Molecular weight effects on solution rheology of pullulan and mechanical properties of its films

Abstract

The effects of molecular weight on solution rheology of pullulan, and on thermomechanichal properties of sorbitol and/or water-plasticized pullulan specimens, prepared by either hot pressing or casting of aqueous solutions, were studied. Pullulan samples differing in molecular weight were characterized by 13C NMR spectroscopy and size exclusion chromatography combined with a multiangle laser light scattering and a refractive index detector. For samples with weight average molecular weight ( M w) ranging between 100 and 560×10 3, the values of limiting viscosity ([η]), critical concentration ( c ∗) and coil overlap parameter ( c ∗[η]) were within the range 0.38–0.70 dl/g, 1.4–3.1 g/dl and 1.0–1.2 dl/g, respectively. The thermomechanical properties of five molecular weight grades of pullulan, either alone or with sorbitol (plasticized at a 10% d.b. level) were examined by dynamic mechanical thermal analysis (DMTA). A large drop in storage modulus E′ (∼10 1.5–10 3 Pa) and a peak in tan δ in the DMTA traces accompanied the glass–rubber transition ( T g) or the α-relaxation ( T α) of pullulan; the magnitude of the drop in E′ and the tan δ peak height increased with increasing water content. The plasticizing action of water and sorbitol was evident in the DMTA curves, and the T g vs. moisture content data were fitted to the Gordon–Taylor empirical model. Within the range of molecular weights tested there was no effect of polymer molecular weight on T g. A β-relaxation detected by DMTA was shifted to lower temperature with increasing moisture content and to higher temperature with addition of sorbitol. Apparent activation energies for α-relaxation ( E αα) and β-relaxation ( E αβ) processes, estimated from multifrequency measurements, were within 171–640 and 118–256 kJ/mol, respectively; the values for E αα and ‘fragility’ parameter decreased with increasing moisture content. Analysis of viscoelasticity data using the time–temperature superposition principle with the Williams–Landel–Ferry equation was successful over the range T g to T g+40 °C, provided that the coefficients C 1 and C 2 are optimized and not allowed to assume their ‘universal’ values. Large deformation mechanical tests demonstrated large decreases in tensile (Young's) modulus ( E) and strength ( σ max), and an increase in percentage elongation with increasing water content and/or addition of sorbitol in pullulan films. Relationships between the tensile parameters ( E and σ max) and water content showed an increase in stiffness of the films from 3 to 7% moisture, and a strong softening effect at higher water contents. The tensile tests revealed some relationships between mechanical properties under uniaxial load and the molecular characteristics of pullulan, e.g. E, σ max, and elongation values increased with increasing molecular weight.