Figure 4.

(a) Titration of printed paper disks coated with the sensory polymer VP:MMA:MCN2:MCAR4 (molar ratio of 49:49:1:1, respectively), with NaNO₂. Each 2-cm-diameter disk was coated with 50 μL of the sensory polymer solution in acetonitrile (100 mg in 1 mL). The coated paper disks were dipped in different NaNO₂ acid solutions (0.4/4/12/40/120/240 and 400 mg NaNO₂/100 mL) and finally dipped in aqueous NaOH (1 M). RGB parameters were extracted from the digital photograph, grouped as the unique variable ΔRGB,²⁶ and represented against the concentration of NaNO₂. (b) Azo-coupling reaction in the chromogenic polymer.^21,22 (c) Colored paper disks after dipping in water and propylene carbonate for 30 min. The picture shows the water-resistant character of the polymer and the solubility in propylene carbonate.