Tailoring the Properties of Optical Force Probes for Polymer Mechanochemistry

Abstract

The correlation of mechanical properties of polymer materials with those of their molecular constituents is the foundation for their holistic comprehension and eventually for improved material designs and syntheses. Over the last decade, optical force probes (OFPs) were developed, shedding light on various unique mechanical behaviors of materials. The properties of polymers are diverse, ranging from soft hydrogels to ultra-tough composites, from purely elastic rubbers to viscous colloidal solutions, and from transparent glasses to super black dyed coatings. Only very recently, researchers started to develop tailored OFP solutions that account for such material requirements in energy (both light and force), in time, and in their spatially detectable resolution. We here highlight notable recent examples and identify future challenges in this emergent field.

Keywords: chemiluminescence; fluorescence; mechanochemistry; polymers; sensors.

Figure 1

Different optical responses of OFPs that alter either absorption, fluorescence, or chemiluminescence. (a) Schematic depiction and (b) structural formula of exemplary OFPs. DABBF is a scissile, reversible OFP that becomes blue colored upon activation. Anthracene‐maleimide Diels‐Alder adducts are scissile, quasi‐static OFPs that can either turn “on” their fluorescence or are dual‐fluorescent. Dioxetanes are scissile, quasi‐static, mechanoluminescent OFPs. Note that force‐induced transitions can be either scissile or non‐scissile as indicated by the dashed lines in (a). Dashed lines in (b) indicate the attachment points to the macromolecular framework.

Figure 2

A selection of bond scission mapping visualizations in OFP‐crosslinked polymer networks: (a) Overlay of CLSM micrographs showing non‐activated (cyan) and activated (yellow) BTD OFP in crosslinked rubber networks, and pixel‐by‐pixel contour map representation of left panel showing the percentage of activated relative to remaining non‐activated BTD upon fracture. Reproduced from Ref. [18]. Copyright 2021, the authors. (b) Progression and extent of a crack front in time and space for dioxetane‐crosslinked PMMA networks under chloroform ingress and swelling‐induced mechanoluminescence. Reproduced from Ref. [19]. Copyright 2017, American Chemical Society. (c) Intensity‐based mapping of bond scission in notched samples of single, double, and triple networks, containing dioxetane crosslinker in the first network, around the tip of a propagating crack and schematic of the bond scission mechanism. Reproduced from Ref. [20]. Copyright 2014, American Association for the Advancement of Science. (d) Stress map around the tip of a propagating crack in acrylate networks crosslinked with spiropyran converted to different merocyanine isomers. Reproduced from Ref. [13]. Copyright 2021, Royal Society of Chemistry.

Figure 3

Examples for different force responses of OFPs undergoing non‐scissile covalent bond isomerization, non‐covalent bond rearrangement, or bond rotation. (a) Schematic depiction and (b) structural formula of exemplary OFPs. Spiropyran is a non‐scissile, reversible OFP that undergoes covalent bond isomerization to merocyanine, which has purple color and is red fluorescent. PBI loops vary between excimer (associated) and monomer (dissociated) emission. Diketopyrrolopyrroles change the emission wavelength based on charge transfer from the planarization of the twisted phenyl moiety.

Figure 4

Different temporal responses of OFPs that are either quasi‐static, transient, or reversible. The measured optical signal intensity I is shown as a function of observation time t and timepoints of force application to the material are indicated with F.

Figure 5

Different spatial resolutions that can be achieved during OFP analysis exemplified on a dog bone‐shaped sample widely employed for the mechanical testing of polymers. Either whole sample monitoring, diffraction‐limited microscopy, or super‐resolved microscopy techniques are used and indicate the optical signal at the region of interest.