Some effects of processing on the molecular structure and morphology of thermoplastic starch
ABSTRACT Hydroxypropylated and oxidised potato starch (HONPS) was used together with glycerol and water to produce thermoplastic starch. The amount of glycerol was kept constant at 22 parts by weight per 100 parts of dry starch. The thermoplastic starch was converted into films/sheets using three different processing techniques; casting, compression moulding and film blowing. The last two methods represent typical thermoplastic conversion techniques requiring elevated processing temperatures. By means of size-exclusion chromatography, it was found that compression moulding and film blowing led to some degradation of high-molecular weight amylopectin as well as of high-molecular weight amylose-like molecules. The degradation was significantly less pronounced for the cast films. The morphology of the specimens was quite complex and phase separations on different levels were identified. In the cast films and, to a lesser extent, in the compression-moulded specimens, a fine network structure could be distinguished. Such a structure could however not be ascertained in the film-blown material and this is discussed in terms of the thermo-mechanical treatment of the starch materials.
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ABSTRACT: The effects of glycerol and water content on the thermal transitions of plasticized barley starch were examined using differential scanning calorimetry. The glycerol contents studied were 14, 20, 29 and 39% and the water content, obtained by conditioning in different relative humidities, varied in the range 1–28%. On the basis of the observed calorimetric glass transition temperatures and corresponding heat capacity increments it was inferred that a single phase system occurred at low water and glycerol contents, while in other cases phase separation occurred and the system was composed of starch-rich and starch-poor phases. Dynamic mechanical thermal analysis on a phase-separated sample showed mechanical loss peaks corresponding to the glass transitions of both phases. Amylopectin crystallization did not occur within 1 week of storage in mixtures having less than 20% water, indicating that glycerol interacted with starch, inhibiting crystallization of amylopectin.Carbohydrate Polymers. 01/1997;
Article: Microstructure of amylose gels[show abstract] [hide abstract]
ABSTRACT: The aim of this investigation was to study the structural characteristics of amylose gels (2–8% w/v) at different scales of organization. Native gels have been studied by electron microscopy, mild acid hydrolysis, differential scanning calorimetry and size-exclusion chromatography. A model for organization of the polysaccharide within the gel structure is proposed.Amylose gels exhibited a macroporous structure (mesh size 100–1000 nm) containing filaments 20 ± 10 nm wide. The filaments resulted from the association of segments of amylose chains (26 < DPn < 31; 56 < DPw < 73) which were orientated obliquely to the filament axis. These amylose fragments were partially organized in a B-type crystalline array.Amylose gels also contained an amorphous fraction consisting of 18–33% of the polysaccharide. This fraction was easily degraded by acid hydrolysis. The amorphous fraction was described as dangling chains (6 < DP < 30) located in the macroporous structure. It was mainly responsible for the hydrodynamic behaviour and the porosity of native amylose gels.The limit between the associated and amorphous regions of the gel was not perfectly defined and an intermediate zone which may recrystallize upon acid hydrolysis was identified.Carbohydrate Polymers. 01/1992;
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ABSTRACT: The dynamic mechanical, dielectric and calorimetric behaviour of amylose films, plasticised with glycerol, was examined as a function of temperature and glycerol content. Two separate relaxation processes were observed. Whereas the temperature of the lower temperature transition, in the vicinity of −50°C, showed a weak dependence on composition, the temperature of the upper transition, in the range 150–50°C, decreased with increasing glycerol content. The activation energy of the lower temperature transition, obtained from the dependence of the frequency of the relaxation peak upon temperature, was typical of a primary α-relaxation or glass transition process. This indicated that although glycerol was an effective plasticiser of amylose, amylose–glycerol mixtures are only partially miscible.Carbohydrate Polymers. 01/2001;