Quasi-isothermal Temperature-modulated Differential Scanning Calorimetry Research Articles

Heat capacity and index of refraction of polyzwitterions

Specific heat capacities and indices of refraction were measured for a series of six sulfobetaine zwitterionic polymers and compared to topological and group contribution models. Poly(sulfobetaine methacrylate) (PSBMA), poly(sulfobetaine acrylate) (PSBA), poly(ethyl sulfobetaine methacrylate) (PESBMA), poly(sulfobetaine methacrylamide) (PSBMAm), poly(sulfobetaine 2-vinylpyridine) (PSB2VP) and poly(sulfobetaine 4-vinylpyridine) (PSB4VP) were specially synthesized with changes of chemical structure in order to study effects of either the polymer backbone or the side group on their properties. Temperature modulated differential scanning calorimetry (TMDSC) as well as quasi-isothermal TMDSC were used to determine the temperature dependent specific heat capacity, cp(T) from 30 to 200 °C. The solid state specific heat capacities were measured for all the polyzwitterions, while liquid state heat capacity measurements were limited to PSBA and PESBMA. Ellipsometry at wavelengths from 300 nm to 3000 nm was used to measure the index of refraction for polyzwitterion thin films at room temperature. Theoretical room temperature values of cp and n were calculated for the polyzwitterions using topological and group contribution methods and compared to the measured values. The measured index of refraction values agree well with predictions for the polyzwitterions. However, for the solid state heat capacity, the model produces accurate prediction only for PSB2VP and PSB4VP. The discrepancy between the measured and predicted cp values is most likely due to the reduction of vibrational degrees of motion in the solid state for some of the polyzwitterions due to physical crosslinking.

The Narrow Thickness Distribution of Lamellae of Poly(butylene succinate) Formed at Low Melt Supercooling

Crystallization of poly(butylene succinate) (PBS) at 100 °C, about 30 K below the equilibrium melting temperature, allowed the preparation of crystals, which were analyzed regarding their zero-entropy-production melting temperature. Irreversible melting occurs in a rather narrow temperature window of only around 8 K, between 101 and 109 °C, revealing a narrow distribution of the thickness of isothermally formed lamellae and a rather low thickening/stabilization factor of less than 1.4. Quasi-isothermal temperature-modulated differential scanning calorimetry suggests significant reversible melting and crystallization during and after crystallization, proving the existence of a large fraction of crystalline phase being at the stability limit at the crystallization temperature. Heating of crystals formed at 100 °C to above their zero-entropy-production melting temperature, followed by isothermal annealing, permitted the analysis of the kinetics of irreversible melting, yielding superheating-dependent rate constants of the order of magnitude of 10² s–¹ 5–10 K above the zero-entropy-production melting temperature. The advanced analysis of the melting behavior of polymer crystals isothermally grown at low melt supercooling allows one to draw conclusions about their (inherently) low thermodynamic stability.

Specific heat of metal oxide nanofluids at high concentrations for heat transfer

In this work isobaric specific heat capacity measurements were performed for five different nanofluid sets designed as dispersions up to 15% mass concentrations of MgO, ZnO and ZrO2 in pure ethylene glycol as well as ZnO and ZrO2 in an ethylene glycol+water mixture at 50:50% in volume. Isobaric specific heat capacities for the three dry metallic oxide nanopowders and the two base fluids were also studied with the aim of contributing to elucidate on the heat capacity behavior of nanofluids for heat transfer applications. Measurements were carried out by using a quasi-isothermal Temperature-Modulated Differential Scanning Calorimetry (TMDSC) method in the temperature range from 243.15 to 473.15K. Base fluid results were compared with literature data to validate the measurement procedure. The specific heats for nano-sized materials here presented were also compared with literature data for bulk-sized of the same metallic oxides. In addition, the experimental values were used to evaluate the goodness of different models existing in literature to predict or correlate the specific heat capacities. A new fitting equation was also proposed to correlate specific heats of nanofluids as a function of the base fluid, nanoparticles as well as nanoparticle concentration.

