Glass-rubber transition measurement

It is clear from the described characteristics that the secondary transitions have specific features, different from those of the glass-rubber transition. Furthermore, to determine these characteristics, as well as the existence of single or multiple motional processes, measurements have to be performed at various temperatures and frequencies. 

In Chapter 6 we have seen that tan 8, a measure of the relative energy dissipation, depends on temperature and frequency, and that it shows maxima at transitions. A strong maximum occurs at the glass - rubber transition, weaker maxima at secondary transitions. In general tan <5is higher when the E(T) curve is steeper. 

Below the glass-rubber transition temperature glassy polymers also show other, secondary transitions. Their effects are smaller and often less obvious, although they are important to the mechanical behaviour (to diminish brittleness). Secondary transitions can be detected by studies of mechanical damping, by NMR or by electric loss measurements over a range of temperatures. 

The acoustic properties of polymers are just as for many properties strongly dependent on temperature around the glass-rubber transition the sound speed decreases rapidly from a relatively high value at T < Tg to a relatively low value at T > Tg. During this transition the absorption shows a maximum value. An example is given in Fig. 14.4, where data for a poly(metacarborane siloxane) are displayed. The measurements were made in the longitudinal mode as a function of temperature at a frequency... 

Dielectric measurements combined with thermal analysis revealed two transitions a, a glass-rubber transition between 295-340 K (10 Hz), and B, a sub-glass transition exhibiting Arrhenius behaviour. 

The DSC is widely used to measure the glass-rubber transition temperature (Tg-value), which is an important parameter for polymer characterisation. The Tg-value represents the temperature region at which the (amorphous phase) of a polymer is transformed from a brittle, glassy material into a tough rubberlike liquid. This effect is accompanied by a step-wise increase of the DSC heat flow/temperature or specific heat/ temperature curve. Enthalpy relaxation effects can hamper the... 

Subsequently, a foam strip of 8 x 12 x 2 mm was carefully clamped in a Polymer Laboratories DMA system and measured using a frequency of 10 Hz. and a heating rate of l8C/minute. Figure 4.7 shows the result of such a measurement. The decrease of the Youngs modulus at temperatures > 120°C, accompanied by clear maxima of the loss modulus and the tan 6 indicate that the glass-rubber transition of such a foam is easily measured by DMA. The loss modulus (E") maximum temperature was chosen to indicate the Tg-value of these rigid foam systems. 

The dielectric loss data calculated from the I(discharge) currents according to equation 5.19 show a clear maximum due to the glass-rubber transition for the experiments at 29°C and 20°C. These data are plotted in Figure 5.7 as a function of the frequency together with the AC data. The results of both measuring systems can be fitted reasonably. The measured dielectric loss and tangent delta maximum temperatures are... 

The results of these measurements are listed in Table 5.2 and plotted in Figure 5.8. The tangent delta maximum value due to the glass-rubber transition is clearly visible in the curves measured at 3°C, -9°C and -17°C. 

The TSD/TMA combination is a typical example of a system developed to measure the same physical property i.e. the glass-rubber transition, on the same sample and at the same time using two independent techniques. 

The thermal expansion of the silver electrodes is neglected if only the temperature location of the glass-rubber transition of the sample is measured. 

The mass increase of an RB 830 syndiotactic 1,2-BR sample was measured as a function of time at 240°C in an air atmosphere. About thirty minutes were necessary to obtain a mass increase of 0.19 %wt. (0.22 %wt. during the non-isothermal experiment), see Figure 8.5. This sample was cooled and then placed in the DSC. A crystalline phase with a Tm- and Hf-value of respectively 9l°C and 23 J/g was detected the Tg-value of the amorphous phase proved to be -13°C. Hence, the Tg-, Tm- and Hf-values were not influenced at all by the samples heat treatment. Even the strength of the glass-rubber transition effect (0.26 J/g.°C) was not changed. Thus, the detected mass increase effect can not be explained by some cyclisation reaction of the BR. 

The (crystalline) a-transition has been disappeared completely for the lower crystalline (at 240°C, compression moulded) sample. The intensity of the glass-rubber ( ) transition, however, has been clearly increased. Moreover, the B-transition E"(max.) temperature is shifted from about 50°C to 15°C. The Tan 6(max.) temperature is used to indicate the (DMA) Tg-value of compression/injection moulded PK co- and terpolymers. This Tg-value proved to be 19°C + 2°C with no significant difference between these values measured for PK copolymer and PK terpolymer. 

Thermomechanical Analysis (TMA) can be defined as the measurement of a specimen s dimensions (length or volume) as a function of temperature whilst it is subjected to a constant mechanical stress. In this way thermal expansion coefficients can be determined and changes in this property with temperature (and/or time) monitored. Many materials will deform under the applied stress at a particular temperature which is often connected with the material melting or undergoing a glass-rubber transition. Alternatively, the specimen may possess residual stresses which have... 

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