M. Pyda

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).

Temperature-modulated differential scanning calorimetry of reversible and irreversible first-order transitions

Abstract Temperature-modulated differential scanning calorimetry of first-order transitions has led to many new observations. Some of these involve non-linear processes or deal with transformations of practically instantaneous response. The latter may cause serious lags within the calorimeter due to limited thermal conductivity of the sample and the instrument. In both cases the “reversing heat capacity” or a “complex heat capacity” is not a precise representation of the transition since both are computed from abbreviated Fourier transforms, limited to the evaluation of the first harmonic component. One has in these cases to work in the time-domain with the raw output. But even from these analyses in the time-domain many interesting new insights about the transition and the calorimeter performance can be generated.

Heat capacity of poly-p-dioxanone

Abstract The heat capacity of poly-p-dioxanone (PPDX), (CH2[sbnd]CH2[sbnd]O[sbnd]CH2[sbnd]COO[sbnd])x, was determined using both differential scanning calorimetry (DSC) and temperature-modulated DSC (TMDSC) from 200 K to 430 K. Based on the new data and literature data, the heat capacity of the solid state was analyzed using an approximate group vibrational spectrum and skeletal vibrations. The 10 skeletal vibrational modes are well represented by a Tarasov function with theta temperatures of θ1 = 478.7 K and θ3 = 50.4 K. The heat capacity of the liquid was fitted to a linear function, C liquid = 0.1484 T + 144.3 in units of J K−1 mol−1, which is close to the sum of equations developed earlier for the liquids of poly(oxyethylene) and polyglycolide. The change in heat capacity of amorphous PPDX at the glass transition temperature (264 K) is 69.9 J K−1 mol−1, and the heat of fusion for perfect crystals at the melting temperature (≈400 K) is 14.4 kJ mol−1. The integral thermodynamic functions were derived, a...

Calorimetric Study of Block-Copolymers of Poly(n-butyl acrylate) and Gradient Poly(n-butyl acrylate-co-methyl methacrylate)

Abstract The nanophase separation in diblock and triblock copolymers consisting of immiscible poly(n-butyl acrylate) (block A) and gradient copolymers of methyl methacrylate (MMA) and n-butyl acrylate (nBA) (block M/A) were investigated by means of their heat capacity, Cp, as a function of the composition of the blocks M/A and temperature. In all copolymers studied, both blocks are represented by their Cp and glass transition temperature, Tg, as well as the broadening of the transition-temperature range. The low-temperature transition of the blocks A is always close to that of the pure poly(n-butyl acrylate) and is independent of the analyzed compositions of the block copolymer, but broadened asymmetrically relative to the homopolymer due to the small phase size. The higher transition is related to the glass transition of the copolymer block of composition M/A. Besides the asymmetric broadening of the transition due to the phase separation, it decreases in Tg and broadens, in addition, symmetrically with increasing acrylate content. The concentration gradient is not able to introduce a further phase separation with a third glass transition inside the M/A block.

Structure-property analysis for gel-spun, ultrahigh molecular mass polyethylene fibers

Abstract The structures of four gel-spun, ultrahigh molecular mass polyethylene fibers have been studied with the techniques of full-pattern x-ray diffraction, small-angle x-ray scattering, powder x-ray diffraction, solid-state 13C nuclear magnetic resonance, differential scanning calorimetry, and optical microscopy. A high molecular mass polyethylene fiber is also studied for comparison. At room temperature these fibers show mainly the common orthorhombic crystals and a small amount of monoclinic crystals in addition to an intermediate, oriented phase and the amorphous phase. The structure parameters of the orthorhombic phase change slightly with fiber processing history. The chains of the intermediate phase have largely a trans-conformation and are oriented preferentially parallel to the fiber axis, but are disordered laterally. The mobility (correlation time) of the carbon atoms of the intermediate phase is higher than that of the crystalline phase by 2 orders of magnitude, but lower than that of the a...

