Heat capacity measurement by modulated DSC at constant temperature

The mathematical equations for step-wise measurement of heat capacity (C _p ) by modulated differential scanning calorimetry (MDSC) are discussed for the conditions of negligible temperature gradients within sample and reference. Using a commercial MDSC, applications are evaluated and the limits explored. This new technique permits the determination ofC _p by keeping the sample continually close to equilibrium, a condition conventional DSC is unable to meet. Heat capacity is measured at ‘practically isothermal condition’ (often changing not more than ±1 K). The method provides data with good precision. The effects of sample mass, amplitude and frequency of temperature modulation were studied and methods for optimizing the instrument are proposed. The correction for the differences in sample and reference heating rates, needed for high-precision data by standard DSC, do not apply for this method. Presented in preliminary from at the 22nd NATAS Conference in Denver, CO 9/19-22/93 (Proceedings, pages 59–64, editor K. R. Williams).     

Glass transition temperature and thermal decomposition of cellulose powder

Cellulose powder and cellulose pellets obtained by pressing the microcrystalline powder were studied using differential scanning calorimetry (DSC), differential thermal analysis (DTA), and thermal gravimetry (TG). The TG method enabled the assessment of water content in the investigated samples. The glass phase transition in cellulose was studied using the DSC method, both in heating and cooling runs, in a wide temperature range from −100 to 180 °C. It is shown that the DSC cooling runs are more suitable for the glass phase transition visualisation than the heating runs. The discrepancy between glass phase transition temperature T _g found using DSC and predictions by Kaelbe’s approach are observed for “dry” (7 and 5.3% water content) cellulose. This could be explained by strong interactions between cellulose chains appearing when the water concentration decreases. The T _g measurements vs. moisture content may be used for cellulose crystallinity index determination.     

Calculation of smoothing factors for the comparison of DSC results

Complex mixtures of long chain organic compounds often show overlapping glass transition temperatures (T _gs) when analyzed by differential scanning calorimetry (DSC) or modulated DSC (MDSC). In such cases, subjective and inconsistent smoothing of data acquired under different conditions can lead to the misinterpretation of results. A quantitative method for the selection of smoothing factors for the analysis and comparison of (M)DSC results is presented. The method is most useful for the analysis of the derivative of the heat capacity, dC _p/dt or dC _p/dT, plots which best highlight overlapping T _gs. Four equations are shown to relate the heating rate and the smoothing factor. The equations allow a comparison of data acquired i) at different heating rates and plotted vs. temperature, ii) at a single heating rate and plotted vs. both time and temperature, i.e., dC _p/dt vs. dC _p/dT, iii) at different heating rates and plotted vs. both time and temperature, and iv) at different heating rates, and shown exclusively in the time domain. Examples of the use of the equations are provided for the analysis of bitumen, a complex mixture of natural origin.     

StepScan DSC

StepScan DSC technique was used for the study of the glass transition phenomenon. This method allows relatively good monitoring of reversing and non-reversing processes, and thus is very useful tool for glass transition studies of wide type of glass-forming materials, including inorganic glasses as well as organic polymers. In this work, experience with the StepScan DSC technique is summarized. Some interesting results of its application are presented, such as determination of glass transition temperature independent on thermal history of glass, discovered relationship between the slope of temperature dependence of C _p in T _g, and Angel’s index of fragility and estimation of viscosity glass transition temperature, T _g,η.     

Water sorption and glass transition of freeze-dried camu-camu (myrciaria dubia (H.B.K.) Mc Vaugh) pulp

Differential scanning calorimetry (DSC) was used to determine phase transitions of freeze-dried camu-camu pulp in a wide range of moisture content. Samples were equilibrated at 25°C over saturated salt solutions in order to obtain water activities (a_w) between 0.11–0.90. Samples with a_w>0.90 were obtained by direct water addition. At the low and intermediate moisture content range, Gordon–Taylor model was able to predict the plasticizing effect of water. In samples, with a_w>0.90, the glass transition curve exhibited a discontinuity and T^’_g was practically constant (–58.8°C), representing the glass transition temperature of the maximally concentrated phase(T_g ).     

