Question

1. Consider a case in which you have a material that support a certain stress at 75^oC and you have several candidate polymers for which you need to determine the Tg using DSC or DMA(both of these are in your lab). Which technique would you select? Please provide a basis for your selection in terms of how the type of information it provides (based on how it works) and how/why it would be best suited for this application.

Answer 1

Both DSC and DMA can be used to determine the Tg. DMA, because of its inherent sensitivity to the glass transition, is an ideal technique for identifying the Tg of highly filled systems. Absolute modulus numbers, both below and above the glass transition temperature, can be determined by plotting the storage modulus signal as a function of sample temperature. The storage modulus above Tg is related to the degree of cure (cross-link density) of the material: the higher the storage modulus above Tg, the higher the degree of cure. Tg is also an indication of degree of cure: the higher the glass transition temperature, the higher the degree of cure.

DMA is particularly useful for evaluating polymeric materials that exhibit time, frequency, and temperature effects on mechanical properties because of their viscoelastic nature.

For DMA we usually use relatively much bigger sample than DSC, so it is very important to use low heating rates, it is highly recommended to use 2 C/min