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Conversely, increasing the plasticizer molecular weight increases the retention because, as the molecular weight increases, the size and length of the plasticizer increase, which makes it more difficult for the plasticizer to diffuse from the PVC molecular structure to the geomem- brane surface.

For this reason, the PVC Geomembrane Institute (PGI) has developed the PGI-1104 Specification, which requires an average plasticizer molecular weight greater than or equal to 400 to ensure long-term plasticizer retention in the PVC geomembrane.

On the other hand, its molecular weight is much lower than the critical molecular weight for entanglement in PLLA, Mc, that was found to be near 9000 g/mol while the molecular weight between entanglements, Me, is near 4000 g/mol . Thus, this plasticizer can be dispersed very well in melt polymers being a liquid and at the same time it does not

The effect of molecular weight of plasticizer on the migration behavior can be analyzed within branched plasticizers, TOTM and TBTM, which have very different molecular weights. At the end of 24 h, TOTM with a higher molecular weight (M w : 546.79 g/mol) has a lower migration rate compared to that of TBTM (M w : 378.5 g/mol) at all plasticizer

Finally, it is recommended that a minimum average plasticizer molecular weight of 400 be used to ensure long-term plasticizer retention and adequate field performance of PVC geomembranes. The weighted-average method for calculating the average molecular weight is recommended when two or more plasticizers are used in the PVC geomembrane formulation.