A three-phase microstructural model to explain the mechanical relaxations of branched polyethylene: a DSC,WAXD and DMTA combined study

The thermal transitions of well-characterised single-site catalysed polyethylenes having various degrees of short chain branching have been studied by differential scanning calorimetry, X-ray diffraction and dynamic mechanical thermal analysis. A critical discussion based on the results obtained by means of the different techniques is presented. The results suggest that the γ transition is independent of the branching content and degree of crystallinity, pointing towards a sub-glass local relaxation mechanism related to both amorphous and crystalline fractions. The temperature of the β transition, T _β from dynamic mechanical measurements, is in agreement with the glass transition temperature obtained by calorimetry, T _g. Moreover, T _γ, and also T _β are directly related to a change in the thermal expansion coefficient of the amorphous phase observed by X-ray scattering. It is found that the corresponding scattering distance of the amorphous halo depends on crystallinity. In addition, the calorimetric heat capacity values at T _β do not account for the total amorphous fraction determined for each material. The relaxation motions assigned to the amorphous phase glass transition seems to parallel the subsequent melting of the crystalline structure, suggesting a hierarchical motion of different structures as temperature increases. Dynamic mechanical thermal analysis supports these observations, showing a broad transition in the phase angle involving first the relaxation of amorphous phase, then the (presumable) more rigid intermediate phase, and finally the crystalline phase, as the temperature increases.     

Glass transition measurements of asphalts by DSC

The glass transition temperature (T_g), measured by Modulated Differential Scanning Calorimetry (MDSC), is related to the flow characteristics of asphalt at low temperatures as is the rate of change of the creep stiffnessm. This study compared the glass transition temperature of different asphalts (neat, chemically modified, and crumb rubber modified asphalts) with the creep stiffness, the rate of change of creep stiffness, and the low specification temperature of the continuous PG grading of those asphalts.From the rheological data (BBR) and the thermal data (MDSC) for the virgin and the modified asphalts, the modified products had the same variations of values ofm, S, and T_g at lower temperatures as those of their corresponding virgin asphalts. A correlation between the T_g andm value was observed for both the modified and unmodified asphalts. Since DSC measurements for asphalt low temperature properties use less operator time, less sample, and have less measurement and operator error than rheological methods, T_g has promise to be considered as a fast and easy laboratory method to obtain the low temperature useful range of asphalts in pavements.The authors would like to acknowledge Susan P. Needham for her work in conducting rheological testing for the modified and unmodified asphalts.     

The rigid amorphous fraction in semicrystalline macromolecules

The first experimental evidence of the existence of the rigid amorphous fraction (RAF) was reported by Menczel and Wunderlich for several semicrystalline polymers. It was observed that the hysteresis peak at the glass transition was absent when these polymers were heated much faster than they had previously been cooled. In the glass transition behavior of poly(ethylene terephthalate) (PET), the hysteresis peak gradually disappeared as the crystallinity increased. At the same time, it was noted that the ΔC _p of higher crystallinity PET samples was much smaller than could be expected on the basis of the crystallinity calculated from the heat of fusion. It was also observed that this behavior was not unique to PET only, but is characteristic of most semicrystalline polymers: the sum of the crystallinity calculated from the heat of fusion and the amorphous content calculated from the ΔC _p at the glass transition is much less than 100% (a typical difference is ~20–30%). This 20–30% difference was attributed to the existence of the “RAF”. The presence of the RAF also affected the unfreezing behavior of the “mobile (or traditional) amorphous fraction.” As a consequence, the phenomenon of the enthalpy relaxation diminished with increasing rigid amorphous content. It was suggested that the disappearance of the enthalpy relaxation was caused by the disappearance or drastic decrease of the time dependence of the glass transition. To check the validity of this suggestion, the glass transition had to be also measured on cooling in order to overlay it on the DSC curves measured on heating. However, before this overlaying work could be accomplished, the exact temperatures on cooling had to be determined since the temperature of the DSC instruments that time could not be calibrated on cooling using the usual low molecular weight standards due to the common phenomenon of supercooling. Therefore, a temperature calibration method needed to be developed for cooling DSC experiments utilizing high purity liquid crystals using the isotropic → nematic, the isotropic → cholesteric, and other liquid crystal → liquid crystal transitions. After the cooling calibration was accomplished, the cooling glass transition experiments indicated that the glass transition in semicrystalline polymers is not completely time independent, because its width depends on the ramp rate. However, it was shown that the time dependence is drastically reduced, and the midpoint of the glass transition seems to be constant which can explain the absence of the enthalpy relaxation. The work presented here has led to a number of studies showing the universality of the rigid amorphous phase for semicrystalline polymers as well as an ASTM standard for DSC cooling calibration.     

