Cellulose propionate/poly(N-vinyl pyrrolidone-co-vinyl acetate) blends: dependence of the miscibility on propionyl DS and copolymer composition

Blend miscibility of cellulose propionate (CP) with synthetic copolymers comprising N-vinyl pyrrolidone (VP) and vinyl acetate (VAc) units was examined, and a data map was constructed as a function of the degree of substitution (DS) of CP and the VP fraction in the copolymer component. Results of differential scanning calorimetry and Fourier transform infrared measurements indicated that the pairing of CP/P(VP-co-VAc) formed a miscible or immiscible blend system according to the balance in effectiveness of the following factors: (1) hydrogen bonding between residual hydroxyls of CP and VP carbonyls of P(VP-co-VAc); (2) steric hindrance of propionyl side-groups to the interaction specified in (1); (3) intramolecular repulsion between the two units constituting the vinyl copolymer; and, additionally, (4) structural affinity between two segmental moieties involving the propionyl group and VAc unit, respectively. The factor 3 inducing intercomponent attraction is responsible for the appearance of a so-called “miscibility window” in the miscibility map, and the factor 4 substantially expands the miscible region whole, wider relative to those in the maps for the corresponding blend series based on cellulose acetate and butyrate. In further refined estimation by dynamic mechanical analysis and T1 ρH quantification in solid-state 13C NMR, it was found that the miscible blends of hydrogen-bonding type (using CPs of DS < 2.7) were completely homogeneous on a scale within a few nanometers, whereas the polymer pairs situated in the window region (using CPs of DS > 2.7) formed blends exhibiting a somewhat larger size of heterogeneity (ca. 5–20 nm).