Thermal stability of poly(vinylidene fluoride) films pre-annealed at various temperatures

This paper investigates the relationship between the pre-annealing conditions and the thermal stability of uniaxially-drawn poly(vinylidene fluoride) (PVDF) films in order to clarify their technical limits in terms of temperatures that can be used for assembly processes and for practical applications. Specimens that are pre-annealed below their melting temperature apparently shrink in the stretch-direction when they are exposed to elevated temperatures above the pre-annealing temperature. Since the content of β-PVDF in the films decreases simultaneously with the shrinkage, their piezoelectric properties also deteriorate. In addition, there is a suggestion that the level of polarization in the remaining β-phase decreases significantly during annealing above 90-100 °C. However, the dimensions and the piezoelectric coefficients of the films remain stable during annealing below the pre-annealing temperature. Therefore, the thermal stability of PVDF films can be controlled practically by using the appropriate pre-annealing temperature. By contrast, the films were softened at 90-100 °C when the pre-annealing treatment was conducted above the melting temperature. The softening of films that are pre-annealed above the melting temperature is a different phenomenon from that observed in specimens that are pre-annealed below the melting temperature.     

Thermal methods for evaluating polymorphic transitions in nifedipine

The thermal behaviour of nifedipine was studied with the view to understand the various phase transitions between its polymorphs. The focus was on polymorph identification, accompanying morphological changes during crystallization and the nature of the phase transformations. These features were compared to the complexity of the crystallization mechanisms, studied by dynamic differential scanning calorimetry (DSC) heating techniques. DSC, thermogravimetry (TG) established the temperature limits for preparation of amorphous nifedipine from the melt. DSC studies identified that metastable form B, melting point ∼163 °C, was enantiotropically related to a third modification, form C, which existed at lower temperatures. Form C converted endothermically to form B at ∼56 °C on heating and was shown by hot stage microscopy (HSM) to be accompanied by morphological changes. Modulated temperature differential scanning calorimetry (MTDSC) showed discontinuities in the reversing heat flow signal during crystallization of amorphous nifedipine (from ∼92 °C) to form B, which suggested that a number of polymorphs may nucleate from the melt prior to form B formation. Identification of the number of nifedipine polymorphs included the use of combined DSC-powder X-ray diffraction (PXRD) and variable temperature powder X-ray diffraction (VTPXRD). The crystallization kinetics studied by dynamic DSC heating techniques followed by analysis using the Friedman isoconversion method where values of activation energy (E) and frequency factor (A) were estimated as a function of alpha or extent of conversion (α). The variations in E with α, from 0.05 to 0.9, for the amorphous to form B conversion could indicate the formation of intermediate polymorphs prior to form B. The form B to form A conversion showed a constancy in E on kinetic analysis from α 0.05 to 0.9, which suggested that a constant crystallization mechanism operated during formation of the thermodynamically stable form A.     

The effects of thermal treatment on the antioxidant activity of polyaniline

The thermal stability of chemically synthesized polyaniline (PANI) was examined, including granular (G) polyaniline powders formed conventionally in an HCl medium, and nanorod (NR) samples prepared using a falling-pH synthesis. The samples were examined before and after dedoping (dd) using thermogravimetric analysis (TGA), which showed small mass losses in the 200-300 °C temperature range, and greater mass losses due to oxidative degradation at higher temperatures. Furthermore, samples were treated thermally at 100, 125, 150, 175, 200, 250 and 300 °C for 30 min in air. SEM images did not show any pronounced effect on the morphologies of the samples from thermal treatment up to 300 °C. The ratios of the intensities (Q/B) of the predominantly quinonoid (Q) and benzenoid peaks (B) from FTIR spectroscopic analysis revealed that NR-PANI and NR-PANIdd underwent cross-linking upon thermal treatment up to 175 °C and were oxidized after treatment above 175 °C. G-PANI and G-PANIdd also underwent the same chemical changes with oxidation occurring above 200 °C. The free radical scavenging capacity of the samples was evaluated using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay, and was found to be independent of the spin concentrations of the samples. All samples exhibited a rapid decline in free radical scavenging capacity when exposed to temperatures above 200 °C, indicating that any polymer processing should be undertaken at temperatures less than this value to achieve high antioxidant activity.     