Thermal Methods Applied to the Glass Transition of Mixed Network AlPO4–BPO4–SiO2 Glasses

Glass transition effect of mixed network AlPO₄–BPO₄–SiO₂ glasses was studied. DTA/DSC and TMA measurements has been applied in the research. It has been found that glass transition effect has structurally sensitive properties. Glass transition temperature T _g, changes of specific heat (Δc _p)and thermal expansion coefficient (α) accompanying the process depend on the nature and the number of components forming the glassy framework. Character of chemical bonds combining them into the glass structure has an influence on the glass transition effect. Its course is dependent on the flexibility of the structure of glasses. This revised version was published online in July 2006 with corrections to the Cover Date.     

Calorimetric and EPR studies of the thermotropic phase behavior of phospholipid membranes

Transmission electron micrographs (TEM) showed that liposome vesicles prepared from DL-α-phosphatidylcholine dimyristoyl (1,2-ditetradecanoyl-rac-glycerol-3-phosphocholine) (DMPC) by the modified reverse-phase evaporation method (mREV) were spherical in shape and in majority of them were less than 100 nm in diameter. Differential scanning calorimetry (DSC) method was used to determine the influence of cholesterol content and pH of Tris-HCl buffer used for the preparation of liposomes on the temperature of phase transition T _C of phospholipids which form the investigated liposome vesicles. The use of DSC method made it possible to determine not only the temperature of the main phase transition of phospholipids but also the temperature of the phospholipid phase transition from the tilted gel phase(L _β′) to the ripple gel phase(P _β′). The results were compared with those obtained with EPR study. EPR study was carried out in the temperature range from 284 to 310 K i.e. below and above the phase transition temperature T _C of DMPC. On the basis of EPR spectra of spin marker 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) incorporated into the liposome, the values of parameters f were determined. Hence TEMPO can be used to observe the change in partition between aqueous and fluid lipid regions. The change in the relative values of f determined for DMPC as a function of temperature shows that this phospholipid undergoes a transition from a ‘gel phase’ to a lamellar smectic liquid crystalline phase in the presence of excess water. The EPR study of TEMPO allowed us to determine the transition temperature T _C. The results were compared with those obtained with DSC method.     

Thermally stimulated current and DSC studies of the broadened glass transition in liquid crystalline polymers

The high sensitivity of the thermally stimulated current, thermal sampling (TS) method is emphasized in a study of the breadth of the glass transition in several liquid-crystalline polymers (LCPs). Differential scanning calorimetry (DSC) was performed on all samples to further quantify the glass transition regions. For “random” copolyester LCPs with widely varying degrees of crystallinity, including highly amorphous samples, very broad glass tran-sition regions were observed. One semicrystalline alternating copolyester and a series of semicrystalline azomethine LCPs were studied as examples of structurally regular polymers. These exhibited relatively sharp glass transitions more comparable to ordinary isotropic amorphous or semicrystalline polymers. The broad glass transitions in the random copolyesters are attributed to structural heterogeneity of the chains. In one example of a moderate-crystallinity random copolyester LCP (Vectra), glass transitions ranging up to ca. 150°C in breadth were determined by the thermal sampling (TS) method and DSC. In other lower crystallinity copolyester LCPs, the main glass transition temperature as determined by DSC was comparable to that determined by TSC although cooperative relaxations of a minor fraction of the overall relaxing species were detected well below the main T_g, by the TS method and not by DSC. Rapid quenches from the isotropic melt to an isotropic glass were possible with one LCP. The anisotropic and isotropic glassy states for this LCP were found to have the same breadth of the glass transition as was determined by the TS method, although TSC and DSC show that T_g is shifted downward by ca. 15°C in the anisotropic glass as compared to the isotropic glass. © 1993 John Wiley & Sons, Inc.     

