Enhanced Thermal Ageing Stability of Mechanophore in Polyurethane Network by Introducing Polyhedral Oligomeric Silsesquioxanes (POSS)

Abstract

Spiropyran (SP) shows excellent ability in light‐ or force‐triggered color and fluorescence generation. However, it has the fatal weakness of poor antithermal bleaching properties in polymers, especially when exposed to severe environments, which will shorten the life of mechanical responsive materials. Here, a strategy is reported to protect SP through incorporation of multifunctional polyhedral oligomeric silsesquioxanes (POSS) as cross‐linkers into a polyurethane network. The introduction of POSS not only improves the mechanical strength of the elastomer, but also enhances the thermal stability of the mechanophore, showing much more sensitive force‐responsive and photoresponsive properties, especially after thermal annealing. POSS contributes to the construction of a 3D network and hinders the mobility of the hard domains of molecular chains, thus enhancing the thermal stability of the mechanophore in the network. The strategy can not only extend the life of the stimuli‐responsive materials, but also promote the application temperature of the mechanically responsive polymers.

The evolution of spiropyran: fundamentals and progress of an extraordinarily versatile photochrome

“…Indeed, mechanochromism in spiropyran-containing materials is now well established, reviewed recently by Li et al in 2017. 22 More recent fundamental contributions include tuning of the robustness of the chromophore by introducing varying degrees of crosslinking 156 and tuning of the colouration threshold over a wide range of tensile strain, 157 while practical applications are found in visible cues for pending failure in soft electronics 158 and a method for electrospinning fibers capable of orientation dependent mechanochromism. 159…”

Mechanochromophore‐Linked Polymeric Materials with Visible Color Changes

“…Afterward, SP‐PU with an enhanced thermal aging stability was developed via employing polyhedral oligomeric silsesquioxanes (POSS) as a cross‐linker. [ 128 ] Wang et al. demonstrated an SP‐containing bio‐based waterborne PU for mechanochromism, where soybean oil modified with hydroxyl groups as polyols were used to synthesize the PU.…”

“…Nanofiller‐like POSS was employed to hinder the mobility of the hard domains via aggregation to improve the strength of SP‐PU. [ 128 ] POSS is comparable to nanofillers such as clay, such that their addition can improve the mechanical properties of polymers. In addition, due to their chemically functionalized structure, they can be covalently linked into polymer systems without compatibility issues.…”

“…[ 173,174 ] Thus POSS was introduced into SP‐linked PU elastomer as chain extenders and cross‐linkers to restrict the chain mobility to enhance the mechanochromic performance. [ 128 ]…”

A Fluorescent Polyurethane with Covalently Cross-Linked Rhodamine Derivatives

“…It is regarded that moderate cross-linking can positively influence these performances [ 16 , 17 ]. Previous research has proved that the thermal stability of cross-linked polyurethanes is higher than that of the linear one [ 18 , 19 ]. Network structures also result in improvement of mechanical properties [ 20 , 21 ].…”