Characterization of Biodegradable Polymer Blends by Inverse Gas Chromatography: Amylopectin and Its Blends

Abstract

Amylopectin (AP), as a potato starch based polymer, with a molar mass of six million gram/mol was blended with several biodegradable polymers: poly(ε-caprolactone) (PCL), Poly (3-hydroxy butyric acid) (PHBA), poly (DL-lactide-co-glycolide) (PLG) and three different molar mass of poly (acrylic acid) (PAA). All pure polymers and the blends were characterized using the Inverse Gas Chromatography Method (IGC), Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD) over a wide range of temperatures (80-260 °C). Nineteen solutes (solvents) were injected into five chromatographic columns containing AP-biodegradable polymer blends. These solutes probed the dispersive, dipole-dipole, H-bonding interactions, acid-base characterstics, the wettability and water uptake of AP-PCL blends. Retention diagrams of these solutes in a temperature range of 80-260 °C revealed at least two zones, crystalline and amorphous. Tg and Tm of the pure polymers and the blends were measured using these zones. The two zones were used to calculate the degree of crystallinity of the pure AP and its blends below the melting temperature which ranged from 81% at 104 °C to 0% at the Tm values. IGC has complimented the DSC method in obtaining the Tg and Tm values of the pure AP and the AP-PCL blends. These values were unexpectedly elevated for the blends from that of the pure AP, and ranged from 105 °C to 152 °C for Tg values, and 166°C to 210 °C for Tm values. The Tm values agreed well with the XRD analysis data. This elevation in Tg and Tm values may be due to the change in heat capacity at Tg, and the dependence of Tg on various variables including molar mass, and the blend composition. Tg values for AP- PHBA, PLG and PAA were also elevated in contrary to the Tm values which showed a depression in the melting points. Polymer blend-solvent interaction parameters were measured using a variety of solutes at a wide range of temperatures which determined the solubility of the blends in these solutes. It was also able to determine the blends compatibility over a wide range of temperatures and weight fractions. χ23 and B23 parameters agreed well on the partial miscibility of the blends studied. The dispersive component of the surface energy of the pure polymers and the blends were measured using alkanes which ranged from 16.00 mJ/m2 for the pure AP to 55.46 mJ/m2 when AP was mixed with PHBA in a 50-50% ratio. This reveals the increase of the surface energy of AP when it was blended with other biodegradable polymers. IGC was effective and versatile in measuring the degree of crystallinity of the pure polymers and the blends at any single temperature below AP's Tm, unlike the DSC method which provides a range of the degree of crystallinity.