Kyungkuk Koh, Younghun Kim, Honglae Sohn
Conventional polysiloxane-based hybrid materials for LED encapsulation suffer from typically low refractive indices, limited optical transmittance, and extended cure times. Therefore, a systematic evaluation of curing behavior, mechanical hardness, refractive-index dispersion, optical transparency, thermal stability, and thermo-optical aging resistance is essential for assessing their applicability as optical encapsulants. To address these limitations, an omnidirectional polysiloxane hybrimer (OPH) was prepared by coupling phenyl-rich phenyl vinyl oligosiloxane (PVO) with omnidirectional cyclotetrasiloxane (OCS) through Pt-catalyzed hydrosilylation and was systematically tested as a high refractive-index optical encapsulant. Spectroscopic ellipsometry and thermogravimetric analysis confirmed that OPH exhibited high refractive indices of 1.64 at 450 nm, 1.61 at 520 nm, and 1.59 at 635 nm, together with a 5% weight-loss temperature (T 5 %) of 352 °C. A controlled Si-H/vinyl ratio enabled the formulation to achieve 96.5% optical transmittance at 450 nm for 2 mm thick films and rapid thermal curing to a Shore D hardness of 82.5 within 1.5 h at 180 °C. DMA further showed that OPH exhibited a higher Tg (54 °C) and rubbery-region storage modulus (356 MPa) than the PSH system (37 °C and 183 MPa), supporting the formation of a more highly cross-linked and mechanically robust network. After thermal aging at 200 °C for 720 h, the transmittance at 450 nm decreased only from 96.5% to 95.5%, and the yellowness-index change remained as low as = 0.02, indicating strong thermo-optical and color stability under accelerated aging conditions. These results demonstrate that the OCS-based omnidirectional cross-linking strategy provides a favorable balance of refractive-index enhancement, optical transparency, curing efficiency, mechanical hardness, and thermo-optical durability, highlighting OPH as a promising material platform for optical encapsulation.