The effect of crystallinity and crosslinking on the depression of the glass transition temperature in nylon 6 by water

The influence of crystallinity and crosslinking on the depression of the glass transition temperature in nylon 6 by water has been investigated by dynamic mechanical methods. Radiation crosslinking by high-energy electrons was effective in preventing morphological changes during the measurement of the incremental change in heat capacity (ΔC_p) at T_g, which was performed by differential scanning calorimetry. The experimentally determined value of ΔC_p, when normalized to account for the crystalline phase, was found to deviate from a linear two-phase relation and was reduced further than would be expected based on this model. It is proposed that nylon 6 is best described by a three-phase model which consists of a crystalline domain, a wholly amorphous domain, and an “intercrystalline” region. The importance of this in explaining the relatively large depression of T_g by small quantities of water is illustrated by applying equations derived to account for the compositional dependence of T_g in polymerdiluent mixtures, based on a classical thermodynamic interpretation of the glass transition phenomenon.     

DSC analysis of human fat tissue in steroid induced osteonecrosis

Osteonecrosis (ON) of the femoral frequently occurs after steroid medication. One of the final pathways leading to steroid induced ON is thought to be pathologic fat metabolism. The pathobiological mechanism underlying the induction of fat metabolism outslides by steroids leading to ON has not been fully elucidated. The purpose of this study was to examine the intraoperative obtained gluteal fat tissue from ON patients with histology, gas chromatography (GC) and differential scanning calorimetry (DSC) and to compare them with otherwise healthy patient’s samples. The histological sections showed no significant differences compared with the control group. GC revealed that fraction of saturated fatty acids decreased in ON samples from mean values of controls of 24% to 21, the polyunsaturated fraction from 20 to 14%. The monounsaturated acids showed an increase from mean rate of 52% of the controls to 65% of steroid treated samples. DSC curves correlate with chromatographic analysis of the tissue fatty acids (Steroid treated, heating between 0–100°C: T _m=5.7°C, ΔH= −15.8J/g⁻¹; heating between −20–100°C: Tm= −9.96 and 5.85°C, ΔH= −59.17 and −16.2 J g⁻¹. Non-necrotic, heating between 0–100°C: two separable transition with Tm=5.7 and 9.9°C, total ΔH= −20.8 J g⁻¹; heating between −20–100°C: Tm= −10.9 and 4.95°C, total ΔH= −75.8 J g⁻¹.) Our preliminary findings are rather tendentious. Further investigations are needed with higher sample rate and under other anamnestic circumstances too.     

The utility of phase correction in modulated DSC

Traditionally, Modulated DSC (MDSC^®) [1] has been used to simultaneously measure the heat capacity and heat flow of a sample in a single experiment. As first proposed by Readinget at. in 1992 [2], this complex heat capacity signal (C_p ^*) can be further deconvoluted into components which are in-phase (C_p) or out-of phase (C_p with the imposed temperature modulation. The vector sum of these components, respectively termed the reversing C_p and kinetic C_p, is equal to the aforementioned complex C_p (C_p ^*).Recent research has centered around the analysis of these signals and their inclusion into MDSC experiments. For most polymer systems, the contribution of the kinetic C_p is negligible, except at the melt. This signal does contain a small peak at the T_g of PET, but the significance of this peak is to date not clear. Examples of further applications will be presented and discussed, as well as the derivation and interpretation of novel MDSC signals.     

Thermal analysis and thermochemistry of vitreous into crystalline state transition

Thermochemistry and structural mechanism of crystallization of MgO-Al₂O₃-SiO₂ glasses with TiO₂ as crystallization activator were studied. Thermal and HREM investigation proved that near the T _g temperature crystallization is going by rearrangement of glass structure elements and part of its components redistribution like at disorder — order phase transition in solid bodies. Nanocrystals of Mg-titanate and high quartz structure solid solution are formed then. Next enstatite and cordierite are crystallizing. Thermochemical and chemical bonds strength analysis indicate that during multistage crystallization of glasses, kind and order of crystal phase formation, is determined by the glass structure decomposition progress and its particular components release accompanying increase of temperature. It has been proved that molar heat capacity change (ΔC _p) accompanying the glass transition is the significant measure of degree of changes in the structure of glass preceding crystallization. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal behaviour of Fe-doped silicate–phosphate glasses

