I have connected a schedule 80 PVC pipe to a 1" copper pipe using a threaded adapter. I have applied teflon tape to the threads; and everything was fine (no leaks) UNTIL the temperature dropped into the low 40's! The PVC appears to have contracted in the cold and cracked where it meets the copper. Result - big time leak!

Is there some kind of connecting union or fitting that would prevent this problem? The PVC is 1 1/2" I.D. and I am reducing to the 1" I.D. copper with a PVC reducer. The PVC is cracking where it comes into contact with the copper which apparently isn't contracting as much in the cold.

There are fittings called "G" couplings that will adapt a copper pipe to a PVC pipe.
When you screwed the PVC and the copper together did you use a male or female PVC adapter? It is very easy to tighten the fitting too tight if you are using a female PVC fitting and screwing it onto a copper male thread. This could be what happened.