Chromophore-Functionalized Glassy Polymers with Large Second-Order Nonlinear Optical Responses. Synthesis, Characterization, and Architecture-Processing-Response Characteristics of Poly(p-hydroxystyrenes) Functionalized with Chiral Chromophoric Side Chains

This paper reports a systematic study of the synthesis, characterization, and second-order nonlinear optical properties of a series of chromophore-functionalized glassy polymers prepared by covalently appending chiral N-(4-nitrophenyl)-(S)-prolinoxy ((S)-NPP) moieties to poly(p-hydroxystyrene) (PHS). Key issues are how macromolecule architecture, chromophore-chromophore interactions, and processing (including corona and contact electric field poling) modulate second harmonic generation (SHG) efficiency as well as the temporal stability thereof. As probed by optical spectroscopy, optical rotation, FT-IR, homochiral versus racemic NPP substituents, and the dependence of SHG response on the chromophore number density, the effects of chromophore-chromophore interactions are found to be relatively minor for 16—90% functionalization of the PHS backbone. The nonresonant second harmonic coefficient, d₃₃, for corona-poled films varies from 12.8 × 10^-9 to 79.0 × 10^-9 esu (λ = 1.064 μm) over this same functionalization range. The relationship of d₃₃ and the contact poling field is approximately linear with the poling field up to ~1.3 MV/cm, beyond which saturation of the response is observed. Increasing NPP functionalization levels are accompanied by declining SHG temporal stability, which approximately tracks polymer T_g values. SHG temporal stability subsequent to poling can be substantially enhanced by prepole film annealing which removes volatile, plasticizing contaminants (and/or minimizes free volume) and by simultaneous cross-linking with diexpoxide reagents. For example, simultaneous poling and cros-linking of (S)-NPP—PHS films with 1,2,7,8-diepoxyoctane effects a 3-fold enhancement in the long-term time constant for SHG decay at 25 °C. However, the influence of various diepoxides on film transparency and SHG temporal characteristics is a sensitive function of stoichiometry and diepoxide conformational mobility. An accompanying contribution explores quantitatively how poling methodology influences macromolecular dynamics and SHG temporal stability.