(Solid + liquid) phase equilibria and heat capacity of (diphenyl ether + biphenyl) mixtures used as thermal energy storage materials

The (solid+liquid) phase equilibrium for eight {x diphenyl ether+(1−x) biphenyl} binary mixtures, including the eutectic mixture were studied by using a differential scanning calorimetry (DSC) technique. A good agreement was found between previous literature and experimental values here presented for the melting point and enthalpy of fusion of pure compounds. The well-known equations for Wilson and the non-random two-liquid (NRTL) were used to correlate experimental solid liquid phase equilibrium data. Moreover, the predictive mixture model UNIFAC has been employed to describe the phase diagram. With the aim to check this equipment to measure heat capacities in the quasi-isothermal Temperature-Modulated Differential Scanning Calorimetry method (TMDSC), four fluids of well-known heat capacity such as toluene, n-decane, cyclohexane and water were also studied in the liquid phase at temperatures ranging from (273.15 to 373.15)K. A good agreement with literature values was found for those fluids of pure diphenyl ether and biphenyl. Additionally, the specific isobaric heat capacities of diphenyl ether and biphenyl binary mixtures in the liquid phase up to T=373.15K were measured.

Apparent heat capacity measurements and thermodynamic functions of d(−)-fructose by standard and temperature-modulated calorimetry

The qualitative and quantitative thermal analyses of crystalline and amorphous d(−)-fructose were studied utilising methods of standard differential scanning calorimetry (DSC), quasi-isothermal temperature-modulated differential scanning calorimetry (quasi-isothermal TMDSC), and thermogravimetric analysis (TGA). Advanced thermal analysis of fructose was performed based on heat capacity. The apparent total and apparent reversing heat capacities, as well as phase transition parameters were examined on heating and cooling. The melting temperature, Tm, of crystalline d(−)-fructose shows a heating rate dependency, which increases with raising the heating rate and leads to superheating. The equilibrium melting temperatures: Tm∘(onset)=370K and Tm∘(peak)=372K, and the equilibrium enthalpy of fusion ΔfusH°=30.30kJ·mol−1, of crystalline d(−)-fructose were estimated on heating for the results at zero heating rate. Anomalies in the heat capacity in the liquid state of d(−)-fructose, assigned as possible tautomerisation equilibrium, were analysed by DSC and quasi-isothermal TMDSC, both on heating and cooling. Thermal stability of crystals in the region of the melting temperature was examined by TGA and quasi-isothermal TMDSC. Melting, mutarotation, and degradation processes occur simultaneously and there are differences in values of the liquid heat capacity of d(−)-fructose with varied thermal history, measured by quasi-isothermal TMDSC. Annealing of amorphous d(−)-fructose between the glass transition temperature, Tg, and the melting temperature, Tm, also leads to crystallization of the sample and shows changes in the total apparent heat capacity. The experimental, apparent heat capacity of fully crystalline and fully or partially amorphous d(−)-fructose was analysed with reference to the solid (vibrational) and liquid heat capacities based on the ATHAS scheme.

Melting, glass transition, and apparent heat capacity of α- d-glucose by thermal analysis

The thermal behaviors of α- d-glucose in the melting and glass transition regions were examined utilizing the calorimetric methods of standard differential scanning calorimetry (DSC), standard temperature-modulated differential scanning calorimetry (TMDSC), quasi-isothermal temperature-modulated differential scanning calorimetry (quasi-TMDSC), and thermogravimetric analysis (TGA). The quantitative thermal analyses of experimental data of crystalline and amorphous α- d-glucose were performed based on heat capacities. The total, apparent and reversing heat capacities, and phase transitions were evaluated on heating and cooling. The melting temperature ( T m) of a crystalline carbohydrate such as α- d-glucose, shows a heating rate dependence, with the melting peak shifted to lower temperature for a lower heating rate, and with superheating of around 25 K. The superheating of crystalline α- d-glucose is observed as shifting the melting peak for higher heating rates, above the equilibrium melting temperature due to of the slow melting process. The equilibrium melting temperature and heat of fusion of crystalline α- d-glucose were estimated. Changes of reversing heat capacity evaluated by TMDSC at glass transition ( T g) of amorphous and melting process at T m of fully crystalline α- d-glucose are similar. In both, the amorphous and crystalline phases, the same origin of heat capacity changes, in the T g and T m area, are attributable to molecular rotational motion. Degradation occurs simultaneously with the melting process of the crystalline phase. The stability of crystalline α- d-glucose was examined by TGA and TMDSC in the melting region, with the degradation shown to be resulting from changes of mass with temperature and time. The experimental heat capacities of fully crystalline and amorphous α- d-glucose were analyzed in reference to the solid, vibrational, and liquid heat capacities, which were approximated based on the ATHAS scheme and Data Bank.