Thermal degradation behavior of poly(4-hydroxybutyric acid)

Thermal degradation behavior of poly(4-hydroxybutyric acid) (P(4HB)) was investigated by thermogravimetric and pyrolysis-gas chromatography mass spectrometric analyses under both isothermal and non-isothermal conditions. Based on the thermogravimetric analysis, it was found that two distinct processes occurred at temperatures below and above 350 °C during the non-isothermal degradation of P(4HB) samples depending on both the molecular weight and the heating rate. From ¹H NMR analysis of the residual P(4HB) molecules after isothermal degradations at different temperatures, it was confirmed that the ω-hydroxyl chain-end was remained unchanged in the residual P(4HB) molecules at temperatures below 300 °C, while the ω-chain-end of P(4HB) molecules was converted to 3-butenoyl units at temperatures above 300 °C. In contrast, the majority of the volatile products evolved during thermal degradation of P(4HB) was γ-butyrolactone regardless of the degradation temperature. From these results, it is concluded that during the thermal degradation of P(4HB), the selective formation of γ-butyrolactone via unzipping reaction from the ω-hydroxyl chain-end predominantly occurs at temperatures below 300 °C. At temperatures above 300 °C, both the cis-elimination reaction of 4HB unit and the formation of cyclic macromolecules of P(4HB) via intramolecular transesterification take place in addition to unzipping reaction from the ω-hydroxyl chain-end. Finally, the primary reaction of thermal degradation of P(4HB) at temperatures above 350 °C progresses by the cyclic rupture via intramolecular transesterification of P(4HB) molecules with a release of γ-butyrolactone as volatile product. Moreover, we carried out the thermal degradation tests for copolymer of 93 mol% of 4HB with 7 mol% of 3-hydroxybutyric acid (3HB) to examine the effect of 3HB units on thermal stability of P(4HB).     

First observation of Chapman rearrangement of a pseudosaccharyl ether in the solid state: the thermal isomerization of 3-(methoxy)-1,2-benzisothiazole 1,1-dioxide revisited

3-(Methoxy)-1,2-benzisothiazole 1,1-dioxide, a pseudosaccharyl ether, was long ago known to undergo a thermal Chapman-like [1,3′]-isomerization to the corresponding N-methyl pseudosaccharin at temperatures above its melting point (ca. 184 °C) [Hettler H., Tetrahedron Lett.1968, 15, 1793]. In the present study, it is shown that this rearrangement can also take place in the solid state, at temperatures as low as 150 °C. This was the first observation of a Chapman-like [1,3′]-isomerization in pseudosaccharyl ethers in the solid state. The study has been carried out by a multidisciplinary approach using temperature dependent infrared spectroscopy, differential scanning calorimetry (DSC), and polarized light thermomicroscopy, complemented by theoretical methods.     

Low-temperature synthesis of SrAl2O4 by a modified sol-gel route: XRD and Raman characterization

Among other alkaline-earth aluminates, the monoclinic (M) polymorph of SrAl₂O₄ can be used as host material for Eu²⁺ luminescence based phosphors. With the aim of reducing the synthesis temperature of this polymorph, we have produced and characterized by XRD and Raman scattering solid solutions of the SrAl_2−xB_xO₄ system (x?0.3) obtained by two different methods, a ceramic route and a modified sol-gel synthesis. Though the addition of boron lowers the temperature of obtention of the M polymorph in both type of samples, lower B contents are needed to stabilize the M form as single phase for samples prepared by the sol-gel method than through the ceramic route. In the sol-gel method, the M polymorph can be obtained at temperatures as low as 1200 °C, with a Boron content of just 1%. Rietveld profile analysis allows us to conclude that coexistence of the monoclinic and hexagonal polymorphs of SrAl₂O₄ occurs for samples synthesized below an onset temperature of about 1000-1100 °C, that depends on the sample composition. Above those temperatures, only the monoclinic phase is formed.     

