Gas Barrier Properties of PLA

The oxygen transport coefficients of PLA, whatever the L/D ratio, are sensitive to the measurement temperature and water activity. Auras et al. [106] showed that the increase in the temperature leads to an increase in the oxygen 

The addition of plasticizer used for the improvement of mechanical properties leads generally to an increase in the oxygen permeability coefficient due to the higher mobility of the polymer chain and higher free volume [102]. On the contrary, the dispersion of nanoclays in PLA makes it possible to divide the permeability coefficient by 2 or 3 depending upon the type of the nanoclays (e.g. organomodified montmorillonite, cloisite 25A or 30B, organomodified synthetic fluorine mica) and exfoliation [129-131]. 

The crystallization of the PLA matrix induces a decrease in the carbon dioxide permeability coefficient [123, 132]. According to Sawada et al, [128] the effect of crystallinity on CO permeability is similar to the one on oxygen, that is to say there is a slight increase in the diffusion coefficient at low crystallinity followed by a decrease of 40%, whereas solubility decreases slowly in relation with the crystallinity degree. 

The L/D ratio might not affect the nitrogen permeability of PLA [134]. The crystallinity effect has also been evaluated for this gas and it seems that the nitrogen permeability coefficient decreases with the increase in the crystallinity degree of PLA [128]. 

The gas selectivity in the PLA membrane depends on the chosen gas. Lehermeier et al. [124] show that the separation factor of CH and CO, equal to 10, is not crystallinity and L/D ratio dependent. Only change in temperature makes it possible to increase this factor. Sawada et al. [128] confirm the inefficiency of the crystallinity on the permselectivity but also show that the diffu-sivity and the solubility selectivity are not influenced by crystallinity. The gas permselectivity in PLA is larger than 120 for H /N, 6 for O /N, 23 for CO / Nj, 27 for COj/CH and 1 for CH /N [128]. The permselectivity of PLA is two times larger than those of LDPE and PVC for O /N and CO /N [122]. 

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