The effect of solution pH on the isomerization of the chromophoric group of P(SPMA-co-MAA) is studied.
Fig. 6a shows the UVeVIS spectra of an aqueous solution (pH 1⁄4 12) of P(SPMA-co- MAA) upon gradual adding dilute HCl solution (pH 1⁄4 1).
When the acid amount is less than 24 mL, no obvious change in the absorption intensity of closed SP at 353 nm is observed, while the absorption band at 515 nm belonging to the characteristic absorption peak of MC form increases with the rising acid amount, suggesting that the acid catalyzes the ring-opening from SP to MC isomer to some extent.
Accordingly, the color of aqueous solution of light-responsive random copolymer changes from colorless to pink (see Fig. 6a inset).
When the volume of added acid exceeds 24 mL, the appearance of precipitate in this system impedes the normal spectrum measurement.
This phenomenon should be associated with typical pH-responsive MAA unit, since the phase separation may occur at pH lower than the pKa of MAA.
Furthermore, the acid-catalyzed ring-opening process discussed above is reversible. Moderate addition of the base can lead to the recovery of closed SP isomer.
It can be observed from Fig. 6b that the intensity of absorption band at l 1⁄4 515 nm decreases with the increasing NaOH amount.
When the volume of added base is 45.6 mL, the intensity is almost as same as that before UV irradiation and the color of the solution changes from pink to light yellow (see Fig. 6b inset), meaning that opened MC has completely transformed to closed SP.
It is worth noting that no absorption peak corresponding to MCH isomer reported by Vamvakaki is found in this study .
This interesting result may be explained by the interaction between the light-responsive spiropyran unit and pH-sensitive MAA unit.
The former becomes more hydrophilic, but the latter unit becomes more hydrophobic upon the introduction of strong acid.
The presence of MAA unit can offset the protonation of MC to a large extent.
In this sense, the two opposite trends may make the acidochromic behaviors of P(SPMA-co-MAA) weaker than that of P(SPMA-co-DMAEMA).