Water Sorption in Glassy Polyvinylpyrrolidone-Based Polymers

Abstract

Polyvinylpyrrolidone (PVP)-based polymers are excellent stabilizers for food supplements and pharmaceutical ingredients. However, they are highly hygroscopic. This study measured and modeled the water-sorption isotherms and water-sorption kinetics in thin PVP and PVP-co-vinyl acetate (PVPVA) films. The water sorption was measured at 25 °C from 0 to 0.9 RH, which comprised glassy and rubbery states of the polymer-water system. The sorption behavior of glassy polymers differs from that in the rubbery state. The perturbed-chain statistical associating fluid theory (PC-SAFT) accurately describes the water-sorption isotherms for rubbery polymers, whereas it was combined with the non-equilibrium thermodynamics of glassy polymers (NET-GP) approach to describe the water-sorption in the glassy polymers. Combined NET-GP and PC-SAFT modeling showed excellent agreement with the experimental data. Furthermore, the transitions between the PC-SAFT modeling with and without NET-GP were in reasonable agreement with the glass transition of the polymer-water systems. Furthermore, we obtained Fickian water diffusion coefficients in PVP and in PVPVA from the measured water-sorption kinetics over a broad range of humidities. Maxwell-Stefan and Fickian water diffusion coefficients yielded a non-monotonous water concentration dependency that could be described using the free-volume theory combined with PC-SAFT and NET-GP for calculating the free volume.

Keywords: NET-GP; PC-SAFT; free volume; water sorption isotherms; water sorption kinetics.

Figure 1

Thickness profile of PVPVA film fixed on a coverslip. The solid line indicates the measurement along the film’s length. In contrast, the dashed line represents the estimated thickness calculated using the film’s diameter, mass, and density of amorphous PVPVA of 1190 kg/m³. We further provide a 3D film model in the upper right corner, where a brighter tone corresponds to a higher thickness.

Figure 2

Water-sorption isotherm of PVP at 25 °C (left y-axis) and glass-transition temperatures of PVP-water mixtures (right y-axis). Water sorption measured in this study is displayed as circles. The PC-SAFT modeling without NET-GP is presented as a dotted line. The PC-SAFT modeling with NET-GP is shown as a dashed line. The combination of NET-GP and PC-SAFT is displayed as a thick solid line. Additionally, isopiestic measurements of PVP-water solutions in the high-humidity range were taken from the literature [39] and are displayed as triangles. The PVP-water mixture’s T_g values were taken from the literature [38] (diamonds). The water concentration resulting in a T_g of 25 °C is displayed as a dash-dotted horizontal line.

Figure 3

Water-sorption isotherms at 25 °C in PVPVA and PVAc (left y-axis) as well as the T_gs of PVPVA-water and PVAc-water mixtures(right y-axis). The water-sorption isotherm of PVPVA (left diagram) measured in this study is displayed as circles. The water-sorption isotherm in PVAc (right diagram) was taken from the literature [40] and is displayed as triangles. The T_gs of PVPVA-water and PVAc-water mixtures were also taken from the literature [40] (diamonds). The water concentration resulting in a T_g of 25 °C is displayed as a dash-dotted horizontal line. PC-SAFT modeling without NET-GP is presented as dotted lines, PC-SAFT modeling with NET-GP is presented as a dashed line and the combined approach is displayed as a thick solid line.

Figure 4

Evolution of the water weight fraction in the investigated polymer films (PVP on the left and PVPVA on the right) at six different RH step changes at T = 25 °C against the square root of time. Each of the step changes is displayed with a different symbol (circles: 0 to 0.1 RH, squares: 0.1 to 0.3 RH, upside triangles: 0.3 to 0.45 RH, stars: 0.45 to 0.6 RH, downside triangles: 0.6 to 0.75 RH, hexagons: 0.75 to 0.9 RH), while the descriptions using the Fickian diffusion model are indicated as solid lines. In addition, the glass transitions taken from the literature [38,40] are displayed as dash-dotted lines for PVP and PVPVA.

Figure 5

Fickian diffusion coefficients $D_{wp}$ (circles) and Maxwell-Stefan diffusion coefficients ${Đ}_{wp}^{″}$. (triangles) of water in polymers at T = 25 °C as function of the average water weight fraction ($0.7w_w^{\infty }+0.3w_w^0)$. of a sorption step. The left diagram displays water diffusion coefficients in PVP, and the right diagram those in PVPVA. The glass transitions are indicated as dashed lines as derived from literatur [38,40].he modeling results of ${Đ}_{wp}^{″}$. obtained from Equation (15) using PC-SAFT without NET-GP are presented as dotted lines, PC-SAFT modeling results with NET-GP are presented as dashed lines, the combined approach is displayed as thick solid line.

Figure 6

The ratio $\frac{v}{v_{0p}^{NE}}$. of the specific volume $v$. of the polymer-water mixture relative to the specific volume $v_{0p}^{NE}$ of the dry polymer in non-equilibrium calculated using PC-SAFT alone (dotted lines), PC-SAFT with NET-GP (dashed lines), and the combined approach (thick solid line) for PVP-water (left) and PVPVA-water (right). The quantity $\frac{\Delta v}{v_{0p}^{NE}}$. describes the difference between the modelings of $\frac{v}{v_{0p}^{NE}}$. using PC-SAFT alone (dotted lines) and the combined approach (thick solid line) and is highlighted as the filled region. The glass transitions are displayed as dashed lines and were derived from literature [38,40].