Sulfur Based Polymers and Polymer Magnetic Nanoparticle Composites: Novel Materials for Next Generation IR & Magneto-Optics

Abstract

This dissertation reports on new material developments as detailed in three chapters discussing the development of high Verdet constant materials containing magnetic nanoparticles (NPs) and one chapter discussing sulfur-containing polymers for infrared (IR) optics applications. The first three chapters will cover the synthesis and characterization of different types of magnetic nanoparticles followed by their incorporation into polymer systems to generate composite materials. Further, the processing techniques utilized to generate magneto-optic (MO) devices from these composites will be discuss as well as their performance related to both the processing methodology used and the properties of the nanoparticles. For chapters 2 and 3, additional experimental information and supporting data can be found in the corresponding appendix chapters. The fourth chapter will cover recent work analyzing the transparency of polymer materials in the IR.The first chapter is the first comprehensive review summarizing the field of high Verdet constant MO materials. The Faraday effect is a type of MO phenomenon where the polarization direction of linearly polarized light is rotated when passing through transparent media under the application of a magnetic field along the direction of the light propagation. At a certain wavelength and temperature, the angle of rotation is directly proportional to the strength of the magnetic field, the path length of the medium, and the Verdet constant of the material. For MO devices, maximizing the rotation angle of the light is desired for device miniaturization and fabrication cost and can be accomplished by increasing any of the aforementioned parameters, but for devices with size constraints and with the limitations of sustainable magnetic fields, high Verdet constant materials are desired. The development of high Verdet constant materials has been ongoing since the discovery of the Faraday effect in the mid-1800s and has come to encompass a variety of material classes from hard matter glasses, crystals, and ceramics, to soft matter small molecules and synthetic polymers, and some composite materials containing both hard and soft matter. The most widely used materials for MO applications have traditionally been glasses and crystals with Verdet constants on the order of 103-104 °/T·m at room temperature when measured in the visible to near-IR range. However, more recent work has shown promising soft matter systems composed of small molecules, polymers, and nanoparticle-polymer nanocomposites with enhanced Verdet constants ranging from 104-106 °/T·m. Chapter 1 is designed to give a broad overview of the different types of high Verdet constant materials with an emphasis on the recent development ultra-high Verdet constant materials such as conjugated polymers and nanoparticle-polymer composites. The second chapter will discuss the preparation of polymer-nanoparticle composite Faraday rotators. As mentioned previously, most applications of Faraday rotators use a variety of hard matter materials to accomplish MO rotation. These materials are however limited by low Verdet constants (when compared to many soft matter systems) which has led to an increased interest in soft matter both for increased Verdet constants and for enhanced processing capabilities. The new type of MO material discussed is based on MO active cobalt ferrite (CoFe2O4) nanoparticles combined with poly(styrene) (PS) to form solution-processable composites. This study examines the importance of exchanging the surface ligands on the CoFe2O4 nanocrystals to polymer ligands that are more compatible with the PS phase of the composite. With surface compatible ligands, the NPs could be dispersed in a polymer matrix at variable loadings from 2.5-15 wt%. The solution processing techniques employed for this system allowed the MO response to be tuned by simply varying the NP loading and the number of layers in the composite. The Faraday rotators were prepared by a multilayer polymer film construct wherein alternating layers of the NP-polymer composite and cellulose acetate were spin coated allowing for an increased path length of the MO active NP-polymer layers with protective layers of cellulose acetate. These multilayered Faraday rotators demonstrated a nearly 10X increase in Verdet constant compared to a terbium gallium garnet reference material at 1310 nm and demonstrated the importance of exploring soft MO materials for enhanced MO responsive materials. The third chapter will discuss an expansion on the previous work with CoFe2O4-polymer composites. The previous work established a construct for how Faraday rotators could be produced by solution-processing using alternating layers of MO active materials to achieve a strong MO response. In this work polymer coated cobalt (Co) nanoparticles were used and offered several advantages over the previously used CoFe2O4 NPs. The synthetic strategy employed to produced Co NPs uses a native polystyrene ligand which eliminated the ligand exchange processing step and provided enhanced dispersion in the polystyrene matrix. This enhanced dispersion allowed for NP loadings up to an astonishing 50 wt% with the same tunability as the previous system. In addition to solution processing, the enhanced dispersion allowed for the formation of composites that could be melt-processed into free standing films with significantly increased thicknesses. The synthetic strategies employed additionally allowed for the synthesis of a range of particle sizes from 9 to 17 nm. This allowed, for the first time, a systematic study Verdet constant with the size dependent magnetic properties of the NPs. Due to both the increased NP loading and a stronger magnetic moment from Co NPs compared to CoFe2O4 NPs the measured Verdet constants were 2-3 orders of magnitude higher than our reference terbium gallium garnet material or our previous NP-polymer composites. The fourth chapter will cover recent work on IR imaging and the importance of uniform reporting techniques for IR transparency. There have been recent reports of new IR transparent polymers that only report the IR transparency measurement as thin films. This chapter demonstrated that thin films of poly(methyl methacrylate) PMMA (a demonstrably poorly IR transmissive polymer) could mistakenly be described as an IR transparent, transmissive optical polymer. To definitively illustrate the non-uniformity of polymer films and windows reported for IR optics, PMMA windows of progressively increasing thickness were prepared for FTIR measurements to quantity optical transmittance in the IR spectrum, along with MWIR imaging experiments. These results were then compared to poly(sulfur-random-diisopropenylbenzene) (poly(S-r-DIB)), our previously established MWIR transparent polymer. The report provided both qualitative and quantitative analysis of PMMA and p(S-r-DIB) and how only reporting on thin film properties of polymers can lead to misrepresentation of the polymer’s IR transparency. These results set a standard for how IR transparency in polymers should be reported.