Abstract
This work is a comprehensive experimental and theoretical study aimed at understanding the photothermal and molecular shape-change contributions to the photomechanical effect of polymers doped with azo dyes. Our prototypical system is the azobenzene dye Disperse Red 1 (DR1) doped into poly (methyl methacrylate) (PMMA) polymer formed into optical fibers. We start by determining the thermo-mechanical properties of the materials with a temperature-dependent stress measurement. The material parameters, so determined, are used in a photothermal heating model—with no adjustable parameters—to predict its contribution. The photothermal heating model predicts the observations, ruling out mechanisms originating in light-induced shape changes of the dopant molecules. The photomechanical tensor response along the two principle axes in the uniaxial approximation is measured and compared with another independent theory of photothermal heating and angular hole burning/reorientation. Again, the results are consistent only with a purely thermal response, showing that effects due to light-induced shape changes of the azo dyes are negligible. The measurements are repeated as a function of polymer chain length and the photomechanical efficiencies determined. We find the results to be mostly chain-length independent.
Highlights
The control of a material’s mechanical properties with light, called the photomechanical effect, is becoming the focus of intense research due to its promise in directly converting light to work [1]
We focus our thermomechanical/photomechanical studies on azo dye-doped polymers that are drawn into fibers to study these effects in the geometry used in polymer optical fiber (POF) technologies, using poly (PMMA) as the host polymer and the azo dye Disperse Red #1 (DR1) as a representative material commonly used in POFs [17]
We develop a semi-empirical theory of the photothermal mechanism, using the thermo-mechanical properties of DR1doped PMMA fibers as a function of the chain transfer agent (CTA) concentration as an input to the model, to determine its absolute contribution to the material’s photomechanical constants in our experimental configuration
Summary
The control of a material’s mechanical properties with light, called the photomechanical effect, is becoming the focus of intense research due to its promise in directly converting light to work [1]. The origins of the idea can be traced to Alexander Graham Bell’s 19th century work, who used photomechanical materials that convert light into sound to demonstrate a photophone [2,3]. Photomechanics saw a rebirth a century later when Uchino used ceramic materials to make small light-actuated “walkers” [4,5,6,7]. In addition to the promise of converting light directly into work—for example, to make motors [8,9,10,11,12,13,14]—materials that combine logic and actuation can underpin ultra-smart morphing technologies that leapfrog present-day capabilities [15,16,17]. The observation that azo dyes can fuel a photomechanical response [24] has made dye-doped polymers an attractive material class for applications
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