Highly integrated surface ligand platform for multifunctional polymer nanocomposites and their applications in LED lighting

Abstract

The first part of this thesis focuses on the synthesis of titania (TiO2) and zirconia (ZrO2) NPs and their surface ligands. The high RI (above 2.2), nanoscale dimension (down to ~4 nm in diameter), and suitable surface property for post-functionalization are the decisive advantages of the synthesized TiO2 and ZrO2 NPs for use as nanofillers in silicone-based LED encapsulants. Meanwhile, the surface ligands play an essential role to overcome the inherent large surface energy discrepancy between the inorganic nanofiller and organic matrix. With minimized Rayleigh scattering losses due to good dispersion of individual high-RI NPs, the highly transparent, high RI silicone nanocomposites can be promising candidates for LED encapsulant materials to enhance the light extraction efficiency of LEDs.

The possibilities of creating highly engineered polymer nanocomposites with stimuli-induced shape changing capability and controlled macro-, micro-, or nano-structures were explored at the end of this thesis as future study. With highly integrated functionalities and optimized surface ligand design, the polymer nanocomposites developed in this thesis open up new opportunities in next-generation LED package geometries and luminaire design.

The last part of this thesis demonstrates exciting implications of the highly integrated surface ligand system for advanced LED lighting applications. Compared to commercial silicone encapsulants, significant light extraction enhancement has been obtained for red and green LEDs encapsulated with the transparent high-RI ZrO2/silicone nanocomposites. The nanocomposite-encapsulated green and blue LEDs exhibit high reliability within 1000 hours at up to 1 A driving current. In addition, the spectrum of a blue LED encapsulated with dye-functionalized NPs can be adjusted from yellowish green to near pure white. Approaches providing confinement against free motions of the conjugated structure of dye molecules were adopted to enhance the stability of the dye-functionalized NPs.

The three-dimensional parametric phase diagram is applicable across many polymer nanocomposite systems, including ZrO2/silicone nanocomposites where the enthalpic core/core attraction is predicted to be smaller than TiO2/silicone systems. With the primary goal being maximizing NP core fraction without inducing macroscopic phase separation in a nanocomposite, a "matrix-free" nanocomposite is presented in the third part of this thesis, where the need for matrix addition is eliminated by cross-linkable polymer brushes grafted on the NP surfaces. To demonstrate the use of NP core as a carrier for additional functionalities, organic dye molecules with suitable end groups are attached to bimodal PDMS brush grafted ZrO2 NPs. The mixed multimodal brush design not only enables homogenous dispersion of high-RI ZrO2 NPs within silicones, but also gives rise to tunable fluorescence behavior of the functionalized NPs via control of intermolecular spacing.

Carefully designed nanoparticle (NP) surface ligands can tune the compatibility between nanofiller and matrix polymer to control filler dispersion. The ligands can also integrate functionality without losing compatibility. In this thesis, a surface ligand toolbox is created for the synthesis, surface modification, and functionalization of high-refractive-index (RI) metal oxide NPs for use in silicone nanocomposites. While this thesis emphasizes the practical applications of the prepared nanocomposites in advanced LED lighting systems, a theoretical model is expanded to predict NP dispersion and is correlated with experiments, providing broad fundamental insight into how to control nanofiller dispersion in any spherical nanofiller/polymer systems with a particle size down to ~4 nm.

However, conventional approaches to dispersing enthalpically incompatible nanofillers using monomodal polymer brushes are often challenged by a delicate balance between allophobic dewetting and autophobic dewetting. The dispersion window becomes even narrower for matrices with higher molecular weight, whose mechanical integrity is important for most applications. The second part of this thesis presents an innovative solution to this challenge using a bimodal polymer brush. TiO2 NPs are grafted with long PDMS brushes at lower graft densities and short PDMS brushes at high graft densities. When dispersed within commercial silicone matrices, the densely grafted short brush shields enthalpic core/core attraction while sparsely grafted long brushes prevent autophobic dewetting. As predicted by a three-dimensional parametric phase diagram modeling the balance between the enthalpic core/core attraction and entropic brush/brush repulsion, the dispersion window for the bimodal systems is largely expanded, which agrees well with experimental observations.