THERMAL CHARACTERISTICS OF POLYLACTIC ACID/WHEAT GLUTEN BLENDS

Abstract

ABSTRACT The interaction between polylactic acid (PLA) and wheat gluten (gluten) has been studied using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, X‐ray diffraction and phase contrast microscope. The primary DSC thermogram of gluten showed one glass transition (Tg) at 63C. Neat (pure) PLA also showed a Tg around the same temperature range followed by crystallization (Cry) and a melting (Mel) transition. Different PLA : gluten blends (20:80, 50:50, 80:20) showed a Tg followed by PLA Cry and immediate Mel transitions. However, subsequent heating‐cooling cycling of the blend demonstrated a decrease in PLA Cry and Mel transitions and the appearance of a second Tg. The presence of gluten reduced the number of cycles needed to change PLA Cry structure to mostly amorphous. The low PLA Cry indicates an interaction between the two polymers. The TGA data showed slower PLA degradation compared with gluten, where 10% was degraded at 316C while 10% of the gluten degraded at 254C. Solid‐state FTIR showed that amide I and II peaks significantly decreased signifying gluten‐heat degradation. A strong peak, corresponding to ester formation between gluten and hydrolyzed gluten molecules, appeared at 1750/cm. The esterification increased in the presence of PLA. The phase contrast microscope demonstrated that the melting of PLA starts at 150C, which means that the continuous phase between 150 and 250C was the melted PLA. At 220C, gluten formed a thin layer; and at about 245C, it aggregated.