Thermal behaviour and structure of glasses from the SiO₂–P₂O₅–K₂O–MgO–CaO system modified by Fe₂O₃ addition were studied by DSC, XRD and FTIR methods. It has been found that the replacement of MgO and CaO modifiers by Fe₂O₃ in the structural network of silicate–phosphate glass results in decrease of the glass transition temperature (T _g) and heat capacity change (ΔC _p) accompanying the glass transformation. Simultaneously, the ability for crystallization, its course and the type of the forming phases depend on the relative proportions between iron and phosphorus as components forming the silicate–phosphate structure. The type of the crystal phases forming in the course of heating the considered glass has been found to be in agreement with the character of the domains occurring in this glass, confirmed by FTIR examinations.     

Quantitative DSC evaluation of phase segregation rate in linear segmented polyurethanes and polyurethaneureas

Linearly segmented polyurethanes and polyurethaneureas are known to be phase separated; one phase is the soft phase, the other, the hard domain. Quantitative evaluation of the degree of phase separation was obtained by DSC. An estimation of the dispersed soft segments outside the soft phase can be made by ΔC_p measurements and the increase in glass transition (T_g) can be attributed to the presence of hard blocks in the soft phase. The segregation rate depends on the nature of the soft segments which decrease from hydrogenated polybutadiene to polybutadiene to polyether. It also depends on the nature of the chain extender from aliphatic to aromatic diol and from aliphatic to aromatic diamine. The presence of soft segments in hard domains can be related to the thermoplastic behavior of the polyurethanes and polyurethaneureas.     

Rigid amorphous fraction in poly(ethylene terephthalate) determined by dilatometry

Volumetric thermal analysis of semicrystalline poly(ethylene terephthalate), PET, with different content of crystalline phase was carried out using mercury-in-glass dilatometry. The effect of crystals on the thermal properties of amorphous phase (glass transition temperature, T _g, thermal expansion coefficients, α) were determined. At cold-crystallization (106°C, up to 4 h), crystalline content of 2.4–25.3 vol.% was achieved. Increasing content of crystalline phase broadens the glass transition region and increases T _g. The change of thermal expansion coefficient during glass transition is lower than that predicted by the two-phase model, which indicates the presence of a third fraction — rigid amorphous fraction (RAF), whose content steadily increases during crystallization. However, its relative portion (specific RAF) is significantly reduced. Further significant decrease in specific RAF appears after annealing at a higher temperature.     

How did we find the rigid amorphous phase?

The first experimental evidence of the existence of the rigid amorphous phase was reported by Menczel and Wunderlich [1]: when trying to clarify the glass transition characteristics of the first main chain liquid crystalline polymers [poly(ethylene terephthalate-co-p-oxybenzoate) with 60 and 80 mol% ethylene terephthalate units] [2], the absence of the hysteresis peak at the lower temperature glass transition became evident when the sample of this copolymer was heated much faster than it had previously been cooled. Since this glass transition involved the ethylene terephthalate-rich segments of the copolymer, we searched for the source of the absence of the hysteresis peak in PET. There, the gradual disappearance of the hysteresis peak with increasing crystallinity was confirmed [1]. At the same time it was noted that the higher crystallinity samples showed a much smaller ΔC _p than could be expected on the basis of the crystallinity calculated from the heat of fusion (provided that the crystallinity concept works). Later it was confirmed that the hysteresis peak is also missing at the glass transition of nematic glasses of polymers. When checking other semicrystalline polymers, the sum of the amorphous content calculated from the ΔC _p at the glass transition, and the crystallinity calculated from the heat of fusion was far from 100% for a number of semicrystalline polymers. For most of these polymers, the sum of the amorphous content and the crystalline fraction was 0.7, meaning that ca. 30% rigid amorphous fraction was present in these samples after a cooling at 0.5 K min⁻¹ rate. Thus, the presence of the rigid amorphous phase was confirmed in five semicrystalline polymers: PET, Nylon 6, PVF, Nylon 66 and polycaprolactone [1]. Somewhat later poly(butylene terephthalate) and bisphenol-A polycarbonate [3] were added to this list.