Competition of phase dissolution and crystallization in poly(ε-caprolactone)/poly(styrene-co-acrylonitrile) blend

Blend of poly(ε-caprolactone) (PCL) and poly(styrene-co-acrylonitrile) (SAN) containing 27.5 wt% of acrylonitrile was studied. The PCL/SAN blend having LCST (lower critical solution temperature) phase boundary above the melting point T_m of PCL offered an excellent opportunity to investigate the competition of liquid-solid phase transition (crystallization) and liquid-liquid phase transition (phase dissolution). A blend with the critical composition (80/20 PCL/SAN) underwent a temperature-jump above LCST to proceed spinodal decomposition, yielding a regularly phase-separated structure (SD structure). Then, it was quenched to the temperatures below T_m at which both the crystallization and the phase dissolution could occur. By transmission electron microscopy it was found that during isothermal annealing after quenching to high temperatures close to T_m (e.g. 51 °C), the SD structure gradually disappeared, and then the crystallization started from a single-phase mixture to yield normal crystalline structure similar to that of a neat crystalline polymer. At lower temperatures (e.g. 40 °C), crystallization quickly occurred and the SD structure was preserved, implying that the crystallization prevailed over the dissolution yielding a bi-continuous structure consisting of amorphous (SAN-rich) and crystalline (PCL-rich) regions. At intermediate temperatures (e.g. 45 °C), the phase dissolution competed with the crystallization, resulting in a bi-continuous structure with longer periodic distance and a broad boundary having a gradient in composition of amorphous region between PCL crystal lamellae. Light-scattering analysis quantitatively revealed a competition of the crystallization and the phase dissolution in terms of the crystallization rate (from H_v scattering) and the apparent diffusion coefficient for dissolution (from V_v scattering).     

Morphology of polytetrafluoroethylene before and after irradiation

Supramolecular structure and morphology of as-polymerized, sintered, and gamma-irradiated suspension PTFE were studied with scanning electron microscopy. Irradiation was performed both below and above melting point of crystal phase. Fibrillar supramolecular structure of as-polymerized PTFE is preserved after its sintering. In contrast to as-polymerized PTFE, in the sintered polymer some segments of fibrils form lamellae of thickness 100-300 nm and length up to several microns, with fibrils arranged perpendicularly to a lamella. Irradiation below the melting point (20 and 200 °C) does not change quantitatively PTFE morphology. In both cases and also in the case of pristine PTFE, dense and loose (porous) regions are present in its morphology. Dense regions are packages of irregular shape and consist of densely packaged fibrils. Loose regions consist of individual ribbons and fibrillar lamellae. Irradiation at 200 °C increases greatly the width of lamellae. PTFE structure rearrranges drastically under irradiation above the melting point. New morphology units, spherulites of size about 50 μm, are formed, the spherulites consisting of radially extending fibrils, and porosity decreases substantially. Formation of spherulites is ascribed to radiation-induced chain scission and decrease in molecular mass and viscosity of polymer.     

Porosity development in chars from thermal decomposition of poly(p-phenylene terephthalamide)

The main objective of this work was to investigate the development of porosity in solid residues from the thermal decomposition of the polymer, poly(p-phenylene terephthalamide) (PPTA). PPTA chars were prepared at different temperatures and characterized by X-ray diffraction and physical adsorption of CO₂ at 0 °C. The carbonization temperatures were selected on the basis of thermogravimetric analysis results. The effect of introducing an isothermal treatment at 500 °C on the characteristics of the resulting chars was also studied. It was found that this pre-treatment lowers the decomposition temperature of PPTA and yields a somewhat less ordered material than in the case of pyrolysis under a constant heating rate. The micropore volume increases with increasing heat treatment temperature for both series of samples. The mean micropore size decreases for the two series of chars until the 700-800 °C interval; above these temperatures, this evolution is reversed. The micropore volume of the samples submitted to the isothermal treatment is higher than when PPTA is treated under a constant heating rate. Likewise, the pore size distribution is more heterogeneous when the intermediate isothermal treatment at 500 °C is introduced during PPTA pyrolysis. Some differences between porosity development in chars from PPTA and other high thermal stability polymers were explained on the basis of different mechanistic features in polymer pyrolysis.     

