Glass transition and physical properties of polyol-plasticised pullulan-starch blends at low moisture

The effects of water and polyols, at low weight fractions, on water sorptio n behaviour, thermal and mechanical properties, and gas permeability of ble nds of pullulan and gelatinised corn starch (ps), prepared by either hot pr essing or casting aqueous solutions, were studied. Incorporation of sorbito l or xylose in the ps blends resulted in lower equilibrium moisture content s in the low to medium a(w) range, and much higher moisture contents at a(w ) > 0.75; the Guggenheim-Anderson-DeBoer isotherm model adequately describe d the sorption data up to the a(w) of 0.9. Water and polyols exerted a stro ng plasticising action, lowering the T-g of the blends and allowing enthalp y relaxation events to occur during aging of the amorphous specimens. At th e low polyol levels examined (10 and 20% dry basis), a single glass transit ion temperature for the polymeric constituents was identified in all sample s by DSC and DMTA; apparent activation energies of 226-296 kJ mol(-1) for t he alpha-relaxation were estimated from multifrequency mechanical measureme nts. Large deformation mechanical tests demonstrated sharp decreases in You ng's moduli with increasing levels of polyol and water, typical of the glas s-rubber transition of amorphous polymers; the relationship of flexural mod ulus and moisture content was quantified using the Fermi's model. The relat ionship between maximum stress (sigma(max)) and water content showed an inc rease in stiffness of the blends from 7 to 11% moisture, and a strong softe ning effect when the water content exceeded this range. The Arrhenius plots of O-2 and CO2 permeability data showed distinct changes in slope in the g lass transition region of the blends. Analysis of viscoelastic data with th e time-temperature superposition principle and treatment of gas permeabilit y data with the Williams-Landel-Ferry equation suggested that application o f this model is meaningful if the coefficients, C-1 and C-2, are allowed to vary instead of assuming their 'universal' values. (C) 1999 Elsevier Scien ce Ltd. All rights reserved.