Dose-dependent evolution of weakened boundary layers on cycloolefin polymer surfaces under vacuum ultraviolet irradiation

This study investigates the dose-dependent evolution of weakened boundary layers (WBLs) formed on cycloolefin polymer (COP) surfaces under vacuum ultraviolet (VUV) irradiation using a xenon excimer lamp, addressing the limited understanding of WBL formation in VUV-induced surface functionalization. Mechanical weakening over a broad VUV dose range was quantified using erosion rates obtained from microslurry-jet erosion testing, where higher erosion rates indicate greater mechanical weakening. The results reveal a systematic three-stage evolution of the WBL with increasing VUV dose. At low VUV doses (≤200 mJ/cm2), mechanical weakening decreases monotonically with depth, indicating a single-layer structure characterized by a gradually weakened layer (GWL). At a threshold VUV dose (∼300 mJ/cm2), mechanical weakening increases sharply within the near-surface region, resulting in the formation of a rapidly weakened layer (RWL) with an estimated thickness of approximately 100 nm above the underlying GWL and resulting in a two-layer structure. At higher VUV doses (≥600 mJ/cm2), both the RWL and GWL continue to undergo progressive mechanical weakening with increasing VUV dose. Overall, this framework provides quantitative insights into WBL evolution on COP surfaces and contributes to optimizing VUV irradiation conditions by balancing surface functionalization and mechanical weakening in adhesion-critical applications.