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Name: James
Status: student
Grade: 6-8
Location: IL
Country: N/A
Date: 1/20/2006

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.

Vince Calder

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