Application of cinchona-sulfonate-based chiral zwitterionic ion exchangers for the separation of proline-containing dipeptide rotamers and determination of on-column isomerization parameters from dynamic elution profiles

The interconversion of cis and trans isomers of dipeptides containing C-terminal proline was studied by dynamic chromatography on zwitterionic chiral stationary phases at temperatures ranging from −15 °C to +45 °C The cis–trans isomers could be separated below 0 °C and above 0–10 °C plateau formation and peak coalescence phenomena occurred, which is characteristic for a dynamic process at the time-scale of partitioning. At and above room temperature, full coalescence was observed, which allowed separations of enantiomers without interference from interconversion effects. Analysis of the dynamic elution profiles of the interconverting peptides allowed the determination of isomerization rate constants and thermodynamic activation parameters (isomerization enthalpy, entropy and activation energy). In accordance with established results, isomerization rates and thermodynamic parameters were found to depend on the nature of the N-terminal amino acid. Isomerization barriers were only slightly lower than values determined with other methods but significant differences in the relative contributions of the activation enthalpy and entropy as well as isomerization rates pointed toward selector-moderated isomerization dynamics.     

New ionic liquids with the bis[bis(pentafluoroethyl)phosphinyl]imide anion, [(C2F5)2P(O)]2N—Synthesis and characterization

A new series of low melting and hydrophobic ionic liquids (ILs) containing the bis[bis(pentafluoroethyl)phosphinyl]imide anion, [(C₂F₅)₂P(O)]₂N⁻ (FPI), and ammonium, phosphonium, imidazolium, pyridinium or pyrrolidinium cations were prepared and characterized. Their density, viscosity, melting point, glass transition temperature, decomposition temperature and conductivity are discussed. Many of these ionic liquids are liquids at room temperature with melting points below 15 °C, viscosities below 110 mm² s⁻¹ and thermal stabilities above 300 °C.     

Characterization, crystallization kinetics and melting behavior of poly(ethylene succinate-co-21 mol% trimethylene succinate) copolyester

A copolyester was synthesized and characterized to have 78.6 mol% ethylene succinate unit and 21.4 mol% trimethylene succinate unit by using NMR. The value of the random parameter is 0.97 that can be considered to be a random copolymer. The melting behavior after isothermal crystallization was studied using differential scanning calorimeter by varying the crystallization temperature, the heating rate and the crystallization time. Triple melting peaks were observed. The melting behavior indicates that the upper melting peaks are primarily due to the melting of lamellar crystals with different stability. The Hoffman-Weeks linear plot gives an equilibrium melting temperature of 94.0 °C. The spherulite growth of this copolyester from 72 °C to 30 or 15 °C at a cooling rate of 1 or 2 °C/min was monitored and recorded using an optical microscope equipped with a CCD camera and a DVD recorder. These experiments including the self-nucleation pretreatment took 72 min and 60 min, respectively. Continuous growth rates between melting and glass transition temperatures can be obtained after curve-fitting procedures. These data fit well with those data points measured in the isothermal experiments, which is time consuming. These isothermal and continuous data were separately analyzed with the Hoffman and Lauritzen theory. A regime II-III transition was detected at about 51.5 ± 0.1 °C.     

Highly fluorinated graphite prepared from graphite fluoride formed using BF3 catalyst

Fluorinated graphites (CF_0.47) were obtained by reaction at room temperature of fluorine gas with graphite in the presence of boron trifluoride and hydrogen fluoride as catalysts. Their thermal treatments under fluorine at temperatures up to 600 °C lead to a progressive increase of the fluorine level resulting in an highly fluorinated graphite (CF_1.02). Whatever the fluorination level, a stage one fluorine-graphite intercalation compound is obtained. The sp² carbon hybridization is maintained for treatment temperature below 300 °C and two types of structure coexist for T_T in the range 350-550 °C. Finally, above 550 °C, carbon hybridization is sp³.The resulting materials were studied by ¹¹B, ¹H, and ¹⁹F NMR and EPR at different experimental temperatures giving informations about the intercalated fluoride species, the temperature of their removal from the host fluorocarbon matrix, as well as their mobility.     

Preparation and nonlinear optical properties of novel Y-type polyesters with highly enhanced thermal stability of second harmonic generation

3,4-Di-(2′-hydroxyethoxy)benzylidenemalononitrile (3) was prepared and condensed with terephthaloyl chloride and adipoyl chloride to yield novel Y-type polyesters (4-5) containing 3,4-dioxybenzylidenemalononitrile groups as NLO-chromophores, which constituted parts of the polymer main-chains. The resulting polymers 4-5 are soluble in common organic solvents such as acetone and N,N-dimethylformamide. They showed thermal stability up to 300 °C in thermogravimetric analysis with glass-transition temperatures obtained from differential scanning calorimetry in the range 89-91 °C. The second harmonic generation (SHG) coefficients (d₃₃) of poled polymer films at the 1064 nm fundamental wavelength were around 2.47 pm/V. The dipole alignment exhibited high thermal stability even at 10 °C higher than T_g, and there is no SHG decay below 100 °C due to the partial main-chain character of polymer structure, which is acceptable for NLO device applications.     

