Why does a 1:1 mixture of Polyvinyl Alcohol (PVA):H2O decompose/carbonize during heating?
I experienced similar phenomena in my experiments.
In my case, I put PVA pallets (M.W. is 84000~12500, 87~89% hydrolyze degree) in the oven at 120∘C. After over 12 hours, the color became yellowish (partially brown). But after dissolving the pallets in the water, there was no difference between control sample(without heat treatment).

There are several papers about formation of polyene in PVA chain under acid catalyst. And, almost commericial PVA product has always small amount of acetic acid which hydrolyzed from PVAc spontaneously. Therefore partial polyene-nization (semicarbonization) was possible with heating under melting temperature.

Also, I made acidic PVA film by solution casting (10% PVA, pH 2 by adding HCl). Then, the color of PVA film was changed to black at 120∘C in the oven. 

So color change of PVA is common while experiments. Maybe it is related with carbonization.


maybe polyenenization or carbonization.
I remembered that PVA becomes viscous near 150-180∘C. Measuring Tm of polymer exactly is not easy. Especially hot plate is not suitable tool to analysis physical properties of polymers. Maybe due to cooling by air, melting of PVA is indistinguishable to glass-transition. I recommand to use DMA instead of your experiment.
To measure Tm, you don't have to mix PVA with water. DSC or DMA is better.