Network formation studied by temperature scanning brillouin scattering and differential scanning calorimetry techniques. II.

The reaction of 1,4‐butanediol diglycidyl ether (EP) with cis‐1,2‐cyclohexanedicarboxylic anhydride (CH) and triethylamine (TEA) as an initiator was studied with temperature scanning Brillouin spectroscopy (TSBS) and differential scanning calorimetry (DSC). The evolution of the reaction process (liquid–gel–solid) was investigated as a function of the epoxy molar fraction (x_EP), for sample compositions varying from an epoxy excess to an anhydride excess. The dependence of the final conversion factors αr_DSC and αr_TSBS and the kinetic parameters E_DSC and E_TSBS on x_EP is presented. A comparison of the experimental gelation point (Pgel) behavior and the expected theoretical one, described by the Flory theory, is also reported. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1326–1336, 2001     

Variation of Hypersound Velocity in the Isotropic Phase of Liquid Crystal Mixtures

The hypersound velocity in a series of binary mixtures containing the constituents 4-n-pentyl-4'-cyanobiphenyl and 4-cyano-4'(2-methyl)butoxybiphenyl has been measured using Brillouin scattering techniques. Measurements have been made in the isotropic phase of the mixtures over the temperature range 30°C to 80°C. A simple theory for the observed velocity variations is proposed and compared with the experimental data. Agreement is found to be excellent over the temperature range studied.     

The cure of elastomers by dicumyl peroxide as observed in differential scanning calorimetry

Differential scanning calorimetry (DSC) has been used to determine the enthalpy changes accompanying the thermal homolysis and subsequent radical reactions occuring in the dicumyl peroxide (DCP) cure of elastomers (NR, EPDM, SBR, BR, NBR, and EPM). The thermal degradation of DCP alone, dispersed on kaolin clay in a hydrocarbon solvent, was also studied.The degradation of DCP alone results in an exothermic enthalpy change of ～ 215 kJ mole^?1. In elastomer systems, the observed enthalpy exchange can be ordered BR >; SBR ; NBR (34% ACN) > NBR (27% ACN) > NR > EPDM ～ EPM.Low enthalpy values are associated with systems containing predominantly secondary hydrogen atoms (EPDM, EPM). The high enthalpy of cure for BR appears associated with the known ability of the polybutadienyl radicals to propagate a limited chain reaction. The relative extent of cumyloxy radical disproportionation to hydrogen abstraction was determined in each system and this has no apparent effect on the observed enthalpy change.The method of Borchardt and Daniels was used for data reduction and calculation of apparent activation energies. For DCP degradation alone the calculated activation energy is in good agreement with literature values. In elastomer systems, the calculated activation energies must be treated with caution because, as pointed out by Borchardt and Daniels, their method does not apply to solid state reactions.     

Differential scanning calorimetry (DSC) of semicrystalline polymers

Differential scanning calorimetry (DSC) is an effective analytical tool to characterize the physical properties of a polymer. DSC enables determination of melting, crystallization, and mesomorphic transition temperatures, and the corresponding enthalpy and entropy changes, and characterization of glass transition and other effects that show either changes in heat capacity or a latent heat. Calorimetry takes a special place among other methods. In addition to its simplicity and universality, the energy characteristics (heat capacity C _P and its integral over temperature T—enthalpy H), measured via calorimetry, have a clear physical meaning even though sometimes interpretation may be difficult. With introduction of differential scanning calorimeters (DSC) in the early 1960s calorimetry became a standard tool in polymer science. The advantage of DSC compared with other calorimetric techniques lies in the broad dynamic range regarding heating and cooling rates, including isothermal and temperature-modulated operation. Today 12 orders of magnitude in scanning rate can be covered by combining different types of DSCs. Rates as low as 1 μK s⁻¹ are possible and at the other extreme heating and cooling at 1 MK s⁻¹ and higher is possible. The broad dynamic range is especially of interest for semicrystalline polymers because they are commonly far from equilibrium and phase transitions are strongly time (rate) dependent. Nevertheless, there are still several unsolved problems regarding calorimetry of polymers. I try to address a few of these, for example determination of baseline heat capacity, which is related to the problem of crystallinity determination by DSC, or the occurrence of multiple melting peaks. Possible solutions by using advanced calorimetric techniques, for example fast scanning and high frequency AC (temperature-modulated) calorimetry are discussed.     

