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

Abstract

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.

Directional Epitaxy Intensifying Intermolecular =C–H···O=C Hydrogen Bonding to Expedite Bulk Crystallization of Biobased Poly(butylene 2,5-furandicarboxylate)

“…33 Meanwhile, a molecular-level comprehension of the epitaxial mechanism remains controversial and is still far from conclusive. 15,[21][22][23][24][25][26][27][28][29]32,33 Therefore, understanding and controlling polymer crystallization via epitaxy especially in bulk materials on foreign surfaces have been and remain to be a serious challenge in polymer physics. [13][14][15][16][17]33 Furan-aromatic polyesters produced from biomass-derived furandicarboxylic acid (FDCA) are considered the renewable alternatives of petrol-based benzene-aromatic polyesters.…”

“…Moreover, the important effect of secondary nucleation upon epitaxial crystallization between polymers was also highlighted . Very recently, Tong and co-workers have approved that lattice matching is indeed required but insufficient for successful epitaxy and the critical nuclei size dominates the epitaxial growth. , Despite the extensive studies with diverse systems over the past decades, regulating polymer crystallization via epitaxy is nowadays hardly desirable. ,,− More importantly, due to the very short range of interfacial epitaxial interaction, the past works involve polymer epitaxy focused on very thin films with the main evidence of crystal morphologies and orientation relationships obtained by atomic force microscopy, transmission electron microscopy, and electron diffraction, ,− while polymer epitaxy from bulk crystallization is seldom concerned and the featured evidence is essentially missing . Meanwhile, a molecular-level comprehension of the epitaxial mechanism remains controversial and is still far from conclusive. ,− ,, Therefore, understanding and controlling polymer crystallization via epitaxy especially in bulk materials on foreign surfaces have been and remain to be a serious challenge in polymer physics. − , …”

Supertoughened Polylactide via the Addition of Low Content Poly(ε-caprolactone) and Tensile Deformation above the Glass Transition Temperature

“…Moreover, highly oriented crystallites act as physical crosslinking points to bear the load and thus further strengthen PLA. Several reports have explored the effects of orientation regarding PLA on the evolutions of crystallization and/or mechanical properties via imposing flow field. − Li et al fabricated a set of PLA films by casting-thermal stretching to balance the tensile strength and tensile toughness. The toughening was manifested by the increase of elongation at break to 295% corresponding to a yield strength of 69.5 MPa at the relatively low pre-drawn ratio ascribed from the gauche–gauche conformation of the oriented amorphous phase.…”

Temperature-Dependent Polymorphism and Phase Transformation of Friction Transferred PLLA Thin Films

“…In addition, they proposed the stress-induced phase transition mechanism α-form with 10/3 helices to β-form with 3/1 helical conformation via α′- (δ-) form under deformation at different temperatures. In our recent study, it was found that β-form fibrils reorganized and perfected upon further annealing by utilizing orientated PLLA ultrathin melt-draw films [ 41 ].…”