Blends of Poly(vinyl alcohol) and Poly(vinyl pyrrolidone): Interrelation between the Degree of Hydration and Thermal and Mechanical Properties

Blends of poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) (PVP) readily absorb water when exposed to external humidity. The plasticizing action of water affects several properties of the blends that are critical for the performance of the materials in various applications. Blend films of PVA and PVP, with the compositions 0, 20, 30, 40, 50, 60, 80, and 100% PVP, were thermally annealed, equilibrated at four different humidities, and the effect of increasing humidity levels on the degree of hydration and the thermal and tensile mechanical properties was studied. A linear relation between the weight fraction of absorbed water at a specific external humidity, and the composition of the blend was observed. Both the compositional dependence of the glass transition temperature (Tg) of dry blend films and the depression of the Tg of each blend with increasing amounts of absorbed water, as studied by differential scanning calorimetry, were equally described well by the Couchman–Karasz and Gordon–Taylor equations. The physical meaning of the respective fitting parameters was evaluated in terms of measured or literature values of PVA and PVP properties. The satisfactory fitting of the Tg depression data allowed us to establish a linear relation between the composition of the blend and the minimum amount of absorbed water needed to transform the initially glassy material to a rubber. For all blend compositions, the Young modulus of hydrated samples suffers a sharp drop of approximately 1 order of magnitude when the degree of hydration was sufficient to transform the initially dry, glassy films to rubbery materials.