We want to show the surface changes to PET and PLA plastics (in pulverized form) as a proof of the UV treatment. From what I understand ATR is useful for spectral analysis of a solid, but I read that it cannot be used for "hard" polymers/powders. Are PET and PLA compatible with this method or is there something else I should look into?
If your powders are fine enough and homogeneous enough, there's no reason why you shouldn't try ATR, despite that "you can't". But these surface IR methods require some experience in sample preparation, so don't be disappointed if you don't get a reasonable spectrum the first time.
Neither of those polymers are too hard to obtain good ATR spectra. If you do the experiments carefully and run control samples you should be able to see the chemistry changes. There are a few things to keep in mind. A diamond ATR will allow you to apply a lot more pressure to the sample and using a high pressure clamp will help. Depending on the amount of treatment, you may need to be careful of the penetration depth. If the ATR element you use gets too deep a penetration, the bulk material may "wash out" the spectrum of the treatment. And if you are planning to compare to library spectra make sure you take into account (or correct for) the wavelength dependent depth of penetration. I would strongly suggest reading this
https://www.piketech.com/files/pdfs/PIKE_ATR_Theory-Applications.pdf
as primer on ATR.
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