Ask A Scientist Chemistry Archive

name         Raina F.
status       other
age          20s

Question -   From a previously asked question, Plexiglass was
identified as consisting of polymethyl methacrylate with rubber
copolymers to maintain flexibility and prevent cracking.  However,
Plexiglass does crack when simple solvents such as isopropyl alcohol are
applied to the Plexiglass surface.  Do you know what is happening
chemically to the polymer to cause the cracking?
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Plexiglas (one "s") is polymethyl methacrylate (PMMA). It comes is a
number of grades and types. One is impregnated with rubber particles to
improve its impact strength. The inclusions are designed to have the same
index of refraction as the PMMA; however, that is only true at room
temperature. At lower temperatures that grade of impact resistant PMMA turns
hazy because the index of refraction no longer matches -- but that is beside
the point of your inquiry.

Plexiglas embrittles when it is exposed to solvents because of a phenomenon
called
"solvent crazing" -- the solvent softens and / or swells exposed areas that
give rise to microscopic stress fractures. These micro-cracks serve as point
weakening of the sheet in a manner similar to scoring the sheet with a glass
cutter, except on a much smaller scale. In fact, PMMA can be cut just like
glass using a glass cutter, but be careful, the edges are "razor sharp".

Vince Calder
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