Quasi-isothermal temperature-modulated differential scanning calorimetry (TMDSC) for the separation of reversible and irreversible thermodynamic changes in glass transition and melting ranges of flexible macromolecules

Abstract With standard differential scanning calorimetry (DSC), it is possible to derive calorimetric data for equilibrium or metastable samples. The introduction of temperature-modulated DSC (TMDSC) permits in its quasi-isothermal (non-scanning) mode (TMDC), long-time apparent heat capacity measurements of high precision (±1 %). For flexible molecules, heat capacity measurements from the various calorimetric methods could be combined in the ATHAS Data Bank, which now contains experimental data for over 200 materials. These data were linked to the vibrational and large-amplitude motion of the constituent atoms and molecules, to provide a base for the judgement of the thermal analyses, extending outside the range of equilibrium or metastability with an error of only 2-5 %. The TMDC together with DSC is now able to quantitatively assess the reversibility of thermal processes. A sufficient number of systems have been analyzed in this fashion to develop better understanding of macro-, micro-, and nanophases of flexible macromolecules. The new concepts discussed are: (1) multiple glass transitions due to possible rigid-amorphous fractions (RAFs) and glass transitions within crystals, both observed in semicrystalline macromolecules, and (2) locally reversibly melting on the surface of chain-folded crystals. The locally reversible melting decreases with crystal perfection and also disappears when the chains become rigid.

Calorimetric Evidence for Two Distinct Molecular Packing Arrangements in Stable Glasses of Indomethacin

Indomethacin glasses of varying stabilities were prepared by physical vapor deposition onto substrates at 265 K. Enthalpy relaxation and the mobility onset temperature were assessed with differential scanning calorimetry (DSC). Quasi-isothermal temperature-modulated DSC was used to measure the reversing heat capacity during annealing above the glass transition temperature Tg. At deposition rates near 8 A/s, scanning DSC shows two enthalpy relaxation peaks and quasi-isothermal DSC shows a two-step change in the reversing heat capacity. We attribute these features to two distinct local packing structures in the vapor-deposited glass, and this interpretation is supported by the strong correlation between the two calorimetric signatures of the glass to liquid transformation. At lower deposition rates, a larger fraction of the sample is prepared in the more stable local packing. The transformation of the vapor-deposited glasses into the supercooled liquid above Tg is exceedingly slow, as much as 4500 times slower than the structural relaxation time of the liquid.

Effect of water on the thermal properties of silk fibroin

Silk fibroin films cast from water solution, and containing bound water, are quantitatively studied in this work. First, to obtain the solid and liquid heat capacities of the pure dry silk fibroin, cyclic heat treatment was used to monitor the process of removing the bound water. After water removal, the glass transition of pure non-crystalline silk was observed at 451 K (178 °C). The solid and liquid heat capacities of the pure silk fibroin were then measured using differential scanning calorimetry (DSC), temperature-modulated DSC (TMDSC), and quasi-isothermal TMDSC, and found to be: C p ( T) solid = 0.134 + 3.696 × 10 −3 T J/g K and C p ( T) liquid = 0.710 + 3.47 × 10 −3 T J/g K over the temperature region from 200 to 450 K. These heat capacities were used to construct the underlying baseline heat capacity for the combined silk–water system. When the combined silk–water system is studied, bound water is lost from the film during heating, and the loss of mass is quantified using thermogravimetric analysis (TGA). Bound water in the silk film acts as a plasticizer, and a lower glass transition of the silk–water system is observed. Comparison of the measured heat capacity of the silk–water system to the calculated total baselines was made in the vicinity of the water-induced glass transition. Results show that the total solid specific heat capacity is in good agreement with the calculated solid baseline in the low-temperature region below about 240 K. As temperature increases above the lower glass transition, all bound water eventually leaves the silk, and the free volume and the silk mobility are reduced. This allows the upper glass transition of the dried silk to be observed.

