Broadband antireflection of block copolymer/homopolymer blend films with gradient refractive index structures

Abstract

The factors that influence the broadband antireflection of block copolymer/homopolymer blend films were systematically investigated. The gradient refractive index structure, which was generated by removing the PMMA content from a film of PS-b-PMMA/PMMA blends, contributed to the broadband antireflection. The volume fraction of the PMMA block (f_PMMA), the total degree of polymerization (N) and the weight percent of the homopolymer played a key role in the formation of the gradient porosity index inner structure. By increasing χN, the PS porous structure was kept from collapsing after removal of the PMMA domain, which led to a stable gradient porosity ratio structure and higher transmittance. While increasing f_PMMA, the PMMA domains changed from a dispersed phase to a continuous phase, which caused the residual PS structure to collapse more easily and the transmittance decreased. The best broadband antireflective properties were achieved from the 45 mg mL⁻¹ solution of PS-b-PMMA (M_n = 68 000–33 500, f_PMMA = ∼25%) blended with 30 wt% PMMA: a mean transmittance of 98.4% at full spectrum, and >98.0% in the visible–NIR range between 600 nm and 2000 nm.