Hydrophilic Polysiloxane Microspheres and Ceramic SiOC Microspheres Derived from Them
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In this overview article, the research on polysiloxane microspheres performed in the authors’ laboratory is briefly reviewed. These microspheres are prepared in water emulsion from polyhydromethylsiloxane (PHMS). This polymer is cross-linked in the emulsion process by hydrosilylation using various low molecular weight cross-linkers having at least two vinyl functions. The microspheres contain a large number of silanol groups which give them hydrophilicity and a broad possibility of functionalization by condensation with reactive silanes bearing a functional group in the organic radical. Further transformation of these functions leads to materials for practical use, such as catalysts and biocidal powders. The hydrophilic-hydrophobic properties of the microspheres may be fine-tuned by silylation or modification of the precursor PHMS polymer. Pristine microspheres are highly hydrophilic and well-dispersed in water. They do not adsorb proteins and hydrophobic organic substances. Macropores may be generated in these particles by a simple modification of the emulsion procedure. These microspheres are also very good precursors for ceramic silicon oxycarbide microsphers because they retain their shape in pyrolytic processes even at high temperatures; and they give a high yield of ceramic material. The polysiloxane microspheres heated at 600 °C give micro and mezo porous materials with specific surface above 500 m2/g. When pyrolysed at temperatures 1000–1400 °C, they form solid ceramic microspheres of high strength. They retain spherical shape at 1500 °C although cracks are formed at their surfaces. Etching them with HF(aq) solution gives porous microspheres with specific surface above 1000 m2/g that is almost devoid of SiO2.
KeywordsPolysiloxane microspheres Hydrophilic polysiloxane Functional microspheres Silicon oxycarbide microspheres Porous microspheres Polysiloxane derived ceramics
Microspheres are spherical particles with a range of diameter from 0.1 to 200 μm and have many types of topologies and diverse chemical structures. Polymer microspheres are a broad area of research, which has been the subject of many review papers, e.g. [1, 2, 3, 4, 5]. Thousands of articles are devoted to the description of methods of their generation and their applications in areas of medical science and technology. Among them, polysiloxane microspheres occupy important position due to some unusual properties that make them desirable materials for many applications. Polysiloxanes themselves are known for their thermal and chemical stability, interesting reological and surface properties, and appropriate optical and dielectrical characteristics. In addition, they may be easily modified and are available at moderate prices . Polysiloxane microspheres are commonly used in medicine and biology for drug delivery systems [7, 8], biological probes and biosensors , carrier of proteins , supports in enzymatic catalysis , and carriers of biocides . They are exploited in the catalysis of various chemical reactions [13, 14] and as components of composite materials . The synthesis and use of polysiloxane microspheres of various morphologies and chemical structures have been the subject of extensive research devoted to polysiloxane core–shell [16, 17], hollowed [18, 19] and porous  microspheres as well as to beads containing other materials dispersed or dissolved in polysiloxane matrixes [20, 21]. The present overview is limited to specific kinds of polysiloxane microspheres, which were obtained from polyhydromethylsiloxanes developed in our laboratory. Their distinctive features are large number of reactive silanol groups that render them hydrophilic and easy their facile functionalization. They are also excellent precursors of ceramic silicon oxycarbide.
2 Synthesis of Hydrophilic Polysiloxane Microsphers
The dependence of the average size of microspheres on the rate and time of stirring in emulsification by MPW-120 homogenizer and on the concentration of PAV
Rate of stirring (rpm)
Time of stirring (s)
PAV (g/100 mL of water)
Average diameter of microspheres (μm)
In the spectrum of the precursor polysiloxane, in addition to a small signal assigned to the end groups, only one strong peak appears at − 38 ppm that is assigned to silicon bonded to hydrogen. This signal is strongly reduced in the spectrum of the microspheres (Fig. 3). However, a dominant signal at − 58 ppm for the silanol group-containing unit is observed. Some of these silanol groups undergo heterocondensation with SiH to give rise to siloxane bridges (resonance at − 68 ppm), which together with the bridges formed by hydrosilylation (− 22 ppm and + 8 ppm) yield the cross-linked structure that stabilizes the microspheres.
