Glass transition temperature factors influencing measurement

In adhesive joints, for example with epoxy resin adhesives, internal stress in the adhesive layer may result from different coefficients of expansion in the glued materials, whereby their moduli of elasticity are important factors. The glass transition temperature, and thus the curing temperature, also play a role. The reaction shrinkage of the resin is another source of internal stress. Suitable formulations with added fillers, oligomers or copolymers are among the measures taken to reduce these influences. 

Another factor that has influenced realizable electro-optic coefficients is conductivity (which attenuates the poling field strength, Ep), Figure 14 illustrates this problem. As the glass transition temperature of the polymer lattice is approached, ionic contaminants become free to move the resulting conductivity attenuates the poling field and the optical nonlinearity decreases. A practical solution is frequently simply to purify chromophores by techniques such as repeated recrystallization. Moreover, the effect can be anticipated by performing conductivity measurements on spin-cast films. 

In practice TMA is mostly used for measurement of the glass transition temperature by measuring CLTE and on occasion volume expansion. Therefore, it is appropriate to review those chemical factors (effects of pendant groups, molecular mass, tacticity, crystallinity, crosslinking, etc.) that have the largest influence on the glass transition temperature and the expansion of a polymer. 

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