Study of the physical aging of an epoxy/cycloaliphatic amine resin modified with abs

Using differential scanning calorimetry (DSC) we have studied the physical aging of an epoxy resin based on the diglycidyl ether of bisphenol A (DGEBA) modified by two different contents of an acrylonitrile-butadiene-styrene (ABS) and cured with 1,3-bisaminomethylcyclohexane (1,3-BAC). Samples fully cured were annealed at temperature of 125°C for periods of time of 72 and 120 h, to determine the process of physical aging. The apparent activation energy for the enthalpy relaxation, Dh*, is determined as the sample is heated at 10°C min^-1 following cooling at various rates through the glass transition region. DSC studies suggested that the presence of thermoplastic inhibits the process of relaxation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enthalpy relaxation in a partially cured epoxy resin

The enthalpy relaxation of a partially cured (70%) epoxy resin, derived from diglycidyl ether of bisphenol-A cured by methyl-tetrahydrophthalic anhydride with accelerator, has been investigated. The key parameters of the structural relaxation (the apparent activation energy Δh*, the nonlinearity parameter x, and the nonexponentiality parameter β) are compared with those of the fully cured epoxy resin. The aging rates, characterized by the dependences of the enthalpy loss and peak temperature on log(annealing time), are greater in the partially cured epoxy than they are in the fully cured resin at an equivalent aging temperature (T_a = T_g − 20°C). There is a significant reduction in Δh*, from 1100 kJ mol⁻¹ for the fully cured system to 615 kJ mol⁻¹, as the degree of cure is reduced. The parameter x determined by the peak-shift method appears essentially independent of the degree of cure (x = 0.41 ± 0.03 for the partially cured resin compared with 0.42 ± 0.03 obtained previously for the fully cured resin), and does not follow the usually observed correlation of increasing x as Δh* decreases. This invariability of the parameter x seems to indicate that it is determined essentially by the local chemical structure of the backbone chain, and rather little by the supramolecular structure. On the other hand, the estimated nonexponentiality parameter β lies between 0.3 and 0.456, which is significantly lower than in the fully cured epoxy (β ≅ 0.5), indicative of a broadening of the distribution of relaxation times as the degree of cross-linking is reduced. Like the parameter x, this also does not follow the usual correlation with Δh*. These results are discussed in the framework of strong and fragile behavior of glass-forming systems, but it is difficult to reconcile these results in any simple way with the concept of strength and fragility. © 1996 John Wiley & Sons, Inc.     

Physicochemical Properties of Plant Protection Substances: I Polymorphism and Binary Systems of Triadimenol

 The fungicide triadimenol consists of a mixture of two diastereoisomers. Diastereoisomer A (1RS,2SR) could be obtained from the mixture by fractionated crystallization from ethanol/water and toluene, successively, whereas diastereoisomer B (1RS,2RS) could be separated by column chromatography on a silica gel column using ethylacetate as eluent. Four different crystal forms of diastereoisomer A could be derived. The modifications were characterized by means of thermal analysis (thermomicroscopy, DSC), FTIR-spectroscopy, FT-Raman-spectroscopy and powder X-ray diffraction, as well as pycnometry. The thermodynamic relationships are illustrated in a semischematic energy/temperature-diagram which provides information about the relative thermodynamic stabilities and physical properties of the four crystal forms. Mod. II (m.p. 132 °C, ΔH_f 33.1±0.2 kJ mol⁻¹, density 1.271±0.001 g cm⁻³) was obtained from toluene after the separation of diastereoisomer A and is enantiotropically related to mod. I (m.p. 138 °C, ΔH_f 32.0 ± 0.2 kJ mol⁻¹, density 1.243±0.001 g cm⁻³). The transition point of mod. II with mod. I was determined between 30 and 40 °C, which means that mod. II is thermodynamically stable at ambient conditions. Mod. III (m.p. 112 °C, ΔH_f 25.1±0.5 kJ mol⁻¹) and mod. IV were obtained from the melt. Furthermore, the phase diagrams of the binary systems of diastereoisomer B and the four modifications of diastereoisomer A were calculated by means of the experimentally obtained thermodynamical data. Received September 30, 1999. Revision July 30, 2000.     