The heat capacity of polyethylene fibers measured by multi-frequency temperature-modulated calorimetry

The apparent heat capacity of polyethylene fibers in the melting region was measured by quasi-isothermal, temperature-modulated differential scanning calorimetry (TMDSC) and compared with results from standard differential scanning calorimetry (DSC) and the solid and liquid thermodynamic heat capacity as references from the ATHAS Data Bank. Using a multi-frequency, complex sawtooth modulation in the quasi-isothermal mode disclosed for the first time that the uncorrected apparent heat capacity Cp=AΦ/(ATsω) of the liquid polyethylene fiber increases with increasing frequency (AΦ is the differential heat-flow rate and ATs is the sample temperature). The frequency-dependent heat capacity cannot be represented by the expression: Cp=AΦ/(ATsνω)[1+(τνω)2]0.5 because of a negative τ2. The results were later confirmed by independent measurements on single sinusoidal quasi-isothermal TMDSC on the same material. The error is caused by shrinking of the fiber, which deforms the sample pan.

Evaluation of the reversible contribution to the reversing heat capacity in isotactic polystyrene

We report the results of time dependent analysis of the reversing heat capacity peaks in isotactic polystyrene (iPS). Solution grown crystals (SGCs) and bulk films of iPS were examined using quasi-isothermal temperature modulated differential scanning calorimetry (QI-TMDSC). Both types of iPS samples showed two reversing heat capacity peaks as reported before [H. Xu, P. Cebe, Macromolecules 37 (2004) 2797; H. Xu, P. Cebe, Macromolecules 38 (2005) 770], and the two peaks are associated with melting of different portions of crystal lamellae with different degree of thermal stability [T. Liu, J. Petermann, Polymer 42 (2001) 6453]. Here, the behavior of the reversing heat capacity is studied as a function of holding time from the establishment of steady state, until the end of the quasi-isothermal holding period. The reversing heat capacity decreases with time at each of the two peak positions. For both types of iPS (SGC and bulk film) the time dependence of each of the reversing heat capacity peaks is fitted using Cp(t)=Cp∞+A1exp(−t/τ1)+A2exp(−t/τ2). The fit parameters (equilibrium reversible heat capacity, Cp∞; prefactors, A1 and A2; relaxation times, τ1 and τ2) provide a means to compare the decay of the dual reversing heat capacity peaks of iPS solution grown crystals with bulk cold-crystallized film. Results show that iPS bulk films have longer relaxation times (τ1 and τ2) and smaller Cp∞ value than the iPS SGCs. Both results suggest that the iPS bulk films have higher degree of crystal perfection than SGCs.

Structural Reorganization in Poly(butylene terephthalate) during Fusion

A detailed investigation of the reorganization processes occurring in the crystalline phase of poly(butylene terephthalate) (PBT) during fusion was performed by temperature-modulated differential scanning calorimetry (TMDSC). Quasi-isothermal TMDSC analyses were conducted for extended times at different temperatures in the whole melting range, revealing different degrees of reversible melting. The latent heat involved in the fusion/recrystallization/crystal-perfection processes was found to decrease with time for the quasi-isothermal analyses performed at temperatures below 500 K. At higher temperatures, on the contrary, recrystallization prevailed with a continuous increase of the apparent reversing heat capacity as a function of time. The multiple melting behavior of PBT, often reported in the literature, was correlated with the results of the present quasi-isothermal TMDSC investigation.

