TG-IR for Degradation Studies

One of the more common uses of TG-IR is to look at the decomposition or degradation of a material. Due to the relatively high detection limits of this method compared to TG-MS or TG-GCMS, it isn’t as useful for detecting trace contaminates or leachates as other methods. However, it does have advantages with looking at the large fragments that come the degradation of polymer or material on burning. This is particularly true for fragments containing functional groups like halogens, oxygen, and nitrogen.

Depending on the material, the fragments that come off from the burn under nitrogen can range from small molecules like CO2 to distinct pieces of the backbone. ECO rubbers for example release monomer molecules on combustion and these are easily seen in the FTIR. In contrast, something like a straight, unmodified polyethylene is going to give CO, CO2 and H2O. Happily, libraries for TG-IR are now available and include both the major polymer types and commonly used additives like antioxidants and slip agents.

In general, the ability of the FTIR to detect a component is determined by the concentration in the gas phase (normally needs about 0.5-1%), the symmetry of the molecule (IR only sees asymmetric stretches), and the intensity of the band.  So both sample size and chemistry come into play.  Using a TGA with a large capacity balance will help increase the amount of sample as will heating faster.  One can also play with gas flow rates. The chemistry of the molecule is what it is. A symmetric molecule is going to be invisible and you’ll need Raman to see that. The strength of the absorption band is also not something we can change, but it is important to be aware of it.  It’s a lot easier to detect stronger bands.  Halogens tend to be very visible because of this and the distinctiveness of the spectrum.