# Tuning of Morphology and Surface Properties of Porous Silicones by Chemical Modification
## 3 Results and Discussion
The premises of the chemical modification were that, taking the polysiloxane chain as a reference, the OH groups would provide hydrophilic behavior, while dimethyl units and hexyl and trimethylsilyl groups would increase the hydrophobic behavior. Powder wetting experiments were performed in order to obtain contact angle values, but the Washburn method which was applied here has several limitations and is not suitable for hydrophobic materials [31; 32]; thus, it could not be fully used as initially intended. Nevertheless, the water sorption isotherms shown in Figure 3 can be used to compare the samples. It is worth noting that all samples were packed in the same way: the same amount of powder was placed between identical pieces of filter paper. The isotherms are depicted as they were registered, without extracting the effect of the filter paper, since they are used only for comparison. The wetting experiments revealed some unexpected outcomes, and it turned out that the morphology of the materials had a significant influence on their surface properties. In Figure 3, the samples can be roughly divided into three categories, according to the amount of water adsorbed: below 0.12 g, around 0.2 g, and above 0.3 g. In the dehydro-coupling sample series, very little difference was registered for DH100, DH70, and DH44, although their dimethylsiloxane content increased. DH100 set the trend, being hydrophobic, in spite of some Si-OH groups being present. The other two samples containing copolymers had similar water sorption and slightly lower hexane sorption capacities (Figure S3). The sample modified with hexene (DHH) exhibited the lowest water sorption and a slightly higher affinity for hexane (Figure S3). Unexpectedly, the water sorption capacities of the DH30 sample, with the highest content of PDMS, and the DHS modified with trimethylsilyl were significantly higher than that of the unmodified DH100, placing these in the middle range of the graph. Their sorption curves had different aspects, showing a stepwise increase before leveling. Sample UV50, which is also placed in the \"middle\" zone of the wetting graph, showed a very slow increase in adsorbed water after a small initial \"burst\". In the attempt to verify this apparently peculiar behavior and to measure the contact angle, drops of water (ca. 10 \(\upmu\)L) were placed on the porous surfaces of the initial monoliths. In all these three cases, the \"petal\" effect was observed, i.e., a water drop did not fall off when the piece of material was tilted or even turned upside down (Supplementary Materials S2; Figure S4). This behavior is assigned to superhydrophobic materials with very high adhesion (high contact angle  
Figure 3: Comparative water adsorption isotherms of porous silicones. hysteresis), in the so-called Cassie impregnating state [21]. In the wetting measurements, water was first adsorbed on the bottom paper layer, then it adhered to the material, filling gradually the pores. This can be seen in the slowly increasing region of the graph, which seems to characterize only the materials showing the petal effect in our experiments. The contact angle hysteresis (calculated as the difference between the CA values on the two sides of the droplet) at a 90\({}^{\circ}\) tilt angle (Figure S4) [19] had values of 45-60\({}^{\circ}\). It is worth mentioning that, since the samples were crushed, the observed behavior characterizes the bulk material and not necessarily the surface of the monoliths or films; thus, it could be best correlated with the morphology of the \"inner\" part of the materials, which will be discussed further. The static water contact angle (CA) was measured for broken samples in order to correlate the data. As for the surfaces, contact angle values could be misleading in such cases due to several effects: the migration/orientation of PDMS chains to the surface, the presence of pores larger than the water droplets, different morphologies on the two surfaces due to stratification during cross-linking [13], and so on.