Mechanochromic materials

The mechanochromic LC-type materials, 17 and 18, show emission colour change on application of shear stress (Scheme 5 and Fig. 8). Since LCs more readily change their molecular arrangement than crystals, they are suitable as mechanochromic materials. In the case of polymer... 

In this chapter, we will discuss the design, synthesis, and the properties of mechano-responsive polymeric materials with foeus on meehanoehromie systems. Initially, natural meehanoehromie events will be deseribed in order to show how these mechanisms can serve as inspiration in the design of artificial devices. Following, meehanieal responsiveness in the eontext of mechanochromic materials will be reviewed. The effeet of meehanieal forces on the molecular, supramolecular, microscopic and macroscopic levels will be taken into consideration, as well as the use of meehanieally responsive biological macromolecules for the preparation of self-reporting polymer-protein hybrid materials. 

Abstract We describe mechanochromic and thermochromic photoluminescent liquid crystals. In particular, mechanochromic photoluminescent liquid crystals found recently, which are new stimuli-responsive materials are reported. For example, photoluminescent liquid crystals having bulky dendritic moieties with long alkyl chains change their photoluminescent colors by mechanical stimuli associated with isothermal phase transitions. The photoluminescent properties of molecular assemblies depend on their assembled structures. Therefore, controlling the structures of molecular assemblies with external stimuli leads to the development of stimuli-responsive luminescent materials. Mechanochromic photoluminescent properties are also observed for a photoluminescent metallomesogen and a liquid-crystalline polymer. We also show thermochromic photoluminescent liquid crystals based on origo-(/ -phenylenevinylene) and anthracene moieties and a thermochromic photoluminescent metallocomplex. 

Abstract The past 10 years have seen a resurgence of interest in the field of polymer mechanochemistry. Whilst the destructive effects of mechanical force on polymer chains have been known for decades, it was only recently that researchers tapped into these forces to realize more useful chemical transformations. The current review discusses the strategic incorporation of weak covalent bonds in polymers to create materials with stress-sensing and damage-repairing properties. Firstly, the development of mechanochromism and mechanoluminescence as stress reporters is considered. The second half focuses on the net formation of covalent bonds as a response to mechanical force, via mechanocatalysis and mechanically unmasked chemical reactivity, and concludes with perspectives for the field. 

Although the property of interest in all of the above-mentioned cyano-OPV-containing materials was a mechanically induced change of the photoluminescence color (i.e., a mechanically induced transformation from an excimer-rich to a monomer-dominated emission), certain cyano-OPVs also display a changed of their absorption color upon (dis)assembly. Thus, Kunzelman et al. reported mechanochromic blend films based on C18-RG and either poly(ethylene... 

Fig. 7 (a) Picture of a mechanochromic elastomer made by integrating C120H-RG into a thermoplastic polyurethane backbone in the unstretched state, (b) Picture of the same material in the stretched state. Both pictures were taken under illumination with ultraviolet light, (c) Ratio of monomer to excimer emission 7m//e (circles) and tensile stress (solid line) under a triangular strain cycle between 0% and 500% at a frequency of 0.0125 Hz. Adapted with permission from [41]. Copyright 2006 American Chemical Society... 

Many other piezochromic or mechanochromic organic materials are now known, some of which (23, 24, 25 and 26 ) are shown in Scheme 7. It is worth noting that the mode of applied stress can be distinguished by the different luminescent response. The yellow luminescence (2em = 556 nm) of a tetrathiazolylthiophene (27) crystal is blue-shifted to green (Agm = 490 nm) by anisotropic grinding but is red-shifted to orange (Agm = 609nm at 3.2 GPa) under isotropic compression, which is explained by decreased and increased contributions of excimer emission, respectively. 

The third and last section of our book deals with polymeric materials with bio-inspired functionality. The opening chapter by Waite gives a biological perspective of how mussels adhere to interfaces and emphasizes the importance of first understanding the biology before new, tmly bio-inspired materials can be achieved. The second chapter in this section, written by Lee, describes the synthetic achievements of mussel glue-inspired polymers, and depicts the polymer chemist s approach to mimic the mussel adhesion described by Waite in the previous chapter. Bruns and co-authors present a chapter on self-reporting polymeric materials with mechanochromic properties. Such materials mimic nature s ability to report, repair, and improve... 

Self-reporting Polymeric Materials with Mechanochromic Properties... 

As a general rule, polymers can be characterised in solution, as solid materials, or in the melt. In the following section, the commonly applied methods to study the response of polymers to mechanical forces are introduced, starting with solution measurements and then moving on to bulk measurements. The effect of shear forces on polymer chains in melts is fascinating, but has not been studied with the help of mechanochromic systems and is therefore not covered in this chapter. 

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