Measurement of the enthalpies of vaporization and sublimation of solids aromatic hydrocarbons by differential scanning calorimetry

An experimental procedure is proposed for direct measurement of the heat involved in the vaporization of a solid organic compound above its normal melting temperature. This technique consists on the fusion of a solid aromatic hydrocarbon, which is then vaporized by a sudden decrease of the pressure. The direct register of heat flow as function of time by differential scanning calorimetry allows the quantifying of the enthalpy of vaporization of compounds such as phenanthrene, β-naphthol, pyrene, and anthracene. Enthalpies of vaporization were measured in an isothermal mode over a range of temperatures from 10 to 20 K above the melting temperatures of each compound, while enthalpies of fusion were determined from separate experiments performed in a scanning mode. Enthalpies of sublimation are computed from results of fusion and vaporization, and then compared with results from the literature, which currently are obtained by calorimetric or indirect techniques.     

Differential scanning calorimetric measurement of cartilage destruction caused by Gram-negative septic arthritis

The treatment of the bacterial arthritis of the joints is still a great challenge for orthopedic surgeons and rheumatologists. Aerobic Gram-negative bacteria are involved only in 20–25 % of cases. The inadequate therapy can cause cartilage destruction and can result in severe osteoarthritis of the affected joint. The aim of this study was to demonstrate and follow the destruction of the joints’ hyaline cartilage by calorimetric method. We induced experimental septic arthritis in knee joints of seven New Zealand rabbits by a single inoculation of Escherichia coli ATCC 25922 culture (0.5 mL cc. 10⁸ ± 5 % c.f.u.). The duration of this experiment was 7 days from the first to the last injection. After euthanizing the first subject, all other animals were given an overdose of anesthetics and samples were isolated from the cartilage of the femurs by surgical intervention for calorimetric measurements. The DSC scans clearly demonstrated the development of infective structural destruction in the cartilage from the first to the tenth day of incubation. In case of healthy control the melting temperatures (T _m) were: 57 and 63.1 °C and the total calorimetric enthalpy change (ΔH) was 0.37 J g^?1. After the third day, the enthalpy increased extremely (3.67 J g^?1), the two transition temperatures shifted toward lower temperature: 47.7 and 62.3 °C. At the fifth day, the effect of infection is culminated with T _m = 62.2 °C and a further elevation in ΔH (3.75 J g^?1). These results can indicate a dramatic change of the structure of rabbit cartilage between the third and fifth days. Therefore, the time elapsed seems to be critical and possesses clinical relevance, since by the sixth day, ΔH decreased to 2.6 J g^?1 with a practically unchanged melting temperature. Between the sixth and tenth days, significantly increased melting temperatures (64.9 °C) were observed with decreased (3.38 J g^?1) calorimetric enthalpy. In conclusion, calorimetric measurements have been proven to be a reliable method in the measurement of cartilage destruction, caused by Gram-negative septic arthritis.     

Differential scanning calorimetry (DSC) analysis of human plasma in melanoma patients with or without regional lymph node metastases

Melanoma malignum (MM) is a common type of skin cancer, and its incidence is increasing in the general population. We aimed to detect blood plasma components with differential scanning calorimetry (DSC) in 15 white adult MM patients, who had histopathologically diagnosed, operable cutaneous MM without any distant metastases. We observed that thermal changes (second T _m , calorimetric enthalpy) in blood plasma showed correlation with tumor thickness and the extent of regional invasion. Further studies are needed to elucidate these relationships, but our preliminary work has provided DSC should be a new tool for the early diagnosis and monitoring of MM patients.     

