Effects of temperature on the weathering lifetime of coated polycarbonate

The lifetime of polycarbonate (PC) coated with silicone hardcoats containing UV absorber is shorter at elevated temperatures. The activation energy (E_a) for delamination was found to be 18 ± 2 kJ/mol (4.3 ± 0.5 kcal/mol) at the 95% confidence level in this study. This E_a is the consequence of the sensitivity of the substrate and the UV absorber to temperature. The E_a for PC photodegradation was previously found to be 17-21 kJ/mol (4-5 kcal/mol). The E_a for loss of absorbance in the second-generation silicone hardcoat was found to be 28.5 ± 5.4 kJ/mol (6.8 ± 1.3 kcal/mol) at the 95% confidence level. Results are consistent with experimental findings when these activation energies are used in published predictive models. Since the E_a for coating delamination depends on the E_a of UV absorber loss, coating systems different from the one in this study will need to be investigated separately.     

Effects of temperature on the weathering of engineering thermoplastics

We have determined the activation energies (E_a) of yellowing and gloss loss for a large number of engineering thermoplastics and blends under accelerated weathering conditions. The E_a often depend on the property measured and exposure conditions, although they vary over a fairly small range. Under the CIRA/sodalime-filtered xenon arc conditions most likely to be representative of outdoor exposure, the E_a for gloss loss was ≤5 kcal/mol for all samples tested. The E_a for yellowing was also ≤5 kcal/mol except for SAN and ABS. Evidently the color bodies formed from photo-oxidation of SAN are more sensitive to temperature. A reaction with an E_a of 5 kcal/mol will increase its rate by about 33% for each 10 °C increase in temperature near room temperature. Temperature is an important, but not overwhelming, variable in the weathering of most engineering thermoplastics.     

Determination of Activation Energies and the Optimum Temperatures of Hydrolysis of Starch by α-Amylase from Porcine Pancreas

The present paper reports the determination of the activation energies and the optimum temperatures of starch hydrolysis by porcine pancreas α-amylase. The parameters were estimated based on the literature data on the activity curves versus temperature for starch hydrolysis by α-amylase from porcine pancreas. It was assumed that both the hydrolysis reaction process and the deactivation process of α-amylase were first-order reactions by the enzyme concentration. A mathematical model describing the effect of temperature on porcine pancreas α-amylase activity was used. The determine deactivation energies E_a were from 19.82 ± 7.22 kJ/mol to 128.80 ± 9.27 kJ/mol, the obtained optimum temperatures T_opt were in the range from 311.06 ± 1.10 K to 326.52 ± 1.75 K. In turn, the values of deactivation energies E_d has been noted in the range from 123.57 ± 14.17 kJ/mol to 209.37 ± 5.17 kJ/mol. The present study is related to the starch hydrolysis by α-amylase. In the industry, the obtained results the values E_a, E_d, T_opt can be used to design and optimize starch hydrolysis by α-amylase porcine pancreas. The obtained results might also find application in research on the pharmaceutical preparations used to treat pancreatic insufficiency or prognosis of pancreatic cancer.     

Second-order kinetics of enthalpy recovery in polystyrene

Second-order kinetics were found applicable to the isothermal enthalpy release after quenching from a high temperature. The kinetic behavior was unaffected by different amounts of initial excess enthalpy. The activation enthalpy obtained from the temperature dependence of the second order rate constant was 200 kcal/mol or 837 kJ/mol which could be compared with 736 kJ/mol for volume recovery in a similar temperature range relative to T_g.     

Influence of thermal treatment on the glass transition temperature of thermosetting epoxy laminate

The influence of accelerated thermal treatment of thermosetting epoxy laminate on its glass transition temperature was studied. Lamplex^® FR-4 glass fibre-reinforced epoxy laminate (used for printed circuit board manufacturing) was used in these experiments. The composite was exposed to thermal treatments at temperatures ranging from 170 °C to 200 °C for times ranging from 10 to 480 h. The glass transition temperature (T_g) was analysed via dynamic mechanical analysis (DMA). It has been proven that the glass transition temperature rapidly decreases in reaction to thermal stress. The obtained T_g data were used for Arrhenius plots for different critical temperatures (T_g-crit. = 105–120 °C). From their slopes (?E_a/R), the activation energy of the thermal degradation process was calculated as 75.5 kJ/mol. In addition to this main relaxation mechanism, DMA also recorded one smaller relaxation process in the most aged samples. Microscopic analysis of the sample structure showed the presence of pronounced small regions of degradation both on the surface and in the inner structure, which are probably the causes of microscopic delamination and the smaller relaxation process.     

