Viscoelastic and thermal properties of collagen/poly(vinyl alcohol) blends

Blends of poly(vinyl alcohol) (PVA) with collagen and gelatin, prepared from aqueous solution by solvent casting, were investigated by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). After conditioning at 51% relative humidity, collagen and PVA show nearly coincident glass transition temperatures (Tg approximately 35 degrees C), while gelatin has a higher Tg (approximately 70 degrees C). Gelatin/PVA blends show two invariant Tgs, whose temperature and associated specific heat increment clearly indicate the coexistence of two amorphous phases composed of the pure components. Owing to similarity of the Tgs of collagen and PVA after humidity conditioning, DSC offers no indication on miscibility of collagen/PVA blends. In DMTA experiments, where absorbed water freely evaporates from the samples during the thermal scan, PVA shows a glass transition relaxation at about 50 degrees C, while both gelatin and collagen display an intense glass transition in the vicinity of 230 degrees C. The DMTA spectra of collagen/PVA and gelatin/PVA blends show two invariant glass transition relaxations at about 50 and 230 degrees C. Absence of any Tg shift with composition demonstrates that the blend components are immiscible. However, blends of PVA with collagen and gelatin form optically clear films with good mechanical properties over the whole range of compositions. It is found that at T > Tg (PVA) the elastic modulus (E') of the blends strongly increases with increasing content of the biopolymer. In the case of collagen/PVA blends, experimental E' values agree with the predictions of a simple two-phase composite model with phases connected in parallel. It is concluded that, though thermodynamically immiscible with both native and denatured collagen, PVA forms mechanically compatible blends with collagen and gelatin.