Optical Sensing of Stress in Polymers
This chapter discusses recent approaches towards the optical detection of stress and deformation in polymeric materials, an important tool in monitoring material integrity and in the study of failure mechanisms of polymeric materials. Optical sensing has specific advantages based on the ease of detection, high sensitivity and spectral resolution of light. In this chapter, a classification of sensing mechanisms is used that distinguishes between the molecular phenomena of isomerization, bond scission, change in conjugation and collective phenomena such as changes in chromophore aggregation and photonic band gap tuning. Molecular mechanisms are discussed that have been used to obtain stress-induced changes in absorption and fluorescence properties and recent work is presented in which the chain scission of dioxetanes is used to produce a luminescent signal with high detectability. Pi-conjugated systems play an important role in optical detection of stress and damage in polymers because their optical properties are very sensitive to changes in conformation and aggregation state. Finally, photonic band gap polymers and cholesteric liquid crystals are discussed, in which the periodic organization of structural features at the scale of the wavelength of light leads to strain-dependent reflection and absorption bands.