Study of thermal transitions in polymers by a multifrequency modulated DSC technique

Resumo

Differential Scanning Calorimetry (DSC) is one of the most widely used thermal analysis techniques for the study of transitions and relaxation processes in polymers and also in other materials. It measures the heat flow as a function of time and/or temperature, and determines the energy released or absorbed by a sample when it is heated (cooled) or maintained at a constant temperature. Its advantages are that it is fast and sensitive, the amount of sample needed is small (~mg), it could be in the liquid or solid state and the sample preparation is easier than for some other techniques. Nevertheless, it present some drawbacks, such as not being able to separate different overlapping transitions that happen in the same temperature range, the difficulty of detecting very weak transitions, or the requirement for multiple experiments to determine heat capacities that increases the experimental time needed. Temperature modulated DSC techniques (TMDSC) were introduced in the market more than 16 years ago. In these techniques a periodic temperature modulation of small amplitude is superimposed on the underlying rate of conventional DSC. This superposition gives two different heating rates: the underlying and the instantaneous due to the sinusoidal signal, thus permitting a better resolution and sensitivity (in DSC a better resolution gives a worse sensitivity and vice versa) and the evaluation of the heat capacity in real time. TMDSC is able to determine not only the total heat flow but also its two individual components usually referred to as “reversing” and “non-reversing”, thus it is possible to separate overlapped transitions. Although TMDSC offers more information than that available by DSC it also shows some drawbacks as the use of lower underlying heating rates than in conventional DSC which leads to longer experiments, or the requirement of making a separate scan for each frequency if one wishes to study, for example, the frequency dependence of a certain phenomenon. More recently, in 2005, TOPEM, a new technique of TMDSC from Mettler-Toledo, was commercialised. Instead of being based upon a periodic modulation of the heating rate, as is the situation with other TMDSC techniques, TOPEM uses a stochastic modulation of the heating or cooling rate by means of random pulses of temperature. This stochastic perturbation introduces a broad frequency spectrum in the response, which implies that TOPEM is apparently able to determine the complex heat capacity over a range of frequencies in a single scan. In the present thesis, a detailed description of the bases and operation of TOPEM is presented, showing the differences between the other calorimetric techniques, DSC and TMDSC. All the parameters which define an experiment and the parameters needed to make an evaluation of the experimental response are explained. The glass transition of polycarbonate is selected to make an initial study of the influence of all the different parameters (experimental and calculation) on the results of an experiment. Different samples are employed and different experiments by TOPEM and ADSC (TMDSC technique specific of Mettler- Toledo) are performed and compared. The limits and advantages offered by TOPEM are observed and presented here, and with a more profound understanding of the technique as a result, additional materials and transitions are studied. In the next part, an epoxy resin with a diamine is selected to be studied. The vitrification during the isothermal cure is studied by TOPEM. The results are again compared with those obtained by TMDSC and the additional information obtained is analyzed and compared with additional results obtained from the literature. With the aim to confirm some experimental data observed, a simulation is made with MATLAB to confirm the experimental results. It is found that the theoretical results not only predict the calorimetric data, but also those obtained from dielectric analysis at higher frequencies which were extracted from the literature. In the next part, the same epoxy resin with a diamine is selected. The vitrification and devitrification during the non-isothermal cure is analyzed by TOPEM. The results obtained permit to construct a CHT diagram (conversion-time-temperature) which characterized the system. As in the previous case a simulation is made with MATLAB and the theoretical results obtained compare well with the experimental data. Finally, some preliminary experiments related to other transitions are presented and are planned to be studied in the future.