Synthesis, characterization, and thermal degradation of Poly(L-Lactide)s

In this project, 1, 2, 3, and 4-armed poly(L-lactide)s (PLs) were synthesized by ring opening polymerization (ROP) of L-lactide in the presence of an alcohol and stannous dioctoate. The resultant polymers were characterized by Gel Permeation Chromatography (GPC) and Nuclear Magnetic Resonance spectroscopy (NMR). The dynamic thermal degradation of these polymers was studied by Thermogravimetric Analyzer (TGA). In order to study the effect of end-groups on thermal degradation, the synthesized OH functional polymers were reacted with succinic anhydride to obtain COOH functional polymers. It was found that the thermal degradation temperatures of acid functional polymers are 25oC higher than those of OH functional ones at the same heating rate. Therefore, they are more stable than the OH counterparts. The average activation energies (Ea) of thermal degradation of OH and COOH functional polymers were also determined using Ozawa’s and Reich’s approaches. According to Ozawa’s approach, Ea values of OH functional PLs changing between 73.7 kJ/mol and 76.5 kJ/mol while Ea values of COOH functional PLs changing between 77.9 kJ/mol and 81.8 kJ/mol. According to Reich’s approach, Ea values of OH functional PLs changing between 67.8 kJ/mol and 70.7 kJ/mol while Ea values of COOH functional PLs changing between 72.2 kJ/mol and 75.8 kJ/mol. Crystallinities of resultant PLs were characterized by X-Ray Diffraction (XRD). From the diffraction line broadening, it was concluded that the OH and COOH functional PLs have the same crystalline structure. However, some differences exist between the crystallite sizes of linear and multi-arm PLs as well as PLs with OH and COOH endgroups.