Manipulating the Relative Rates of Reaction and Diffusion in a Holographic Photopolymer Based on Thiol–Ene Chemistry

Properly balanced reaction and diffusion kinetics are crucial for achieving optimal performance in holographic photopolymers. However, a comprehensive study on the effect of reaction and diffusion on the performance of thiol–ene-based holographic photopolymers has not been performed previously. To determine the relationship between reaction and diffusion, the refractive index modulation (Δ n ) of holographic gratings recorded in a model thiol–ene photopolymer system was evaluated with controlling the rates of reaction and diffusion processes. By changing the molecular weight of the polymer binder, the diffusion rate was varied over orders of magnitudes with the highest Δ n of 0.026 achieved at a molecular weight of 2.9 × 10 4 Da. Meanwhile, the haze was significantly reduced in binders of higher molecular weight. Similarly, as the reaction rate was reduced in accordance with lowering the light intensity, the Δ n reached a peak value of 0.023 at 7 mW/cm 2 and was found to decrease at both higher (2500 lines/mm) and lower (1000 lines/mm) spatial frequencies. In particular, Δ n approaching 0 was observed at a very low intensity (2 mW/cm 2 ) and when the binder with a molecular weight as low as 0.5 × 10 4 Da was used. An analogous formulation incorporating a secondary thiol, which has slower reaction kinetics and a more stable thiyl radical relative to the primary thiol, exhibited a lower Δ n, especially at higher spatial frequencies. Through one-step thiol-Michael addition, the functionality of a tetrathiol monomer was reduced to various extents to obtain a series of thiol–ene photopolymers with precise control over gel point conversions, among which the thiol with an average functionality of 3.5 realized the highest Δ n of 0.028. An enhanced reactive binder with norbornene pendant groups was also synthesized, and holographic gratings recorded in it showed notably higher Δ n at low light intensities compared to those recorded in non-reactive or less reactive binders.