Hydrothermal Aging of Polycarbonate and Its Polycarbonate-Polydimethylsiloxane Copolymer Blends: Desorption Behavior Related to Hydrolytic Stability

The moisture uptake process in polymers involves a complex interplay between diffusion-driven sorption and structural relaxation. While the moisture sorption behavior of bisphenol-A polycarbonate has been extensively characterized under hydrothermal conditionings, its structural relaxation behavior remains unclear. This study investigates the long-term moisture uptake characteristics and tensile/impact property evolution of commercial polycarbonate and its blends with polycarbonate-polydimethylsiloxane copolymer under hydrothermal conditions. Following prolonged hydrothermal aging and moisture saturation, samples with lower desorption rates show enhanced hydrolytic stability. Here, we propose that chemical aging-induced chain scission facilitates structural relaxation (physical aging), which dominates over moisture diffusion during prolonged aging. Molecular chain rearrangement associated with structural relaxation drives the observed desorption behavior. Gel permeation chromatography confirms chain scission in polycarbonate under hydrothermal exposure, while polycarbonate-polydimethylsiloxane copolymers exhibit superior hydrolytic stability relative to neat polycarbonate. Incorporating polycarbonate-polydimethylsiloxane copolymer (7.5 to 22.5 wt.%) reduces desorption rate by 25 to 50 %, with an inverse correlation between desorption rate and property retention observed in blends. These findings establish the relationships among composition, moisture sorption/desorption characteristics, and hydrolytic stability in polycarbonates, providing insights for the design of hydrothermal-resistant engineering thermoplastics.