By tracking shrinkage, color, chemistry, and mechanical behavior through controlled heating, researchers followed the hidden sequence of changes inside a 3D-printed hybrid material as it moved toward glass.
Paper: Uncovering the thermolysis conversion mechanisms of 3D nanoprinted silica glass. Image Credit: Hendy Herdiansyah / Shutterstock
In a recent research article published in the journal Communications Materials, researchers investigated the microscale mechanisms governing the thermal conversion of two-photon polymerization (2PP)-printed hybrid organic-inorganic structures into high-strength silica glass.
Thermolysis of POSS-Printed Silica
Advances in two-photon polymerization (2PP) nanoscale 3D printing have enabled the fabrication of highly detailed polymer micro- and nanostructures. Extending this approach to inorganic materials like silica glass expands the functionality of 3D-printed components for applications in micro-optics and nanotechnology.
Hybrid organic-inorganic resists based on polyhedral oligomeric silsesquioxane (POSS) combine inorganic silicon-oxygen cage structures with cross-linkable organic groups, allowing initial polymerization into well-defined nanostructured templates.
Converting these templates into pure silica glass through thermal treatment, or thermolysis, remains poorly understood at the microscale, particularly the sequence of microstructural changes and their links to dimensional stability, optical clarity, and mechanical strength.
Addressing these knowledge gaps is needed to reliably produce complex, high-fidelity inorganic micro- and nanostructures with targeted functionalities.
Correlative Thermal and Mechanical Analysis
This study employed a combined analytical approach using thermal, optical, mechanical, and spectroscopic measurements to investigate the thermal conversion of 2PP-printed POSS-based microstructures into silica glass.
Microscale disk and pillar specimens with characteristic dimensions from about 15 to 100 μm were 3D-printed using 2PP from a hybrid organic-inorganic resist incorporating POSS cages and acrylic monomers.
Mechanical and O-PTIR specimens underwent controlled thermal treatments in air at 300 °C, 440 °C, 520 °C, 700 °C, and 1000 °C to trigger organic component removal and silica network formation. The in situ optical sample was heated to 700 °C.
The evolution of microstructure and properties was monitored via in situ optical microscopy combined with digital image correlation to track dimensional shrinkage and color changes, thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) for reaction kinetics and mass changes, in situ scanning electron microscopy (SEM), tension and compression testing of micrometer-scale specimens, and optical photothermal infrared (O-PTIR) spectroscopy to reveal chemical composition and bonding changes.
The TGA/DSC samples were separately prepared by drop casting and flood curing, while the O-PTIR spectra were collected from printed micropillars after their thermal treatments.
Together, these measurements enabled detailed correlation of microstructural transformations with changes in optical appearance and mechanical behavior during thermolysis.
Microstructural Transformations and Properties
The thermolysis of 2PP-printed POSS structures proceeds through distinct microstructural transformation stages. Initially, the hybrid organic-inorganic network contains densely packed inorganic POSS cages embedded in a crosslinked polymer matrix.
Up to about 300 °C, thermally induced cross-linking competes with decomposition, and the material remains rubber-like while becoming stiffer and stronger.
Above 300 °C, thermal treatment drives progressive degradation and removal of organic components, detected as mass loss and observed as chromatic changes, from yellowing and darkening to eventual lightening, under optical microscopy.
The main mass-loss and contraction rates peak between about 350 and 400 °C. Organic removal and the accompanying microstructural reorganization produce an isotropic linear shrinkage near 40%, consistent with densification of the remaining material.
O-PTIR spectroscopy revealed that many organic signatures had weakened by 300 °C. By 440 °C, C-H stretching from the organic backbone had disappeared, and at 520 °C only trace carbonyl-containing species remained.
At the same time, the characteristic silicon-oxygen signatures of the POSS cages disappear by 440 °C as new bands associated with tetrahedral amorphous silica form, signaling the transformation of isolated nanoclusters into a connected silica glass network.
The process involves cleaving the discrete POSS cages and reorganizing silicon-oxygen species into the glass network.
At higher temperatures, continued strengthening aligns with further short-range reorganization of ring-like Si-O structures toward a more tetrahedral silica network, as the authors reported in previous work.
In situ micromechanical testing uncovered a strong correlation between these chemical and structural changes and the material's mechanical behavior. Initially, the rubberlike organic matrix dominates load-bearing.
As thermolysis progresses, this load shifts to the emerging inorganic silica network, with measured Young's modulus rising from about 0.5 to 0.6 GPa in the as-printed material to more than 10 GPa after treatment at 440 °C. The modulus then increased to 63.83 ± 2.29 GPa after 700 °C and 76.59 ± 2.67 GPa after 1000 °C.
The transition to ceramic-type brittle behavior is already clear by 440 °C. Above this temperature, oxidation progressively removes residual carbonaceous material; substantial transparency returns above about 575 °C, and at 700 °C the material shows the spectral and mechanical characteristics of dense fused silica.
By selecting the treatment temperature, the process enables the selection of intermediate material states with tailored deformability, stiffness, and strength.
After treatment at 700 °C, the converted silica glass reached 4.17 ± 0.46 GPa tensile strength; after 1000 °C, 5.94 ± 0.45 GPa, which the authors describe as the tensile-strength benchmark among 2PP-derived materials.
The observed dimensional contraction and network evolution help guide efforts to preserve structural fidelity during conversion, aiming to maintain high mechanical integrity and optical clarity at the microscale.
The multi-technique microscale analysis clarified the mechanistic pathways that transform hybrid POSS-containing organic-inorganic networks into pure silica glass.
These findings inform the design of thermal post-processing routes for complex inorganic microstructures with controlled properties and have implications for nanoscale 2PP structures.
The study also identifies the early formation of a mechanically self-supporting inorganic network as a key condition for high-fidelity structures.
Understanding Precision Silica Manufacturing
This work establishes a detailed mechanistic picture of the thermal conversion process by which 2PP-printed hybrid organic-inorganic POSS structures become mechanically strong and optically clear silica glass.
Using integrated thermal, chemical, mechanical, and optical measurements, the study shows how controlled removal of organic components and progressive assembly of silica networks govern dimensional shrinkage, mechanical strengthening, and chromatic evolution.
The range of accessible intermediate material states allows treatment temperature to tune deformability, stiffness, and strength before full conversion to dense glass. The results show that useful mechanical properties emerge well before complete conversion, while higher-temperature treatment produces dense silica with tensile strength approaching 6 GPa.
These findings support precision manufacturing of durable, high-fidelity free-form inorganic structures for applications including micro-optics and micromechanical devices.
The analysis approach developed here is expected to guide thermal conversion protocols for other feedstock resists and process routes, but this study did not test those routes.