Thermoreversible gelation of isotactic poly(methyl methacrylate) in butyl acetate: a differential scanning calorimetry study

The thermal behaviour of thermoreversible gels prepared well below room temperature from isotactic poly(methyl methacrylate) (i-PMMA) in butyl acetate (BAC) was investigated by means of differential scanning calorimetry. Gel-formation and gel-melting enthalpies together with melting enthalpy of the solvent were determined as functions of polymer concentration. The results are consistent with the existence of a polymer/solvent complex with a stoichiometry of 1,5–2 BAC molecules per i-PMMA monomeric unit.     

Starch-lipid interactions studied by differential scanning calorimetry

The amylose-lipid complex shows an endothermic transition around 100 °C in excess water. Complexes were prepared by adding lipids to an amylose-solution, and the precipitated complex was studied in the DSC during a heating and cooling sequence. The thermal stability of the complex depends on the lipid part, and the reversibility during cooling depends on presence of excess lipids.The influence of lipids on the gelatinization of starch was studied by adding lipids to wheat and potato starch, respectively, before the DSC-analysis. Depending on the lipid, an earlier as well as a delayed gelatinisation could be obtained.     

Thermoreversible gelation and phase separation in aqueous methyl cellulose solutions

We derived typical phase diagrams for aqueous solutions of methyl cellulose (MC) of different molecular weights via micro‐differential scanning calorimetry, small‐angle X‐ray scattering, and visual inspection. The phase diagrams showed the cooccurrence of gelation and phase separation and qualitatively agreed with the theoretically calculated diagrams. The sol–gel transition line and phase separation line of a lower critical solution point type shifted toward lower temperatures and lower concentrations with an increase in the MC molecular weight. The sol–gel transition line intersected at a temperature higher than the critical point of the phase separation; therefore, both sol–gel phase separation and gel–gel phase separation were possible, depending on the temperature. Specifically, through visual inspection of a high molecular weight MC sample in the critical temperature region, we observed phase separation into two coexisting gels with different polymer concentrations. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 91–100, 2001     

Thermal behaviour of emulsions studied by differential scanning calorimetry

This article is a review about the ways in which solidification and the melting may occur within emulsions submitted to steady cooling and heating performed in a differential scanning calorimeter. Simple, multiple and mixed emulsions are considered. Due to nucleation phenomena creating supercooled and supersaturated liquids, the DSC curves obtained during cooling and heating are quite different. The influence of a solute in the disperse phase is described in detail. Some implications about the instabilities of emulsions due to mass transfer phenomena are described.     

Interactions between poloxamers in aqueous solutions: micellization and gelation studied by differential scanning calorimetry, small angle X-ray scattering, and rheology

Poloxamers F88 (EO97PO39EO97) and P85 (EO27PO39EO27) are triblock copolymers of ethylene oxide (EO) and propylene oxide (PO), which have the same hydrophobic PO block. We studied aqueous solutions of these two copolymers by the conjoint use of differential scanning calorimetry (DSC), rheology, and small-angle X-ray scattering (SAXS). The results showed that the temperature-induced micellization of aqueous solutions of F88 and P85 was a progressive process followed by gelation for sufficiently concentrated samples. Gelation was due to the ordered packing of micelles under a hexagonal compact (HC) structure for P85 and a body-centered cubic (BCC) phase for F88. Importantly, the phase diagram of F88/P85 mixtures in water was elucidated and showed the destabilization of the HC phase upon addition of small amounts of F88.     

Hydration of thermally denatured lysozyme studied by sorption calorimetry and differential scanning calorimetry

We have studied hydration (and dehydration) of thermally denatured hen egg lysozyme using sorption calorimetry. Two different procedures of thermal denaturation of lysozyme were used. In the first procedure the protein was denatured in an aqueous solution at 90 degrees C, in the other procedure a sample that contained 20% of water was denatured at 150 degrees C. The protein denatured at 90 degrees C showed very similar sorption behavior to that of the native protein. The lysozyme samples denatured at 150 degrees C were studied at several temperatures in the range of 25-60 degrees C. In the beginning of sorption, the sorption isotherms of native and denatured lysozyme are almost identical. At higher water contents, however, the denatured lysozyme can absorb a greater amount of water than the native protein due to the larger number of available sorption sites. Desorption experiments did not reveal a pronounced hysteresis in the sorption isotherm of denatured lysozyme (such hysteresis is typical for native lysozyme). Despite the unfolded structure, the denatured lysozyme binds less water than does the native lysozyme in the desorption experiments at water contents up to 34 wt %. Glass transitions in the denatured lysozyme were observed using both differential scanning calorimetry and sorption calorimetry. Partial molar enthalpy of mixing of water in the glassy state is strongly exothermic, which gives rise to a positive temperature dependence of the water activity. The changes of the free energy of the protein induced by the hydration stabilize the denatured form of lysozyme with respect to the native form.     

