So what's the verdict: does polymer degrade over time or not?
Well, as an aerospace engineer, I work with metals embedded in polymers in all sorts of sophisticated, top-secret military applications. So, I am very well-qualified to answer this concern....
Yes, age alone will steadily de-polymerize the plastic in a GLOCK, until it is reduced to dust. The process is called "quantum-mechanical entropic decay," and it attacks all engineering materials that are more complex than single atoms.
Oxygen, especially ozone, viciously attacks the polymer frame of a GLOCK from the moment that it comes out of the mold. This damage can be retarded only by storing your GLOCKs in a "vacuum safe" -- a gun safe with a built-in high-efficiency vacuum pump that removes the air down to no more than 10.3 micro-millibars of atmospheric pressure.
(Try to shoot your GLOCKs only on indoor ranges that don't have much airflow, or better yet, on ranges without any air present at all. Your bullets will lose velocity more slowly on such air-less ranges as well.)
Metal embedded in polymer (the frame rails on your GLOCK) is constantly being attacked, deep in the frame where you can't see it, by "molecular acids" that form at the interface between the polymer and the metal. This hidden failure mode can cause the frame rails to tear out of the frame when you least expect it. You can tell how badly this is happening to your GLOCK by looking for it -- if you don't see anything, the hidden damage is getting very serious, and catastrophic failure is likely imminent.
Also, stray electrical currents are set up in your GLOCK by the contact between the electrically insulative polymer and the electrically conductive metal. This eats away at the metal parts of the GLOCK. This is the same micro-galvanic decay process that destroys your home's water heater -- but unlike water heater makers, GLOCK intentionally does not provide a replaceable sacrificial annode on his guns. Planned obsolescence by G2, don't you know....
All of these processes are constantly at work in your GLOCK, degrading it, and will destroy it totally in about, oh, say, 50,000,000 years -- give or take a few tens of thousands of years....
(In case the above is too subtle, I am kidding -- except about the 50,000,000 years part.
Yes, age and exposure to moisture, light, and air will slowly degrade the engineering polymers used in handguns, but not at a rate that is of any concern on human timescales.)
I'm not sure if you are kidding about being an aerospace engineer as well as the rest. If not, maybe you could answer the question I've had about how much heat the polymer in a Glock can withstand on a day after day basis. Say, from being left in a car that is in the sun in 110 - 120 degree heat. I haven't been able to find anything that tells me just how hot the interior of a car gets in those conditions...
Yes, I really am an aerospace engineer -- although I am not formally trained as a polymer specialist.
If you want a serious specific answer about effects of "temperature soaking" on GLOCK polymer, here it is -- I can't answer specifically about the exact polyamide that GLOCK uses, because it is proprietary and Gaston won't tell anyone exactly what it is.
But, I can answer generically, since I believe that GLOCK probably uses a proprietary varient of cast (not extruded) Nylon 6. This substance, which in its cast form is actually partially crystalline like a metal, can be manipulated to obtain a wide range of balance between such physical properties as "glass transition point" (G-sub-T), melting point, and max % of water absorbtion.
Since G2 himself is a polymer chemist, no doubt he manipulates his Nylon 6 variant (itself a varient of DuPont's proprietary Nylon 6,6 -- remember the Remington Nylon 66 rifle?) to achieve a balance of these properties that he feels is ideal for his firearms.
Ever notice the rock-hard yet still slightly "waxy" feel to the surface of a new GLOCK right out of the box? To a polymer chemist, that feel tells you that Gaston Glock really knows his polymers.
From the surface feel, I suspect that Gaston Glock probably found a way to push his GLOCK frame material well towards the polyphthalamide (PPA) point, thus reducing the tendency towards water absorbtion while still also elevating BOTH the melting point and the T-sub-G point.
Both yield-strength and the stiffness of the finished material are enhanced by this. This would be done by chemically manipulating the separation of the amide groups within the polymer, thus enhancing the final strength of the resulting hydrogen-bonding within the cast crystal matrix.
Now, back to your question: ordinary cast Nylon 6 melts at around 500 degrees F. Gaston's custom-blended meterial is probably more like a Nylon 4/6, or even close to a PPA, which means that it is stiffer and melts higher, around 550 degrees F or even more. (By the way if somebody is willing to melt their GLOCK and tell us the exact temp at which it melts, we will know more about it.)
So the temps inside your enclosed car (probably about 140 degrees F max inside in summer) will not come anywhere near melting your GLOCK. The bigger concern is that a prolonged "soak" in that temperature range would degrade the primers and powder in your onboard ammo.
Another variable with the effect of a 140 degree F heat soak on your GLOCK frame is the temperature at which the GLOCK factory molds are run. This would determine the degree of solid-state crystallization achieved upon curing, and the residual internal stresses left within the frame -- i.e., if there are a lot of internal stresses left over from casting, the gun might warp during a prolonged heat soak long before if melted.
But, we have to assume that Gaston, clever boy that he is, runs his molds at a fairly high temperature matched to his polymer blend, so as to achieve both sufficient solid-state crystallization and also some decent degree of stress-relief upon cooling of the cast frames.
See why not just everybody can make a polymer gun like GLOCK? Sort of gives you a newfound respect for our boy G2, no?
So after all of this, the short answer to your question is, your GLOCK will survive temps far beyond anything your ammo can, but you should not leave your ammo in a hot car for too long.
(By the way, never ever put your GLOCK in a dishwasher on the hot cycle. All polyamides, even PPAs, will absorb up to about 2 or 3 percent by weight of hot water, when heated above about 145 degrees F. So while the dishwasher water won't melt anything, it will permanently add water inside the polymer matrix between the polymer strands, which while it will not wreck it, is not especially good for it.)