High Performance Graded Rainbow Holograms via Two-Stage
Sequential Orthogonal Thiol–Click Chemistry
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Abstract
Orthogonal, sequential “click”
reactions were implemented
to yield novel polymeric substrates with the ability to record holographic
data. The base-catalyzed thiol–acrylate Michael “click”
reaction was implemented to yield a writable, stage 1 polymeric substrate
with glass transition temperatures (<i>T</i><sub>g</sub>) ranging from 0 to −26 °C and rubbery storage moduli
(<i>E</i>′) from 11.1 to 0.3 MPa. The loosely cross-linked
matrix also contained a novel high refractive index monomer 9-(2,3-bis(allyloxy)propyl)-9<i>H</i>-carbazole (BAPC) that did not participate in the thiol–Michael
reaction but allowed for large index gradients to be developed within
the network upon subsequent exposure to coherent laser beams and initiation
of the radical-mediated thiol–ene reaction. The holographic
gratings were recorded with 96% diffraction efficiency and ca. 2.4
cm/mJ of light sensitivity in 2 s under a 405 nm exposure with an
intensity of 20 mW/cm<sup>2</sup>. Subsequent to pattern formation,
via a thiol–allyl radical “click” photopolymerization
initiated by flood illumination of the sample, holographic materials
with high <i>T</i><sub>g</sub>, high modulus, diffraction
efficiency as high as 82%, and refractive index modulation of 0.004
were obtained. Graded rainbow holograms that displayed colors from
blue to red at a single viewing angle were readily formed through
this new technique