Mechanisms governing the enhanced thermal stability of acid and base treated polypyrrole

One of the major problems associated with the use of polypyrrole (PPy) in a practical engineering application is its poor thermal stability at elevated temperatures, especially in the presence of oxygen and moisture. Several authors have shown that enhanced stability can be achieved through treatment with simple acids and bases. This paper presents a summary of the possible structural changes which occur as a result of these treatments and those that appear to be related to enhanced conductivity stability. A slight increase in conductivity (10–20%) is observed for acid treated PPy films which is found to be the result of protonation of the pyrrole structure. This effect is dramatically enhanced by treatment at high temperatures where an increase in conductivity of >84% can be achieved. Base treatment of the PPy films results in the deprotonation of the pyrrole structure leading to the loss of conductivity (>40%). Preliminary X-ray Photoelectron Spectroscopy (XPS) results indicate that both acid and base treatment resulted in the elimination of reactive sites for oxygen. Long term thermal ageing of these treated films were conducted at 150 °C in air. The conductivity decay behaviour was found to follow multiple first order chemical reaction kinetics.

Ion exchange, anisotropic structure and thermal stability of polypyrrole films

The thermal stability of electrochemically prepared polypyrrole (PPy) films with p-toluene sulfonate (pTS) or perchlorate (CIOP₄⁻) counter ion (PPy/pTS and PPy/ClO₄⁻) is improved by simple treatment with aqueous sulfuric acid, sodium sulfate or sodium bisulfate. The degree of stabilization achieved depends on the solution, temperature and duration of treatment. PPy/pTS is easily stabilized and thick films (43μm) retain 90 % of the initial conductivity after long period (300 h) at 150 °C, while thinner films (12 μm) retain slightly less (70 %). A model for the conductivity decay has been proposed. Although the mechanism for improved stability is not yet clear it is apparent that the level of ion exchange and the original polymer microstructure are important. The early stages of ion exchange are not symmetrical and diffusion is facilitated at the electrode side of the film. Furthermore, X-ray diffraction shows no evidence of morphological change after treatment of thick PPy/pTS but in thin PPy/pTS and PPy/ClO₄⁻ films an additional peak is indicative of more ordered structure following treatment. These observation may imply that there is a higher density of crosslinks and branching at the growth side than at the electrode side of the film.

Thermal stability of the Al70Ni10Ti10Zr5Ta5 amorphous alloy powder fabricated by mechanical alloying

An Al₇₀Ni₁₀Ti₁₀Zr₅Ta₅ amorphous alloy powder was fabricated by mechanical alloying. The phase structure and characteristic temperatures of the alloy were determined by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The glass transition behavior and crystallization kinetics were analyzed using Lasocka and Kissinger functions. The results show that the alloy has a higher crystallization temperature, a higher effective activation energy of crystallization and a wider supercooled liquid region than the previously reported values, suggesting a high thermal stability and promising applications.

Structure and thermal stability of natural rubber reinforced with in situ generated zinc sorbate

In situ prepared zinc disorbate (ZDS) in natural rubber (NR) by the reaction of zinc oxide and sorbic acid was used to reinforce the dicumyl peroxide-cured NR vulcanizate. The changes in mechanical properties of NR vulcanizates after ageing and were determined and the structures and thermal stability of vulcanizates were also analyzed using scanning electron microscope and thermal gravimetric analyzer. The change ratios in tensile strength and elongation at break of NR vulcanizate with theoretic formation of ZDS of 21phr can be increased to -33 from -44 and -27 from -38 after ageing and the initial weight loss temperature of NR vulcanizate can be increased for about 7°C as compared to un-reinforced NR vulcanizate, indicating that the antioxidative behavior and thermal stability of NR can be improved significantly with theoretic formation of ZDS of 21phr.

