Draft:Conversion of the polymer perhydropolysilazane to silica

The conversion of perhydropolysilazane to silica is a key reaction in preceramic polymer chemistry and plays an important role in producing functional inorganic materials via solution-based methods. Perhydropolysilazane (PHPS) is an inorganic polymer that can be converted to amorphous silica (SiOₓ) without the need for traditional alcoholic silane precursors or acidic or basic catalysts. This makes the process safer, more environmentally friendly, and compatible with temperature-sensitive materials.

The conversion of PHPS into SiOx with only UV-light

A notable feature of PHPS is its high reactivity and solubility. It contains Si–H groups that readily react with water and oxygen to form siloxane bonds (Si–O–Si). This allows silica networks to form under relatively mild conditions, such as room temperature, ambient humidity, UV light or ozone. The resulting silica is chemically inert, transparent, porous and thermally stable – properties that make it suitable for applications ranging from microelectronics to medical coatings and catalysts. Recent studies have demonstrated the PHPS can be used to create encapsulated particles or structures with nanometer resolution via patterning, highlighting its potential in nanotechnology.

Chemical structure of perhydropolysilazane (PHPS)

See also: Polysilazane

Perhydropolysilazane (PHPS) is an inorganic polymer, which consist of repeatedly varied of Si–N and Si–H. Due to the presence of reactive Si–H bonds, the polymer is particularly suitable for further functionalisation and conversion to silica. The general formula can be represented as [HSi(NH)O]ₙ or similar derivatives, depending on synthesis and processing. A unique feature of PHPS is the presence of these Si–H groups, which are highly reactive and therefore form the basis for hydrolysis and further condensation. This makes PHPS particularly suitable for conversion to silica and functionalisation with other groups or nanoparticles.

The molecular weight distribution and degree of polymerisation of PHPS can vary depending on the synthesis conditions. Due to the incomplete crosslinking of the structure, PHPS remains soluble in apolar solvents, making it attractive for wet processing methods. The presence of Si-H groups makes the material not only reactive, but also potentially useful for further functionalisation. PHPS is usually supplied as a viscous, colorless to pale yellow liquid, dissolved in solvents such as dibutyl ether, toluene, or hexane. In commercial formulations, a typical composition is 20 wt% PHPS in dibutyl ether. Storage under inert gas (e.g. nitrogen or argon) is essential to prevent premature hydrolysis by atmospheric moisture. Processing should ideally take place in a drying cabinet or under glove box conditions.

Important features of the inorganic polymer perhydropolysilazane are:

• High reactivity: The Si–H bonds readily undergo hydrolysis and oxidation.
• Solubility: Soluble in organic solvents, which facilitates film formation via wet-processing.
• Storage sensitivity: Must be kept under an inert atmosphere to prevent premature crosslinking and degradation.

Mechanism of conversion

Hydrolysis and condensation

The first step in the conversion of PHPS to silica is the hydrolysis of the Si–H groups. This usually occurs under the influence of air humidity or by direct addition of water vapour or aqueous solutions. The formed silanol groups (Si–OH) then react with each other or with the remaining Si–H groups to form Si–O–Si bonds and split off water. This leads to a gradual network formation of silica.

This reaction shows how the inorganic polymer PHPS is converted into silica after coming in touch with water (direct water or indirectly with water in the air)

The extent of hydrolysis and condensation is influenced by environmental parameters such as air humidity, temperature, the nature of the substrate, and the layer thickness. For thin films (<1 µm), the hydrolysis usually proceeds completely within a few hours at room temperature and a humidity of 40–90%. At thicker films or at low water diffusion rates, the conversion may be incomplete, resulting in residual groups that affect chemical stability and porosity.

Alternatively, silanol groups can react with residual Si–H groups:

HO–Si–O + H–Si–O → Si–O–Si + H₂

These reactions result in the formation of a three-dimensional Si–O–Si network through sequential hydrolysis and condensation processes. This transformation can proceed at room temperature in the presence of adequate humidity. Several factors influence the reaction rate, including the thickness of the applied layer, the availability of moisture, interactions with the substrate, and the diffusion rate of water. Additionally, catalytic residues—such as acids or bases—present on the substrate or in the surrounding environment can accelerate the reaction. However, such catalysts may also alter the morphology and uniformity of the resulting material.