Computation of heat capacities of solids using a general Tarasov equation

The general Tarasov function is fitted to the skeletal heat capacities of materials with widely different crystal structures. Examples are chosen from flexible macromolecules (polyethylene, polypropylene, poly(ethylene terephthalate), selenium, rigid macromolecules (diamond and graphite), and a small molecule (fullerene, C60). A new optimization approach using the MathematicaTM software is developed. It results in one-, two-, and three-dimensional Debye temperatures, Θ1, Θ2 and Θ3 the fitting parameters of the Tarasov function. In addition to the Tarasov function, the evaluation of the heat capacities makes use of approximate group-vibrational spectra. The results support the earlier assumption that Θ2=Θ3 for simple, solid, linear macromolecules. In more complicated bonding situations, Θ1, Θ2 and Θ3 are used as averaging fitting parameters. This general approach provides an improvement in the quantitative thermal analyses of polymers and other substances included in the ATHAS Data Bank. Sufficient programming information is provided to enable anyone the computation with a copy of the popular MathematicaTM software. The programming file is also downloadable from the WWW.

CAN ONE MEASURE PRECISE HEAT CAPACITIES WITH DSC OR TMDSC? A study of the baseline and heat-flow rate correction

SummaryDuring a prior study of gel-spun fibers of ultrahigh-molar-mass polyethylene, a substantial error was observed on calculating the heat capacity with a deformed pan, caused by the lateral expansion of the fibers on shrinking during fusion. In this paper, the causes of this and other effects that limit the precision of heat capacity measurements by DSC and TMDSC are explored. It is shown that the major cause of error in the DSC is not a change in thermal resistance due to the limited contact of the fibers with the pan or the deformed pan with the platform, but a change in the baseline. In TMDSC, the frequency-dependence is changed. Since irreversible changes in the baseline can occur also for other reasons, inspections of the pan after the measurement are necessary for precision measurements.

Heat capacity of solid-state biopolymers by thermal analysis

Heat capacities in the solid state of four globular proteins (bovine β-lactoglobulin, chicken lysozyme, ovalbumine, and horse myoglobin) and of the poly(amino acid) poly(L-tryptophan) have been determined using the Advanced THermal Analysis System (ATHAS). The experimental measurements were performed with adiabatic and differential scanning calorimetry over wide temperature ranges. The heat capacities were linked to an approximate vibrational spectrum by making use of known group vibrations and of a set of parameters, Θ1 and Θ3, of the Tarasov function for the skeletal vibrations. Good agreement was found between experiments and calculations with root mean square errors mostly within ±3%. The experimental data were analyzed also with an empirical addition scheme using the known data for poly(amino acid)s measured earlier. Based on this study, vibrational heat capacities can now be predicted for all proteins with an accuracy comparable to common experiments.

Multi-frequency heat capacity measured with different types of TMDSC

Abstract The heat capacities of sapphire (Al2O3) and sodium chloride (NaCl), have been measured to establish the accuracy and precision of two different temperature-modulated differential scanning calorimeters operated in diverse multi-frequency modes. The calorimeters have then been applied to find the apparent, reversing heat capacity of polystyrene as a function of frequency in the glass transition region. The first modulation mode consisted of a series of linear heating and cooling segments and produced four harmonics with practically equal temperature amplitudes (1st, 3rd, 5th, and 7th), one of lower amplitude (9th), and almost negligible higher harmonics. The second modulation mode is a rather sharp step ending in an isotherm or slow temperature-decrease and leads to a controlled spike in the heat-flow rate response which produces Fourier components of similar amplitudes for all harmonics of the rates of changes of temperature. The apparent, reversing heat capacity is evaluated from the amplitudes of the heat-flow rates and the corresponding sample temperatures or heating-rates. A time-constant or calibration constant which accounts for thermal conductivities and resistances within the calorimeters can be evaluated from the different harmonics of each run. Measurements in the glass transition region have a slow response of the sample. They are evaluated by separating the sample effect from the calorimeter response which can be extrapolated from data gained outside the transition. One measurement is thus sufficient for the evaluation of the frequency dependence of the heat capacity in the glass transition region.

A Study of Temperature-modulated Differential Scanning Calorimetry with High-resolution Infrared Thermography

Temperature gradients within the sample and furnace in temperature-modulated differential scanning calorimetry (TMDSC) are studied for a power-compensation calorimeter of the Perkin Elmer type. The temperature measurements were made with a high-speed, high-resolution infrared camera. Differences between programmed and actual temperature amplitudes are determined as a function of sample thickness for a sawtooth modulation with up to 48 K min−1 heating and cooling rates. Phase angles have been established, and the effect of open and sealed sample pans has been analyzed. A simple one-dimensional description of the observed effects is made and a three-dimensional one is suggested based on a model available in the literature.