Multiscale Elasticity of Epoxy Networks by Rheology and Brillouin Light Spectroscopy

Area of Science:

• Materials Science
• Polymer Physics
• Spectroscopy

Background:

• Soft materials exhibit frequency-dependent responses, typically studied at lower frequencies (10⁻²–10² rad/s).
• Conventional methods like rheology and atomic force microscopy are often contact-based or invasive.
• Understanding the relationship between low-frequency viscoelasticity and high-frequency elasticity is crucial but remains unclear.

Purpose of the Study:

• To compare mechanical moduli of polymer networks across different frequencies using complementary techniques.
• To investigate the correlation between shear rheology and Brillouin light spectroscopy measurements.
• To elucidate the elasticity of soft materials at varying scales.

Main Methods:

• Utilized solvent-free epoxy polymer networks with varying cross-link densities as model systems.
• Employed shear rheology to measure the storage modulus (G') in the MPa range.
• Applied Brillouin light spectroscopy (BLS) to determine the longitudinal modulus (M') in the GPa range at GHz frequencies.

Main Results:

• Storage modulus (G') showed a significantly stronger increase with cross-link density (~3.5x) compared to the longitudinal modulus (M').
• Observed unexpectedly fast hypersonic dispersion in cross-linked epoxy networks compared to their precursors.
• Correlated G' with the phantom network model and M' with Wood's inverse rule of mixtures.

Conclusions:

• The distinct responses of G' and M' to cross-linking highlight the importance of multi-scale elasticity measurements.
• BLS provides a noncontact, optical method for probing GHz-frequency mechanical properties of soft materials.
• Findings encourage further research into bridging the understanding of soft material elasticity across different frequency regimes.