Glass-rubber transition time effects

It has to be mentioned that the effect of dilution is in the terminal zone (long times) greater than in the glass-rubber transition zone (short times), which is specified by log ac. This becomes also clear from Fig. 16.11 the values of log ac in Figs. 16.12-16.14 were taken in the transition zone. It is beyond the scope of this book to go into more detail into this subject. For more details of the time-concentration superposition principle the reader is referred to the monograph by Ferry (General references, 1980, Chap. 17). 

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. 

It will be clear that the mechanical properties of polyketones at ambient temperatures are sensitive for these ageing and these moisture absorption effects especially due to the presence of the glass-rubber transition in that temperature region. The influence of both effects is in general opposite to each other the stiffness increases due to ageing and decreases due to moisture absorption. Moisture absorption effects are time and object dimensions dependent whereas ageing effects are only time dependent. 

The time and temperature dependent properties of crosslinked polymers including epoxy resins (1-3) and rubber networks (4-7) have been studied in the past. Crosslinking has a strong effect on the glass transition temperature (Tg), on viscoelastic response, and on plastic deformation. Although experimental observations and empirical expressions have been made and proposed, respectively, progress has been slow in understanding the nonequilibrium mechanisms responsible for the time dependent behavior. 

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