Determination of the activation energies for α and β transitions of a system containing a diglycidyl ether of bisphenol a (DGEBA) and 1,3-bisaminomethylcyclohexane (1,3-BAC)

Using dynamic mechanical analysis (DMA) we have studied the variation with the frequency of the dynamic mechanical properties (storages modulus,E'; loss modulus,E'' and loss tangent or tan σ) for a system containg a diglycidyl ether of bisphenol A (DGEBA) and 1,3-bisaminomethylcyclohexane (1,3-BAC). These properties were measured both in the glass transition and β transition regions. An increase in frequency caused a shift of tan σ peak positions in both regions toward higher temperature. Finally, we report the activation energies of a DGEBA/1,3-BAC expoxy system for α and β transitions.     

Acridinium Ion Ionization at Elevated Temperatures and Pressures to 200 °C and 2000 bar

The temperature and pressure dependences of pK for acridine ion ionization were determined up to 200 °C and 2000 bar. The UV-Vis measurements at high temperatures and pressures were conducted in flow-through spectrophotometric cells. Two independent series of experiment were performed: one in a Ti–Pd cell with silica quartz windows for measurements in the ultraviolet region, and another in a Ti grade 5 cell with sapphire windows for use at higher pressures, which permitted measurements in the visible region. Combined chemometric and thermodynamic analyses of the UV-Vis spectrophotometric data were used to extract the ionization constants as well as the changes in molar volume ΔV° for acridine protonation as functions of temperature and pressure. Values of pK decrease from 5.52 to 3.74 with increasing temperature from 25 to 200 °C at saturated water-vapor pressure. The pressure dependence of acridinium ion ionization is small (e.g., pK=5.5 at 25 °C and 2000 bar) and is characterized by positive ΔV°≤1.2 cm³⋅mol⁻¹, which is not surprising for this type of isocoulombic reaction involving a large molecule.     

New dielectric relaxation process reveals mesomorphic ordering in rapidly cooled poly(ethylene naphthalate)

A curious, strong dielectric relaxation process (δ) was found in rapidly cooled poly(ethylene naphthalate). This process, which is located between two known β and β* relaxations of PEN, appears predominantly after rapid cooling and remains present even after heating above the glass transition temperature. In view of its very low activation energy of ∼10 kJ/mol, its markedly high relaxation strength of up to Δɛ=5, and its Debye-like peak shape, a collective relaxation mechanism is proposed, which involves collective crankshaft motions of the -O-CH₂-CH₂-O- sequences in a regular arrangement of the main chains. The analogy between this δ-relaxation and an ultra-slow relaxation recently found in the smectic E phase of a side-chain liquid crystalline polymer suggests a (close-to) hexagonal smectic ordering in PEN. The very existence of liquid-crystalline order in PEN is corroborated by the observation of a thermo-reversible discontinuity in the relaxation parameters around −90 °C, which resembles a broadened LC-LC phase transition. Re-evaluation of experimental data of the β* relaxation, which occurs in the non-crystalline fraction of PEN, suggests that this relaxation is sensitive to the local orientational order, which extends from nematic to isotropic. The shift in the temperature of the β* peak and even the splitting of this peak found by other authors can be ascribed to the lowering of the activation energy by the local ordered packing of the PEN chains in line with a lower activation energy in the nematic order. The coexistence of isotropic and nematic regions in PEN is put in the context of orientational order fluctuations during the induction period of cold crystallisation of semi-flexible polymers. Received: 31 August 2000 Accepted: 30 October 2000     

Structural relaxation of amorphous Ge38S62 studied by length dilatometry and calorimetry

The structural relaxation of Ge₃₈S₆₂ glass has been studied by length dilatometry and calorimetry. The Tool-Narayanaswamy-Moynihan model was applied on obtained data of structural relaxation and parameters of this model were determined: Δh*= 483±2 kJ mol^-1, ln(A/s)= -81±1, β= 0.7±0.1 and x=0.6±0.1. Both dilatometric and calorimetric relaxation data were compared on the basis of the fictive relaxation rate. It was found that the relaxation rates are very similar and well correspond to the prediction of phenomenological model. This revised version was published online in July 2006 with corrections to the Cover Date.     

