Should I use polypropylene or polystyrene plates to store my library (siRNA, miRNA, shRNA, cDNA, ORF)?

There is a trade-off between polypropylene and polystyrene for storage at -80C. With polypropylene, the plates can warp making use with automated/robotics systems problematic. With polystyrene, the plate can be very brittle while frozen at such a low temperature, but the problem is not permanent - once the plate is thawed it is not as brittle any longer. So you must decide which features are more important to you. We tend to use polystyrene for most things as it is more conducive to our automated manufacturing environment. For synthetic siRNA and miRNA libraries, a storage temperature of -20C can be used to minimize the problems associated with storage of plates at lower temperatures.