We investigated the thermal stability of the diglycidyl ether of bisphenol A (DGEBA)-methylhexahydrophtalic anhydride (MHHPA) modified with phenylmethylsiloxane-modified epoxy (PMSE) and the effects of this hybrid on the performance of light emitting diodes (LEDs). The optical and dynamic mechanical properties of the materials, and the effects of these properties on the light output from the encapsulated LEDs, were studied following long-term thermal aging at 150 degrees C. The DGEBA-PMSE hybrids were highly transparent and its optical stability was shown in the stable light output of the LED, which was 19% higher than that of LEDs encapsulated by DGEBA-MHHPA after 30 days. Molecular rearrangement occurred in the nanoscale domain of the hybrids after thermal aging, which caused the dynamic mechanical effect on the LED performance, as demonstrated by TEM. We observed the hybrid modified using 0.2 equivalents of PMSE possessed both optical and dynamic mechanical stability, and resulted in the most stable LED performance. (c) 2012 Elsevier B.V. All rights reserved.