Polystyrene, Polypropylene and Microwaves
I am doing an experiment to see what happen when
plastic containers with water inside are heated in the
microwave. When heated with water inside, polystyrene
containers sagged and deformed, but polypropylene containers did not.
I try to explain my results based on the learning that each
polymer has different glass transition temperature and melting
temperature. I want to know whether my reasoning below is correct.
Polystyrene has glass transition temperature of 100(C) and melting
temperature of 240(C). When microwaved with water
inside and the water reached the boiling point of 100C,
polystyrene reached its glass transition temperature and lost the
rigid structure and sagged. It did not melt because temperature
did not reach 240C. On the other hand, polypropylene has glass
transition temperature of -18C and melting temperature of
175C. Polypropylene container was at the softer state above the
glass transition temperature at room temperature before heating
and the heating did not raise the temperature to 175C, so it did
not show any sagging/deform or melting, except it felt
softer. Am I correct in reasoning? Is there other way to explain the result?
Your reasoning is pretty much on target. The difference is that
polypropylene, while it has a lower glass transition temperature,
also has crystalline domains imbedded in its structure. Polystyrene
has an essentially random structure in its chains. So even though
polypropylene softens the hard domains provide hard regions that do
not "melt". So while it softens it maintains its structural
integrity. You have to heat it to about 175C for the crystal
domains to "melt". An approximate analogy would be the comparison
of "pure" polystyrene compared to polystyrene containing a hard
filler, like sand. In this case, even though the polystyrene
softens, the hard filler allows it to resist sagging.
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Update: June 2012