Atomistic modeling of electric field poling of PMMA-based polymer material with guest quinoxaline chromophores

Abstract

The effect of applied electric field strength on the chromophore alignment in composite PMMA-based material is studied by atomistic modeling. Polymer material of real density (1.07 g/cm3), established in the course of modeling using NPT protocol, is considered along with model materials with reduced density (0.6 and 0.9 g/cm3), applied fields ranging from 100 to 1000 V/μm. Glass transition temperature, at which materials simulation was performed, was estimated in the course of multistage simulation. Simulation was performed with OPLS3e force field. Both centric, 〈P2〉, and acentric, 〈cos3Θ〉, chromophores order parameters were estimated. For the polymer material with high density, ρ = 1.07 g/cm3, the value of the order parameter 〈cos3Θ〉 grows from 0.25 to 0.30 for field strength varying from 300 to 1000 V/μm; the values of 〈P2〉 for chromophores are in agreement with the results of measurements by UV–vis spectroscopy. The obtained values of order parameters (both 〈P2(ch)〉 and 〈cos3Θ〉 for model materials with decreased density (0.6 and 0.9 g/cm3) are quite close to each other and seem to be overestimated compared to the values for the material with high density; this observation is in agreement with calculated values of free volume in the composite. The applied approach may provide guidelines for the elaboration of optimal poling protocols used at the development of composite polymer materials with quadratic nonlinear optical properties, containing quinoxaline chromophores-guests possessing high dipole moments (more than 20 D).