Stable α-heteroatom-free dialkylcarbenes: a DFT study

The stability of the singlet dicyclopropylcarbene (ΔE _S–T = 15.3 kcal/mol, 1) is increased not only by cyclization to 2,5-dicyclopropylcyclopentanylidene (ΔE _S–T = 20.3 kcal/mol, 2), but even more so by unsaturation to 2,5-dicyclopropylcyclopentenylidene (ΔE _S–T = 22.5 kcal/mol, 3). In a further attempt to pave the way toward synthesis of new stable dialkylcarbenes, we introduced different substituents on the α-cyclopropyls of 3, where the stability was increased over twice of 1 (ΔE _S–T = 37.8 kcal/mol) for 2,5-bis(2,3-dihydroxycyclopropyl)-3,4-dinitrocyclopentenylidene, 3_{\textOH-\textNO 2} {\mathbf{3}}_{{{\text{OH}}{-}{\text{NO}}_{ 2}}} .     

Solubilities of cholesterol and desmosterol in binary solvent mixtures of n-hexane + ethanol

The solubilities of cholesterol and desmosterol in binary solvent mixtures of n-hexane + ethanol at temperatures of 293.2–323.2 K were determined by a static equilibrium method. The solubilities increase with temperature and go through a maximum at a specific solvent composition. The fusion enthalpy Δ_fusH and the melting point T_m, determined by differential scanning calorimeter (DSC), are 28.5 kJ/mol, 421.7 K for cholesterol and 15.9 kJ/mol, 388.2 K for desmosterol, respectively. The solubilities of cholesterol and desmosterol in pure n-hexane or ethanol follow a linear Van’t Hoff relation with temperature. Activity models, such as Wilson, NRTL and UNIQUAC models were used to correlate and predict the solubilities of cholesterol and desmosterol in n-hexane + ethanol mixed solvents. The interaction parameters were expressed as a function of temperature.     

Spectra and structure of silicon containing compounds: XXIX. Conformational studies of methyl vinyl difluorosilane from temperature dependent FT-IR spectra of xenon and krypton solutions

Variable temperature (−55 to −150°C) studies of the infrared spectra (3500–400 cm⁻¹) of methyl vinyl difluorosilane, CH₂CHSiF₂CH₃, dissolved in liquid xenon and krypton have been recorded. Utilizing three sets of conformer doublets due to the cis and gauche rotamers from the krypton solution and two pairs from the xenon solution, the enthalpy difference has been determined to be 67±7 cm⁻¹ (0.80±0.09 kJ/mol) and 83±11 cm⁻¹ (0.99±0.14 kJ/mol), respectively, with the gauche conformer the more stable form. However, in the crystalline solid only the cis conformer is present. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311+G(2d,2p) with full electron correlation by the perturbation method from which the conformational stabilities have been determined. With the largest basis set MP2/6-311+G(2d,2p), the cis conformer is predicted to be the more stable conformer by 10 cm⁻¹ which is inconsistent with the experimental results; however, this value is so small that the ab initio prediction cannot be relied on to give the correct conformer stability. The spectroscopic and theoretical results are compared to the corresponding quantities for some similar molecules.     

Calorimetric measurements and first principles to study the (Ag-Li) liquid system

Integral molar enthalpies of mixing were determined by drop calorimetry for (Ag-Li) liquid alloys at two temperatures (1253 and 873) K. The integral molar enthalpies of mixing are negative in the entire range of concentrations. For the mole fraction of lithium X_Li = 0.5664, minimum value of the integral enthalpy of mixing of at ΔH_m = −11.679 kJ/mol was observed. For (Ag-Li) liquid alloys, between T = (873 and 1253) K no temperature dependence was observed. Ab initio molecular dynamics was used to simulate liquid phase structures at T = 873 K (Li-rich side) and at T = 1250 K (Ag-rich phase) for subsequent calculation of the vibrational energy, respectively. Our measured and calculated data were compared with literature data.     

