Influence of the Rigid Amorphous Fraction and Crystallinity on Polylactide Transport Properties

Abstract

The effects of the crystallinity and polymorphism of PLLA and PDLA–PLLA 50:50 blends on the free volume and transport properties have been studied. In the case of PDLA–PLLA 50:50 the increase on crystallinity promotes a process of densification and a reduction in free volume fraction and rigid amorphous fraction (RAF), contrary to PLLA in which the enlargement of a rigid amorphous fraction led to a dedensification. This result offers a unique opportunity to analyze separately the opposite influence in the transport properties of the crystallization and free volume associated with RAF. Overall, these findings provide a better understanding on the relationship between crystallinity and transport properties and would explain also the controversial data reported in the literature.

Crystallization-induced formation of rigid amorphous fraction

“…If the path is extended, permeation can be delayed by a decrease of diffusivity . Conversely, solubility and permeability are favored by the high free volume of the RAF . The influence of the RAF on barrier properties was deeply investigated for PET and PLLA .…”

“…Conversely, solubility and permeability are favored by the high free volume of the RAF . The influence of the RAF on barrier properties was deeply investigated for PET and PLLA . For PLLA, crystallization was found to decrease in the oxygen permeability, but not in linear proportion with the decrease in amorphous volume, which was attributed to the low density rigid amorphous layer at the crystal/amorphous boundary .…”

Improving the barrier character of poly(caprolactone): Transport properties and free volume of immiscible blends

“…The cell, which is made of polytetrafluoroethylene, is a small container partially filled with water and a membrane sealing its top. The measurements were carried out in a Sartorius BP 210 D balance with 10 −5 g readability . The measurements were repeated five times to obtain the average values and the error bars shown in this paper correspond to their standard deviations.…”

“…After the adequate data treatment, the solubility and diffusion coefficients, S and D , respectively, can be determined. The absorbed gas concentration, C , is calculated from the equilibrium weight gain, M ∞ , by the following equation, where M ∞ is the total absorbed mass at equilibrium in g, 44 g/mol is the molecular weight of carbon dioxide, and V plm is the volume occupied by the sample. The solubility coefficient can be calculated from the following equation, where p is the pressure.…”

“…This equation depends on the geometry of the sample and concentration conditions. In the case of a thin film geometry in which the diffusion on the edge of the film can be considered negligible and considering that the pressure of the penetrant and the diffusion coefficient are constant, the mass uptake is described by this equation, where M t is the absorbed gas at t time, l is the thickness of the film, and D is the diffusion coefficient (in cm 2 /s). This equation can be simplified to obtain the so‐called short time approximation (up to 50% of M ∞ ) and long time approximation (50–90% of M ∞ ) …”