Heat Capacity Study of Isotactic Polystyrene: Dual Reversible Crystal Melting and Relaxation of Rigid Amorphous Fraction

Isotactic polystyrene (iPS) is demonstrated to have unique thermal behavior, showing dual reversible crystal melting and irreversible enthalpic relaxation of its rigid amorphous fraction (RAF). Quasi-isothermal temperature-modulated differential scanning calorimetry (TMDSC) and standard DSC were used to study the heat capacity of cold-crystallized iPS. IPS shows two or three endotherms depending upon cold crystallization temperature, Tc. Crystal melting causes the higher temperature endotherm(s), and under quasi-isothermal conditions, we report for the first time observation of dual locally reversible melting endotherms. Quasi-isothermal TMDSC shows that the RAF is established at the crystallization temperature in iPS. Furthermore, we show that the lowest temperature endothermic peak, called the annealing peak, represents the transition of the RAF. For cold-crystallized iPS the annealing peak is an irreversible, enthalpy-involved relaxation of RAF, which transforms solidlike RAF into liquidlike mobile amorphous fraction. Depending upon Tc, RAF may be the sole contributor to the lowest temperature endotherm. To accommodate the relaxation of RAF, the experimentally determined heat capacity should be written in terms of the underlying linear baseline heat capacity plus the enthalpic terms relating to crystal melting and to relaxation of RAF.

The study of photo- and thermally-induced diffusion and dissolution of Ag in As 30S 70 amorphous films and its reaction products

Kinetics of photo- and thermally-induced diffusion and dissolution (PIDD and TIDD) of silver in a-As 30S 70 films were measured by monitoring the change in thickness of the undoped chalcogenide using a modified computer-controlled reflectivity technique. Silver-concentration profiles during photo- and thermally-induced solid-state reactions were traced by means of Rutherford backscattering spectroscopy (RBS). The enthalpy of photo- and thermally-induced reactions was measured by means of differential photocalorimetry and quasi-isothermal temperature-modulated differential scanning calorimetry (DSC), which are shown to be connected with the chemical reaction: Ag+As 3S 7=AgAsS 2+As 2S 5.

Melting and Crystallization of Polyethylene of Different Molar Mass by Calorimetry

Crystallization and melting of n-paraffins of chain lengths up to C60H122 can be largely reversible, and practically no supercooling is seen in differential scanning calorimetry (DSC) and temperature-modulated DSC (TMDSC). To find the changes with chain length for this effect, polyethylene fractions of oligomers of mass 560, 1150, and 2150 Da and a polymer of mass 15 520 Da were analyzed. The mode of analysis was quasi-isothermal TMDSC with an amplitude of 0.5 K about a fixed series of temperatures. For the oligomer of 560 Da, a mainly reversing melting was seen, similar to the behavior of paraffins of the same molar mass. The oligomer of 2150 Da as well as the polymer shows an irreversible crystallization with a crystallization of more than 5.0 K below the melting temperature, and the oligomer of 1150 Da is intermediate. Typical for polymers, a small amount of reversing material remains in the melting range of the polymer. The 1150 and the 2150 Da oligomers grow to extended-chain crystals, and the polymer yields folded-chain crystals. This research reveals that there is little difference in reversibility between monodisperse paraffins and fractions with similar average mass. There is no major difference in supercooling behavior for extended- and folded-chain crystals. The critical chain length for which a substantial supercooling is observed is 12.5 nm. The three types of reversible latent heat observed in macromolecules are traced to the short-chain molecules.

Reversible and irreversible heat capacity of poly(trimethylene terephthalate) analyzed by temperature-modulated differential scanning calorimetry

The heat capacity of poly(trimethylene terephthalate) (PTT) has been analyzed using temperature-modulated differential scanning calorimetry (TMDSC) and compared with results obtained earlier from adiabatic calorimetry and standard differential scanning calorimetry (DSC). Using quasi-isothermal TMDSC, the apparent reversing and nonreversing heat capacities were determined from 220 to 540 K, including glass and melting transitions. Truly reversible and time-dependent irreversible heat effects were separated. The extrapolated vibrational heat capacity of the solid and the total heat capacity of the liquid served as baselines for the analysis. As one approaches the melting region from lower temperature, semicrystalline PTT shows a reversing heat capacity, which is larger than that of the liquid, an observation that is common also for other polymers. This higher heat capacity is interpreted as a reversible surface or bulk melting and crystallization, which does not need to undergo molecular nucleation. Additional time-dependent, reversing contributions, dominating at temperatures even closer to the melting peak, are linked to reorganization and recrystallization (annealing), while the major melting is fully irreversible (nonreversing contribution). © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 622–631, 2000