A determination of the effective temperatures for the dissociation of the proton bound dimer of dimethyl methylphosphonate in a planar differential mobility spectrometer

The dissociation of the proton bound dimer of dimethyl methylphosphonate to the protonated and neutral molecule has been studied in a planar differential mobility spectrometer. The internal energy of the ions is the sum of the thermal component and the electric field component and results in an effective temperature, T_eff, that is significantly greater than that of the ambient atmosphere temperature, T. The measured rate constant for dissociation, which rises exponentially with field strength, together with activation energy and pre-exponential factor previously determined under thermal conditions, allow the calculation of T_eff as a function of electric field strength. T_eff is a linear function of electric field intensity at constant T over the range of T from 60 °C to 140 °C. The efficiency of the available collision energy in causing dissociation decreases with increasing T, from 52% at 60 °C to 40% at 140 °C.     

Conformational and Structural Studies of 1-Fluoropropane from Temperature Dependant FT-IR Spectra of Rare Gas Solutions and Ab Initio Calculations

Variable temperature (–55 to –150°C) studies of the infrared spectra (3500 to 400 cm^–1) of 1-fluoropropane, CH₃CH₂CH₂F, dissolved in liquid krypton and xenon have been recorded. Utilizing three conformer pairs in the krypton solution and four conformer pairs in the xenon solution, enthalpy differences of 104±6 cm^–1 (1.24±0.07 kJ/mol) and 99±5 cm^–1 (1.16±0.06 kJ/mol) were obtained from the krypton and xenon solutions, respectively, with the gauche form the more stable conformer. From these data it is estimated that 24% of the trans forms is present at ambient temperature. The conformational stabilities, harmonic force constants, fundamental frequencies, infrared intensities and Raman activities have been obtained from RHF/6-31G(d) and/or MP2/6-31G(d) ab initio calculations and these quantities have been compared to the experimental values when appropriate. The optimized geometries have also been obtained with several different ab initio basis sets up to MP2/6-311+G(2d,2p). The r₀ structural parameters have been obtained by combining the ab initio data with the previously reported rotational constants for both conformers. The results are compared to the corresponding results for some similar molecules.     

Direct ab initio MD study on the hydrogen abstraction reaction of triplet state acetone from methanol molecule

Solvent re-orientation process of triplet acetone/methanol complex and intermolecular hydrogen atom abstraction reaction on the triplet state energy surface, (CH₃)₂C=O (T₁) + CH₃OH → (CH₃)₂C–OH + CH₂OH in gas phase, have been investigated by means of density functional theory (DFT) and direct ab initio molecular dynamics (MD) methods. The static DFT calculation of hydrogen abstraction reaction at the T₁ state showed that the transition state is 16.4 and 30.9 kcal/mol lower than the energy levels of S₁ and S₂ states, respectively, and 9.2 kcal/mol higher than the bottom of T₁ state. The product state, (CH₃)₂C–OH⋯CH₂OH, is 8.4 kcal/mol lower in energy than the level of T₁ state. The direct ab initio MD calculation showed that the product is rapidly formed within 150 fs and the separated products (CH₃)₂C–OH + CH₂OH were formed. The mechanism of reaction dynamics of the triplet acetone/methanol complex was discussed on the basis of theoretical results.     

Thermodynamic properties and oxygen stoichiometry of Ba2Cu3O5 + δ

The thermodynamic properties and oxygen stoichiometry of Ba₂Cu₃O_{5 + δ} are studied by means of the electromotive force (EMF) with a fluoride electrolyte, dissolution calorimetry, and thermogravimetry. It is shown that the temperature dependence of the Gibbs energy of the formation of barium cuprate from simple oxides and oxygen in the temperature range of 860–1120 K can be described by the polynomial Δ_{f, ox} G ^°(Ba₂Cu₃O_{5 + δ}) ± 0.1 (kJ/mol) = ?291.78 + 1.127T ? 0.13207 TlnT (kJ/mol).     