热动力学的滴定量热发研究 I.一级反应的热动力学

用滴定量热法分别建立了滴定期和滴反应期一级反应热动力学的数学模型，根据这两种模型，均可由一次实验的滴定量热曲线同时解析出一级反应的速率常数和摩尔反应焓。用滴定量热法研究了去离子水溶剂中乙酸乙酯皂化反应的热动力学，实验结果验证了本文用滴定量热法研究一级反应热动力学的适用性。     

Reaction thermodynamics of amine‐cured epoxy systems: Validation of the enthalpy and heat capacity of reaction as determined by modulated temperature differential scanning calorimetry

The reaction enthalpy and reaction heat capacity of three aromatic epoxy–amine systems have been determined with modulated temperature diffential scanning calorimetry (MTDSC), mostly in quasi‐isothermal conditions, over a wide temperature range (33–140 °C) and for different mixture compositions. The reaction enthalpy is only slightly dependent on the epoxy–amine chemistry, from ?111 to ?98 kJ/mol epoxy functionality. With the model system phenyl glycidyl ether (PGE)+aniline, the reaction enthalpy of the secondary amine–epoxy reaction step is equal to that of the primary amine–epoxy reaction. Group contributions needed to calculate the reaction heat capacity with an additivity approach are evaluated, and a new value of 37.2 J mol^?1 K^?1 for the group N? (H)(C)(CB) is proposed. With this group contribution, the additivity method predicts almost equal values for the reaction heat capacity of both amine–epoxy reaction steps at 298.15 K (Δ_rC_p,prim = 15.7 J mol^?1 K^?1 and Δ_rC_p,sec = 14.6 J mol^?1 K^?1), whereas the experimental value of Δ_rC_p,sec is about three times larger than that of Δ_rC_p,prim at 100 °C. These results are confirmed experimentally for PGE+aniline as a different temperature dependence of both reaction heat capacities. MTDSC therefore is potentially interesting for differentiating between reactive species in an epoxy–amine reaction, a benefit previously assigned to spectroscopic methods only. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 594–608, 2003     

Studies of irreversible heat denaturation of green fluorescent protein by differential scanning microcalorimetry

Heat denaturation of green fluorescent protein (the GFP-cycle3 mutant) was studied by the method of differential scanning microcalorimetry. Activation energy values for two stages of GFP unfolding were calculated from the calorimetric data using the model of irreversible denaturation. Dependences of activation energy and denaturation enthalpy on the temperature of the maxima of corresponding stages of denaturation were obtained, which allow estimating the corresponding increments of heat capacity. Based on the known correlations of the structure and energy parameters, it was concluded that the first transition state is close to the native state, whereas the second transition state is close to the denatured state, judging by the exposure of hydrophobic groups to the solvent.     

A Method for Indirectly Measuring the Second Sound Velocity in Smectic a Liquid Crystals

We present a simple method by which the variation of the hypersound velocity with the angle between the liquid crystal director and the scattering plane is fitted to theory. Combinations of bulk elastic constants can be obtained together with a value for the second sound velocity. This method is used to study the smectic A phase of 4-n-octyl-4'-cyanobiphenyl. We compare our results with previous work.     

Fast method for the experimental determination of vaporization enthalpy by differential scanning calorimetry

A simple method is proposed to estimate the vaporization enthalpy of the palmitic acid (hexadecanoic acid) at its normal boiling temperature. Differential scanning calorimetry (DSC) was the technique used to directly measure these thermodynamic properties. The advantages of this method are its speed and small amount of sample required. In order to avoid evaporation and to ensure equilibrium conditions, the experiments were carried out including a-alumina in contact with the fatty acid. The effect of the alumina concentration is discussed. The obtained experimental data (T_bp=625.4±0.5 K, D_vap H=237.6±5.9 J g^-1) is compared with that obtained by using thermodynamic equations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Brillouin scattering in amorphous polymeric solids