Reversible Surface Melting of PE and PEO Crystallites Indicated by TMDSC

Quasi-isothermal temperature-modulated differential scanning calorimetry was employed in a study of thermoreversible structural changes in the melting range of semicrystalline polymers. The results indicated a reversible melting and crystallization process occurring at the fold surfaces of crystallites of polyethylene and poly(ethylene oxide). For polyethylene, good agreements were found with reported small-angle X-ray scattering data and Fischer's theory. Surface melting and crystallization depend on the ability of the chains in the crystals to carry out a sliding diffusion. This was shown by a comparison of polyethylene and poly(ethylene oxide) with other polymers such as poly(ethylene terephthalate), polycaprolactone, isotactic polypropylene, and syndiotactic polypropylene. When the longitudinal chain mobility in the crystals is much reduced or completely absent, only a small or no excess reversing heat capacity is observed. The special performance of short-chain poly(ethylene oxide) is indicative for the metastability of the crystals built of once-folded chains.

Crystallization of polymers studied by temperature-modulated techniques (TMDSC, TMDMA)

Quasi-isothermal temperature-modulated differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) measurements (TMDSC and TMDMA, respectively) permit determination of heat capacity and shear modulus as a function of time during crystallization. Nonreversible and reversible phenomena in the crystallization region of polymers can be observed. The combination of TMDSC and TMDMA yields new information about local processes at the surface of polymer crystals, like reversible melting. Reversible melting can be observed in the complex heat capacity and in the amplitude of sheer modulus in response to temperature perturbation. The fraction of material involved in reversible melting, which is established during primary crystallization, remains constant during secondary crystallization for polycaprolactone (PCL) and polyether ether ketone (PEEK). This shows that, also after long crystallization times, the surfaces of the individual crystallites are in equilibrium with the surrounding melt. Simply speaking, polymer crystals are “living crystals.”

Measurement of Thermal Conductivity using TMDSC: Solution to the Heat Flow Problem

The dependence of the apparent heat capacity obtained from quasi-isothermal temperature-modulated differential scanning calorimetry (TMDSC) experiments and the thermal conductivity is determined for several cases. The relationships are based on the solution of the heat conduction equation which gives the temperature profile in the TMDSC sample. The temperature profile is then used to calculate the sinusoidal heat flow to the sample. We compare our results with those of other researchers. We also show the effect of thermal contact resistance on the results.

Isotropization of nematic liquid crystals by TMDSC

Temperature-modulated differential scanning calorimetry (TMDSC) and traditional DSC are used to study the transition between the nematic liquid crystalline state and the isotropic liquid for two small molecules [4,4′-azoxyanisole and N,N′- bis(4- n-octyloxybenzal)-1,4-phenylenediamine] and one macromolecule (4,4′-dihydroxy-α-methylstilbene copolymerized with a 1 : 1 molar mixture of 1,7-dibromoheptane and 1,9-dibromononane). The DSC measurements with 4,4′-azoxyanisole were used for temperature calibration with varying heating and cooling rates. The breadths of the transitions were analyzed with quasi-isothermal TMDSC, using a small temperature-modulation amplitude and standard TMDSC with underlying heating and cooling rates. It could be verified that the isotropization transition of a nematic liquid crystal is, indeed, reversible for all three molecules. The nature of the transition changes, however, from relatively sharp for small and rigid molecules, to about 3 K wide for the small molecule with flexible ends, and to as broad as 20 K for the macromolecule. It was also demonstrated that quantitative heats of fusion of sharp transitions can be extracted from TMDSC, but only from the heat-flow signal in the time domain.