Application of the van der Pauw Methodto Conductivity Relaxation Experimentson YBa2Cu3O6+δ

Summary.  The van der Pauw method has been applied to conductivity relaxation experiments on YBa₂Cu₃O_6+δ at 600°C in order to determine the chemical diffusion coefficient as a function of the oxygen partial pressure in the surrounding atmosphere (10⁰ > p _{O 2}/bar > 10⁻³). It is shown that the van der Pauw technique is suitable for monitoring the conductivity relaxation when the oxygen diffusion is perpendicular to the direct current flowing through the sample in accordance with the van der Pauw geometry using thin tablets as samples. The oxygen partial pressure is changed stepwise (generally Δlogp _{O 2} ≤ 0.5) by employing appropriate gas mixtures as well as an electrochemical oxygen pump device. An evaluation formula is given for the determination of the chemical diffusion coefficient neglecting slow surface processes. In addition, the electronic conductivity of YBa₂Cu₃O_6+δ has been measured at 600°C as a function of oxygen partial pressure of the ambient atmosphere (10⁰ > p _{O 2}/bar > 10⁻⁵) by means of the van der Pauw method applying the same experimental set-up. Typical values of the chemical diffusion coefficient are in the range of 10⁻⁶ cm²·s⁻¹; the results of the conductivity measurements are interpreted in terms of an appropriate defect model. Received May 30, 2000. Accepted June 8, 2000     

Micellization of Pentanediyl-1,5-bis(hydroxyethylmethyl hexadecylammonium Bromide) as a Cationic Gemini Surfactant in Aqueous Solutions: Investigation Using Conductometry and Fluorescence Techniques

Pentanediyl-1,5-bis (hydroxyethylmethylhexadecylammonium bromide) was synthesized and characterized as a type of novel gemini cationic surfactant. Its solution properties were determined at various temperatures by conductivity measurements and the fluorescence quenching technique. The CMC increased in the range of 1.85 to 2.77 μmol⋅L⁻¹ as the temperature increased. The aggregation number was determined at various concentrations of NaBr solutions by the fluorescence quenching of pyrene. The thermodynamic parameters of micellization were determined using the mass law equation and the values of ΔG °, ΔH °and ΔS ° were determined for the micellization process.     

A Dehydrogenated Cycloadduct of N-Phenylmaleimide and the Azomethine Ylide Derived from Ninhydrine and Phenylalanine: Structure and Conformation

Summary.  The structure of the dehydrogenation product 1′,3a′-dihydro-3′-((1,3-dioxoindan-2-ylidene)-phenyl-methyl)-5′-phenyl-spiro-(indan-2,1′-pyrrolo[3,4-c]pyrrole)-1,3,4′,6′-(5′H, 6a′H)-tetrone derived from the cycloadducts (±)-(3a′S,6a′R)-1′,3a′-dihydro-3′-((R)-α-(1,3-dioxoindanyl)-benzyl)-5′-phenyl-spiro-(indan-2,1′-pyrrolo[3,4-c]pyrrole)-1,3,4′,6′(5H,6a′H)-tetrone and/or (±)-(3a′S,6a′R)-1′,3a′-dihydro-3′-((S)-α-(1,3-dioxoindanyl)-benzyl)-5′-phenyl-spiro-(indan-2,1′-pyrrolo[3,4-c]pyrrole)-1,3,4′,6′(5H,6a′H)-tetrone, which were synthesized by 1,3-dipolar cycloaddition of N-phenylmaleimide to 2-((2-(1,3-dioxoindan-2-yl)-2-phenyl-ethenyl)-imino)-indan-1,3-dione, was determined by X-ray analysis. Crystal data (CCD, 180 K): rhombohedral, R&3macr;;, a = 34.0871(7), c = 13.9358(5) ?, Z = 18; the structure was solved by direct methods and refined by full-matrix least-squares procedures to R(F, I ≥ 3σ(I)) = 0.053. The molecule contains a central folded ring system of two cis-fused 5-membered heterocyclic rings; each ring is nearly planar, and the angle between the rings amounts to 59.0°. Dynamic ¹H NMR spectroscopy of the product revealed an exchange process caused by restricted rotation of the double bonded 1,3-indandione moiety and the phenyl group about the C_sp2-C_sp2 single-bonds. Molecular modeling and complete lineshape analysis indicated a four site exchange process for which free energies of activation and free energies could be established. ΔG ^‡ values for the barriers of rotation are in the range of 57–59 kJ · mol^− 1 at 273 K, which is unusually high for an unsubstituted phenyl group. Received May 3, 2001. Accepted (revised) June 8, 2001     

