Low polymerization-shrinkage nanoparticle-polymer composite films based on thiol-ene photopolymerization for holographic data storage

Stoichiometric thiol-ene ratio dependence of refractive index modulation and polymerization shrinkage in holographic nanocomposite materials

“…As a result, gelation takes place late in conversion in the step-growth polymerization process, leading to reduced volume (bulk) shrinkage and stress [12]. Indeed, using holographic NPCs with organic nanoparticles and the stoichiometric mixture of multifunctional mercaptopropionate thiols and allyl ether enes, we showed shrinkage reduction as low as 0.3% with Δn and S as high as 8×10 −3 and 1014 cm/J, respectively, in the green [9,10].…”

“…The dispersion of 25 vol.% silica nanoparticles having the lower refractive index (1.46) would lead to large Δn since the refractive index of the thiol-ene monomer mixture is between 1.52 and 1.53 in the solid phase [8][9][10]. In addition, silica nanoparticles would provide better thermal stability of a recorded hologram than the case of using organic nanoparticles [9,10] since the coefficient of linear thermal expansion for inorganics is much smaller than that for organics and the sign of the thermo-optic coefficient dn/dT is opposite between inorganics and organics [7]. To sensitize in the green, we employed a titanocene organo-metallic complex, Irgacure 784 (Ciba), in combination with benzoyl peroxide, BzO 2 (Aldrich), as a visible initiator/sensitizer system [13].…”

“…To further reduce shrinkage of NPCs, we recently proposed the use of thiol-ene photopolymerizations that proceed via a step-growth radical addition mechanism together with nanoparticle dispersion in thiol-ene monomers [8][9][10]. The main idea of using the step-growth radical addition polymerization for volume holographic recording stems from the fact that while high-molecular-weight polymer is formed immediately in chain-growth radical polymerization of crosslinking (meth)acrylate monomers, it is not obtained until the later stage of the polymerization in the step-growth polymerization process [11].…”