^{1}and V. Teboul

^{1,2,a)}

### Abstract

We investigate the effect of the isomerization rate f on the microscopic mechanisms at the origin of the massive mass transport found in glass-formers doped with isomerizing azobenzene molecules that result in surface relief gratings formation. To this end we simulate the isomerization of dispersed probe molecules embedded into a molecular host glass-former. The host diffusion coefficient first increases linearly with f and then saturates. The saturated value of the diffusion coefficient and of the viscosity does not depend on f but increases with temperature while the linear response for these transport coefficients depends only slightly on the temperature. We interpret this saturation as arising from the appearance of increasingly soft regions around the probes for high isomerization rates, a result in qualitative agreement with experiments. These two different physical behaviors, linear response and saturation, are reminiscent of the two different unexplained mass transport mechanisms observed for small or large light intensities (for small intensities the molecules move towards the dark regions while for large intensities they move towards the illuminated regions).

We acknowledge support from Université d'Angers under an “ARIANE-Aides à la mobilité” funding grant. We are grateful to David Chandler's group in Berkeley for interesting discussions.

I. INTRODUCTION

II. CALCULATIONS

III. RESULTS AND DISCUSSION

IV. CONCLUSION

### Key Topics

- Isomerization
- 61.0
- Diffusion
- 28.0
- Viscosity
- 17.0
- Materials properties
- 14.0
- Relaxation times
- 13.0

## Figures

(a) Diffusion coefficient of the host molecules versus the isomerization frequency f = 1/τ p . The temperatures are from bottom to top: T = 100 K (black circles solid and open); T = 140 K (solid and open red circles); T = 200 K (open red and solid grey triangles). These temperatures are effective temperatures of the coarse grain model. The lowest temperature (T = 100 K) is below T g ≈ 105 K in the model while the two others are above T g but below T m ≈ 300 K. As D driven reaches D thermal around T m , 55 the curves must decrease at some temperature in between 200 K and 300 K. However, the uncertainty of the difference (D driven − D thermal ) increases with temperature in our simulations making 200 K the largest temperature that could be displayed here. The D thermal values in this figure are Å2/ns, Å2/ns, and Å2/ns. In order to evaluate a possible aging mechanism associated with the isomerization we show two sort of data on the figure. For each temperature the open symbols stand for direct simulations from an equilibrated configuration without isomerizations while the solid symbols stand for simulations that follow a 10 ns first run with the isomerization set on. The dashed lines are fit to the points with the following equation: , with γ = 1. (b) As (a) but in a logarithmic scale.

(a) Diffusion coefficient of the host molecules versus the isomerization frequency f = 1/τ p . The temperatures are from bottom to top: T = 100 K (black circles solid and open); T = 140 K (solid and open red circles); T = 200 K (open red and solid grey triangles). These temperatures are effective temperatures of the coarse grain model. The lowest temperature (T = 100 K) is below T g ≈ 105 K in the model while the two others are above T g but below T m ≈ 300 K. As D driven reaches D thermal around T m , 55 the curves must decrease at some temperature in between 200 K and 300 K. However, the uncertainty of the difference (D driven − D thermal ) increases with temperature in our simulations making 200 K the largest temperature that could be displayed here. The D thermal values in this figure are Å2/ns, Å2/ns, and Å2/ns. In order to evaluate a possible aging mechanism associated with the isomerization we show two sort of data on the figure. For each temperature the open symbols stand for direct simulations from an equilibrated configuration without isomerizations while the solid symbols stand for simulations that follow a 10 ns first run with the isomerization set on. The dashed lines are fit to the points with the following equation: , with γ = 1. (b) As (a) but in a logarithmic scale.

Inverse of the α relaxation time 1/τα versus the isomerization rate f = 1/τ p for various temperatures. The α relaxation time is obtained from the relation F S (Q, τα) = e −1. τα is equivalent to the local viscosity here and can be used as a measure of that viscosity. From bottom to top the temperatures are T = 100 K, 140 K, and 200 K. As in Figure 1(a) the open and solid symbols correspond to different aging procedures. We relate f = 1/τ p to the light intensity I with the rough formula: 1/τ p ≈ (πλ3/4hC) I.

