Search Results
Abstract
An inherent challenge with polymer blends is the difficulty in resolving the glass transition, T g, for the smaller mass fraction component. The objective of this work was to determine the practical scanning conditions for identifying the dual T g’s for a 75:25 polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) blend using a thermomechanical analyzer (TMA). Scanning rates up to 20°C min−1 using dilatometer and expansion modes were studied. Heating and cooling rates were found to affect both T g values but the effects were not simple relationships. T g values could either increase or decrease depending on the scanning rate applied. Higher rates resulted in large thermal lags which opened the accuracy of measurements to question. Generally, higher rates tended to display only one T g but the duality of T g’s can be detected with scanning rates between 0.5 and 5°C min−1 for both modes.
Calibration of micro-thermal analysis for the detection of glass transition temperatures and melting points
Repeatability and reproducibility
Abstract
Micro-thermal analysis (μTATM) is a technique in which thermal analysis is performed on surfaces of test specimens on a small (ca. 2×2 μm) scale. Like any thermal analysis technique, interpretation of results benefits from accurate temperature information and knowledge of the precision of the resultant measurement. However, temperature calibration for such methods is more challenging than with its macro relatives since the calibrant comes into direct contact with the AFM sensor. This paper describes suitable calibration procedures for different types of transitions namely for first order transitions (melting points) and for glass transition temperatures using organic chemicals and polymers.
Differential scanning calorimetry (DSC) measurements have been carried out on a series of ABA poly(styrene-b-isoprene) triblock copolymers with 30% polyisoprene content and various molecular weights. The DSC data show an inward shift for the glass transition temperatures (T g) of the blocks compared to the corresponding homopolymers. As a function of the molecular weight, one to three transitions were found. The additional thirdT g gives some further evidence of the existence of an interphase between the microdomains.
In view of the lack of precise experimental data in the literature concerning the determination of a definedTg value for natural rubber (NR) and the differences when such data are given, a reference definition ofTg(Vo) is offered and a procedure for obtaining it described in detail.
Abstract
The measurement of the maximum glass transitionT g∞ of a thermosetting resin is usually performed by differential scanning calorimetry in the second scan (T g2scan), after a previous scan by heating up the sample to a temperature where the exothermic curing reaction has been completed. However, this method can eventually produce thermal degradation, decreasing the crosslinking density and theT g of the sample. Values ofT g2scan between 95† and 102†C were found in an epoxy resin based on DGEBA cured with phthalic anhydride. Thermal degradation effects can be avoided if the measurement is performed by isothermal curing and further determination ofT g. AT g∞ value of 109†C is achieved, which is the maximum value ofT g according to the topological limit of conversion.
The glass transition of an irradiated, ultra-high molecular weight, linear polyethylene was investigated by means of the Perkin-Elmer DSC-2 differential scanning calorimeter. The experimental specific heat data were compared with those of the nonirradiated sample, obtained by DSC and adiabatic calorimetry.
Abstract
Glass transitions of several non-ionic cellulose ethers differing in molecular mass and nature and amount of substituents were analyzed (as compressed probes) by differential scanning calorimetry (DSC), modulated temperature differential scanning calorimetry (TMDSC@®), and oscillatory rheometry. In general, the low energy transitions accompanying the Tg of methylcellulose (MC), hydroxypropyl methylcellulose (HPMC), and hydroxypropylcelluloses of low (L-HPC) or medium-high (HPC) degree of substitution were difficult to characterize using DSC. Non-reversing heat flow signals obtained in TMDSC experiments were more sensitive. However, the best resolution was obtained using oscillatory rheometry since these cellulose ethers undergo considerable changes in their storage and loss moduli when reaching the Tg. Oscillatory rheometry also appears as a useful technique to characterize the viscoelastic behavior and thermal stability of pharmaceutical tablets. Tg values followed the order HPC (105°C)<HPMC (170-198°C)<MC (184-197°C)<L-HPC (220°C). For HPMCs, the Tg increases as the methoxyl/hydroxypropoxyl content ratio decreases. The results indicate that Tg depends strongly on the structure of the cellulose ethers. In general, increasing the degree of substitution of cellulosic hydroxyls, the hydrogen bonding network of cellulose decreases (especially when the substituents cannot form hydrogen bonds) and, in consequence, Tg also decreases.
Abstract
Values of the Flory-Huggins interaction parameters c were predicted on the base of mixing enthalpy H M for nitrocellulose-s-diethyldiphenylurea system. The phase diagram of the system and the glass transition temperature of mixtures T g12 were estimated using calculated c parameters. The predicted glass transition temperatures were in accordance with values determined experimentally.