A new molecular precursor route for the synthesis of Bi-Y, Y-Nb and Bi-doped Y-Nb oxides at moderate temperatures

Yttrium-based multimetallic oxides containing bismuth and/or niobium were prepared by a method starting from pre-isolated stable water-soluble precursors which are complexes with the ethylenediaminetetraacetate ligand (edta). The cubic Bi_1−xY_xO_1.5 (x=0.22, 0.25 and 0.3) and Y₃NbO₇ oxides were obtained in a pure form in a range of moderate temperatures (600-650 °C). This preparation method also allowed to stabilize at room temperature, without quenching, the tetragonal YNbO₄ oxide in a distorted form (T′-phase) by calcining the precursor at 800 °C. When heated up to 1000 °C, this metastable T′-phase transforms into the metastable “high-temperature” T oxide, which converts on cooling down to room temperature into the thermodynamically stable monoclinic M oxide. Doping the YNbO₄ oxide with Bi³⁺ cations (0.5% and 1% Bi with respect to total Bi+Y amount) led at 800 °C to a mixture of the T′-phase and the thermodynamically stable monoclinic one. At 900 °C, the almost pure monoclinic structure was obtained.     

Thermal stability of solid and aqueous solutions of humic acid

The effects of temperature on the stability of a soil humic acid were studied in the present work. Solid samples of Gohy-573 humic acid (HA) and dissolved ones in aqueous solution (pH 6.0, 0.1 mol L⁻¹ NaClO₄) were investigated in order to understand the impact of temperature on the chemical properties of the material. The methods applied to solid samples in the present investigation were thermogravimetric analysis (TGA), temperature-programmed desorption coupled with mass spectrometry (TPD-MS), and in situ diffuse reflectance infrared Fourier transformed spectroscopy (in situ DRIFTS). Humic acid samples were studied in the 25-800 °C range, with focus on thermal/chemical processes up to 250 °C. The reversibility of the changes observed was investigated by cyclic changes to specified temperature ranges (40-110 °C). All measurements were conducted under inert-gas atmosphere in order to avoid samples combustion at increased temperatures. Aqueous solutions were analyzed by UV-vis absorption spectroscopy after storage at temperatures up to 95 °C, and storage times up to 1 week. For temperatures below 100 °C experiments on solid and aqueous samples have shown results which were consistent to each other. The amount of water desorbed is temperature dependent and up to 70 °C this process was totally reversible. Above 70 °C an irreversible loss of water was also observed, which according to UV-vis spectroscopy corresponds to water produced by condensation leading to more condensed polyaromatic structures. The water released up to 110 °C was about 7 wt% of the total mass of the dried humic acid, where less than 50% corresponded to reversibly adsorbed water. At higher temperatures (>110 °C), gradual decomposition resulting in the formation of carbon dioxide (110-240 °C), and carbon monoxide (140-240 °C) takes place. Hence, thermal treatment of Gohy-573 humic acid above 70 °C results in irreversible structural changes, that could affect chemical properties (e.g., complex formation) of the material.     

