Dynamic mechanical analysis superpositioning

The data can he obtained from dynamic mechanical analysis (DMA) tests used to construct the stress relaxation master cnrves at different levels of conversion, within the frame of time-temperature superposition principle. Curves of stress relaxation modulus can he modeled using the stretched Kohlrausch-William-Watts (KWW) exponential function at each level of conversion, as follows ... 

As discussed iu Section 2.16, dynamic mechanical analysis offers an enhanced means of evaluating the performance of polymeric systems at elevated temperatures. It provides a complete profile of modulus versus temperatures, as well as measurement of mechanical damping. Operating in the creep mode and coupled with the careful use of time-temperature superpositioning, projections can be made regarding the long-term time-dependent behavior under constant load. This provides a much more realistic evaluation of the short- and long-term capabilities of a resin system. 

Differential scanning calorimetry (DSC), DMA and TG were used by Tabaddor and co-workersl l to investigate the cure kinetics and the development of mechanical properties of a commercial thermoplastic/ thermoset adhesive, which is part of a reinforced tape system for industrial applications. From the results, the authors concluded that thermal studies indicate that the adhesive was composed of a thermoplastic elastomeric copolymer of acrylonitrile and butadiene phase and a phenolic thermosetting resin phase. From the DSC phase transition studies, they were able to determine the composition of the blend. The kinetics of conversion of the thermosetting can be monitored by TG. Dynamic mechanical analysis measurements and time-temperature superposition can be utilized to... 

Dynamic mechanical analysis is an extremely powerful and widely used analytical tool, especially in research laboratories. In addition to measuring the temperature of the glass transition, it can be used to study the curing behavior of thermosetting polymers and to measure secondary transitions and damping peaks. These peaks can be related to phenomena such as the motion of side groups, effects related to crystal size, and different facets of multiphase systems such as miscibility of polymer blends and adhesion between components of a composite material [24]. Details of data interpretation are available in standard texts [1,2,25]. In the next section, we consider time-temperature superposition, which is another very useful apphcation of dynamic mechanical data. 

Detailed analysis of the isothermal dynamic mechanical data obtained as a function of frequency on the Rheometrics apparatus lends strong support to the tentative conclusions outlined above. It is important to note that heterophase (21) polymer systems are now known to be thermo-rheologically complex (22,23,24,25), resulting in the inapplicability of traditional time-temperature superposition (26) to isothermal sets of viscoelastic data limitations on the time or frequency range of the data may lead to the appearance of successful superposition in some ranges of temperature (25), but the approximate shift factors (26) thus obtained show clearly the transfer viscoelastic response... 

The principles of time-temperature superposition can be used with equal success for dielectric measurements as well as dynamic mechanical tests. Analysis of the frequency dependence of the glass transition of the adhesive in the system described above shows that it follows a WLF type dependence whereas the transition of PET obeys Arrhenius behaviour. This type of study can be used to distinguish between different types of relaxation phenomena in materials. 

Table 5.3 lists the principal experimental methods used in dynamic mechanical testing. Of the experiments considered below, the thermal scan mode (method 1) is the technique most commonly used by thermal analysts. Here typical applications in quality control or processing look for differences in material batches, thermal history, different grades, reactivity, and other characteristics. The stepped isotherm (or step isothermal) experiment (method 2) is used mainly in studies involving detailed mechanical property determination for structural analysis, vibration damping applications, and for determining time-temperature superposition master curves. Method 3 (fast scan or single isotherm) is application specific. 

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