The formation of a large number of silanol groups requires a large quantity of water. Since the molecular mass of the siloxane unit in PHMS is 60 and that of water is 18, more than 20 w% of the water in the polymer droplets is needed to form a large number of the silanol groups. How this water gets into the hydrophobic polymer is puzzling. Hypothetically, a slow penetration of water combined with hydrolysis of the subsequent layer of the polymer particle is possible during microsphere consolidation. However, the result of simple experiments contradict this mechanism . Microspheres from one synthetic run were fractionated and analysed. It turned out that the larger microspheres contained more silanol groups that the smaller microspheres, although the hydrosilylation reaction proceeded at the same rate in all the microsphere fractions. Thus, water does not diffuse into, but out of the microspheres. The double emulsion of water/oil–oil/water is formed during the homogenization process. Each polymer microdroplet in the formed emulsion contains dispersed nanodroplets of water, which is the source of water that is used in SiH hydrolysis. Water nanodroplets have tendency to escape from the polymer droplets, which is the reason for the failure of attempts to obtain submicron size microspheres that contain a large number of SiOH groups even by using an ultra high-speed homogenizer.
3 Functionalization of Hydrophilic Microspheres
4 Hydrophilicity and Tuning of the Polysiloxane Microspheres
5 Generation of Macropores in the Hydrophilic Polysiloxane Microspheres
The micospheres prepared in our laboratory are cross-linked polysiloxane elastomers that are solids. Generation of pores in such elastomer particles carried out in other laboratories required special methods in their preparation. Thus, macroporous polysiloxane microspheres were produced by using a double emulsion system involving a microfluidic techniqe . A two-step double emulsion technique was also used to obtain spheroidal particles with a size greater than 10 mμ . Spheroidal polysilsesquioxane macroporous particles of several hundred micrometers were formed by an electrodynamic method . Another method employed was that based on emulsion-ice templating .
6 Fabrication of Core–Shell Polysiloxane Microcapsules Containing Phase Change Materials (PCM)
Polysiloxane containing alkyl and hydroxyl group in its siloxane unit behaves as a surfactant in aqueous media and allows the encapsulation of some hydrophobic materials in the polysiloxane shell.
n-Paraffines, such as n-eicosane, are commonly used as phase change materials (PCM) applied for thermoregulation and thermal energy storage [46, 47]. The capsules containing n-eicosane, which have a high latent heat of fusion and a melting point close to the physiological temperature of the human body, are particularly interesting materials for use in the textile industry. If dispersed in cloth, it gives skin the feeling of thermal comfort and protection against thermal shock. Polysiloxanes are also attractive materials for use as a capsule shell because of their physiological neutrality, chemical and thermal stability and resistance to oxygen and water vapor. They could replace materials that are based on carcinogenic formaldehyde. The polysiloxanes are mechanically strong and elastic enough to withstand the volume change during a phase change. Nonwoven textile material that is modified with selected polysiloxane PCM capsules was subjected to thermoregulation studies . The modified textile had good and stable thermoregulating properties and showed a high-energy storage coefficient.
7 Generation of SiOC Ceramic Microspheres
Silicon-based polymer-derived ceramics have been intensively developed in various laboratories over past 40 years [49, 50]. Making ceramic products from polymer precursors has many advantages over the classical methods. One of the advantages is the possibility of shaping ceramics on nano and micro scale, which opens the route to ceramic fibers, thin films and specifically shaped regular nano- and microparticles. Amongst the latter, microspheres are gaining particular importance [49, 51]. They could find application as catalysts for high temperature processes, hot-gas purification, gas separation and generation of composites. The ceramic microspheres develop their shape at the precursor stage. However, the basic problem is the retention of their shape during the ceramization process . The chemical and physical structures of the material are heavily changed during the thermal process and may lead to cracking and disruption of the spherical particles. Very slow temperature increase is required to prevent these phenomena.
Our polysiloxane microspheres are very good precursors for the preparation of ceramic silicon oxycarbide microspheres. Due to the relatively loose structure and the presence of functional groups capable of condensation in elevated temperature, they retain their spherical structure during heating at high temperature and can withstand a high rate of the temperature increase [52, 53]. They do not need a separate cross-linking step. Moreover, since the organic group contribution to their weight is low, the yield of ceramic material is high, in some cases as much as 90%.
