Migration of LMW Molecules

The migration of the low molecular weight PDMS (6 to 15 siloxane monomers) molecules is considered the primary mechanism responsible for the recovery of hydrophobic-ity [71, 72, 75, 77], These molecules are formed during the polymerization process. Due to their small size they are quite flexible and are not included (connected) in the crosslinked network. Consequently, considering the weak intermolecular forces observed, they are free to move within the material volume and diffuse to the surface. When a contamination layer is formed they can penetrate into the layer and change the initial hydrophilic behavior to hydrophobic. Thus, although the surface is covered by a hydrophilic film, eventually a hydrophobic behavior is achieved. 

The speed of recovery and the endurance of the migration mechanism are affected from the amount of LMW molecules available. The initial population and type (linear or cyclic) depends on the polymerization process and the vulcanization that follows. Further, during service a possible loss of the LMW molecules can be assumed since they are exposed on the surface. Consequently, a probable depletion of the LMW reservoir, which corresponds to the end of the material lifetime, becomes the primary user concern, and thus the parameters that influence the diffusion process have to be considered. 

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