Studies on Molecular Origin of Polylactide Mesophase and its Thermal Stability

An annealing study of oriented amorphous fibers was conducted to uncover the molecular origin of polylactide (PLA) mesophase and its thermal stability. A quiescent and isothermal annealing treatment was carried out at temperature < T_g to avoid crystallization but suffice to activate the concerned structure. WAXS and FTIR analyses were used to detect the structure and related molecular interactions. DSC analysis was used to examine the thermal response and stability. The results showed that the mesophase was developed readily in the annealed fibers, along with enhanced dipole-dipole interactions between the C=O groups. These were realized through the conformational rearrangement during which the distance between the C=O groups was shortened sufficiently. It is the stress-induced orientation in the annealed fibers that reduces the kinetic barrier and promotes the energetically more favorable conformational rearrangement. Upon heated above T_g, the mesophase formed in the annealed fibers was not destructed but grew up during the subsequent heat-treatment.