I did some googling looking for definitive answers, but didn't find any. I did find this post from another forum from someone who sounds pretty knowledgeable:
I've mentioned in a few other places here that I'm a polymers guy. Here are a few thoughts:
The shaft resin and adhesives used in golf clubs are thermosets - they do not "melt" in the sense that they can be reworked. They do, however, soften - lose their strength - at higher temperatures. Think of a how stiff a rubber garden hose is in winter vs summer. But it won't "melt." Epoxies soften too, just at higher temperatures. The epoxy adhesives used in golf clubs soften enough to yank the shaft from the clubhead at about 250 degrees F under force. That is hotter than the interior of a car gets on the hottest day, and it is hotter than you can hold in your hands. Most reports I have read say the interior temperature gets to about 50-60 degrees above the outdoor temperature, So on a 100 degree day, the interior of a car can approach 160 degrees. I have seen claims of 200 degrees, but even this is likely too low to soften the resin enough under a full swing, even if you could hold the club. The other interesting thing is that the trunk is 5-10 degrees cooler than the interior, because the trunk has no glass/greenhouse effect.
That said, moderate temperatures are less harmful to clubs than extremes - hot or cold - so I tend to keep the clubs out of the car unless I know I'll be playing that day. How much more harmful is hot storage? I would drive across the South on a sunny 90 degree week-long road trip with my clubs in my trunk, but if I don't need to keep them in there, I don't. I don't dispute that some of you saw clubheads fly, but temperatures were probably at most a contributing factor, not the primary factor.
Grips are a little different. They are softer at lower temperatures, and they may contain additives to tackify the grip, or at least things like stabilizers/antioxidants. Sometimes, these additives are designed to be incompatible with the rubber used in the grips, so that the tackifier or whatever slowly bleeds to the surface so the tackiness is always being renewed - until it gets really hot, then the bleeding occurs much faster and the grips get, to use a technical term, yucky. Even compatible additives can migrate more to the surface in really hot weather and create some of that same yuckiness. And if the grips are made of the right (or wrong) type of rubber, trunk temperatures could allow irreversible deformation of a grip that is being pushed by another shaft. It didn't actually melt, but it was soft enough to allow the deformation to go far enough that it wouldn't bounce back. Those of a certain age who used to pull six-pack rings probably did just that with the plastic. For the rest of you, think of a tree branch. Under gentle winds, the branch yields and recovers. Given too much wind or ice, the branch will bend or break and not rebound. It takes less force to deform when the rubber is hot - or more force/longer times at lower temperatures.
Bottom line - IMHO, I avoid hot or cold storage, but I consider it more of a good-to-do than a must-do.