Brush Particle-Based Building Blocks for High Refractive Index Composites

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

Non-Technical Summary Optical devices that emit, absorb, or bend light require mechanical protection, as scratches or cracks can cause undesirable light scattering. Such coatings must have a range of different properties, including both mechanical resilience and high refractive indices; the former is necessary to protect the underlying device, while the latter ensures maximum optical performance, because through the protection layer the light gets bent towards the device. However, most materials can only be optimized for one of these target criteria. With this project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research, Prof. Rob Macfarlane and his research group develop new strategies to synthesize composite materials consisting of high refractive index nanoparticles embedded within a mechanically robust polymer matrix. These materials can be easily formed into appropriate macroscopic shapes at low temperatures, then thermally crosslinked into mechanically stable solids. The researchers study how and why significantly larger fractions of the high refractive index components can be incorporated in the matrix with this synthesis method, which then results in materials that have ideal mechanical and optical performance. The simple synthesis methods for these materials potentially allow for the fabrication of more complex devices in the future that can be used to tailor material reflection to either induce structural coloration or anti-reflective properties. Such materials have benefit as adhesives, coatings, and sensors coupled to optical devices. In addition, the project provides opportunities for community college students from underrepresented groups to participate in STEM research, allowing them to gain both experience in current chemistry research techniques and insights into potential career opportunities. This program enables real-world, hands-on experiences to aid these students in pursuing higher education opportunities or STEM careers. Technical Summary With this project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research, polymer/nanoparticle composites are synthesized using crosslinkable polymer-grafted nanoparticles (PGNPs) containing large volume fractions of high refractive index nanoparticles. Systematic investigations of fundamental structure-property relationships are conducted to explain how the optical and mechanical properties of these materials are affected by the large amount of filler content. The target materials possess tunable refractive indices due to the controlled amount of inorganic content (up to 85 wt.%), but also possess superior mechanical properties that surpass the bare polymer alone. Specifically, the researchers study the following aspects I) Developing materials-versatile protocols to synthesize and crosslink PGNPs with different inorganic nanoparticle and polymer compositions, II) Fully characterizing the mechanical and optical properties of the resulting structures, and III) Developing methods to induce photonic band gap architectures through ordered nanoparticle organization or multi-layer stacks of different composite compositions. The basic synthetic science and materials characterization efforts conducted enable a new class of optical materials, and also provide better design parameters for both grafting polymer brushes to inorganic particles and inducing covalent bond formation on nanoscale surfaces. This work advances both fundamental chemistry in solid state materials development and provides highly useful composites for optical devices. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.