Chemical and thermal stability of surface‐modified porous polyethylene membranes

In this paper, we describe the surface modification of porous polyethylene by the adsorption of polyelectrolyte mutilayers on plasma‐activated polyethylene surfaces. We use the migration rates of deionized water as an effective alternative to contact angle measurements in order to probe the interfacial energy of the modified surface. The newly acquired surface properties that result from the surface modification are monitored with respect to several key chemical and environmental variables. These variables were chosen so that they will reflect some of the common handling procedures in a laboratory or health care environments, such as exposure to solvents of different pH and polarities, and fluctuations of ambient temperature over an extended period, i.e., “shelf‐life” duration. The stability of these surface properties of the modified membranes is a fundamental requirement for their potential use in a variety of applications involving lateral flow and binding media for bio‐assays. In this paper, we show that a membrane modified by a polyelectrolyte monolayer is more stable than a membrane that has undergone plasma activation alone, while a membrane modified by a polyelectrolyte bilayer exhibits retention of the enhanced surface hydrophilic properties under various conditions and over a long period of time.

The thermal stability and structural site-distribution of Eu3+ ions in the red-emitting phosphors Ca9Eu2W4O24 and Sr9Eu2W4O24

The red-emitting phosphors Ca₉Eu₂W₄O₂₄ and Sr₉Eu₂W₄O₂₄ were synthesized by the solid-state reaction method. The crystal phases were characterized by X-ray powder diffraction. The photoluminescence excitation and emission spectra were investigated. The luminescence excitation and emission spectra confirm that the phosphors are efficiently excited by near UV light. The dependence of luminescence intensities on the heating temperatures was investigated. The Ca₉Eu₂W₄O₂₄ phosphor exhibits higher thermal stability than that of Sr₉Eu₂W₄O₂₄. The crystallographic sites for Eu³⁺ ions in Ca₉Eu₂W₄O₂₄ and Sr₉Eu₂W₄O₂₄ are investigated by the site-selective excitation spectra in the ⁵D₀ → ⁷F₀ wavelength region. It is identified that the Eu³⁺ ions occupy only M sites (statistically occupied by 0.5Eu and 0.5Ca) in Ca₉Eu₂W₄O₂₄ and, however, the Eu³⁺ ions can substitute both M sites (Eu³⁺ + Sr²⁺) and Sr²⁺ sites in Sr₉Eu₂W₄O₂₄. The luminescence spectra and the thermal stability are discussed on the basis of the crystal structure, Eu³⁺ site-distributions and the energy transfer.

Enhanced thermal stability and lifetime of epoxy nanocomposites using covalently functionalized clay: Experimental and modelling

The present work aims at finding a relationship between kinetic models of thermal degradation process with the physiochemical structure of epoxy-clay nanocomposites in order to understand its service temperature. In this work, two different types of modified clays, including clay modified with (3-aminopropyl)triethoxysilane (APTES) and a commercial organoclay, were covalently and non-covalently incorporated into epoxy matrix, respectively. The effect of different concentrations of silanized clay on thermal behaviour of epoxy nanocomposites were first investigated in order to choose the optimum clay concentration. Afterwards, thermal characteristics of the degradation process of epoxy nanocomposites were obtained by TGA analysis and the results were employed to determine the kinetic parameters using model-free isoconversional and model-fitting methods. The obtained kinetic parameters were used to model the entire degradation process. The results showed that the incorporation of the different modified clay into epoxy matrix change the mathematical model of the degradation process, associating with different orientations of clay into epoxy matrix confirming by XRD results. The obtained models for each epoxy nanocomposite systems were used to investigate the dependence of degradation rate and degradation time on temperature and conversion degree. Our results provide an explanation as to how the life time of epoxy and its nanocomposites change in a wide range of operating temperatures as a result of their structural changes.

Enhancing thermal stability and mechanical properties of lyotropic liquid crystals through incorporation of a polymerizable surfactant.

We present a facile method to prepare thermally stable and mechanically robust crosslinked lyotropic liquid crystals (LLCs) through incorporation of a polymerizable amphiphile into a binary LLC system comprising commercially available surfactant Brij 97 and water. Thermal stability and mechanical properties of the polymerized LLCs were significantly enhanced after polymerization of the incorporated polymerizable surfactant. The effect of incorporating a polymerizable amphiphile on the phase behavior of the LLC system was studied in detail. In situ photo-rheology was used to monitor the change in the mechanical properties of the LLCs, namely the storage modulus, loss modulus, and viscosity, upon polymerization. The retention of the LLC nanostructures was evaluated by small angle X-ray scattering (SAXS). The ability to control the thermal stability and mechanical strength of LLCs simply by adding a polymerizable amphiphile, without tedious organic synthesis or harsh polymerization conditions, could prove highly advantageous in the preparation of robust nanomaterials with well-defined periodic structures.

