initiator upon cross-linking using V-50 to 3 °C at 10:1 N:initiator. At compositions near 60:1 N:initiator, an unusual decrease in the rate at which T_g changes with cross-linking was observed, both with and without LiTf present, indicating that some undefined morphological changes occur. The effect of this morphological change resulted in the highest Ac conductivities at 60:1 N:initiator for all LiTf concentrations studied. At 20:1 N:Li⁺ LiTf and 60:1 N:initiator, the room temperature Ac conductivity was 1×10⁻⁸ S/cm which increased to 1×10⁻⁵ S/cm at 80 °C, the highest conductivity observed in the concentration ranges studied. Infrared spectroscopy (IR) showed that the concentrations of the individual ionic species present were largely independent of either LiTf concentration or cross-linking density, suggesting that changes in ion mobility, likely resulting from morphological changes, substantially control the ionic conductivity.     

Superfast responsive ionic hydrogels with controllable pore size

A series of strong polyelectrolyte hydrogels was prepared from the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) as the monomer and N,N′-methylene(bis)acrylamide (BAAm) as a crosslinker in aqueous solutions. The gel preparation temperature (T_prep) was varied between −22 and 25 °C. It was found that the swelling properties and the elastic behavior of the hydrogels drastically change at T_prep=−8 °C. The hydrogels prepared below −8 °C exhibit a discontinuous morphology consisting of polyhedral pores of sizes 30-50 μm, while those formed at higher temperatures have a non-porous structure. The pore size of the networks increased by decreasing the charge density of the hydrogels, while addition of low molecular weight salts into the gelation system reduced the size of the pores. Calculations based on the equilibrium between the ice and unfrozen gel phases in the reaction system at low temperatures explain the results of observations. It was also shown that the hydrogels formed below −8 °C exhibit superfast swelling properties as well as reversible swelling-deswelling cycles in water and acetone.     

Reversible thickness control of polymer thin films containing photoreactive coumarin derivative units

The reversible control of the thickness of polymer thin films was investigated using (meth)acrylic polymers containing photoreactive coumarin derivative units in the side chain. Coumarin derivative units underwent dimerization and the reverse-dimerization by photoirradiation and were used as a reversible cross-linking point. The homopolymer of 7-methacryloyloxy-4-methylcoumarin (T_g = 194 °C) did not cause changes in film thickness after photoreactions. The homopolymer of 7-(2′-acryloyloxyethoxy)-4-methylcoumarin (AEMC) (T_g = 89 °C) decreased 19% of film thickness by photodimerization and 73% of the decreased thickness was recovered after the reverse-dimerization and the subsequent thermal annealing at 130 °C. The reverse-dimerization of the copolymer of AEMC and n-butyl acrylate (AEMC content = 19 mol%, T_g = 11 °C) resulted in 53% of recovery from the decreased film thickness without annealing. The mobility of polymer main-chain was revealed to be essential factor to change film thickness by photoreactions. Photodimerization of coumarin derivative units in low glass transition temperature (T_g) tended to proceed faster than in high T_g polymers and resulted in larger decrease in film thickness.     

Orientation of graphitic planes during annealing of “dip deposited” amorphous carbon film: A carbon K-edge X-ray absorption near-edge study

Annealing effect of amorphous carbon thin films on Si(1 0 0) substrates is studied by normal incidence and angle dependent carbon K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The angle dependence of the XANES signal shows that the graphitic basal planes are oriented perpendicular to the surface when the film is annealed at 1000 °C. Micro-Raman spectroscopy reveals two well-separated bands the D band at 1355 cm⁻¹ and G band at ∼1600 cm⁻¹, and their I_D/I_G intensity ratio indicates the formation of more graphitic film at higher annealing temperatures. X-ray diffraction pattern of 1000 °C temperature annealed film confirms the formation of graphite structure.     

Electrochemical properties of novel ionic liquids for electric double layer capacitor applications

An aliphatic quaternary ammonium salt which has a methoxyethyl group on the nitrogen atom formed an ionic liquid (room temperature molten salt) when combined with the tetrafluoroborate (BF₄⁻) and bis(trifluoromethylsulfonyl)imide [TFSI; (CF₃SO₂)₂N⁻] anions. The limiting oxidation and reduction potentials, specific conductivity, and some other physicochemical properties of the novel ionic liquids, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEME-BF₄) and DEME-TFSI have been evaluated and compared with those of 1-ethyl-3-methylimidazolium tetrafluoroborate. DEME-BF₄ is a practically useful ionic liquid for electrochemical capacitors as it has a quite wide potential window (6.0 V) and high ionic conductivity (4.8 mS cm⁻¹ at 25 °C). We prepared an electric double layer capacitor (EDLC) composed of a pair of activated carbon electrodes and DEME-BF₄ as the electrolyte. This EDLC (working voltage ∼2.5 V) has both, a higher capacity above room temperature and a better charge-discharge cycle durability at 100 °C when compared to a conventional EDLC using an organic liquid electrolyte such as a tetraethylammonium tetrafluoroborate in propylene carbonate.     

