Thermochromism in Polydiacetylene/Poly(vinyl alcohol) Hydrogels Obtained by the Freeze–Thaw Method: A Theoretical and Experimental Study

The chromatic transition from blue to red in polydiacetylene (PDA) membranes can be used for optical sensing in various applications ranging from use as chemosensors to the quality control of foods and medicines. The preparation of PDAs embedded in poly(vinyl alcohol) (PVA) can be considered one of the simplest and most viable methodologies for obtaining membranes with colorimetric responses. However, it is still a scientific challenge to understand how the polymeric matrix affects the chromatic transition of PDAs and how to control it. In this work, PDA/PVA hydrogels were prepared to study its thermochromism. The colorimetric responses of these hydrogels were compared with that of PDA vesicle suspensions in order to show how the matrix of PVA affects the color transition. Theoretical calculations have been carried out to provide atomistic insights into the involved mechanism, showing how the noncovalent interactions with the PVA matrix decrease the rotational energy barrier of the PDA. Finally, in an attempt to modulate the thermochromism of the hydrogels, membranes were obtained by gelling PDA/L64 blends in the PVA matrix, using different concentrations of the L64 copolymer. The results showed that the chromatic transition of the PDA/L64/PVA membranes is more sensitive at lower temperatures with increasing L64 concentration.