# The Effect of Nanoparticle Shape, Orientation, and Heterogeneity on the Optical Birefringence of Polymer Nanocomposites

## TL;DR

This paper introduces a method to measure how nanoparticles are oriented in polymer nanocomposites using optical birefringence, enabling better control over material properties.

## Contribution

A novel technique using optical birefringence to independently determine nanoparticle orientation and aggregation in polymer nanocomposites.

## Key findings

- Optical birefringence measurements correlate with nanoparticle orientation order in polymer nanocomposites.
- The method is robust against nanoparticle size and shape heterogeneity.
- Variable-angle spectroscopic ellipsometry effectively measures orientation order and aggregation.

## Abstract

Embedding plasmonic
nanoparticles (NPs) into polymer
nanocomposites
(PNCs) is a facile method for integrating them into functional devices,
whose properties are tunable through varying NP size, shape, and loading.
Using anisotropic NPs adds an additional degree of tunability to their
orientation order in the PNC, as properties such as conductivity and
charge transport can be enhanced in specific directions. In thin films,
the film thickness and block copolymer self-assembly can affect the
degree of NP orientation, which can be used as a method of control
over these properties. However, large-scale control of orientation
order in randomly distributed NPs, with both anisotropic NP shapes
and heterogeneous shape distributions, remains a challenge. This is
partly due to the lack of cost-effective, ensemble-level characterization
methods that can independently determine the orientation order and
degree of aggregation of anisotropic NPs. Here, we model the complex
index of refraction of PNCs with plasmonic NP inclusions in the optical
frequency domain by using an effective medium approximation. We quantitatively
relate the simulated optical birefringence of the medium to the orientation
order parameter of plasmonic nanorods and nanodisks in a robust manner
insensitive to heterogeneity in simulated NP size and shape. Experimentally,
we measure this orientation order parameter through the birefringent
index of refraction using variable-angle spectroscopic ellipsometry
(VASE). We demonstrate that we can independently determine the orientation
order and degree of aggregation for various PNCs with gold nanorods
and nanosphere inclusions. This facile technique provides a powerful
method to broadly measure the average orientation order of anisotropic
particles in PNCs, which can be correlated to their functional properties.