Dynamic Polymer Binder with Light-Regulated Molecular Weight via Addition-Fragmentation Chain Transfer for Holographic Recording

Dynamic covalent chemistry was employed to improve the holographic grating performance by facilitating diffusion of monomers and network rearrangement in a holographic photopolymer system. Because of the high degree of spatiotemporal control afforded by the radical-mediated addition-fragmentation chain transfer reaction between allyl sulfide (AS) moieties and thiols, this chemistry proved well-suited for the formation of high-quality holographic photopolymers. Through exchange with a monofunctional thiol, polymer chains incorporating AS along the backbone exhibited a decrease of molecular weight of more than 80% upon the light exposure. The dramatic molecular weight reduction resulted in a corresponding viscosity drop of the whole resin system up to 1 order of magnitude. To take advantage of the covalent bond exchange, the polymers with AS (termed AS polymers) functional groups were utilized as “dynamic binders” in thiol-ene based holographic photopolymers designed to delay and reduce the viscosity increase of recording media during the holographic exposures. Such light-regulated viscosity reduction was found to greatly improve the recording formulations with limited diffusion of monomer during the recording. The systems utilizing AS polymer demonstrated a notable improvement of the refractive index modulation as large as 70% compared with the nondynamic analogue. Additionally, the haze in these dynamic systems was almost unchanged compared to control groups where no exchange occurred.