# Liquid Crystallinity in Epoxy Networks: A Systematic Study of Thermal Conductivity and Structure

## TL;DR

This study examines how liquid crystalline structures in epoxy materials affect their thermal conductivity, finding that higher conductivity values reported in previous studies could not be replicated.

## Contribution

The study systematically evaluates epoxy formulations and finds no strong evidence of crystallinity-induced thermal conductivity improvements under their experimental conditions.

## Key findings

- All epoxy formulations showed thermal conductivity between 0.13 and 0.32 W/(m·K), with no values exceeding 1 W/(m·K).
- No strong evidence of crystallinity was observed in the tested epoxy systems.
- Variations in thermal conductivity measurements across methods were significant, suggesting methodological factors may influence results.

## Abstract

Epoxy resins are valuable in aerospace, electronics, and high-performance industries; however, their inherently low thermal conductivity (TC) limits applications requiring effective heat dissipation. Recent reports suggest that certain liquid crystalline or partially crystalline epoxy formulations can achieve higher TC, even exceeding 1 W/(m·K). To investigate this, 17 epoxy formulations were prepared, including the commonly used diglycidyl ether of bisphenol A (DGEBA) and two custom-synthesized diepoxides: TME4, which contains rigid aromatic ester linkages with a C4 aliphatic spacer, and LCE-DP, featuring rigid imine bonds. Thermal conductivity was measured using four techniques: laser flash analysis (LFA), modified transient plane source (MTPS), time-domain thermoreflectance (TDTR), and displacement thermo-optic phase spectroscopy (D-TOPS). Additionally, small-angle and wide-angle X-ray scattering (SAXS/WAXS) were performed to detect crystalline or liquid crystalline domains. All formulations exhibited TC values ranging from 0.13 to 0.32 W/(m·K). The TME4–DDS systems, previously reported to be near 1 W/(m·K), consistently measured between 0.26 and 0.30 W/(m·K). Thus, under our synthesis and curing conditions, the elevated TC reported in prior studies was not reproduced, and no strong evidence of crystallinity was observed; indications of local ordering did not translate into higher conductivity. Variations in TC among methods often matched or exceeded the gains attributed to mesophase formation. More broadly, evidence for crystallinity in epoxy thermosets appears weak, consistent with the notion that crosslinking suppresses long-range ordering.