In this work, PET foils with different degrees of crystallinity were
plasma-treated with a DBD operating in dry air at medium pressure.
By submerging 2 types of commercially available PET foils (amorphous
and biaxially orientated) into liquid nitrogen, PET samples with
4 different degrees of crystallinity could be obtained. From modulated
DSC results, it was found that the degree of crystallinity of the PET
samples employed in this work varied from 40.7% down to 2.1%. After
plasma treatment, the surfaces of the 4 different PET foils became
more hydrophilic as indicated by a decrease in water contact angle.
After the samples had received an energy density of 150 mJ/cm², the
contact angle however saturated. Contact angle results have clearly
shown that this saturated contact angle value decreases with
increasing crystallinity. Moreover, it was also observed that the
ageing behavior tends to be more pronounced for PET samples with a
low degree of crystallinity. Based on AFM results, it was also found
that plasma treatment increases the roughness of samples with a high
degree of crystallinity, while the surface of samples with a low degree
of crystallinity becomes smoother. These latter surfaces are homogenously
etched, while for samples with a high degree of crystallinity
the etching effect is more at random. As a result, it is most likely that
amorphous polymer regions are more prone to plasma etching than
crystalline regions. All these previously mentioned results thus
suggest that PET crystallinity has a great influence on plasma
treatment outcome and on the subsequent ageing process.