Structure Evolution of Oriented Poly(l‑lactic acid) Ultrathin Films during Deformation

Poly­(l-lactic acid) (PLLA) ultrathin films with a highly oriented chain-fold lamellar structure in its α phase with a 10/3 helical chain conformation were prepared by a melt-draw technique. The structure evolution of the oriented α PLLA films during deformation along the original chain direction was studied by electron microscopy combined with electron diffraction and Fourier transform infrared (FTIR) spectroscopy. The results clearly illustrate that further deformation always enhances the orientation of chain segments in amorphous regions regardless of the draw ratio, which reduces slightly after thermal annealing, whereas the orientation status of chains in the crystalline phase is less affected owing to their already perfect alignments. Moreover, the combined results of electron diffraction and FTIR demonstrate that stretching the oriented thin film up to a draw ratio of λ = 1.5 reduces the crystallinity of α PLLA due to the distortion of pre-existing crystals and transition of 10/3 helical chains into 3/1 ones of some chain segments. The packing of 3/1 helical chains characteristic of the β form PLLA into an ordered crystal unit cell is, however, not realized even after thermal annealing. Further stretching up to λ = 2 results in the formation of β crystals, which are perfected by annealing at 150 °C but melt and recrystallize into oriented α crystals again after annealing at 175 °C. These results clearly shed more light on the strain-induced α-β transition of oriented PLLA thin films.