An Investigation of the Use of Cerium and Polyhedral Oligomeric Silsesquioxanes for the Protection of Polymeric Epoxy Compounds in the Low Earth Orbit Environment

Low Earth orbit presents many hazards for composites including atomic oxygen, UV radiation, thermal cycling, micrometeoroids, and high energy protons. Atomic oxygen and vacuum ultraviolet radiation are of concern for space-bound polymeric materials as they degrade the polymers used as matrices for carbon fiber composites, which are used in satellites and space vehicles due to their high strength to weight ratios. Epoxy-amine thermosets comprise a common class of matrix due to processability and good thermal attributes. Polyhedral oligomeric silsesquioxanes (POSS) have shown the ability to reduce erosion in polyimides, polyurethanes, and other polymers when exposed to atomic oxygen. The POSS particle is composed of a SiO1.5 cage from which up to eight organic pendant groups are attached at the silicon corners of the cage. POSS reduced atomic oxygen impact on polymers by a process known as glassification wherein the organic pendants are removed from the cage upon atomic oxygen exposure and then the cage rearranges to a passive silica network. In addition, POSS shows good UV absorbance in the UVb and UVc ranges and POSS can aid dispersion of titanium dioxide in a nanocomposite. In this work, Chapter 1 focuses on hazards in low Earth orbit, strategies for protecting organic material in orbit, and the capabilities of POSS. Chapter 2 details the experimental practices used in this work. Chapter 3 focuses on work to induce POSS phase separation and layering at the surface of an epoxy-amine thermoset. Generally, POSS is dispersed throughout a nanocomposite, and in the process of erosion by atomic oxygen some polymer mass loss is lost before enough POSS is exposed to begin glassification. Locating POSS at a surface of composite could possibly reduce this mass loss and the objective of this research was to investigate the formation of POSS-rich surfaces. Three POSS derivatives with different pendant groups were chosen. The POSS derivatives had a range of miscibilities with the epoxy-amine matrix. A sedimented layer of the most incompatible POSS moiety was observed at the bottom of bars at the highest loading level of 5 wt% POSS. It was concluded that POSS could form a sedimented layer in this epoxy during cure. Epoxy amine materials containing POSS derivatives were tested by exposure to atomic oxygen at NASA Glenn Research Center with each POSS derivative present in separate samples at 2.5 wt% loading levels. Mass loss did not decrease against an unfilled control and glassification was not observed, leading to the conclusion that POSS could not be effectively concentrated at a surface to reduce degradation given the methods used. Taking this into account, the study transitioned into seeking ways to integrate highly UV absorbent cerium compounds with POSS. This part of the study is reported in Chapter 4. It was anticipated that POSS with a polar pendant group would interact through intermolecular forces with cerium (IV) oxide and produce a suspension that could be cured at the surface of polymers. However, in every experiment the cerium (IV) oxide was not dispersed. However, a homogeneous dispersion of a cerium-containing compound was achieved by combining trisilanol phenyl POSS with cerium (III) nitrate hexahydrate. NMR and mass spectrometry showed that the mixture of Cerium nitrate and trisilanol phenyl POSS did not result in the formation of a chemical compound but FTIR studies indicated the presence of hydrogen bonding between the POSS silanols and cerium-associated water. The resulting material was termed “CePOSS”. CePOSS was more UV absorbent in the UVc region than POSS or other cerium compounds as measured by solution UV-vis spectroscopy. In addition, CePOSS could be mixed into a POSS-epoxy coating, after pre-blending with poly(ethylene glycol) POSS, to produce films that were essentially opaque in the UV region below a wavelength of about 300 nm, and transparent in the visible region above 300 nm. The discovery of a ‘window of transparency’ in the visible region is significant in view of the fact that the epoxy-amine polymers, sans the POSS and cerium additives, were opaque across the entire UV/ visible range. The investigation of the UV transmittance and glassification response of these CePOSS-POSS-epoxy films is described in Chapter 5. UV transmittance of the POSS-epoxy coating was predicted to decrease below 275 nm with the presence of CePOSS given the solution UV-vis spectroscopy results. However, there was no difference seen in transmittance between coatings with and without CePOSS below 275 nm. The transparent region above 300 nm was seen in all samples with any type of POSS. In addition, UV/ozone exposure was completed on epoxy, POSS-epoxy, and CePOSS-POSS-epoxy coatings to examine the effect of cerium on POSS glassification. Oxidation was achieved even in the presence of CePOSS as verified by x-ray photoelectron spectroscopy, scanning electron microscopy, and contact angle. Finally, UV transmittance was done on pre and post exposed materials

Identification Of Post-Translationally Modified (O-GlcNAc) Hepatic Proteins In Fasted And Re-Fed Mice

Diabetes mellitus type 2 affects over eight percent of all Americans. However, despite the prevalence of the disease, much is unknown. Previous studies have shown that O-linked N-acteylglucosamine (O-GlcNAc) modification of proteins increases in frequency when blood glucose levels are elevated. We sought to determine which proteins receive the O-GlcNAc modification as well as the function of those proteins in response to hyperglycemia. To identify the modified proteins, we created a unique methodology based on a combination of established techniques. Samples from a control group of fasted mice modeled lower blood glucose levels and from an experimental group of fasted then re-fed mice modeled higher blood glucose levels. Dimethyl labeling was applied to distinguish the experimental and control mice. A lectin weak affinity chromatography (LWAC) column was used to isolate O-GlcNAc modified proteins. The identity of the modified proteins was determined with a combination of heavy and light mass spectrometry. The initial steps of this novel protocol were successful. Dimethyl labels appeared on the mass spectra and enabled us to confirm an increase in O-GlcNAc modification in response to glucose in the re-fed mice. Future studies will utilize this labeling technique in combination with chromatography and mass spectrometry with larger sample sizes to further understand the role of O-GlcNAc modification in Diabetes mellitus

Effects of POSS Addition On Bisphenyl-E Cyanate Ester Network

Polyhedral oligomeric silsesquioxanes (POSS) and cerium (IV) oxide were added to a bisphenol-E cyanate ester matrix as protective filler for UV/atomic oxygen exposure. These additions did not adversely affect properties such as thermal degradation and glass transition temperature. However, cure conversion decreased at 30 wt% glycidyl POSS from on average 95% to 89 wt% conversion indicating a potential loading limit. Scanning electron microscopy revealed 1-5 μm POSS aggregations located uniformly throughout sample cross sections. Ceria also was well dispersed but did form an approximately 50 μm layer on the bottom edge of samples. Altering the cure to introduce an isothermal hold to take advantage of a low viscosity region did not change this particle distribution and there was no difference between the location of POSS and ceria specifically as detected by energy dispersive x-ray spectroscopy. Future work will target development of a dual layer combination of matrix and a CeO2/POSS-rich overlay to avoid issues with control of filler aggregate location