The Potential Energies of Cohesion of a Crystalline Organic Enantiomer and the Racemate; on the Energy for the Liquid Racemate [1]

Summary. The cohesion potential energy of the crystal of one enantiomer of ethyl 3-cyano-3-(3,4-dimethyloxyphenyl)-2,2,4-trimethylpentanoate, −47.7 ± 0.1 kJ mol⁻¹ (0–90°C), was found out from the heat of sublimation (123.2 ± 5.1 kJ mol⁻¹, 78.6°C) and the kinetic energies for the gas phase and the crystal. It was found that the entropy function of Debye’s theory of solids mathematically agreed with the vibrational entropy of the gas (variationally obtained), allowing to disclose the vibrational energy using the Debye energy function (E _vib 835.0 kJ mol⁻¹ (78.6°C), E ⁰ included). E _kin for the crystal (771.1 kJ mol⁻¹ (78.6°C)) was obtained by Debye’s theory with the experimental heat capacity. The cohesion energy represented a moderate part of the sublimation energy. The cohesion energy of the racemic crystal, −44.2 kJ mol⁻¹, was obtained by the heat of formation of the crystal in the solid state (3.0 kJ mol⁻¹, 83.3°C) and E _kin for the crystal (by Debye’s theory). The decrease in cohesion on formation of the crystal accounted for the energy of formation. The change in potential energy on liquefaction of the racemate from the gas state was disclosed obtaining added-up E _{vib + rot} for the liquid in the way as to E _vib for the gas, the Debye entropy function being increasedly suited for the liquid (E _{vib + rot} 763.4 kJ mol⁻¹ (115.4°C)). Positive ΔE _pot, 13.0 kJ mol⁻¹, arised from the increase in electronic energy (Δ _l ν_mean − 154.3 cm⁻¹, by the dielectric nature of the liquid), added to the cohesion energy.     

The Potential Energies of Cohesion of a Crystalline Organic Enantiomer and the Racemate; on the Energy for the Liquid Racemate [1]

The cohesion potential energy of the crystal of one enantiomer of ethyl 3-cyano-3-(3,4-dimethyloxyphenyl)-2,2,4-trimethylpentanoate, −47.7 ± 0.1 kJ mol⁻¹ (0–90°C), was found out from the heat of sublimation (123.2 ± 5.1 kJ mol⁻¹, 78.6°C) and the kinetic energies for the gas phase and the crystal. It was found that the entropy function of Debye’s theory of solids mathematically agreed with the vibrational entropy of the gas (variationally obtained), allowing to disclose the vibrational energy using the Debye energy function (E _vib 835.0 kJ mol⁻¹ (78.6°C), E ⁰ included). E _kin for the crystal (771.1 kJ mol⁻¹ (78.6°C)) was obtained by Debye’s theory with the experimental heat capacity. The cohesion energy represented a moderate part of the sublimation energy. The cohesion energy of the racemic crystal, −44.2 kJ mol⁻¹, was obtained by the heat of formation of the crystal in the solid state (3.0 kJ mol⁻¹, 83.3°C) and E _kin for the crystal (by Debye’s theory). The decrease in cohesion on formation of the crystal accounted for the energy of formation. The change in potential energy on liquefaction of the racemate from the gas state was disclosed obtaining added-up E _{vib + rot} for the liquid in the way as to E _vib for the gas, the Debye entropy function being increasedly suited for the liquid (E _{vib + rot} 763.4 kJ mol⁻¹ (115.4°C)). Positive ΔE _pot, 13.0 kJ mol⁻¹, arised from the increase in electronic energy (Δ _l ν_mean − 154.3 cm⁻¹, by the dielectric nature of the liquid), added to the cohesion energy.     

