Mechanoactivation of Spiropyran Covalently Linked PMMA: Effect of Temperature, Strain Rate, and Deformation Mode

Polymer multifunctionality can be designed through the incorporation of chemical groups termed. mechanophores that have a specific chemical transformation in response to applied force. The behavior of mechanophore-linked polymers depends on synthetic factors such as the choice of the mechanophore, the polymer chemistry, and the mechanophore linking architecture and on externally imposed factors such as temperature, loading mode, and loading rate. While many papers have explored changing polymer architecture, relatively few have systematically looked at these external factors, particularly temperature and loading mode. These external factors are critical for practical application of mechanophore-linked polymers, particularly for damage detection in engineering materials. Here, we use a single synthetic system to quantify the influence of these externally imposed factors. In particular, the mechanophore spiropyran (SP) is covalently bonded into lightly cross-linked poly(methyl methacrylate) (PMMA). SP is a mechanophore that has a distinct color and fluorescence change when activated through force to the merocyanine state, making it ideal for in situ studies. We monitor and analyze the full field fluorescence of SP-PMMA samples during mechanical loading under tension and compression, over 3 decades of strain rate, and over a 60 degrees C range in temperature. Typical SP mechanoactivation response exhibits three distinct regimes: minimal change through yield, followed by rapid intensity increase, and approach to a steady state. Stress has a strong influence on the rate of increase in SP activation, where stress increase by temperature decrease or strain rate increase substantially raises the SP activation rate. Uniaxial compression displays a qualitatively similar response to that of uniaxial tension. However, a longer flat region is observed in the case of compression as compared to tension corresponding with the larger yield strain.