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Solid propellants are energetic materials used to launch and propel rockets and missiles. Although their history dates to the use of black powder more than two millennia ago, greater performance demands and the need for "insensitive munitions" that are resistant to accidental ignition have driven much research and development over the past half-century. The focus of this review is the material aspects of propellants, rather than their performance, with an emphasis on the polymers that serve as binders for oxidizer particles and as fuel for composite propellants. The prevalent modern binders are discussed along with a discussion of the limitations of state-of-the-art modeling of composite motors.

The development of mechanochromic molecules has opened new pathways for the study of localized stress and failure in polymers. Their application as stress or temperature diagnostics, however, requires suitable measurement techniques capable of detecting the force- and temperature-sensitive chemical species with high spatial resolution. Confocal imaging techniques offer excellent spatial resolution but the energy input during these measurements can itself affect the activation state of the mechanochromic species. Here, we present a systematic study of the effects of laser-based imaging on the activation and fluorescence behavior of mechanochromic spiropyran (SP) integrated into poly( methyl acrylate) (PMA) and poly(methyl methacrylate) matrices using a confocal Raman microspectrometer. Localized stress and temperature activation were studied by means of high-rate compressive loading and dynamic fracture. Laser illumination of SP in PMA revealed a strong excitation wavelength- and power-dependence. Suitable correction functions were established and used to account for the observed laser effects. The presented study demonstrates that confocal imaging using conventional Raman spectrometers is a powerful characterization tool for localized stress analysis in mechanochromic polymers, offering quantifiable information on the activation state with high spatial resolution. However, laser-mechanophore interactions must be well understood and effects of laser excitation and exposure times must be taken into consideration when interpreting the obtained results.

The development of mechanochromic molecules has opened new pathways for the study of localizedstress and failure in polymers. Their application as stress or temperature diagnostics, however,requires suitable measurement techniques capable of detecting the force- and temperature-sensitivechemical species with high spatial resolution. Confocal imaging techniques offer excellent spatial re-solution but the energy input during these measurements can itself affect the activation state of themechanochromic species. Here, we present a systematic study of the effects of laser-based imagingon the activation and fluorescence behavior of mechanochromic spiropyran (SP) integrated into pol-y(methyl acrylate) (PMA) and poly(methyl methacrylate) matrices using a confocal Raman micro-spectrometer. Localized stress and temperature activation were studied by means of high-ratecompressive loading and dynamic fracture. Laser illumination of SP in PMA revealed a strong excita-tion wavelength- and power-dependence. Suitable correction functions were established and used toaccount for the observed laser effects. The presented study demonstrates that confocal imaging usingconventional Raman spectrometers is a powerful characterization tool for localized stress analysis inmechanochromic polymers, offering quantifiable information on the activation state with high spatialresolution. However, laser-mechanophore interactions must be well understood and effects of laserexcitation and exposure times must be taken into consideration when interpreting the obtained results.

We report the high strain-rate response of a spiro-pyran (SP) mechanophore in poly(methylmethacrylate). Previ-ous work on this system has demonstrated a reversible bondscission in the SP under local tensile force, converting it to afluorescent merocyanine form. A Hopkinson bar was used toapply fast compressive loads at rates from 102to 104s21,resulting in significant activation of the SP near fracture surfa-ces. However, comparison with a similar thermochromic SPreveals that much of the observed activation likely arises fromthermal effects during high-rate fracture. These results showthe importance of a thermally active control system in distinguishing mechanochromic response during high-rate loading. Microscale fluorescence mapping of the fracture surfaces using a confocal Raman microspectrometer suggests that some dis- tinct mechanical activation may be occurring in craze-like regions during fibril rupture. The thermal response of the SP is useful in its own right for characterizing plastic heating regions during dynamic fracture.