[JSUCY232]

Does anyone have a reference to a DSC for polymethyl methacrylate? I have done DSC on PMMA, and a copolymer, and I am trying to compare. The problem is that I don't have the ability to perform further runs at this time, due to the fact that the primary operator of this instrument will be out of town for a month. I understand that the Tg is in the range of 150C. I have a couple of peaks that I can't definitively identify. I also thought that the Tm was also in the same range, but some references say around 280. I'd like to know the approximate Tm for PMMA for comparison to my copolymer and any other information I can infer. I am not very familiar with DSC. Thanks.

[Corribus]

Measuring glass transition temperatures by DSC can be challenging. I don't know much about DSC of PMMA in particular, but I do know that picking out the glass transition by DSC - which can often be a tiny little sigmoidal feature - requires a nice flat baseline, and the baseline here is kind of a mess. There are some specialized modes (they have different names depending on the instrument manufacturer sometimes - like modulated DSC) that are supposed to help deconvolute the glass transition features from, e.g., relaxation heating transitions that can be a problem in old samples. Bear in mind that DSC isn't really a fingerprint technique - the size and shape of transitions can depend quite a bit on input parameters like heating and cooling rates. Use the wrong heating/cooling rate, and you may not see the glass transition at all. It is not necessarily surprising that you aren't seeing what you're looking for based on a single scan acquired by a different operator.

The melting transition will be a large endotherm. But you haven't indicated which direction in the exothermic direction (or, for that matter, the temperature units), so it's not possible to tell.

[wildfyr]

The trick with measuring such things by DSC is to cycle them several times. Typically the material is heated up above its melting point and cooled down in the DSC several times to remove all "memory" then only the final run is taken. Going up to 400C+ is not necessary for this polymer, and cycling it at temperature this high may cause degradation, even under inert atmosphere.

Do you know if this cycling was done for your materials? If not the measurements are not trustworthy. I would look in the Polymer handbook for a literature value for the Tg of PMMA. Its a very standard measurement on a very standard polymer, it should be easy to find.

By definition the Tg is below the Tm. The Tm of PMMA is 160C so any value above that is automatically wrong. In about 4 seconds of googling ("Tg of PMMA") I found this paper https://pubs.acs.org/doi/abs/10.1021/ma000302l?journalCode=mamobx which states that the bulk Tg of MMA is 108C. Seems trustworthy, but you should download the paper and see if it has a couple corroborating sources.

[JSUCY232]

OK. Thanks. And yes the temp is Celsius. Forgot to add the label. The original scan was done to achieve degradation, but I did not realize at the time that wasn't an optimal goal. It was the choice of the operator running the scan. I'm learning a lot about DSC, and am finding it interesting, so hopefully next time will be better.

[wildfyr]

You can run it to a high degradation temp on thr last run, but for the first two cycles I would only run it to a bit above Tm

[JSUCY232]

Thanks Wildfyr. It looks like I will need to wait until the operator returns from vacation to re-run the sample.