Intermediate species may exist, such as silanol-terminated chains, cyclic oligomers, or partially cross-linked networks. These species can be identified using spectroscopic techniques such as FTIR, which show a characteristic Si–OH stretching around 950 cm⁻¹ and Si–O–Si vibrations around 1080–1100 cm⁻¹. A decreasing Si–H stretching around 2250 cm⁻¹ is an indicator of ongoing conversion.

Conversion can occur at room temperature but is more complete under elevated temperature or UV light (e.g., 185 nm). UV exposure for 15 minutes can lead to near-complete conversion, as evidenced by FTIR and XPS.

Thermal treatment (pyrolysis)

Whereas room temperature hydrolysis and condensation is feasible, thermal post-treatment is typically necessary to force the condensation to completion, remove residual groups, and densify the silica network.

The pyrolysis reaction to convert PHPS into silica

Important processes during heating:

• Removal of unreacted Si–H and Si–OH groups

• Densification of the silica network to a glassy structure

• Removal of organic residues and traces of solvent

Temperatures between 150 and 350 °C are common for this. Between 200 and 250 °C exothermic reactions are observed indicating accelerated network formation. The atmosphere plays an important role during heating:

• In air: complete oxidation to amorphous silica and elimination of hydrogen and nitrogen-containing fragments

• In nitrogen (N₂), vacuum or other inert atmospheres (e.g. argon (Ar)): silicon oxynitride (SiOₓNᵧ) network formation, partial conversion, and residual nitrogen groups.

Thermal UV-curing at 120 °C greatly improves the material's conversion degree, as reflected by a rise in the O/Si ratio from approximately 1.75 to 1.89, compared to curing at room temperature. The improved conversion results in improved material properties such as increased optical transparency, increased chemical resistance, and improved mechanical strength.

However, the thermal treatment is also associated with a high volumetric shrinkage, typically in the range of 20% to 40%, depending on the film thickness and the applied temperature ramp. The shrinkage can introduce internal stresses that may result in cracking or delamination of the coating or film. To reduce such an effect, there is a need for a properly controlled thermal profile. This is achievable through stepwise heating protocol and close monitoring of the reaction by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).

Characterisation techniques

To monitor the progress of the conversion and assess the quality of the silica formed, several advanced characterisation techniques are applied:

The left spectrum is the conversion of silica before & after UV-irradiation for different time at room temperature. The right spectrum show what happens with silica UV-irradiation at different temperatures for 15 min

• FTIR spectroscopy (Fourier transform infrared): Analysis of Si–H stretching (~2250 cm⁻¹), Si–OH (~950 cm⁻¹), and asymmetric Si–O–Si stretching (~1080–1100 cm⁻¹). The decrease of the Si–H peak and the increase of the Si–O–Si band confirm the conversion.

• UV-Vis spectroscopy: Changes in optical absorption or transparency provide information about film integrity and network growth.
• XPS: Shows uniform Si and O distribution; nitrogen absence confirms complete conversion.

• SEM/TEM (scanning/transmission electron microscopy): Visualisation of morphology, fiber structure (in electrospinning), porosity, and composition at the nanoscale.

Applications

The broad applicability of PHPS conversion to silica is due to the versatile processing methods and the ceramic character of the end product:

• Antibacterial coatings: By adding silver nanoparticles to PHPS solutions, hybrid coatings with antimicrobial properties can be formed after conversion. These coatings are interesting for medical devices and food packaging.

• Printable electronics: PHPS is compatible with techniques such as inkjet printing, spray coating, and spin coating. The solutions can be applied to flexible substrates and then converted to electrically insulating silica at low temperature.
• Electrospinning: Blends of PHPS with poly(ethylene oxide) can be electrospun into fibers, which are then thermally or chemically converted to porous silica fibers. These structures have applications in filtration, catalysis, and sensory.
• Barrier layers and optical coatings: The high optical transparency and chemical inertness of SiOₓ make PHPS-derived films suitable for use as protective layers in displays or as gas barriers in packaging materials.
• Biodegradable substrates: Unlike polyethylene terephthalate (PET), cellulose-based coatings with PHPS maintain environmental compatibility.