Tantalum (oxy)nitrides prepared using reactive sputtering for new nonplatinum cathodes of polymer electrolyte fuel cell

Tantalum (oxy)nitrides (TaO_xN_y) have been investigated as new cathodes for polymer electrolyte fuel cells without platinum. TaO_xN_y films were prepared using a radio frequency magnetron sputtering under Ar + O₂ + N₂ atmosphere at substrate temperatures from 50 to 800 °C. The effect of the substrate temperature on the catalytic activity for the oxygen reduction reaction (ORR) and properties of the TaO_xN_y films were examined. The catalytic activity of the TaO_xN_y for the ORR increased with the increasing substrate temperature. The ORR current density at 0.4 V vs. RHE on TaO_xN_y prepared at 800 °C was approximately 20 times larger than that on TaO_xN_y prepared at 50 °C. The onset potential of the TaO_xN_y for the ORR was obtained at the ORR current density of −0.2 μA cm⁻². The onset potential of the TaO_xN_y prepared at 800 °C was ca. 0.75 V vs. RHE. The X-ray diffraction patterns revealed that Ta₃N₅ structure grew as the substrate temperature increased. While, the ionization potentials of all specimens were lower than that of Ta₃N₅, and decreased with the increasing substrate temperature. The TaO_xN_y which had Ta₃N₅ structure and lower ionization potential might have a definite catalytic activity for the ORR.     

Multi-month thermal aging of electro-optic polymer waveguides: Synthesis, fabrication, and relaxation modeling

The preparation of polyimides containing side-chain chromophores and the long-term aging performance of poled films are described. These materials were compared to guest-host polycarbonate films. Mach-Zehnder optical interferometers were fabricated from these polymers that contained CLD- and FTC-type chromophores. Changes in optical properties were monitored for months at four temperatures ranging from ambient to 110 °C. The isothermal relaxation data were modeled using both a stretched exponential equation and a power law in time equation. The temperature dependency of the time constants of these equations was modeled using a new activation-energy equation: ln(τ/τ_p) = E_R(1 + tanh[(T_c − T)/D])/2RT + E_p/RT where T_c is the central temperature of the transition zone, D is the breadth of the zone, and Es are the activation energies of rigid and pliable materials. Multi-year high-temperature stability of the poled guest-host and side-chain materials was predicted.     

Structural,electrical, and optical properties of Sb-doped SnO2 transparent conductive oxides fabricated using an electrospray technique

Sb-doped SnO₂ (ATO) thin films, for use as transparent conductive oxides (TCOs), were synthesized using an electrospray technique, and their structural, electrical, and optical properties were investigated. To elucidate the optimum fabrication conditions for the best electrical and optical properties, ATO thin films were calcined using four different temperatures, 450 °C, 550 °C, 650 °C, and 750 °C. When calcined at 650 °C, ATO thin films exhibit excellent resistivity (~8.14×10⁻³ Ω cm), superior transmittance (~91.4% at 550 nm), and good figure of merit (~11.4×10⁻⁴ Ω⁻¹) compared to the other samples. The enhanced properties of ATO thin films are attributed to high densification without formation of cracks, and the increased grain size of ATO nanoparticles.     

The influence of axial pressure on relaxor properties of BaBi2Nb2O9 ceramics

On the basis of our studies it results that dielectric properties of BaBi₂Nb₂O₉ ceramics are sensitive to axial pressure applied. The pressure causes an increase of dispersion in the real part of dielectric permittivity ?′(T,f) and a rise in the temperature T_m at which the maximum in ?′(T,f) dependence occurs. The applied pressure induces in the ?′(T) dependence an additional step-like anomaly, which appears at the temperature T_A < T_m. The applied pressure shifts both T_m and T_A at the same rate, i.e. dT_A/dX = dT_m/dX = +14 °C/kbar at high axial pressure range, above the threshold pressure X_thresh. The Vogel–Fulcher relationship is employed to determine the axial pressure influence on relaxor properties of BBN ceramics. The simulated order parameter q takes non-zero values below Burn‘s temperature T_B, where the polar clusters appear on cooling. For pressures higher than 0.8 kbar, the T_B changes at the rate dT_B/dX = −200 °C/kbar. The decrease in the difference between Burn's T_B and the freezing T_f temperatures induced by the applied axial pressure is observed. This could be ascribed to the narrowing of temperature range of relaxor behavior.