Thermal stability of whey proteins studied by differential scanning calorimetry

The thermal stability of the bovine whey proteins.; β-lactoglobulin (β-1g), α-lactalbumin (α-1a) and serum albumin (BSA) was studied individually and in mixtures in the temperature range 25–140°C by differential scanning calorimetry. The thermal denaturation temperature (T_D) and the transition enthalpies (ΔH_app) were determined at different pH-values (3.0–10.0) in simulated milk ultrafil-trate (SMUF).β-Lg was, except at pH 9.0 and 10.0, the most thermostable protein at all pH-values. At acidic pH-values BSA was the least thermostable. At alkaline pH-values, however, α-la had lower thermal stability than BSA. α-La exhibited double peak behaviour at acidic pH-values and ΔH_app was dependent on Ca-content. Mixtures of the proteins were studied at pH 4.0, 5.0 and 6.6. In general, when mixed, the proteins seemed to denaturate independently of each other.     

Phase transformations in sodium molybdate studied by differential scanning calorimetry

Differential scanning calorimetry (DSC) was used to study the thermal behavior of sodium molybdate in the range of elevated temperatures. Parameters (temperatures and enthalpies) of phase transformations in Na₂MoO₄ were determined.     

Study of gelation using differential scanning calorimetry (DSC)

A new method for determining the degree of conversion of gelation (_gel) and gel time (t _gel) at gel point using a single technology, DSC, is discussed in this work. Four kinds of thermoset resins are evaluated. It is found that the mutation points of reduced reaction rate (V _r ) vs. reaction conversion () curves, corresponding with the changes of reaction mechanism, represents the gelation of the reaction. The at the mutation point is defined as _gel. From isothermal DSC curves, the point at _gel is defined ast _gel. Traditional techniques (ASTM D3532 and DSC method) are also used to determine _gel andt _gel in order to demonstrate this new method. We have found that the results obtained from this new method are very consistent with the results obtained from traditional methods.     

Demixing and gelation behavior of ternary cellulose acetate solutions

The demixing behavior on cooling of ternary systems of cellulose acetate/solvent/water has been examined for CA concentrations up to 40 wt% CA in several solvents. Cloud points have been measured as a function of cooling rate. The rapid process of liquid—liquid demixing can be discriminated from the slow process of aggregate formation by examining the dependence of the cloud point on the cooling rate and by structure analysis of quenched solutions with scanning electron microscopy. The appearance of aggregate formation depends strongly on the type of solvent. Slow cooling of ternary solutions in which acetone is the solvent leads to aggregate formation long before liquid—liquid demixing occurs. In addition, isothermal sol—gel transitions have been measured for quenched solutions at varying gelation times. It is concluded that gelation is not always preceded by aggregate formation.     

Thermoreversible gelation of solutions of vinyl polymers

The thermoreversible gelation of solutions of isotactic poly(methyl methacrylate) is investigated. Amorphous gels can be prepared in l-butanol by a combination of a liquid-liquid demixing with an upper critical demixing temperature and a glass transition. Annealing of the demixed solutions above their glass transition temperature T_G, results in the formation of a crystalline gel. In oxylene, crystalline gels are formed by a liquid-liquid demixing with an lower critical demixing temperature and an annealing at room temperature. Very fast gelation is observed to occur far below room temperature in solvents like 2-butanone.     

Water transfer in mixed water-in-oil emulsions studied by differential scanning calorimetry

Water transfers are observed within complex systems containing aqueous phases separated by a membrane or an oil phase, such as biological cells or multiple emulsions. In order to better understand water transfer mechanism, a system made of a mixed water-in-oil (W/O) emulsion containing two kinds of aqueous droplets — pure water and a 30 % urea solution — was developed. Water transfer from pure water droplets to urea solution droplets was evidenced by Differential Scanning Calorimetry (DSC). Finally the mixed emulsion contains one kind of droplets made of a diluted urea solution which composition is in agreement with formulation and data obtained from experiments performed on single W/O emusions which dispersed phase is a diluted urea solution of the same composition. These mixed emulsions have been pictured as a three-fluid phases system containing two aqueous phases separated by a plane oil membrane. From a homogeneous solubility-diffusion model applied to a quasi-stationnary regime, the water intra-diffusion coefficient has been obtained and compared to the value calculated from the Stokes-Einstein equation. A factor ten makes the discrepancy between the two values, the value deduced from the model being the highest. A possible influence of the emulsifier molecules has been evoked.     