Bioinspired strategy for tuning thermal stability of PVA via hydrogen-bond crosslink

Although many approaches have been employed to enhance thermal stability of PVA, developing a facile and effective strategy remains highly attractive. Herein, we demonstrate a highly effective approach to strikingly improve thermal stability of PVA by selecting the types of multiamines molecules to tune the hydrogen-bond crosslink density. Results show that only adding 0.5 wt% of 2,4,5,6-tetraaminopyrimidine can make the initial degradation temperature (Ti) and maximum degradation temperature (Tmax) of PVA increase by ~55 °C and 98 °C due to the formation of 3D physically H-bond crosslinked network, resulting in superior thermal stability property to those of PVA nanocomposites. Moreover, thermal stability strongly depends on the H-bond crosslink density, and Ti and Tmax basically obey the linear hydrogen-bond relations despite some deviations. This work opens up a novel biological methodology for creating thermally stable polymeric materials.

High-performance composite from epoxy and glass fibers : morphology, mechanical, dynamic mechanical, and thermal analysis

Diglycidyl ether of bisphenol-A type epoxy resin cured with diamino diphenyl sulfone was used as the matrix for fiber-reinforced composites to get improved mechanical and thermal properties for the resulting composites. E-glass fiber was used for fiber reinforcement. The morphology, tensile, flexural, impact, dynamic mechanical, and thermal properties of the composites were analyzed. The tensile, flexural, and impact properties showed dramatic improvement with the addition of glass fibers. Dynamic mechanical analysis was performed to obtain the Tg of the cured matrix as well as the composites. The improved thermal stability of the composites was clear from the thermogravimetric analysis. Scanning electron micrographs were taken to understand the interfacial adhesion between the fiber and the matrix. The values of mechanical properties were compared with modified epoxy resin composite system. Predictive models were applied using various equations to compare the mechanical data obtained theoretically and experimentally.

Ordering and stability in conducting polypyrrole

X-ray diffraction (XRD) was employed to characterize electrochemically synthesized polypyrrole (PPy) films with 1,5-naphthalene disulfonate (1,5-NDS) counterions treated with simple acid and base. Results show that the as-synthesized film is amorphous with short-range ordering in the polymer backbone. This ordering is soon lost after thermal ageing at 150°C for 60 days and there is evidence of counterion degradation. Base treatment of the PPy/1,5-NDS films has similar effects leading to a complete loss of ordering in the polymer backbone and dedoping of the polymer. Acid treatment at high temperatures increases the ordering of the polymer backbone and results in the development of a secondary interdopant peak confirming that ion exchange has occurred. Conductivity of the PPy was also increased substantially. The enhanced ordering was maintained even after thermal ageing. Room-temperature acid treatment also results in improved ordering of the polymer as well as the counterion but the increase in conductivity is only marginal and most of the ordering is soon lost after thermal ageing. Increase in ordering of the polymer structure seems to lead to better conductivity, although not necessarily improved thermal stability.

Enhanced thermal properties and morphology of ion-exchanged polypyrrole films

The thermal stability of electrochemically prepared polypyrrole (PPy) films with p-toluenesulfonate (pTS) or perchlorate (ClO₄⁻) counterion (PPy/pTS and PPy/ClO₄⁻) is improved by simple treatment with aqueous sulfuric acid, sodium sulfate or sodium bisulfate. The degree of stabilization achieved depends on the solution, temperature and duration of treatment. Although the mechanism for improved stability is not yet clear, it is apparent that the level of ion exchange and the original polymer microstructure are important. A model for the conductivity decay as a function of thickness has been proposed. The early stages of ion exchange are not symmetrical, and diffusion is facilitated at the electrode side of the film. Furthermore, X-ray diffraction shows no evidence of morphological change after treatment of PPy/pTS (43 μm), but in PPy/pTS (12 μm) and PPy/ClO₄⁻ (41 μm) films an additional peak is indicative of more ordered structure following treatment. The glass transition temperature, T_g, of PPy/pTS and PPy/ClO₄⁻ films obtained by modulated differential scanning calorimetry is approximately 155°C.

