3.1. Surface structures of the coatings
The SEMimages of Fig.2revealthe effect ofthe SiO2 nanoparticle
concentration on the surface structures of the coatings, deposited
on silk. When the silk substrate is covered by pure siloxane
(Fig. 2a) there is no significant surface structure originated by the
deposited coating. The use of nanoparticles results in the formation
of microscale clusters with nanostructures, which in turn induce a
surface roughness atthe micrometer and nanometer scale. Initially,
the clusters are separated by large areas with no special structure
(Fig. 2b and c). Larger, coalesced and higher clusters are formed
gradually as the particle concentration increases (Fig. 2d–f). The
scenario revealed in Fig. 2 is in perfect agreement with previously
published studies which described the effect of nanoparticles on
the surface of polymer-nanoparticle composite films [51–54].