Thus, during heating at 400 °C, homo and hetero-condensation processes occur; i.e., signals of SiOH and SiH containing units at − 58 and − 38 ppm, respectively, decrease while the signal of the units linking the polymer chains by siloxane bridges at − 68 ppm is strongly enhanced. The material becomes harder. Further heating at 600 °C causes partial cleavage of the organic groups with the formation of gaseous products, mostly methane and hydrogen. The material becomes hard as new carbon and oxycarbon bridges are formed between the polymer chains. The material also becomes porous because the evolution of gaseous products produces micro- and mezopores with specific surface areas up to 550 m2/g. It is not a ceramic material since the 29Si MAS spectrum is typical for a polymer. Such a material is called a ceramer . True SiOC ceramics are obtained in the pyrolysis that is carried out at 1000 °C [49, 54]. The 29Si MAS NMR spectrum of the polysiloxane microspheres that are heated for a longer time at this temperature is typical of a ceramic SiOC material (Scheme 5). The broad signals represent all silicon atoms being in the center of tetrahedra, which differ in the number of carbon and oxygen atoms at the tetrahedral corners. The signals are centered at following shifts (in ppm): − 108 for SiO4 representing silica, − 70 for SiCO3, − 37 for Si C2O2, + 1 for SiC3O and − 18 for SiC4 representing silicon carbide. The segregation of free carbon occurs during the ceramization process, already below 1000 °C [57, 59]. The aromatic sp2 carbon gives an intensive broad peak in the 13C MAS NMR spectrum with a maximum of ~ 130 ppm . This carbon forms disordered turbostratic domains that have a tendency to graffitization when heated to higher temperatures. This free carbon segregation makes the material black and is possibly the reason that the microspheres lose their porosity.
Comparison of hardness and Young modulus of polysiloxane microspheres ceramized at various temperatures (data selected from Ref. )
Temperature of pyrolysis (°C)
Young modulus (GPa)
8 Generation of Pores in the SiOC Ceramic Microspheres
Porosity of ceramic microspheres derived from polyhydromethylsiloxane pyrolized in various conditions
Temperature of ceramization (oC)
Time of heating (h)
Specific surface area (SSA) (m2/g)
Pore volume (V) (cm3/g)
Pore size (Dav) (nm)
On the other hand, silica is almost fully removed from the microspheres ceramized for 1 h at 1500 °C by etching performed under the same conditions as seen from the comparison of 29Si MAS NMR spectra displayed in Fig. 9a, b, . The volume of pores is high and their specific surface is over 1000 m2/g. Heating for longer periods of time at 1500 °C leads to a decrease in the specific surface to about 700 m2/g. At the same time the average diameter of pores and their volume increase (Table 3), which means that fewer micropores and more mezopores are formed. This observation allows us to conclude that small silica nanodomains have a tendency to disappear at the cost of the increase of larger domains when the material is heated for a longer time at 1500 °C.
Aqueous emulsion processing of polyhydromethylsiloxane in the presence of Pt(0) Karstedt catalyst replaces partly the SiH bonds by silanol groups. The process described here allows for the substitution of SiOH groups with as much as 80% SiH bonds in the polymer. The resulting polysiloxanes shows very interesting hydrophilic-hydrophobic properties. In addition, the SiH and SiOH reactive groups can be very easily modified. The hydrolysis of SiH takes place during the consolidation of microspheres and results in the cross-linking of the polymer by the hydrosililation of a diolefinic reactant with the SiH groups. The method provides multiple possibilities of manipulation with solvents, cross-linkers, additives and reaction parameters to generate microspheres that have various average sizes containing diverse numbers of silanol, hydrosilane and alkoxysilane groups to achieve a variety of cross-linking densities as well as structures containing bridges that link polymer chains. Macroporous spheroidal particles and core–shell capsules may be also formed. Chemical modification allows for the introduction of a variety of functional groups to the microspheres and fine-tunes their hydrophilic-hydrophobic properties. These polysiloxane microspheres are useful preceramic materials for the generation of silicon oxycarbide microspheres. In their ceramization processes carried out at temperatures 1000–1500 °C the spherical shape of the particles is retained. The additional step of the preceramic polymer cross-linking is not needed and a high yield of ceramic material is obtained. Highly porous structures of the formed ceramic microspheres may be developed by a HF etching process.
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