*3.1. Characterization of Synthesized Polymers*

Four PVA–Ac copolymers of different composition were synthesized varying PVA-to-acetaldehyde molar ratio. The reaction scheme and chemical structure of the obtained PVA copolymers are illustrated in Figure 1a. The copolymer composition and aqueous solution properties were studied by NMR and UV-VIS spectroscopy, respectively. The results are summarized in Table 1.

**Figure 1.** (**a**) Schematic presentation of acetalization reaction of PVA; (**b**) 1H NMR spectrum of copolymer AC18 (solvent DMSO-d6).


**Table 1.** Acetal content and cloud point (TCP) of modified PVA copolymers.

\* As measured for 5 g/L aqueous copolymer solution*;* \*\* No TCP detected up to 90 ◦C.

Typical proton NMR spectrum of PVA–Ac is shown in Figure 1b. Copolymer composition expressed as a content of acetal groups was calculated by comparing the area of the peak assigned to the methine protons from the acetal group (e) to those assigned to the methine protons from the PVA main chain (b+d).

The synthesized PVA–Ac copolymers, although very water soluble at room temperature, undergo phase transition when increasing the temperature of the aqueous solutions and turn water insoluble. This is due to the introduced fractions of hydrophobic acetal groups and reflects in the reduced hydrogen bonding between copolymers and water molecules as compared to pure PVA. To evaluate the hydrophilic–hydrophobic balance of the copolymers, expected to influence the humidity-sensing properties of the corresponding thin films, *T*CP in dilute aqueous solutions were measured. Clouding curves of the copolymer aqueous solutions were registered, and *T*CP were estimated at 50% transmittance. As seen in Table 1, the higher the acetal content, the lower the *T*CP.
