Epoxy free volume void

The results of the PALS investigations of the aging behavior of thin epoxy films are presented here. The decisive parameters of the PALS analysis for polymers are the formation probability lo.ps of ortho-positronium and its average lifetime To-ps- While lo-Ps predominantly represents the effectivity of the electron acceptor groups in the polymer, Tq.ps is directly correlated with the sizes of the free-volume voids. 

Regarding the average lifetime of ortho-positronium to.ps for the unaged epoxy film on Au substrate in Fig. 29.2, Tq-ps is constant within the experimental scatter for positron implantation depths above approx. 20 nm. This reflects constant free-volume void sizes, independent of the sample depth. A shallow maximum around 150 nm and a decrease in x. p above 400 nm are not significant. On the other hand, for very low depths up to 20 nm, a significant increase in To-Ps is observed. This can be interpreted as the effect of a surface region in the order of less than 20 nm wide where the sizes of the free-volume voids increase [19]. However, a diffusion of ortho-positronium to the epoxy surface and into vacuum or an impediment of the positronium formation due to a reduced... 

Whereas Tq-ps is in the range 1710 10 ps inside the unaged epoxy films on Al and Au, it is only 1695 10 ps on the Cu substrate. As described below, these films on Cu contain fewer residual oxirane groups than on Al and Au. This could be the reason for the lower size of the free-volume voids in the epoxy network on Cu. 

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