Brillouin scattering of laser light has been used to study the temperature dependence of phonon velocity in a variety of amorphous polymeric systems, particularly internally and externally plasticized methacrylates. Discontinuities in the temperature coefficient of the hypersound velocities are observed at the glass transition temperatures (T_g). This phenomenon is related to changes in the temperature behavior of the specific volume accompanied by corresponding discontinuities in certain second-order thermodynamic quantities. This method was also used to examine the temperature dependence of the Landau-Placzek ratio. This ratio is relatively large in polymer systems and appears to be independent of temperature in the region of the glass transition, provided that there are no internal strains in the sample at the temperature of measurement. Evidence is presented which suggests that the abrupt changes in this ratio at T_g reported by earlier workers were due to kinetic effects related to the relaxation of internal strains above T_g, and the results of recent studies by other investigators, both corroborating and supplementing the present work, are reviewed.     

The antimalarial agent halofantrine perturbs phosphatidylcholine and phosphatidylethanolamine bilayers: a differential scanning calorimetric study.

The interaction of halofantrine with phosphatidylcholine and phosphatidylethanolamine bilayers has been investigated by differential scanning calorimetry. Halofantrine caused a broadening of the gel to liquid crystalline phase transition endotherm of the phosphatidylcholines. A decrease in the transition temperature Tm and enthalpy (delta H) of transition was also observed. This varied with the chain length of the phospholipid and was more pronounced with short chain members. Halofantrine-induced changes to the thermotropic characteristics of dipalmitoylphosphatidylcholine (DPPC)/cholesterol bilayers suggested that the penetration of halofantrine into the bilayer was diminished in the presence of cholesterol. A more complex calorimetric profile was observed in the interaction of halofantrine with phosphatidylethanolamines and the results suggested that halofantrine did not disrupt the cooperativity of the phosphatidylethanolamine bilayers to the same extent as that observed with the phosphatidylcholines. Halofantrine caused significant perturbation of phospholipids and this property might have an important bearing on its pharmacodynamic effects.     

The search for kinetic reference materials for adiabatic and differential scanning calorimetry

No reference materials are currently available to study thermoanalytical kinetic methods, apparatus, or software. The ASTM International Committee E27 on Hazard Potential of Chemicals seeks to identify possible calorimetric reference materials for evaluating kinetic parameters, including activation energy (E), log pre-exponential factor (log Z), and reaction orders (m and n), as well as reaction enthalpy (H). Six candidate materials are examined including di-tertiary-butyl peroxide (DTBP), trityl azide, azobenzene, azobisisobutyronitrile (ABIN), cumene hydroperoxide (CHP), and phenytetrazolthiol. No single material appears to meet all needs. The merits and applicability of each candidate are discussed and recommended kinetic reference values are presented.     

Investigation of copper(II) complexation by glycylglycine using isothermal titration calorimetry

Isothermal titration calorimetry (ITC) and potentiometric titration methods have been used to study the process of proton transfer in the copper(II) ion-glycylglycine reaction. The stoichiometry, conditional stability constants, and thermodynamic parameters (ΔG, ΔH, and ΔS) for the complexation reaction were determined using the ITC method. The measurements were carried out at 298.15 K in solutions with a pH of 6 and the ionic strength maintained with 100 mM NaClO₄. Carrying out the measurements in buffer solutions of equal pH but different enthalpies of ionization of its components (Mes, Pipes, Cacodylate) enabled determination of the enthalpy of complex formation, independent of the enthalpy of buffer ionization. The number of protons released by glycylglycine on account of complexation of the copper(II) ions was determined from calorimetric and potentiometric measurements.     