The p–T phase diagram for ferroelectric bis-thiourea pyridinium nitrate

The effect of temperature and pressure on physical properties of the ferroelectric bis-thiourea pyridinium nitrate inclusion compound has been studied by dielectric spectroscopy and nuclear magnetic resonance (NMR). At ambient pressure the ferroparaelectric phase transition observed at T₂ = 216 K is continuous in contrast to the nonferroelectric phase transition observed at T₁ = 273 K. Under small pressures, the temperatures of the phase transitions T₁ and T₂ increase with increasing pressure. Starting from about 250 MPa, T₁ temperature decreases with increasing pressure, while T₂ temperature increases with increasing pressure. At 450 MPa and 245 K a triple point is observed. Bis-thiourea pyridinium nitrate undergoes a continuous phase transition from the ferroelectric to paraelectric phase under 450 MPa, while above this pressure the phase transition from the ferroelectric to paraelectric phase is discontinuous. The change in the phase transition character is related to the crystallographic change in the group–subgroup relation between the ferro- and paraelectric phases taking place with increasing pressure.     

Novel Co complex with high transformation temperature of valence tautomerism

The preparation and characterization of [Co^II-HS(dpa)(3,5-DBSQ)₂] are presented first time. From the magnetic curve, the phase transition temperatures, T_c is 380 K. To our knowledge, the T_c value is the highest among the species exhibiting thermal valence tautomerism. After illumination with 532 nm light at 5 K, the magnetization values increase from μ_eff=1.73 to 2.0μ_B, showing valence tautomerism. The metastable state will recover to the original state with the increase of temperature. The alternate illumination with 532 and 830 nm light at 5 K can induce a reversible change in magnetization. The quick response to light means that this compound can be used for the development of optical–magnetic switch.     

A photon transmission study for film formation from poly(vinyl acetate) latex particles with different molecular weights

The photon transmission technique was used to monitor the temperature evolution of film formation from poly(vinyl acetate) (PVAc) latex particles with two different molecular weights. Two sets of latex films were prepared below the glass transition temperature (T_g) of PVAc, which are named as low (LM) and high molecular weight (HM) films. These films were annealed at elevated temperatures above the T_g of PVAc for various time intervals. It is observed that transmitted photon intensity (I_tr) from these films increased as the annealing temperature was increased. Onset temperatures (T_H) at given times (τ_H) for starting the optical clarity of LM and HM films were measured and used to calculate the healing activation energies (ΔH) for the PVAc minor chains, and found to be as 28.1 kcal/mol and 27.7 kcal/mol, respectively. The increase in the transmitted photon intensity, I_tr above T_H was attributed to the increase in the number of disappeared interfaces between the deformed latex particles. Prager–Tirrell (PT) model was employed to interpret the increase in the crossing density of chains at the junction surfaces. The interdiffusion (backbone) activation energies (ΔE) were measured and found to be 177.5 kcal/mol and 210.7 kcal/mol for a diffusing PVAc chains across the junction surface of LM and HM latex films, respectively. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2918–2925, 2007     

Preparation and thermal degradation kinetics of terpolymer poly(?-caprolactone-co-1,2-butylene carbonate)

Poly(?-caprolactone-co-1,2-butylene carbonate) (PBCCL) was successfully synthesized via terpolymerization of carbon dioxide, 1,2-butylene oxide(BO) and ?-caprolactone (CL). A polymer-supported bimetallic complex (PBM) was used as catalyst. The influences of various reaction conditions such as reaction content, reaction time and reaction temperature on properties of terpolymers were investigated. When CL content increased, the viscosity-average molecular weights (M_v), glass transition temperature (T_g) and decomposition temperature (T_d) of PBCCL improved relative to those of poly(1,2-butylene carbonate) (PBC). Prolonging the reaction time resulted in increase in M_v and T_g. As reaction temperature increased, the molar fractions of CL (fCL) increased obviously. When the reaction temperature went beyond 80 °C, the resulting copolymers tended to be crystalline. The thermal properties and degradation behaviors of PBCCL were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The apparent activation energy and thermal degradation model of PBCCL was estimated by means of Ozawa-Flynn-Wall method and Phadnis-Deshpande method, respectively. The results showed that T_g and T_d of the terpolymer PBCCL were much higher than those of PBC. The thermal degradation behavior of PBCCL was evidenced by one-step thermal degradation profile. The average apparent activation energy is 77.06 kJ/mol, the thermal degradation kinetics follows the power law thermal decomposition model.     

