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Thermo-gravimetrc analysis, often refered to as TGA, measures the mass of an unknown solid substance as it heats up, of course for the elemental analysis of the very substance. When the substance is heating up, due to special chemical or physical reactions, the mass is decreasing. e.g.: Dehydration. Anyhow, we know that in special circumstances the mass of the substance will increase. When? That is the question. ( This is not quantum physics! I am talking about rest mass, if you had wondered. As a result, the increase will have nothing to do with Einstein's relativity formulas.) The diagram below is an example of some normal polymer tested in normal conditions, no relations with the exceptions!

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update: There also is a special type of gas that is blown above the surface of the substance. The system is designed in a way that doesn't allow the gas to "interfere" with the measurements. And I just got lucky to find another substance experimented. I estimate it is going to be a liquid alkanol, maybe ethanol. Diagram two displays the experiment's results. I know it seems a little bit vague, but all the info that I provided at above is all we have got to answer the question.

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  • $\begingroup$ By "mass" you mean density aka mass per cm3 or do you mean literally total mass of some plastic object? Like Klaus written bellow,there is nothing from nowhere,if that gas reacts with that polymer and puts some of its atoms into it,that will be the source of the extra mass.If you mean density,it could perhaps be some kind of polymer that gets denser whem it transitions from solid to liquid phase.I have never heard about any polymer like that,I am not even sure if it is possible,thats my best guess given your description. $\endgroup$ – wav scientist Mar 27 '18 at 16:21
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There's nothing from nowhere: Heating of a polymer in vacuum will never lead to an increase in mass.

In the presence of an acidic catalyst and water (or alkanols, etc.), however, polyesters might partly undergo cleavage and/or transesterification with a net mass uptake.

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The only "special " circumstance I can think of is when the polymer can actually absorb or adsorb moisture or just any gas when it is heated and thus "increasing" the mass somehow.

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