Visualization and quantitative evaluation of covalent bond scission in polymeric materials are highly important for understanding failure, fatigue, and deterioration mechanisms and improving the lifetime, durability, toughness, and reliability of the materials. The diarylbibenzofuranone-based mechanophore radical system enabled, through electron paramagnetic resonance spectroscopy, in situ quantitative evaluation of scission of the mechanophores and estimation of mechanical energy induced along polymer chains by external forces. The coagulation of polymer solutions by freezing probably generated force but did not cleave the mechanophores. On the other hand, cross-linking led to efficient propagation of the force of more than 80 kJ mol(-1) to some mechanophores, resulting their cleavage and generation of colored stable radicals. This mechanoprobe concept has the potential to elucidate other debated issues in the polymer field as well.
The
development of a multicolor mechanochromic polymer/silica composite
is achieved by using two distinct types of mechanochromophores. The
multicolor mechanochromism of the composite containing diarylbibenzofuranone
in silica-rich domains and naphthopyran in the polymer-rich domain
is observed. The obtained composite shows blue, green, and orange
colors according to the intensity of applied mechanical stimuli, solvent
addition, and lapse of time. This unique multicolor mechanochromic
behavior is evaluated by solid-state UV–vis absorption spectroscopy,
ab initio steered molecular dynamics simulations, and computed minimum
energy paths on force-modified potential energy surfaces. The unique
mechanochromism is attributed to the difference in properties, activated
colors, and domain locations between the two mechanochromophores.
Polymer–inorganic
composites with diarylbibenzofuranone
(DABBF) moieties, dynamic covalent mechanochromophores, were prepared,
and their mechanochromic behavior was systematically investigated.
The central C–C bonds in DABBF moieties can be cleaved by mechanical
force to form the corresponding stable blue radicals, which can be
quantitatively evaluated by electron paramagnetic resonance (EPR)
spectroscopy. One controversial issue but attractive property in the
DABBF system is the equilibrium between the activated and deactivated
states. Although the deactivation process decreases the sensitivity
of some equilibrium mechanophores, the equilibrium has rarely been
considered when establishing molecular and/or material design of these
systems. Herein, a rational macromolecular design to suppress the
deactivation of activated dynamic mechanophores and improve sensitivity
by limiting their molecular motion is proposed. Polymer–inorganic
composite materials with rigid networks prepared from DABBF alkoxysilane
derivatives exhibited significant activation of the incorporated DABBF
linkages by grinding, with sensitivities more than 50 times as high
as that of DABBF monomers. The increased sensitivity is due to the
effective transmission of mechanical force to the DABBF moieties in
the network structures and suppression of the recombination of the
generated radicals by the rigid frameworks. Furthermore, when the
rigid frameworks were incorporated into elastomers as inorganic hard
domains, the DABBF mechanophores at the interface between the organic
and inorganic domains were preferentially activated by elongation.
Visualization and quantitative evaluation of covalent bond scission in polymeric materials are highly important for understanding failure, fatigue, and deterioration mechanisms and improving the lifetime, durability, toughness, and reliability of the materials. The diarylbibenzofuranone-based mechanophore radical system enabled, through electron paramagnetic resonance spectroscopy, in situ quantitative evaluation of scission of the mechanophores and estimation of mechanical energy induced along polymer chains by external forces. The coagulation of polymer solutions by freezing probably generated force but did not cleave the mechanophores. On the other hand, cross-linking led to efficient propagation of the force of more than 80 kJ mol À1 to some mechanophores, resulting their cleavage and generation of colored stable radicals. This mechanoprobe concept has the potential to elucidate other debated issues in the polymer field as well.
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