Characterization of Environmentally Friendly Polymers by Inverse Gas Chromatography: I Amylopectin

Amylopectin, as a potato starch based polymer, with a molecular weight of six million gram/mol was characterized using the Inverse Gas Chromatography Method (IGC). DSC method was also used to measure the glass and melting temperatures. Both DSC and IGC agreed well on a T_g of 105°C and T_m of 160–166°C. Nineteen solutes (solvents) were injected onto a chromatographic column containing amylopectin. These solutes revealed the interaction of alkanes with AP, and the wettability and water intake of AP. Alkanes showed exothermic values of interaction parameters which were increased as the temperature increased and as the number of carbons in the alkane series decreased. Retention diagrams of these solutes in a temperature range of 80–200°C revealed two zones, crystalline and amorphous. T_g and T_m were measured using these zones which complimented the DSC values. The two zones were used to calculate the degree of crystallinity below the melting temperature which ranged from 85% at 104°C to 0% at 161°C. The dispersive component of the surface energy of amylopectin was measured using alkanes which ranged from 25.35 mJ/m² at 80°C to 8.47 mJ/m² at 200°C. This is attributed to the weak crystalline surface of AP at 80°C and when the surface melted at 160°C the surface energy decreased due to the thermal expansion of the surface.     

Effect of Water Content in Potato Amylopectin Starch on Microwave Foaming Process

In this study we investigated the role of the water content of extrudates had in foaming capacity and searched for the water content giving the greatest expansion of starch extrudates. Porous structures based on potato amylopectin starch were prepared by extrusion followed by a microwave foaming process. Starch was first extruded with water, in order to incorporate water in the granular structure and achieve gelatinization. Extrudates were conditioned at humidities ranging from 11% to 97%. The water content in the starch extrudates was studied by a water vapor sorption isotherm study. Extrudates were analyzed with light microscopy and wide angle X-ray scattering studies to determine degree of crystallinity. In the second step, extrudates were foamed in a microwave oven. As the water started to boil, it acted as a blowing agent, leaving a porous closed-cell starch structure. The densities and the expansion ratios of the foamed samples are determined. Porosity was studied with environmental scanning electron microscopy. Mechanical properties as a function of the surrounding humidity were analyzed with dynamic mechanical analysis. We found that the maximal degree of expansion was in extrudates conditioned at 33% and 54% RH and having water content of 11.2% and 13.4%, respectively. This level of water is sufficient to expand the extrudate to a maximum level but not high enough to plasticize the starch and cause cell collapse after treatment.     

The Effect of Starch Composition on Structure of Foams Prepared by Microwave Treatment

A microwave technique was used to prepare foams from different potato starches in granular form, with varying amounts of amylose content, and water. In addition to native potato starch (PN), high amylose potato starch (HAP) and potato amylopectin (PAP) were used, as well as mixtures thereof. In all cases the native crystallinity of starch granules was lost upon microwave treatment and an amorphous material was created. An increased concentration of starch in the initial water dispersion resulted in a less dense foam structure. The potato amylopectin formed open cell foams, whereas increased amylose content, as in native potato starch, yielded a more compact structure with irregular pore shapes. The high amylose potato starch yielded a structure with hardly any porosity. Foaming experiments were done to compare pre-gelatinized and granular starches dispersed in water. The pre-gelatinisation did not affect the pore formation process. These experiments indicated that the molecular architecture of starch polymers is more important for foam formation than starch polymer organization in the granules. Studies of temperature profile and dry matter content during microwave treatment showed that water evaporates more rapidly from a high amylose starch solution than native potato starch and potato amylopectin solutions. Rheological measurements showed that the amylose solution had much lower viscosity than starch and amylopectin. This confirms that polymer – water interaction, such as in amylopectin solution, favours stabilization of bubbles formed upon boiling and evaporation of water, which yields high porosity materials.