## 5 Conclusions  
We have investigated the effect of adding dye and annealing on the thermomechanical and photomechanical properties of PMMA fibers by measuring the intensity-, polarization-, and temperature-dependent Young's modulus and stress. The results show that adding DR1 to PMMA leads to antiplasticization by increasing the stiffness and lowering the \(T_{g}\) in the fiber geometry. Moreover, annealing the fibers at 90 \({}^{\circ}\)C for several days decreases their stiffness and increases their \(T_{g}\) since it reduces chain alignment and, thereby, increases entropy. The change in stress in DR1-doped PMMA fibers is greater than in plain ones because of their lower \(T_{g}\), which leads to a more facile rearrangement of the polymer chains.  
The fibers' stiffness is not affected by light illumination at a wavelength of 488 nm, independent of polarization, but the absolute stress changes in response to light. The process is found to be reversible, returning the stress to its original value after the pump light is turned off.  
Models are proposed to fit the temperature-dependent Young's modulus and stress. The data fit the models well and are used to determine the critical temperatures, which, in annealed fibers, is greater than in unannealed ones. This observation is consistent with the decrease in \(T_{g}\) upon annealing and adding a dopant dye.  
Figure 19: Figure of merit as a function of temperature for PMMA and DR1/PMMA fibers derived using Equation (11). The inset shows the average FOM over all intensities as a function of fiber composition and processing. The lines are fits to each composition and processing condition.  
A comparison of dye-doped fibers with plain fibers is used as a control. We find that the dominant photomechanical mechanism in DR1/PMMA is photothermal heating. The time response derived using stress versus time data fits a single exponential, which suggests that only one mechanism is involved.  
The measured temperature-dependence of Young's modulus and the photomechanical constant are used to determine the temperature-dependent figure of merit, which defines the efficiency of a material in converting light energy to mechanical work. The results show that increasing the temperature increases the FOM and suggest that higher temperatures are desirable for having more efficient photomechanical materials.  
This set of results suggest ways in which the thermomechanical and photomechanical properties of a dye-doped polymer can be controlled to optimize it for a particular application. The techniques introduced here can also be used to study the physics of the photomechanical response and how it is affected by the rheological properties of the material.  
Conceptualization, M.G.K.; methodology, M.G.K.; software, B.Z.; validation, B.Z., Z.G. and M.S.K.; formal analysis, M.G.K. and Z.G.; investigation, Z.G., M.S.K., M.G.K., B.Z. and R.O.; resources, M.G.K.; data curation, B.Z., Z.G. and M.G.K.; writing--original draft preparation, Z.G. and M.G.K.; writing--review and editing, M.G.K., Z.G., B.Z., R.O. and M.S.K.; visualization, Z.G. and M.G.K.; supervision, M.G.K.; project administration, M.G.K.; funding acquisition, M.G.K. All authors have read and agreed to the published version of the manuscript.  
This research was funded by The National Science Foundation, Directorate for Engineering (ENG) grant number EFRI-ODISSEI:1332271.  
Not applicable.  
The authors thank the National Science Foundation (EFRI-ODISSEI:1332271) for supporting this work.  
The authors declare no conflict of interest.  
Samples of the compounds are available from Z.G., M.S.K., or R.O.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} DR1 & Disperse Red 1 \\ PMMA & Poly(methyl methacrylate) \\ \(T_{g}\) & Glass Transition Temperature \\ FOF & Polymer Optical Fiber \\ FOM & Figure of Merit \\ MMA & Methyl methacrylate \\ INT & Initiator \\ CTA & Chain Transfer Agent \\ \end{tabular}