A molecular dynamics study of siloxane diffusion in a polyester-melamine solution

This study examines the diffusion of small methylated and hydroxylated siloxanes through solutions of polymer and crosslinker molecules. The diffusivity of the siloxanes was calculated for bulk diffusion and for surface segregation. Our simulations demonstrate that there is significant interaction between 3,3-dimethylhexahydroxytrisiloxane (hexa-OH-TS) and the polar solvent components of the paint matrix at 298 K. The nature of these associations is largely due to hydrogen bonding between the siloxane and the polar solvent molecules. At 573 K the diffusivity of solvent molecules is sufficiently high to disrupt interactions. The smaller number of hydroxyl groups in 1,5-dihydroxyhexamethyltrisiloxane (hexa-Me-TS) results in a weaker interaction with the polar solvent. Surface segregation studies indicate that there is a slight increase in the concentration of hexa-OH-TS in the surface layer of the system as a result of curing and relaxation processes. However, for hexa-Me-TS there is in fact a decrease in the surface concentration of the siloxane relative to the bulk concentration.