In general, infrared spectrometry provides a unique "fingerprint" for a material. Qualitatively, it can tell you if your sample is polyethylene, or polyamide, etc. It can be difficult for the technique to tell you that the sample is 80 % polypropylene plus 10 % talc plus 5 % fire retardant plus 5 % colorant. However, in absorbance mode rather than transmittance, quantitative results are possible through Beer's Law.
There's limited information that's one can glean from an FTIR. An FTIR measures the spectral signature of specific molecular bonds. If whatever "degradation" which you've neglected to describe, causes breakage of such bonds, then there may be some indication of the degradation in the FTIR.
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No, the question was what was the suspect degradation mechanism? If there is a physical or chemical change to the composition of the material, then there may be some change in the FTIR results. However, if the change is due to something that's purely physical, without any chemical composition change, then the FTIR might not show a change, necessarily.
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If the degradation results in the production of some chemical species that isn't present in the original polymer, the presence of some new dips in the IR spectrum may allow you to tell it's there.
I suspect that the "limited quantitative" bit comes from the fact that the vast majority of IR spectra are published in normalised form. You'll see where the dips are, and how deep they all are relative to the deepest dip in the spectrum allowing you compare spectra to see what you might have. What you don't get (apart from a relatively small library of common targets) is any indication of the overall strength of absorption from different substances and it's this you need if you want to tell how much contaminant/degradation product you've got in your sample.
