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Name: Natalie C.
Status: student
Age: 18
Location: N/A
Country: N/A
Date: Monday, January 20, 2003


Question:
What is the molecular structure of polystyrene? When we did the disappearing coffee cup experiment, we had a thick sticky residue left over that floated on the top of the acetone. We were wondering exactly what that was, and why it is there? We thought maybe the brand of the cup or something else?


Replies:
Natalie,

It is not yet easy to draw chemical structures on this web-site. However, an organic chemistry book will likely have an illustration for the monomer (styrene) and the polymer, polystyrene.

If the cup to which you refer was clean, pure polystyrene, it should have dissolved in acetone and resulted in a water-clear solution. Perhaps the material floating atop your solution was some part of the cup that was insoluble in the acetone. Your supposition (a brand label?) might be correct.

I hope you did not pour the solution down the drain. If you do, it will make a terrible clog that will not make the maintenance men happy. LOL.

Regards,
ProfHoff 554


What you observed was probably polystyrene with occlusions of air that made the overall density less than acetone -- hence it floated. Polystyrene (and other polymers as well) have extremely high molecular weight -- 10^5, 10^6 or even higher. They also are often cross linked, that is, the long chains of polymer units are connected to one another by chemical bonds, forming a 3-dimensional network, so that the entire polymer is a single molecule.

This is probably true of Polystyrene foam used in coffee cups. In the case of these cross linked polymers, they cannot dissolve in the usual sense of the word. Rather, they are swollen by the solvent and form an insoluble "glob". This is not unique to polystyrene. Corn starch and gelatin, for example, behave similarly when added to warm water and cooled.

The "swelling" is not due to the brand of cup. It is characteristic of all polystyrene polymers "dissolved" in a good solvent like acetone.

Vince Calder


Polystyrene is a repeating polymer with the formula (CH2-CH[Ph])n, where Ph is a phenyl (C6H5) ring. Chances are that's what was in the residue. Did you weigh the cup before hitting it with acetone, and weigh the residue after letting it dry? That would give you an idea of what fraction of the original cup ended up in the residue.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois



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