  

  Here is shown how the inorganic polymer (poly ethylene oxide) transforms from a solid fibre after dissolving the polymer powder in deionised water

Environmental and Safety Considerations

While PHPS offers a safer alternative to alkoxysilanes, it remains highly reactive. Moisture sensitivity can lead to premature polymer degradation or uncontrolled gelation. Proper storage (under argon or nitrogen, low humidity) is essential. Thermal treatment can release ammonia or hydrogen; ventilation is required during processing.

Additionally, PHPS-derived silica is free of residual organic contaminants and is suitable for eco-friendly coatings. The conversion route does not require toxic solvents or catalysts, aligning with green chemistry principles.

Advantages and limitations

The conversion of PHPS to silica offers both unique advantages and some limitations, depending on the intended application. This dual nature requires balancing performance, processing conditions, and cost-effectiveness when selecting PHPS for specific end uses.

The advantages to convert PHPS into SiO_x are:

• Low processing temperatures: In contrast to classical sol-gel, the conversion of PHPS can take place at room temperature. This makes it compatible with polymeric or sensitive substrates (room temperature to 120 °C with UV) and catalyst-free conversion.

• Solubility and processing: PHPS is soluble in non-polar solvents, which makes spin coating, dip coating, and electrospinning possible.

• Pure end product: Since PHPS does not contain any organic residues, it produces a virtually carbon-free silica after complete conversion.

• Precise patterning: Due to its compatibility with UV and lithographic techniques, patterns on a micron to nanometer scale can be realized.

There are also some disadvantages to perform this reaction and those are:

• Sensitivity to moisture: PHPS is highly reactive and requires strict moisture-free storage and handling, otherwise uncontrolled hydrolysis occurs. Furthermore it is required to work with PHPS in a controlled humidity or UV for full conversion.

• Limited long-term stability: With long-term storage, the viscosity and reactivity may decrease due to slow polymerisation.

• Shrinkage and Cracking: Significant volume shrinkage occurs during thermal post-treatment, which can induce stresses in the material.

• Limited Commercial Availability: PHPS is not widely available and often requires laboratory synthesis.

These properties make PHPS particularly suitable for applications in research, prototyping and niche production of functional layers, sensors, and coatings.

Related compounds and methods

PHPS belongs to a broader class of polysilazanes used for SiOₓ and ceramic coatings. Related materials include:

• Alkoxysilanes (e.g., TEOS): Traditional sol-gel silica sources.
• Polymethylsilazanes: Lead to Si–C–N ceramics.
• Hybrid systems: PHPS doped with TiO₂, ZnO, or AgNPs enhance barrier, catalytic, or antimicrobial properties.
• UV-initiated conversion: Alternative to thermal pyrolysis, ideal for temperature-sensitive substrates.
• Roll-to-roll coating (R2R): Demonstrated for scalable industrial application of PHPS-based coatings.

See also

• Silsesquioxane
• Sol–gel process
• Silica
• Preceramic polymer
• Spin coating
• Electrospinning
• Polysilazane

Further reading

• Yue, Shuangshuang; Wang, Shuanjin; Han, Dongmei; Huang, Sheng; Xiao, Min; Meng, Yuezhong (October 2022). "Perhydropolysilazane-derived-SiOx coated cellulose: a transparent biodegradable material with high gas barrier property". Cellulose. 29 (15): 8293–8303. doi:10.1007/s10570-022-04746-9.
• Hu, Longfei; Li, Meishuan; Xu, Caihong; Luo, Yongming (November 2011). "Perhydropolysilazane derived silica coating protecting Kapton from atomic oxygen attack". Thin Solid Films. 520 (3): 1063–1068. Bibcode:2011TSF...520.1063H. doi:10.1016/j.tsf.2011.10.011.

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