The study of chemical reactions, absorption and extraction, using apparatuses with serpentine pie for pelliclizing-bubbling, XIV. Hydroperoxidation of 1,3-diisopropylbenzene with oxygen

Experimental results obtained by thermal hydroperoxidation of 1,3-diisopropylbenzene in the temperature region of 102–105°C, using a Vodnár-1 type apparatus with discontinuous operation are presented. Based on the results, the activation energy (E=48.5±2 kJ/mol), enthalpy (ΔH⁺=47±2 kJ/mol), entropy (ΔS⁺) and the activation free enthalpy (ΔG⁺) were calculated. XIII. J. Vodnár, A. Chis, A. Biro, S. Békássy and M. Dragan:Studia Univ. Babes-Bolyai, Chemia, Cluj-Napoca, 1997,42, 2.     

Dissociation Quotients for Citric Acid in Aqueous Sodium Chloride Media to 150°C

The three molal dissociation quotients for citric acid were measured potentiometrically with a hydrogen-electrode concentration cell from 5 to 150°C in NaCl solutions at ionic strengths of 0.1, 0.3, 0.6, and 1 molal. The molal dissociation quotients and available literature data at infinite dilution were fitted by empirical equations in the all-anionic form involving an extended Debye-Hückel term and up to five adjustable parameters involving functions of temperature and ionic strength. This treatment yielded the following thermodynamic quantitites for the first dissociation equilibrium at 25°C: logK _1a=−3.127±0.002, ΔH _1a ^o =4.1±0.2 kJ-mol⁻¹, ΔS _1a ^o =−46.3±0.7 J-K⁻¹-mol⁻¹, and ΔCp _1a ^o =−162±7 J-K⁻¹-mol⁻¹; for the second acid dissociation equilibrium at 25°C: logK _2a =−4.759±0.001, ΔH _2a ^o =2.2±0.1, ΔS _2a ^o =−83.8±0.4, and ΔCp _2a ^o =−192±15, and for the third dissociation equilibrium at 25°C: logK _3a=−6.397±0.002, ΔH _3a ^o =−3.6±0.2, ΔS _3a ^o =−134.5±0.7, and ΔCp _3a ^o =−231±7.     

Physicochemical characterization of a novel surfactant peptide containing an arginine cation and laurate anion

 A novel surfactant peptide consisting of an arginine cation with laurate anion has been synthesized, purified and characterized. The critical micellar concentration (cmc) of peptide in aqueous solutions has been determined using spectroscopic techniques and is found to increase from 0.06 to 0.11 mM with increasing temperature (15–45 °C). Cmc is also determined in the presence of salts like NaCl, KCl and sodium acetate and it is found that these electrolytes hinder aggregation with a significant increase in the case of sodium acetate. The aggregation number of the surfactant peptide has been determined using fluorescence quenching measurements and is observed to decrease from 14 to 6 with increasing temperature (15–45 °C). The standard free energy change (ΔG ⁰ _m) and standard enthalpy change (ΔH ⁰ _m) of the peptide aggregate are found to be negative with a small positive value for standard entropy change (ΔS ⁰ _m). The peptide aggregate seems to undergo phase transition above 50 °C as observed from UV–vis and fluorescence spectroscopy. From pyrene binding studies, it is shown that the interior dielectric constant increases from 5.08 at 34 °C to 8.77 at 50 °C and further decreases with increase in temperature indicating a phase change at 50 °C. Also, the ratio of excimer intensity to monomer intensity, which is a measure of microviscosity of the aggregate, decreases with increase in temperature with a change at 50 °C indicating a phase change. Received: 14 February 1997 Accepted: 13 August 1997     

A temperature study on critical micellization concentration (CMC), solubility, and degree of counterion binding of α-sulfonatomyristic acid methyl ester in water by electroconductivity measurements

 For a sodium salt of α-sulfonatomyristic acid methyl ester (14SFNa), one of the α-SFMe series surfactants, critical micellization concentration (CMC), solubility and degree of counterion binding (β) were determined by means of electrocon-ductivity measurements at different temperatures (at every 5 °C) ranging from 15 to 50 °C. The phase diagram of 14SFNa in pure water was constructed from the CMC- and solubility-temperature data, in which the Krafft temperature (critical solution temperature) was found around 0 °C. The changes in the Gibbs energy, ΔG ⁰ _m, enthalpy, ΔH ⁰ _m, and entropy, ΔS ⁰ _m, upon micelle formation as a function of temperature were evaluated taking βvalues into calculation. Received: 28 August 1996 Accepted: 5 November 1996     

