# Tuning of Morphology and Surface Properties of Porous Silicones by Chemical Modification
## 1 Introduction  
Porous polymers have very diverse applications, such as absorption, separation, decontamination, microelectromechanical systems, optical interference coatings, tissue engineering, and (bio)sensing [1; 2]. The generation of pores in polymers may be performed by \"top-down\" approaches, such as nanoimprinting or etching, or by \"bottom-up\" approaches, which rely on phase separation and block copolymer self-assembly. Methods for producing pores involve the use of porogens in the form of gas, liquid, or solid dispersed phases and combinations of various templates, which leave voids of different sizes after being removed. Other technologies are also used for pore generation, such as electrospinning, 3D printing, freeze drying, or injecting, sometimes combined with template-based ones, aiming to generate hierarchical porous materials. Silicones, with their most-known representative, polydimethylsiloxane (PDMS), have unique properties, dictated by the particularities of the siloxane bond [3]. PDMS is an insulator and has a low viscosity, a low glass transition temperature (around \(-120\)\({}^{\circ}\)C), very low surface tension and pronounced hydrophobicity, good thermal stability, optical transparency, stability to UV and corona discharge, and water vapor permeability. Inaddition, silicones are chemically stable, resistant to weathering, have good blood compatibility, and are considered biologically inert. Their chemical modification is possible by various methods and confers special properties and utility to the resultant silicone materials, spanning from liquid crystals and surfactants to electroactive dielectrics [4]. Creating pores in silicones is another way to expand their use, since silicone rubber foams bring together the properties of silicone elastomers and those of foams in materials with low density, heat and cold resistance, good electrical insulation, and biocompatibility [5]. Porous silicones are proposed for applications as sensors [6], sorbents [7], acoustic metamaterials [8], and cell-growth scaffolds [9]. The methods for pore generation in silicones are diverse. The most-known ones rely on crystalline materials with a narrow particle size distribution as sacrificial direct templates [9,10]; employ the emulsion template method, where water droplets act as the porogen [11, 12, 13]; or use various foaming agents. The latter can be, for example, supercritical CO\({}_{2}\)[14] or sodium bicarbonate as a CO\({}_{2}\) source [6], alkane by-products in Piers-Rubinsztajn reactions [15], or hydrogen gas. The in situ evolution of hydrogen in dehydrocooupling reactions occurring between Si-H and OH groups in the presence of a platinum catalyst specific for hydrosilylation curing was explored long ago [16]. Additives, such as water, ethanol, ethylene glycol, and 2-methoxyethanol, have been used in order to obtain pores within the membranes, which are possibly useful in ultrafiltration. It was reported that the pore size depends on the additive's reactivity, which controls the rate of H\({}_{2}\) evolution (foam formation). When poly(methylhydrogen)siloxane (PMHS) is crosslinked with a vinyl silicone, pores form due to dehydrocoupling side reactions, and the hydrophilic-hydrophobic properties can be modified [17]. Adjusting the surface properties of polymeric materials is a key aspect for their practical use. For example, foaming issues in the petroleum industry are in close connection to the surface tension and surfactant-polymer interactions [18]. In the biomedical field, the hydrophobic surfaces increase the protein adsorption, which may be undesired in a biological environment, but useful in other applications, like protein separation [17]. Superhydrophobic surfaces have contact angles (CAs) larger than 150\({}^{\circ}\), but an essential parameter that defines their behavior against water is the contact angle hysteresis (CAH) [19]. When the CAH is very low, the surfaces mimic lotus leaves, where water drops roll off [20], and are of interest in self-cleaning coatings, for example.