Mechanochromic cholesteric liquid crystal devices for mechanical strain detection.

The properties of functional soft materials have opened the doors for the development of a wide range of stimuli-responsive smart devices. The ability to react to factors such as humidity, temperature or pressure, inducing changes in the materials’ properties, for instance, colour, constitutes the basis of smart sensing. For such applications, liquid crystals (LCs) present a particularly compelling and innovative class of materials that have been extensively studied. Amongst LCs, cholesteric LCs (CLCs) have been gaining much attention for different applications, owing to their selective and tuneable reflection of circularly polarised light. This work was focused on the application of CLC-based smart sensing devices for mechanical strain detection through the production of mechanical stress-sensitive devices. These sensors may be used, for instance, as standalone sensors to evaluate a crack propagation in concrete structures, having a visual readout. For this effect, CLC elastomer (CLCE)-based mechanochromic devices were produced. PDMS was embedded with 3D CLCE structures, obtained through the crosslinking of an acrylate-terminated CLCE (ACLCE) precursor solution, followed by shaping either by dropwise addition onto a silicon oil bath – with and without agitation – or by moulding. The sensorial capabilities of the devices were tested by subjecting them to mechanical deformation to assess whether this leads to a change in colour. It was concluded that, considering the main goal of achieving mechanochromic behaviour, the ACLCE structures obtained from dropwise addition, under no agitation, exhibited the strongest and most uniform mechanochromism.