Abstract
Molecular motion in polymers is frozen below the glass transition temperature T_{g} and changes of viscoelastic functions are most spectacular near T_{g}. Exceptional enhancement of molecular mobility and a decrease of polymer viscosity, by several orders of magnitude, down to the viscous flow regime, are observed way below T_{g} by light absorption. Relaxation processes, which take decades to centuries in some high-T_{g} polymers, are reduced to minute timescales by sub-T_{g} light absorption. Here we develop a model for this intriguing albeit spectacular action of light on glass forming materials and we propose experiments to relate light absorption to materials properties. The model provides a solution to a long-lasting problem of how molecular mobility is enhanced in solid polymers by photoisomerization and provides a tool for a better understanding of the relationship between light absorption and material properties and developing photosensitive polymers for light to mechanical energy transduction.
Highlights
If a polymer is cooled, from the viscous flow to the glassy regime, its viscosity η is dramatically increased by several orders of magnitude, much like a supercooled liquid
We demonstrate that the decrease of viscosity and relaxation time in solid films of polymers can be explained by an increase in the free volume of the polymer by light absorption
The dyes, which are embedded into polymers by, for example, chemical attachment, are in perpetual motion during light absorption, i.e., they undergo billions of shape changes per second owing to picosecond isomerization under absorption of light of the visible wavelength, and they cause molecular displacement of the surrounding medium in their nanocogent environment [10,11] and create additional free volume, thereby reducing friction and viscosity and enhancing molecular mobility
Summary
If a polymer is cooled, from the viscous flow to the glassy regime, its viscosity η is dramatically increased by several orders of magnitude, much like a supercooled liquid. Exceptional enhancement of molecular mobility and a reversible change from solid to viscous regimes occur athermally in photoactive polymers when units of the polymer undergo structural rearrangement upon light absorption [2,3,4,5,6]. The dyes, which are embedded into polymers by, for example, chemical attachment, are in perpetual motion during light absorption, i.e., they undergo billions of shape changes per second owing to picosecond isomerization under absorption of light of the visible wavelength, and they cause molecular displacement of the surrounding medium in their nanocogent environment [10,11] and create additional free volume, thereby reducing friction and viscosity and enhancing molecular mobility. Many polymers containing photochromic dyes, either as guests or as pendant side groups or even cross-linked, exhibit a fractional free volume in the range of 0.02–0.03 [14]
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