Isothermal dynamic relaxation test

Winter (Winter and Chambon, 1986, Winter, 1987) also described an isothermal dynamic relaxation test to measure the gel point. He noted that the gel point coincides with a power-law relationship between the relaxation modulus (G) and relaxation time (t) G = St ", where S and n are constants. An isothermal step strain test measures the relaxation modulus as a function of time after an instantaneously applied strain. The gel time can be measured as the point at which the profile of the relaxation modulus can be expressed by this power law. This model is equivalent to tan <5 being independent of frequency. 

Arrelano et al. (1989) evaluated the gel points of various DGEBA-based epoxy-resin systems via the crossover method. They also defined the vitrification point as the maximum in G" in an isothermal dynamic time test. In all these gel-point measurements the frequency must be chosen such that the relaxation of the network is enabled during data sampling. This is represented as... 

Construction of a scale model must be accompanied with an analysis to determine test conditions that ensure the test results from the scale model are representative of the processes in the prototype. In combustion applications, although most of the processes are inherently at elevated temperatures, physical modeling is usually carried out under isothermal conditions. Isothermal physical modeling technique is based on the principle of relaxation. Under this principle, the variables that are important for the phenomena under study are stressed. The variables that are stressed are duplicated as necessary to obtain a representative result. No scale physical model can be an exact model of the reality unless an exact full-scale prototype is made. However, by using accurate correlations the modeling work can provide a good qualitative understanding of the fluid dynamics in the prototype. This chapter attempts to answer the question How does one ensure that the scale model test results are representative of the actual processes in the prototype ... 

Let us first consider, as extreme cases, the possibility that the molecular dynamics is controlled solely by (1) thermal energy fluctuations, or (2) local density fluctuations (i.e., free volume). Both cases are illustrated schematically in Figure 12 by plotting the isobaric and isothermal dependencies of the stractural relaxation times on volume. The characteristic feature of the isothermal dependence is the absence of any volume dependence of r indicating that the stmctural relaxation process is purely thermally activated. In the second case, the isobaric and isothermal curves superimpose into a master curve and such a behavior is consistent with free-volume approaches. A critical test of the origin of the different processes and of the relative influence of volume and temperature in each... 

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