We report on the use of hyperbranched polymers (HBPs) acting as mobile organic nanoparticles doped in methacrylate photopolymers for highly efficient volume holographic recording. Two types of reduced HBPs having the average size of 10nm are synthesized. The diffraction efficiency near 100% in the green (532nm) and substantive polymerization-shrinkage suppression in this photopolymer system are demonstrated. Periodic assembly of HBPs under holographic exposure is also observed.
Highly branched poly(methyl methacrylate) (PMMA) stars were prepared by one-pot approach
based on N,N-diethyldithiocarbamate- (DC-) mediated living radical photopolymerization. Soluble alternating
hyperbranched microgel formation was initiated fast with preferential consumption of inimer 4-vinylbenzyl N,N-diethyldithiocarbamate (VBDC) and 4,4‘-bismaleimidediphenylmethane (BMIM) in the presence of an excess of
methyl methacrylate (MMA). PMMA arms could then grow from DC groups of microgel as macroinitiator.
Next, star−block copolymers consisting of diblock arms [(AB)
f
-DC; arm: PMMA-
block-poly(n-butyl methacrylate)
(PBMM)] were synthesized by photoinduced atom transfer radical polymerization (ATRP) of n-butyl methacrylate
(BMM) initiated by the functionalized PMMA star (A)
f
-DC in the presence of CuCl/bipyridine (bpy) catalyst.
Subsequently, star−block copolymers consisting of triblock arms [(ABA)
f
-DC; arm = PMMA-
block-PBMM-
block-PMMA] were synthesized by similar method initiated by (AB)
f
-DC macroinitiator. It was found from light
scattering data that such (AB)
f
and (ABA)
f
star−block copolymers (arm number f = 28) not only took spherical
shape but also formed a single molecule in solution. We performed the construction of nanocomposites by using
these (ABA)
f
stars as fillers. Free radical polymerization products from the mixture of MMA and (ABA)
f
star
provided transparent fims. It was found from electron microscopy observations that (ABA)
f
stars distributed
molecularly and homogeneously in a PMMA matrix, because such star−block copolymers promoted the ordering
strucrure.
Hyperbranched polystyrenes (HPS) were prepared by living radical polymerization of 4-vinylbenzyl N,N-diethyldithiocarbamate (VBDC) as an inimer under UV irradiation. These HPS exhibited large amounts of photofunctional diethyldithiocarbamate (DC) groups on their outside surfaces. We derived star-HPS (SHPS) by grafting from such HPS macroinitiator with methyl methacrylate (MMA) or ethyl methacrylate (EMA). The ratios of radius of gyration to hydrodynamic radius R g /R h for HPS and SHPS in tetrahydrofuran (THF) were in the range of 0.74 -0.90 and 1.05-1.12, respectively. HPS and SHPS behaved in a good solvent as hard and soft spheres, respectively. We demonstrated the structural ordering of both branched polymers in THF through small-angle X-ray scattering (SAXS), by varying the polymer concentration. As a result, HPS and SHPS formed face-centered-cubic (fcc) and body-centered-cubic (bcc) structures, respectively, near the overlap threshold (C*).
We demonstrate a new class of holographic recording constructed by hyperbranched polystyrene (HPS) or poly(ethyl methacrylate) (HPEM) dispersed in methacrylate photopolymers. Hyperbranched polymers behave as hard spheres in monomer and diffuse easily into gel networks without any aggregation. Holographic gratings are constructed by anisotropic distributions of polymer and hyperbranched nanoparticle layers formed by photo-polymerization of methacrylate monomers following phase separation of hyperbranched nanoparticles in the form of interference patterns by irradiation of two laser beams of different phase. High-contrast refractive index changes ( n) with high recording sensitivity and dimensional stability are achieved for the system of HPEM-[2-methylacrylic acid 2-4-[2-(2-methylacryloyloxy)-ethylsulfanylmethyl]-benzylsulfanyl ethyl ester] (UV3000) as methacrylate monomer.
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