Abstract
The microscopic and semi-macroscopic mechanisms responsible for photoinduced mass transport in functionalized azo-polymers are far from deeply understood. To get some insight into those mechanisms on “microscopic” scale, we studied the directed photoinduced motion of single functionalized polymer chains under various types of polarized light illumination using Monte Carlo bond fluctuation model and our kinetic Monte Carlo model for photoinduced mass transport. We found sub-diffusive, diffusive and super-diffusive regimes of the dynamics of single chains at constant illumination and mostly super-diffusive regime for directed motion in the presence of the gradient of light intensity. This regime is more enhanced for long than for short chains and it approaches the ballistic limit for very long chains. We propose a physical picture of light-driven inscription of Surface Relief Gratings (SRG) as corresponding to a dynamical coexistence of normal and anomalous diffusion in various parts of the system. A simple continuous time random walk model of SRG inscription based on this physical picture reproduced the light-driven mass transport found in experiments as well as the fine structure of SRG.
Highlights
Thin films of azobenzene functionalized polymers [1] may develop a periodic surface corrugation pattern called Surface Relief Grating (SRG) when exposed to interfering polarized laser beams in a Degenerated Two Wave Mixing (DTWM) experiment [2,3]
We have proposed a generic stochastic Monte Carlo (MC) Bond Fluctuation Model (BFM) [39] of the photoinduced mass transport in azo-polymers [38], which mimics the effects of multiple photoisomerization cycles of functionalized dyes in a host polymer matrix, in the presence of spatially inhomogeneous light illumination
The object of main interest in this study is a polymer chain functionalized with azo-dyes (e.g., DR1, see Figure 1) in the presence of spatially inhomogeneous light illumination used for an inscription of SRG in DTWM experiment
Summary
Thin films of azobenzene functionalized polymers [1] may develop a periodic surface corrugation pattern called Surface Relief Grating (SRG) when exposed to interfering polarized laser beams in a Degenerated Two Wave Mixing (DTWM) experiment [2,3]. The origin of photofluidization scenario goes back to Kumar [12], who formulated the concept of light-induced plasticization caused by reorientation of chromophores (via trans ↔ cis cycles). It is pointed out that the phenomenological orientation approach based on the effective potential provides results in a good agreement with experimental data for photodeformation of azobenzene-containing polymers [27,28,29,30,31,32,33,34]. The effective orientation potential can be applied to study the reorientation kinetics, deformation and mass transport of a large variety of azo-materials, e.g., amorphous, liquid-crystalline and cross-linked polymers, azobenzene-functionalized dendrimers and brushes and many others The orientation approach is justified by a theoretical analysis of kinetics of photoisomerization and time evolution of ordering in azobenzene-containing materials [37]
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