What sort of materials should an outdoor chaise lounge be made of to resist mold growth?

As long as the furniture you choose is designed expressly for outdoor use, you can have your choice of materials. Plastic rattan is really low-maintenance, but even treated wicker will resist mildew. A mild bleach solution used for cleaning will help prevent further mildew from forming too.

Having metal frame would be the best idea as far as the frame is concearned. Mold is a product of moisture,soo drying the cushions after rain will help also.