Phase Transitions in Non-ionic Detergent Micelles

Differential scanning calorimetry (DSC) studies of micellar, 60 mM solutions of the octaethyleneglycol alkylethers C₁₄E₈ and C₁₆E₈ provide evidence for a narrow endothermic transition at 41 and 32°C,respectively, characterized by an enthalpy change of 2 kJ mol⁻¹ for both detergents. The observed thermal transition is indicative of a concerted transition of the surfactant molecules, as illustrated on the basis of a simple molecular model. The effect of co-solvents such as different alcohols on the thermal transition is investigated. Glycerol markedly lowers the transition temperature whereas the transition is absent in the presence of at least 10% ethanol. The calorimetric transition correlates with the temperature dependent increase of viscosity and static light scattering as well as with changes observed by small-angle neutron scattering (SANS). The SANS results provide clear evidence for a distinct structural change occurring at the transition temperature, which is interpreted as a sphere-to-rod transition of the detergent micelles. Moreover, the rod length increases with increasing temperature. We suggest that the process causing the thermal transition acts as the prerequisite of the growth process. This revised version was published online in August 2006 with corrections to the Cover Date.     

Structural relaxation processes in polyethylene glycol/CCl4 solutions by Brillouin scattering

We present results of a Brillouin scattering experiment on solutions of poly(ethylene glycol) of mean molecular mass 600 g/mol (PEG600) in CCl4. The relaxation process detected has been assigned to conformational rearrangements of the polymeric chains, triggered by reorientation of the side groups. The concentration dependencies of the hypersound velocity and normalized absorption are compared against the indications from several models proposed in the literature. The concentration evolution of the system is described in terms of two distinct regimes. At high polymer content, the system is dominated by the structure of the dense polymer, where polymer-polymer interactions, together with excluded volume effects, induce the existence of a preferred local arrangement resulting in a narrow distribution of the relaxation times, with the average value of the relaxation time following a simple Arrhenius temperature dependence. As the concentration decreases, the original structure of the hydrogen bonded polymer network is destroyed, and a number of different local configuration coexist, giving rise to a wider distribution of relaxation times or to a multiple relaxation. At low concentrations, the experimental data are well fitted assuming a Vogel-Fulker-Tammon behavior for the average relaxation time. In addition, the observed deviation from the ideal behavior for the refractive index and the density suggests that CCl4 does not behave as an inert solvent, and due to polarization effects, it can develop local hetero-associated structures via electrostatic interaction with the O-H end groups of the polymeric chains. The hypothesis has been successfully tested by fitting the concentration behavior of the hypersonic velocity to a recent three-component model, suitable to describe the concentration dependence of sound velocity in moderately interacting fluids. The indication of the model furnishes a very high value for the association constant of the PEG600, confirming the literature indication that, in polymeric systems capable of developing long liner aggregates via hydrogen bonding interaction, the Brillouin probe is insensitive to the true length of the polymeric chains. The Brillouin scattering experiment just sees an effective hydrogen bonded aggregate that is huge relative to the length of the single polymeric chain and becomes sensitive only to the density fluctuations of the local segmental motions.     

Further study of the viscoelastic phase separation of cyanate ester modified with poly(ether imide)

In this study, the viscoelastic phase separation process was studied further by time‐resolved light scattering, differential scanning calorimetry, and scanning electron microscopy in the system of poly(ether imide)‐modified bisphenol‐A dicyanate. It was observed that the evolution time of phase structure and relaxation time of diffusion flow of the bisphenol‐A dicyanate were similar with the phase diagram of curing conversion versus content of PEI. The results suggested that the viscoelastic phase separation was affected by the curing conversion of the system at the onset point of phase separation. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 517–523, 2006     

Heat-induced conformational transition of cytochrome c observed by temperature gradient gel electrophoresis at acidic pH

Temperature-gradient gel electrophoresis (TGGE) has been used to study the thermal unfolding of ferricytochrome c in low and high concentrations of acetic acid. It has been observed that the mobility of cytochrome c is a linear function of temperature when the system is characterized by a homogeneous population of conformation-state, single molecular species. Within the transition temperature range, the mobility clearly displays the characteristic sigmoidal shape describing the transitions of protein unfolding. The data obtained by TGGE were used to estimate the apparent thermodynamic parameters (enthalpy change deltaHvh and transition temperature Tm), associated with the transition of unfolding. The accuracy of the apparent thermodynamic parameters obtained by this method agrees within error limits with the values obtained by direct calorimetric measurements using differential scanning calorimetry (DSC).