Thermophysical properties of chitosan, chitosan–starch and chitosan–pullulan films near the glass transition

The thermo-mechanical properties of aqueous solution-casted films of chitosan (C), starch–chitosan (SC) and pullulan–chitosan (PC) blends were examined by Dynamic Mechanical Thermal Analysis (DMTA) and large deformation tensile testing. Incorporation of sorbitol (10 and 30% d.b.) and/or adsoprtion of moisture by the films resulted in substantial depression of the glass transition (Tg) of the polysaccharide matrix due to plasticization. For the composite films there was no clear evidence of separate phase transitions of the individual polymeric constituents or a separate polyol phase; a rather broad but single drop of elastic modulus, E′, and a single peak tanδ were observed. The relationship between the Tg and moisture for all films could be modeled with the empirical Gordon–Taylor equation. Apparent activation (Ea) energies for the α-relaxation process, estimated from multifrequency DMTA measurements, were within 225–544 kJ mol−1 depending on film composition; the Ea and ‘fragility’ parameters decreased with increasing moisture content. Analysis of visoelasticity data using the time–temperature superposition (TTS) principle with the Williams–Landel–Ferry (WLF) equation was successful, provided that the coefficients C1 and C2 are optimized and not allowed to assume their ‘universal’ values. Tensile testing of films adjusted at various levels of moisture indicated large drops in Youngʹs modulus and tensile strength (σmax) with increasing level of polyol and moisture; the sensitivity of the films to placticization was in the order of SC>PC>C. Modeling of the modulus data with the Fermiʹs equation allowed comparison among samples for the fall in modulus around the glass transition zone as a function of moisture content under isothermal conditions. Relationships between σmax and water content showed an increase in stiffness of the PCs films from 7–11% moisture, and a strong softening effect at higher water contents. The observed range of σmax values (20–80 MPa) for most films is comparable to many medium-strength commercial films.