Structural Evolution of Silica Gel and Silsesquioxane Using Thermal Curing

Area of Science:

• Materials Science
• Polymer Chemistry
• Spectroscopy

Background:

• Siloxane-based materials like silica gel and silsesquioxane are crucial in various applications.
• Understanding their curing processes at modest temperatures is key to controlling material properties.
• In situ spectroscopic methods offer detailed insights into structural evolution during thermal treatment.

Purpose of the Study:

• To investigate and compare the curing mechanisms of silica gel and hybrid silsesquioxane under modest heating.
• To elucidate the structural transformations, including ring formation/breakdown and silanol condensation, in both materials.
• To identify the influence of organic groups on the curing behavior of silsesquioxanes.

Main Methods:

• In situ heating Fourier transform infrared spectroscopy (FT-IR).
• Perturbation correlation moving window (PCMW) analysis.
• Two-dimensional correlation spectroscopy (2D-COS).
• Positron annihilation lifetime spectroscopy (PALS) for confirmation.

Main Results:

• Silica gel: (Si-O)6 ring formation from Si-O-Si network, followed by silanol condensation, ring transformation, and isolated silanol generation.
• (Si-O)4 to (Si-O)6 ring transition observed and confirmed.
• Silsesquioxane: Immediate silanol condensation without network rearrangement, increased (Si-O)6 fraction, and stable (Si-O)4 structures.
• (Si-O)4 rings show higher stability in silsesquioxanes.
• Silsesquioxane curing results in reduced silanols without isolated silanol formation, attributed to organic groups.

Conclusions:

• Silica gel curing involves significant Si-O-Si network rearrangement and eventual isolated silanol formation.
• Silsesquioxane curing is characterized by direct silanol condensation and greater stability of (Si-O)4 rings.
• Organic groups in silsesquioxane lower cross-linking density and network rigidity, leading to different curing pathways compared to silica gel.