The thermal decomposition of some primary explosives as studied by differential scanning calorimetry

Most currently used stab- and percussion-sensitive mixes consist of lead azide, lead styphnate or lead dinitroresorcinate in admixture with 2–5% tetrazene, which functions as an energetic sensitizer. A major problem with the use of such compositions is the poor thermal stability of tetrazene, depletion of which leads to loss of activity. The thermal decomposition of some 24 compounds, all identified as potential replacements for tetrazene in stab and percussion mixes, has been studied by non-isothermal differential scanning calorimetry, and the physicochemical parameters which characterise these explosive reactions have been determined and compared with data from temperature of ignition measurements.     

Thermokinetics of the ethyl acetate acid and alkaline hydrolysis by differential scanning calorimetry

The kinetic investigation of thermoanalytical curves for the ethyl acetate and alkaline hydrolysis, requires tedious calculations of several mathematical expressions, to establish the correlation between the thermochemical and kinetic parameters. Using the DSC measured data for the heat flow-temperature dependence of the studied reactions, we have obtained the kinetic bulk parameters and the dependencek=k(T) for the basic and acid hydrolysis of ethyl acetate.     

Thermal denaturation ofTrichoderma reesei cellulases studied by differential scanning calorimetry and tryptophan fluorescence

The thermal denaturation of four purified Trichoderma reesei cellulase components, cellobiohydrolase (CBH) I, CBH II, endoglucanase (EG) I, and EG II, has been monitored using a combination of classical temperature/activity profiles, differential scanning calorimetry (DSC), and thermal scanning fluorescence emission spectrometry. Significant correlations were found between the results of enzyme activity studies and the results obtained through the more direct physical approaches, in that both DSC and the activity studies showed EG II (T_m = 75°C) to be much more thermostable (by 10–11 °C) than the other three enzymes, all three of which were shown by both activity profiles and DSC to be very similar in thermal stability. The temperature dependence of the wavelength of maximum tryptophan emission showed a parallel result, with the three enzymes exhibiting less thermostable activity being grouped together in this regard, and EG II differing from the other three in maintaining a less-exposed tryptophan microenvironment at temperatures as high as 73 °C. The DSC results suggested that at least two transitions are involved in the unfolding of each of the cellulase components, the first (lower-temperature) of which may be the one correlated with activity loss.

     

Purity determination by differential scanning calorimetry

A review of the literature on the DSC method for purity determination is presented, with a discussion of the most important aspects, i.e. theory, sample handling, calibration of the instrument, evaluation of melting curves, and the conditions and accuracy of the measurement of eutectic impurities.     

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     

Influence of different initiators on methyl methacrylate polymerization,studied by differential scanning calorimetry

Differential scanning calorimetry (DSC) was used to measure the decomposition rates of four commercially used initiators, 2,2′-azobis(isobutyronitrile) 2,2′-azobis(2,4-dimethylvaleronitrile), dilauroyl peroxide and bis(4-t-butylcyclohexyl)peroxydicarbonate, in dynamic mode, while the courses of methyl methacrylate polymerization with the listed initiators at 65, 75 and 85°C were measured isothermally. From the DSC curves, the polymerization enthalpies, the overall reaction rate constants and the activation energies for the initial steady-state polymerization were calculated. It was found that the polymerization enthalpy and the kinetic parameters depended on the type of the initiator. An initiator with a shorter decomposition half-lifetime shifted the onset of the gel effect to a higher conversion, intensified it and decreased the average molar mass of the polymer.     

Modulated differential scanning calorimetry

Modulated-temperature differential scanning calorimetry was used to measure the glass transition temperature,T _g, the heat capacity relaxation in the glassy state and the increment of heat capacity, C_p, in the glass transition region for several polymers. The differential of heat capacity with respect to temperature was used to analyseT _g and C_p simply and accurately. These measurements are not affected by complex thermal histories.