There is less resistence to applied stress from the decrease in intermolecular friction. The change in molecular mobility is significant enough that molded nylon parts can relieve molded in stresses as they absorb moisture. The absorption of moisture by nylon is a completely reversible physical reaction. Drying in an oven will drive off all but a small percentage of the water molecules which can only be removed through dissolution of the nylon molecular matrix. Addition of fillers reduces the effect of moisture both due to volume reduction of the ammount of nylon polymer in the mixture, and by sharing the attraction of the molecules somewhat reducing polarity and the available space for moisture molecules.
Reinforcements reduce the effects more than fillers due to nylons strong affinity for reinforcement.
In addition to the mechanisms which take place with fillers, the adhesion of the nylon molecular matrix to dimensionally stable reinforcements is stronger than than polar bonding of the water molecules and it dominates. Another area where moisture has significant effects on nylons is in processing. As with most crystalline polymers, the molecular chains are uncluttered by large substituent groups. They are flexible and regular in group spacing and crystallize readily. As with acetals, this crystallinity is responsible for properties of wear resistance, chemical resistance, thermal resistance, and unfortunately, higher mold shrinkage.