Structure and Mechanical Property of Melt-Drawn Oriented PLA Ultrathin Films

Abstract

Ultrathin poly(lactic acid) (PLA) films have been prepared by a melt-draw technique, and their structures and mechanical properties have been studied by atomic force microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and tensile test. It is found that the meltdrawn PLA films consist of highly oriented granular α microcrystallites. The draw rate and temperature exhibit a synergistic effect on regulating the structures, including crystallinity and chain orientation, of the melt-drawn films. Post-thermal annealing leads to a structure reorganization from granular into lamellar structures with enhanced crystallinity up to about 55%. It also increases the chain orientation in the amorphous region, while the orientation status of chains in the crystalline phase remains almost unchanged because its orientation function in the as-prepared samples already close to 1. Mechanical tests of the melt-drawn poly(L-lactic acid) films illustrate a tremendous enhancement in mechanical properties of oriented PLA films compared to their nonoriented counterpart. The highest Young's modulus and tensile strength achieved for the sample prepared at 180 °C under a draw rate of 45 cm/s are about 1.3 and 2.5 times higher than those of its nonoriented counterpart.

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

“…180 °C after annealing. 33 Taking this into account, the vanishing of β crystal diffraction spots in the electron diffraction pattern of the 100% deformed sample after annealing at 175 °C (Figure 4a) demonstrates the melting of β crystals and their recrystallization into α ones. This makes the existence of the 912 cm −1 band characteristic of β crystals in the 100% deformed sample after annealing at 175 °C hard to be understood.…”

“…The structure and morphology of oriented PLLA ultrathin films prepared by melt-draw before and after annealing have been described in our previous work. 33 It consists of highly oriented granular α microcrystallites with the c-axis (chain axis) aligned along the melt-draw direction during film preparation (cf. Figure S2a,b).…”

“…18,55,56 While the 921 and 912 cm −1 bands exhibit perpendicular "⊥" electron vectors with respect to the main chain of the PLLA, the electron vector of the 956 cm −1 band has been determined to be almost parallel to the main chain (with a small angle between the transition moment vector and the chain axis of α = 4.27°). 33 Based on the used absorption bands, the overall crystallinity (X c ) of the sample can be quantitatively evaluated by eq 1, while the ratio of β crystals (φ β ) with respect to their α counterparts can be estimated by eq 2. (2)…”

“…In a previous study, 33 we have studied the structures and morphologies of highly oriented ultrathin films of PLAs (i.e., PLLA and PDLA) produced through a melt-draw technique initially developed by Petermann and Gohil, 34 which has frequently been used to prepare highly oriented ultrathin polymer films. 35−41 It has been found that although the draw speed and temperature synergistically affect the structures of the melt-drawn films, including crystallinity and chain orientation in both crystalline and amorphous regions, highly oriented granular α microcrystallites have always been obtained for both PLLA and PDLA due to the high draw speed used.…”

Flow-Induced Shish Nucleation in Lightly Crosslinked Polyethylene: Connecting Polymer Properties and Strain to the Final Shish–Kebab Structure

“…It is known that external flow can enhance crystallization kinetics of polymers in orders of magnitude , and induce oriented structures. − The flow-induced crystallization (FIC) phenomenon represents a fundamental challenge in kinetics and thermodynamics of semicrystalline polymers. − Moreover, its ubiquity in polymer processing methods, e.g., spinning, blown films, , extrusion, , and injection molding, adds to its importance. Among various studies on FIC, the shish–kebab superstructure − has attracted the most attention since it was found by Blackadder et al, Pennings et al, , and Keller et al , A shish–kebab unit consists of a long central fibrillar backbone (shish) surrounded by chain-folded lamellar crystals (kebabs).…”