Ask A Scientist Chemistry Archive

name         James
status       student
grade        6-8
location     IL

Question -   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?
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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.

Vince Calder
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