Determination of glass transition temperature of poly(ethylene glycol) by spin probe technique

The rotational relaxation data for free nitroxyl radicals in poly(ethylene glycol) samples of mol. wt 3000-22,000 gave for the glass-transition temperature (T_p) a value of ?60°, independent of molecular weight. The rotational activation energy was ～ 2 kJ/mole below T_g and ～ 10 kJ/mole above T_g. This indicated that the mechanism of motion below T_g differs from the above T_g. Evidently the rotating units of polymer are much smaller below T_g than above it. The rotational relaxation time (T) was found to be dependent on the molar volume of PEG (V) and to follow the empirical equations t = A exp (?kV₁) and t = B exp (?kV₁) where V_a and V₁ are the molar volumes (cm³/mol) when T < T_g and T > T_g respectively, and k ～ 1. Therefore the defects in which radicals are located are perhaps the dominant factor determining the dynamic state of probe radicals in polymers at low temperatures.     

Hydrophobic interaction chromatography of proteins: Studies of unfolding upon adsorption by isothermal titration calorimetry

Heat of adsorption is an excellent measure for adsorption strength and, therefore, very useful to study the influence of salt and temperature in hydrophobic interaction chromatography. The adsorption of bovine serum albumin and β‐lactoglobulin to Toyopearl Butyl‐650 M was studied with isothermal titration calorimetry to follow the unfolding of proteins on hydrophobic surfaces. Isothermal titration calorimetry is established as an experimental method to track conformational changes of proteins on stationary phases. Experiments were carried out at two different salt concentrations and five different temperatures. Protein unfolding, as indicated by large changes of molar enthalpy of adsorption Δh_ads, was observed to be dependent on temperature and salt concentration. Δh_ads were significantly higher for bovine serum albumin and ranged from 578 (288 K) to 811 (308 K) kJ/mol for 1.2 mol/kg ammonium sulfate. Δh_ads for β‐lactoglobulin ranged from 129 kJ/mol (288 K) to 186 kJ/mol (308 K). For both proteins, Δh_ads increased with increasing temperature. The influence of salt concentration on Δh_ads was also more pronounced for bovine serum albumin than for β‐lactoglobulin. The comparison of retention analysis evaluated by the van't Hoff algorithm shows that beyond adsorption other processes occur simultaneously. Further interpretation such as unfolding upon adsorption needs other in situ techniques.     

Kinetics of formation of barium tungstate in equimolar powder mixture of BaCO<Subscript>3</Subscript> and WO<Subscript>3</Subscript>

The formation of Barium monotungstate (BaWO₄) particles in equimolar powder mixtures of BaCO₃ and WO₃ was examined under isothermal and non-isothermal conditions upon heating in air at 25–1200 °C, using thermogravimetry. Concurrence of the observed mass loss (due to the release of CO₂) to the occurrence of the formation reaction was evidenced. Accordingly, the extent of reaction (x) was determined as a function of time (t) or temperature (T). The x–t and x–T data thus obtained were processed using well established mathematical apparatus and methods, in order to characterize nature of reaction rate-determining step, and derive isothermal and non-isothermal kinetic parameters. Moreover, the reaction mixture quenched at various temperatures (600–1,000 °C) in the reaction course was analyzed by various spectroscopic and microscopic techniques, for material characterization. The results obtained indicated that the reaction rate may be controlled by unidirectional diffusion of WO₃ species across the product layer (BaWO₄), which was implied to form on the barium carbonate particles. The isothermally determined activation energy (118–125 kJ/mol) was found to be more credible than that (245 kJ/mol) determined non-isothermally.