Electrochemiluminescent monomers for solid support syntheses of Ru(II)-PNA bioconjugates : multimodal biosensing tools with enhanced duplex stability

The feasibility of devising a solid support mediated approach to multimodal Ru(II)-peptide nucleic acid (PNA) oligomers is explored. Three Ru(II)-PNA-like monomers, [Ru(bpy)2(Cpp-L-PNA-OH)]2+ (M1), [Ru(phen)2(Cpp-L-PNA-OH)]2+ (M2), and [Ru(dppz)2(Cpp-L-PNA-OH)]2+ (M3) (bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline, dppz = dipyrido[3,2-a:2′,3′-c]phenazine, Cpp-L-PNA-OH = [2-(N-9-fluorenylmethoxycarbonyl)aminoethyl]-N-[6-(2-(pyridin-2yl)pyrimidine-4-carboxamido)hexanoyl]-glycine), have been synthesized as building blocks for Ru(II)-PNA oligomers and characterized by IR and 1H NMR spectroscopy, mass spectrometry, electrochemistry and elemental analysis. As a proof of principle, M1 was incorporated on the solid phase within the PNA sequences H-g-c-a-a-t-a-a-a-a-Lys-NH2 (PNA1) and H-P-K-K-K-R-K-V-g-c-a-a-t-a-a-a-a-lys-NH2 (PNA4) to give PNA2 (H-g-c-a-a-t-a-a-a-a-M1-lys-NH2) and PNA3 (H-P-K-K-K-R-K-V-g-c-a-a-t-a-a-a-a-M1-lys-NH2), respectively. The two Ru(II)-PNA oligomers, PNA2 and PNA3, displayed a metal to ligand charge transfer (MLCT) transition band centered around 445 nm and an emission maximum at about 680 nm following 450 nm excitation in aqueous solutions (10 mM PBS, pH 7.4). The absorption and emission response of the duplexes formed with the cDNA strand (DNA: 5′-T-T-T-T-T-T-T-A-T-T-G-C-T-T-T-3′) showed no major variations, suggesting that the electronic properties of the Ru(II) complexes are largely unaffected by hybridization. The thermal stability of the PNA·DNA duplexes, as evaluated from UV melting experiments, is enhanced compared to the corresponding nonmetalated duplexes. The melting temperature (Tm) was almost 8 °C higher for PNA2·DNA duplex, and 4 °C for PNA3·DNA duplex, with the stabilization attributed to the electrostatic interaction between the cationic residues (Ru(II) unit and positively charged lysine/arginine) and the polyanionic DNA backbone. In presence of tripropylamine (TPA) as co-reactant, PNA2, PNA3, PNA2·DNA and PNA3·DNA displayed strong electrochemiluminescence (ECL) signals even at submicromolar concentrations. Importantly, the combination of spectrochemical, thermal and ECL properties possessed by the Ru(II)-PNA sequences offer an elegant approach for the design of highly sensitive multimodal biosensing tools.

Thermal degradation kinetics and mechanism of epoxidized natural rubber

Thermal resistance is one of the most dominative properties for polymer materials. Thermal degradation mechanisms of epoxidized natural rubber (ENR) and NR are studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The results show that, the introduction of epoxy groups into the NR molecular main chain leads to a remarkable change in the degradation mechanism. The thermal stability of ENR is worse than that of NR. For the first thermooxidative degradation stage, the thermal decomposition mechanism of ENR is similar to that of NR, which corresponds to a mechanism involving one-dimensional diffusion. For the second stage, the thermal decomposition mechanism of ENR is a three-dimensional diffusion, which is more complex than that of NR. Kinetic analysis showed that activation energy (E?), activation entropy (?H) and activation Gibbs energy (?G) values are all positive, indicating that the thermooxidative degradation process of ENR is non-spontaneous.