Synthesis, spectral studies, and determination of stability constants and thermodynamic parameters for some aromatic diamine transition-metal complexes

The Cu(II) and Zn(II) complexes of aromatic bidentate diamines were prepared and characterized by different analytical and spectral methods. Thermodynamic parameters of complexes of Cu(II) and Zn(II) cations with 1,3-bis(p-aminophenoxy)propane were determined in 60 vol. % DMF-H₂O at various ionic strengths (0.07, 0.13, 0.2 M NaNO₃) and different temperatures (45, 50, 55, 60 ± 0.1°C) using a spectrophotometric method. The NaNO₃ solution was used to maintain the ionic strength. The stability constants show an inverse relationship with ionic strengths. The thermodynamic parameters of 60 vol. % DMF-H₂O (ΔG°, ΔH°, ΔS°) based on these formation constants were determined. The text was submitted by the authors in English.     

The Dissolution Properties of 2-(1,1-Dinitromethylene)-1,3-diazepentane in N-methyl Pyrrolidone

The dissolution properties of 2-(1,1-dinitromethylene)-1,3-diazepentane in N-methyl pyrrolidone(NMP) were studied with a RD496-2000 Calvet microcalorimeter at three different temperatures. The measured molar enthalpies (Δ_sol H) for 2-(1,1-dinitromethylene)-1,3-diazepentane in NMP at T=(298.15,306.15,311.15) K are (5.02, 5.59, 6.67) kJ⋅mol⁻¹, respectively. The differential molar enthalpies (Δ_dif H), the specific enthalpies (Δ_sol h), and the standard heat effect (Q ^Θ) for 2-(1,1-dinitromethylene)-1,3-diazepentane in NMP were obtained at the same time. The kinetic parameters of activation energy E and pre-exponential factor A are 2.26×10⁴ J⋅mol⁻¹ and 10^2.06 s⁻¹, which indicate that NMP is a good solvent for the title compound.     

Changes of mechanical properties in cold-crystallized syndiotactic polypropylene during aging

Many semicrystalline polymers undergo a process of aging when they are stored at temperatures higher than their glass-transition temperature (T _g). Syndiotactic polypropylene was quenched from the melt to −40 °C, crystallized from the glassy state at 20 or 40 °C and stored at the respective temperature for different aging times up to 7200 h. A significant increase in the tensile modulus and stress at yield and a decrease in strain at yield were observed for both aging temperatures. Differential scanning calorimetry (DSC) scans of aged material showed an endothermic annealing peak 15–30 °C above the previous aging temperature, the maximum temperature and enthalpic content of which increased with aging time. The position and the shape of the melting peak were not affected by aging. Scans of the storage modulus obtained from dynamic mechanical analyser measurements indicated a softening process starting at about 20 °C above the aging temperature and correlating with the annealing peak detected by DSC. Density measurements and wide-angle X-ray scattering investigations revealed that neither the crystallinity increased significantly nor did the crystal structure change. So the observed property changes induced by aging are attributed to microstructural changes within the amorphous phase. Furthermore, it could be shown by annealing experiments carried out at 60 °C, that aging above T _g is, analogous to aging below T _g (physical aging), a thermoreversible process. Received: 18 September 2000 Accepted: 2 January 2001     

Dielectric relaxation of sulfolane in benzene solution from microwave absorption data

The electric constant (ɛ′) and dielectric loss (ɛ″) for dilute solutions of sulfolane in benzene solution has been measured at 9.885 GHz at different temperatures (25, 30, 35, and 40°C) by using standard microwave techniques. Following the single frequency concentration variational method, the dielectric relaxation time (τ) and dipole moment (μ) have been calculated. It is found that dielectric relaxation process can be treated as the rate process, just like the viscous flow. Based on the above studies, monomer structure of sulfolane in benzene has been inferred. The presence of solute-solvent associations in benzene solution has been proposed. Energy parameters (ΔH _ɛ, ΔF _ɛ, ΔS _ɛ) for dielectric relaxation process of sulfolane in benzene at 25, 30, 35, and 40°C have been calculated and compared with the corresponding energy parameters (ΔH _η, ΔF _η, ΔS _η) for the viscous flow.