High temperature materials dynamic behaviour

All these possible developments could permit a better understanding of the dynamic behaviour of the scale/substrate interface during scale growth. Such an understanding could, in addition, introduce new opportunities to control the oxidation behaviour of high temperature materials and to improve their oxidation resistance. 

At low temperature the material is in the glassy state and only small ampU-tude motions hke vibrations, short range rotations or secondary relaxations are possible. Below the glass transition temperature Tg the secondary /J-re-laxation as observed by dielectric spectroscopy and the methyl group rotations maybe observed. In addition, at high frequencies the vibrational dynamics, in particular the so called Boson peak, characterizes the dynamic behaviour of amorphous polyisoprene. The secondary relaxations cause the first small step in the dynamic modulus of such a polymer system. 

The dynamic mechanical behaviour of ultra high modulus polypropylene is shown in Fig. 32. As in LPE, the modulus is temperature dependent, rising to a value of 25 GPa at —140 °C, which is rather more than half (he value of 42 GPa obtained from crystal measurements. Although the a and y relaxations of the isotropic polymer can be seen in the highly drawn material, the -relaxation is undetectable. On annealing, the modulus at high temperatures is markedly reduced, and a P-relaxa-... 

A development of the moving die rheometer where the operation of the unit is fully computer controlled. The rate of oscillation, temperature and level of strain can all be run through a series of options. The torque measurements are also highly sophisticated. As a consequence, the unit can be set up to monitor processing parameters, then the cure behaviour and finally the finished dynamic properties of the cured material. It is manufactured by Alpha Technologies. 

Han et al (1997) examined the chemorheology of a highly filled epoxy-resin moulding compound that is characterized by a modifed slit rheometer. Results show that a modified Cox-Merz rule relating dynamic and steady viscosities is established, >7(7 ) = (Tm )-Also the material was shown to exhibit a yield stress at low shear rates and power-law behaviour at higher shear rates. The temperature dependence of the viscosity is well predicted by a WLF model, and the cure effects are described by the Macosko relation. 

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