Photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study

Abstract

We perform molecular dynamics simulations of azobenzene containing side-chain liquid crystalline polymer subject to an external model field that mimicks the reorientations of the azobenzenes upon irradiation with polarized light. The smectic phase of the polymer is studied with the field applied parallel to the nematic director, forcing the trans isomers to reorient perpendicularly to the field (the direction of which can be assosiated with the light polarization). The coupling between the reorientation of azobenzenes and mechanical deformation of the sample is found to depend on the field strength. In a weak field the original smectic order is melted gradually with no apparent change in the simulation box shape, whereas in a strong field two regimes are observed. During the first one a rapid melting of the liquid crystalline order is accompanied by the contraction of the polymer along the field direction (the effect similar to the one observed experimentally in azobenzene containing elastomers). During the slower second regime, the smectic layers are rebuilt to accomodate the preferential direction of chromophores perperdicular to the field.

Reorientation Dynamics of Chromophores in Photosensitive Polymers by Means of Coarse‐Grained Modeling

“…We will show that if the state of orientation order of trans isomers is chosen as the matching criterium, it is possible to introduce the effective potential, which approximates the process of stochastic reorientation rather well. Thism eans that our previous analytical studies [18-20, 26, 27] and computer simulation studies [21,22] are well-grounded and their predictionsc an be safely used for the further developmento ft hese fascinating materials.…”

“…Such an effect has been confirmed in computer simulations of av olumee lement of ap hotosensitive polymer subjectt ol inearly polarized light. [21,22] Different signs of deformation with respect to the polarization vector were observed for polymer systemso fd ifferent molecular architectures. For (e.g.…”

“…[39],e xplainst he formation of the surface-relief gratings. The simulations, [21,22] as well as related theoreticals tudies [18][19][20] did not account for the explicit presence of cis isomers (see Effect 1a bove). Instead, the cis isomers were assumedt op articipate implicitly in the trans-cis-trans photoisomerization cycles leading to Effect 2.…”

“…The rotation potential, U rot ðqÞ,u sed in deterministic simulations [21,22] has asquarec osine form [Eq. (18)]:…”

“…On the coarse-grainedl evel, there exists stochastic Monte Carlo approaches to the isomerization transitionf or the description of polarization gratings as well as surfacer elief gratings. [16,17] Therefore, in our previous analytical [18][19][20] and computer simulation studies, [21,22] we simplified ad escription of the light-induced anisotropy by applying an effective orientation potential to the trans isomerst hat reorients the chromophores perpendicular to the vector E.T his potentialh ave been introduced independently by Chigrinove ta l. [23] and by Saphiannikovae t al. [24] The influence of the cis isomers was assumed to be negligible under visible-light irradiation, owing to their smallf raction (less than 10 %).…”