Glass transition-conversion relationship

A fundamental property that determines the state of a reacting system is its extent of cure or chemical conversion (a). Several papers have shown that there is a unique relationship between the glass-transition temperature (Tg) and a that is independent of cure temperature and thermal history. This may imply that molecular structures of materials cured with different histories are the same or that the changes in molecular structure do not affect Tg. There are generally accepted to be two approaches to modelling glass-transition-conversion relationships, namely thermodynamic and viscoelastic approaches. These are summarized in Table 3.8. 

As discussed in Chapter 4, for many thermosetting polymers a unique relationship may be established between conversion and glass transition temperature, as was also verified for this particular diepoxide-diamine system (Wisanrakkit and Gillham, 1990). So, the left-hand side of Eq. (5.46) may be written as a unique function of Tg, F(Tg)... 

Nevertheless, with the development of new sensors, microdielectrometry constitutes a nondestructive and convenient method for monitoring in situ and in real time the cure of thermosets (Senturia et al., 1982 Kranbuehl et al., 1986). Great efforts have been made and research is still going on to establish relationships between (i) cure index and actual conversion of reactive groups, (ii) conductivity and viscosity, and iii) conductivity and glass transition temperature, in order to obtain models for smart processing. 

The relationship between glass transition temperature and conversion is given by (Williams et al., 1990) ... 

Most of these structural parameters can be derived from a statistical study (Chapter 3), but what is not obvious is the determination of their contribution to Tg. Surprisingly, very simple relationships are available to estimate the evolution of the glass transition temperature with conversion, Tg = f(x) (Chapter 4). 

As can be observed in Figure 4, there appears to be a relationship between the glass-transition temperature and conversion measmed from the residual exotherm. Several workers have shown that for most thermoset systems there is indeed a imique relationship between the chemical conversion of a thermoset and its glass-transition temperature, independent of the cure temperature and thermal history (see, for example. Refs. 41-43). While a few exceptions have been noted, this appears to be the general case (see Ref 2). Thus, measurement of Tg may be assiuned equivalent to direct measurement of conversion. 

Similarly, an increase in intermolecular interactions (ionic interactions, it bonding, H bonding) of different groups in different tacticity polymers also restricts mainchain motions, thus increasing the glass transition temperature— And, of course, Tg increases with crosslink density because of restrictions of mobility in the mainchain. The relationship between Tg and conversion in thermosets is discussed in Section 2.10 (of Chapter 2). 

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