Synergistic short and long spacer fluorinated chain extenders for low refractive index waterborne polyurethane acrylate coatings

Abstract

Fluorination is an effective strategy to reduce the refractive index of polymer coatings for antireflection applications, yet achieving simultaneous enhancement in hydrophobicity and mechanical robustness remains challenging due to the inherent trade-off between fluorine surface enrichment and chain entanglement. Herein, we propose a synergistic spacer group modulation strategy by designing two complementary fluorinated chain extenders—F-DEOA with a short spacer and F-PETMP with a long flexible spacer—and incorporating them into a waterborne polyurethane acrylate (WPUA) system. The optimized DPWPUA-3 coating, containing only 6.79 wt% fluorine, achieves a refractive index of 1.5070 at 550 nm (reduced from 1.5276 for pristine WPUA), high transmittance of 92.84%, and excellent hydrophobicity with a water contact angle of 114.4°. It also exhibits Grade 0 adhesion to PET and good flexibility. Systematic analyses across three sample series reveal that the short F-DEOA chains alleviate physical entanglement among the long F-PETMP chains via a constraint-release effect, promoting efficient surface fluorine enrichment (8.82 wt% surface vs. 6.79 wt% bulk) and controlled microphase separation. Environmental durability tests further confirm that the coating retains its optical and hydrophobic performance after UV aging, solvent wiping, and water immersion. This work demonstrates that by maximizing the functional efficiency of fluorinated moieties through spacer group engineering, competitive optical and surface performance can be achieved with modest fluorine loading—thereby preserving the mechanical integrity, aqueous processability, and long-term durability essential for practical flexible optoelectronic applications.