L-Lactide Unit Content

The ratio of i-enantiomers to n-enantiomers (l/d ratio or l d) is known to affect the properties of PLA, such as melting temperature, degree of crystallinity, and barrier properties. Control of the l d monomer content is an important molecular feature of PLAs that has a large effect on barrier properties. Generally, higher L-lactide content should result in higher barrier properties of the polymer because of greater stereochemical purity [12]. 

FIGURE 12.2 Examples of extrinsic and intrinsic factors affecting P, D, and S in polymers such as PLA. 

2 Water Vapor Water vapor permeability (WVP) of amorphous PLA films (obtained by solution casting) with different L-lactide unit contents (PLLA, P(LLA-DLA) (77 23), and P(LLA-DLA) (50 50)) have been reported as 2.18 xl0 1.90 xl0 and 1.95 x lO- kgm/ (m sPa), respectively, at 25°C [14]. These results indicate that L-lactide unit content had little effect on the water vapor permeability of the PLA films. The values are comparable to those measured by Auras et al. [15] (1.98 x 10 and 

98 X 10 kgm/(m sPa) at 20°C, and 1.65 x 10 and 1.61 X 10 kgm/(m sPa)at30°CforP(LLA-DLA)(98 2) and P(LLA-DLA) (94 6), respectively). The authors also found that water permeability values were similar for the PLA films exposed to different temperatures and RH. The Ep for the water vapor permeation was —9.8 kJ/mol for films made from 98% L-lactide and 10.1 kJ/mol for films made firom 94% L-lactide. 

Caimcross et al. [16] studied moisture sorption and transport in three different PLA films (high percentage l-lactide, a mixture of lactic acid stereoisomers, and a 50 50 blend of PLLA and PDLA) exposed to RH changes from 0 to approximately 25% at 40°C using a quartz crystal microbal-ance/heat conduction calorimeter (QCM/HCC). In addition, moisture transport, crystallization, and degradation in PLA 

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