Inverse of the α relaxation time 1/τα versus the isomerization rate f = 1/τ p for various temperatures. The α relaxation time is obtained from the relation F S (Q, τα) = e −1. τα is equivalent to the local viscosity here and can be used as a measure of that viscosity. From bottom to top the temperatures are T = 100 K, 140 K, and 200 K. As in Figure 1(a) the open and solid symbols correspond to different aging procedures. We relate f = 1/τ p to the light intensity I with the rough formula: 1/τ p ≈ (πλ3/4hC) I.

Inverse of the α relaxation time 1/τα versus the isomerization rate f = 1/τ p for various distance from the isomerizing chromophore. The temperature is T = 100 K. From top to bottom the distances intervals from the chromophore are: 0 < R < 10 Å; 10 < R < 20 Å; 0 < R < ∞; and 20 < R < 40 Å.

Inverse of the α relaxation time 1/τα versus the isomerization rate f = 1/τ p for various distance from the isomerizing chromophore. The temperature is T = 100 K. From top to bottom the distances intervals from the chromophore are: 0 < R < 10 Å; 10 < R < 20 Å; 0 < R < ∞; and 20 < R < 40 Å.

(a) Radial distribution function between host molecules with the origin around the chromophore (0 < R < 10 Å) for various isomerization rates. Green dotted curve: τ p = 5 ps; blue dashed curve: τ p = 100 ps; red continuous curve: τ p = 1000 ps. The temperature is T = 100 K. (b) As (a) but at a larger distance from the chromophore (10 < R < 20 Å). The three curves superimpose showing that the host main structure is unchanged at this distance from the chromophore.

(a) Radial distribution function between host molecules with the origin around the chromophore (0 < R < 10 Å) for various isomerization rates. Green dotted curve: τ p = 5 ps; blue dashed curve: τ p = 100 ps; red continuous curve: τ p = 1000 ps. The temperature is T = 100 K. (b) As (a) but at a larger distance from the chromophore (10 < R < 20 Å). The three curves superimpose showing that the host main structure is unchanged at this distance from the chromophore.

(a) Incoherent intermediate scattering function F s (Q, t) around the chromophore (0 < R < 10 Å) for various isomerization rates f = 1/τ p . The temperature is T = 100 K. (b) Incoherent intermediate scattering function F s (Q, t) for host molecules chosen in the whole simulation box. The different curves correspond to various isomerization rates f = 1/τ p . The temperature is T = 100 K.

(a) Incoherent intermediate scattering function F s (Q, t) around the chromophore (0 < R < 10 Å) for various isomerization rates f = 1/τ p . The temperature is T = 100 K. (b) Incoherent intermediate scattering function F s (Q, t) for host molecules chosen in the whole simulation box. The different curves correspond to various isomerization rates f = 1/τ p . The temperature is T = 100 K.

## Tables

Parameters for the DR1 and coarse grain MMA potentials from Refs. 47,48 . There are no charges in the models. We use the following relations to obtain the interactions between different atoms/grains: σ ij = (σ ii ·σ jj )0.5 and ε ij = (ε ii ·ε jj )0.5. The two molecules are modeled as rigid bodies.

Parameters for the DR1 and coarse grain MMA potentials from Refs. 47,48 . There are no charges in the models. We use the following relations to obtain the interactions between different atoms/grains: σ ij = (σ ii ·σ jj )0.5 and ε ij = (ε ii ·ε jj )0.5. The two molecules are modeled as rigid bodies.

Positions of the atoms and grains (in Å) inside the MMA molecule. The 15 atoms of the MMA molecule are modeled as 4 grains (i.e., center of forces) to increase the simulation efficiency. These 4 grains are located on the positions of the first 4 carbons of the list. However the positions of the masses of the 15 atoms are considered in the equations of motions. Details on this coarse grain model may be found in Ref. 47 . In contrast, the DR1 molecule is modeled with a center of force on each atomic position.

Positions of the atoms and grains (in Å) inside the MMA molecule. The 15 atoms of the MMA molecule are modeled as 4 grains (i.e., center of forces) to increase the simulation efficiency. These 4 grains are located on the positions of the first 4 carbons of the list. However the positions of the masses of the 15 atoms are considered in the equations of motions. Details on this coarse grain model may be found in Ref. 47 . In contrast, the DR1 molecule is modeled with a center of force on each atomic position.

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