Can state of matter from liquid to gas maintain the same molecular structure in PE or in polymers in general, like water does?

If I’ve got a reactor containing PE-Polyethylene $(\ce{{C2}$H\_\text{4})n$}$ (solid) and the rest is filled with nitrogen gas. The system is **closed**. Supposing we have an initial temperature of $30~\mathrm{°C}$ and gradually increase the temperature to $180~\mathrm{°C}$. We’ll observe a change in state of the PE, passing from solid to liquid, at this point the *molecular structure is the same*.


If temperature is increased beyond **$250~\mathrm{°C}$ pyrolysis** will occur on PE and syngas will be produced, at this point the gas molecules will have different structures compared to $(\ce{{C2}$H\_\text{4})n$}$, thus "we’ll not observe" a change of phase from liquid to gas maintaining the same molecular structure. Gas molecules will nave n types of molecular structures.


* Is it possible by **varying PVT** to have a change of phase, from liquid to gas, of PE and **maintain the same molecular structure**. Just like with water->vapor?
* If this is not possible, why? what are the topics to study?


