Water sorption and thermo-mechanical properties of water/sorbitol-plasticized composite biopolymer films: Caseinate–pullulan bilayers and blends

The moisture sorption and thermo-mechanical behavior of pullulan, sodium caseinate and their blend and bilayer films (weight polymer ratio 1:1) were studied. All plain and composite films containing the same level of sorbitol showed very similar equilibrium water content. Incompatibility between both polymers was assumed, since separate tan δ peaks corresponding to those of the two pure components, were observed in the DMTA thermal traces of their blends containing 25% w/w (dry solids) sorbitol. The thermo-mechanical properties of polyol-free blends and bilayers were governed by pullulan, as their behavior was too similar to that of pullulan alone. The plasticizing effect of water was evident in all samples, with the pullulan, blend and bilayer films exhibiting greater glass transition temperature (Tg) depression than the plain sodium caseinate films at similar moisture content levels. This differentiation was attributed to structural variation between the two polymers. Sorbitol addition decreased the Tg of both polymers at water contents up to 10% w/w; however, at higher hydration levels sodium caseinate exhibited an increase in Tg in contrast to pullulan, which showed a continuous decline. The apparent activation energy Ea of the primary relaxation (α-relaxation) decreased with increasing moisture and sorbitol content. In all sorbitol-plasticized films, a low-temperature relaxation (tan δ peak) observed in the region of Tg of sorbitol, shifted to lower temperature with increasing moisture content; the intensity of this transition increased with increasing sorbitol content and the corresponding Ea values were similar to those of a primary relaxation. The low-temperature transition might originate from a coupling of β-relaxation of the polymer and the α-motions of sorbitol. The fragility parameter, m, was calculated for all systems and allowed their characterization as relatively strong materials according to Angell's classification; the fragility decreased with increasing water content. The time–temperature superposition principle, using the Williams–Landel–Ferry equation, applied successfully to plain polymer films as well as to blends and bilayers, assuming that the C1 and C2 constants do not take their universal values, but are optimized for each system separately.