The effect of substitution on the thermo-physical properties of LaMnxV1−xO4−δ

LaMn_xV_1−xO_4−δ(0≤x≤1) samples were characterized using thermogravimetry, thermo-dilatometry, high-temperature X-ray diffraction (HTXRD) and temperature-programmed reduction techniques, with an objective to explore the role of substitution on their thermo-physical properties, which may have a direct bearing on their catalytic behavior. Even though the substituted compositions (x<0.8) were of a single phase, their reduction occurred in two steps, a lower temperature step corresponding to Mn⁴⁺→Mn³⁺/Mn²⁺ and another higher temperature one related to V⁵⁺→V³⁺. The dilatometric measurements gave similar values of linear thermal expansion coefficient (α₁) at temperatures up to 600 °C, both for LaVO₄ and substituted samples. A different behavior was, however, observed at higher temperatures, whereas thermal contraction was observed in case of LaVO₄ for measurements at temperatures above 700 °C, the value of α₁ remained almost constant in case of the substituted samples. Furthermore, the HTXRD data revealed expansion in cell volume for all temperatures up to 950 °C, irrespective of the substitution. These results therefore point to a higher degree of sintering in LaVO₄ as compared to Mn-doped samples on heating at temperatures above 700 °C. It is inferred that the resistance to sintering and the lowering of the reduction temperature are both responsible to the higher catalytic activity of the substituted samples and their compositional stability during the repeated cycles of reduction-reoxidation, as reported earlier [Appl. Catal. A 205 (2001) 295].     

Thermal degradation kinetics of the biodegradable aliphatic polyester, poly(propylene succinate)

The preparation of the biodegradable aliphatic polyester poly(propylene succinate) (PPSu) using 1,3-propanediol and succinic acid is presented. Its synthesis was performed by two-stage melt polycondensation in a glass batch reactor. The polyester was characterized by gel permeation chromatography, ¹H NMR spectroscopy and differential scanning calorimetry (DSC). It has a number average molecular weight 6880 g/mol, peak temperature of melting at 44 °C for heating rate 20 °C/min and glass transition temperature at −36 °C. After melt quenching it can be made completely amorphous due to its low crystallization rate. According to thermogravimetric measurements, PPSu shows a very high thermal stability as its major decomposition rate is at 404 °C (heating rate 10 °C/min). This is very high compared with aliphatic polyesters and can be compared to the decomposition temperature of aromatic polyesters. TG and Differential TG (DTG) thermograms revealed that PPSu degradation takes place in two stages, the first being at low temperatures that corresponds to a very small mass loss of about 7%, the second at elevated temperatures being the main degradation stage. Both stages are attributed to different decomposition mechanisms as is verified from activation energy determined with isoconversional methods of Ozawa, Flyn, Wall and Friedman. The first mechanism that takes place at low temperatures is auto-catalysis with activation energy E = 157 kJ/mol while the second mechanism is a first-order reaction with E = 221 kJ/mol, as calculated by the fitting of experimental measurements.     

Comparative study on hydrolytic degradation and monomer recovery of poly(l-lactic acid) in the solid and in the melt

The hydrolytic degradation of poly(l-lactide) (PLLA) and the formation of its monomer in the solid and in the melt were investigated at 120-150 °C (in the solid), at 160 °C (in the solid up to 40 min and in the melt exceeding 40 min), and at 170-190 °C (in the melt). Such state difference caused the difference in the degradation behavior of PLLA and the behavior of lactic acid formation, although the degradation of PLLA proceeds via a bulk erosion mechanism, regardless of its state. The crystalline residues were formed at the degradation temperatures below 140 °C, but not at the degradation temperatures above 160 °C. The lactic acid yield exceeding 95% can be successfully attained for all the temperatures of 120-190 °C. The activation energy for hydrolytic degradation values of PLLA were 69.6 and 49.6 kJ mol⁻¹ for the temperature ranges of 120-160 °C (in the solid) and 170-250 °C (in the melt), respectively, and are compared with the reported values.     

On the melting temperatures of 3,6-diamino-9H-carbazole and 3,6-dinitro-9H-carbazole

This article records the diversely reported melting temperatures of 3,6-diamino-9H-carbazole (DAC) and 3,6-dinitro-9H-carbazole (DNC), and reports a reinvestigation on their melting phenomena, and associated specific observations. A rationalization of these reports has been attempted on the basis of IR spectroscopic, X-ray diffraction, thermal response, solubility behavior, and differential scanning calorimetric data on DAC and DNC. The data indicate that an exothermic inter-molecular deaminative self-condensation involving N₉H and NH₂ sets in prior to melting of DAC, which obscures the actual melting process. An extrapolated onset melting temperature of 241.0 °C and a peak temperature of 296.5 °C at zero heating rate have been assigned for DAC from DSC measurements. A possible structure of the polymeric self-condensation product is also presented. For DNC, a peak melting temperature of 296.5 ± 1.8 °C, but no onset melting temperature could be assigned.