A PALS Contribution to the Supramolecular Structure of Poly(l-lactide)

Abstract

Positron annihilation lifetime spectroscopy (PALS) was conducted to follow the evolution of free volume during crystallization of PLLA at 100°C. A finite lifetime distribution of three components was used to fit the PALS spectra. The analysis of the longest lifetime component (τ 3 ) indicates that the free volume distribution evolves during crystallization by increasing the number of holes yet decreasing their size; moreover, the free volume fraction increased during crystallization. Following the evolution of the shortest components a correlation was found with crystalline and amorphous phase contents present in PLLA. The first component (τ 1 ) was assigned to positron annihilation in occupied zones of the crystalline phase whereas the second component (τ 2 ) was attributed to annihilation by different amorphous arrangements. A model for the supramolecular arrangement of PLLA chains was devised in terms of free volume enlargement for annealed samples. According to this model transformations occur in mobile amorphous phase (MAP) and rigid amorphous phase (RAP) with PLLA chains evolving from folded or coil conformations in the asquenched samples containing uniquely MAP to opener (more extended) conformations in samples containing larger RAP and crystalline fractions. The proposed model provides a rational for the understanding of some unexpected effects associated with free volume that have been observed in several semicrystalline polymer systems, i.e., the lowering of density during crystallization (dedensification), the acceleration of polymer chains dynamics around the T g (dynamic fragility) due to a rigid amorphous phase confined by crystallites, and also the gas permeability behavior in terms of solubility and diffusion coefficients.

Phase-structure and mechanical properties of isothermally melt-and cold-crystallized poly (L-lactide)

“…(X MA ) and rigid amorphous phases (X RA ) were determined according to a previously reported procedure (Del Rio et al, 2010) with the next equations…”

Impact of crystallinity of poly(lactide) on helium and oxygen barrier properties

“…One reason might be that PLA seems to have lower oxygen solubility compared to PET [4]. Del Rio et al [41] studied very recently the evolution of free volume in crystallized PLLA with the help of Positron Annihilation Lifetime Spectroscopy (PALS) measurements. They observed an increase in free volume upon annealing which they located primarily inside the RAF.…”

“…The free volume theory predicts that diffusion is dominated by the scarcity of free volume and that specific molecular interactions are of secondary importance, particularly in the case of poorly interacting molecules as it is the case in the present system. An ''opener'' [41] structure might thus imply higher connectivity in order to allow transport [42]. Basically speaking, more small voids are more efficient than some large ones.…”

Multi-scale analysis of the impact of polylactide morphology on gas barrier properties

“…The RAF is the intermediate phase connecting the crystalline phase to the mobile amorphous fraction (MAF) [23]. The RAF constitutes a dedensified amorphous phase [24] because of geometrical constraints preventing structure relaxation. Guinault et al [14] showed that the formation of RAF was favoured when PLA was crystallized from the glassy state in a'-form.…”