There is quite a range of variability in PMMA. It's even worthwhile to look at Wikipedia: http://en.wikipedia.org/wiki/Poly(methyl_methacrylate
Scan down to the part that says: "modification of properties". That only hints at the range of property modification that manufacturers practice. The point I'm making is that PMMA can be very different from plexiglass. Then again, maybe plexiglass would work in a panel absorber?
When you look at PMMA membranes, a whole bunch of other properties begin to kick in because blends such as PMA with PMMA and the addition of a dense filler can yield a rubbery product that could easily be mistaken for a filled vinyl like LMV, just based on handling it. Much of the research over the last 15 years on PMMA membranes have involved chemical modification that leads to controlled porosity and other treatments that affect the hydrophylic/hydrophobic balance, behavior in acidic environments, etc.