Rene I. Gonzalez

Reinforcement of Poly(ethylene terephthalate) Fibers with Polyhedral Oligomeric Silsesquioxanes (POSS)

Poly(ethylene terephthalate) (PET)-based composite fibers were prepared by melt spinning three types of PET/polyhedral oligomeric silsesquioxane (POSS) composites. These composites were made by either melt blending POSS with PET at 5 wt% loading level (non-reactive POSS and silanol POSS) or by in-situ polymerization with 2.5 wt% reactive POSS. Significant increases in tensile modulus and tensile strengths were achieved in PET fibers with non-reactive POSS at room temperature. The hightemperature modulus retention was found to be much better for PET/silanol POSS fiber when compared to that of control PET. Although other PET/POSS nancomposite fibers tested did not show this high retention of modulus at elevated temperatures, PET/isooctylPOSS nanocomposite fibers did show increased modulus at elevated temperature compared to that of PET. Higher compressive strengths, compared to PET fibers, were observed for all three nanocomposite fibers. Gel permeation chromatography measurement suggested that there is no significant change in molecular weight during preparation of PET/POSS nanocomposites. SEM observations suggest that there is no obvious phase separation in any of the three PET/POSS systems. Crystallization behavior and thermal stability of the composite were also studied. The fiber spinning and mechanical performance with 10 and 20 wt% of trisilanolisooctyl POSS2 were also investigated1 the composites with higher concentrations of this nanofiller can be spun without any difficulty. At room temperature, the fiber tensile modulus increased steadily with the POSS concentration while fiber tensile strength showed no significant change. The elongation at break decreased significantly with increasing of POSS concentration. The high-temperature moduli of PET/POSS nanocomposite fibers were found to be rather variable, likely due to the modest compatibility between filler and polymers, which can lead to structural anisotropy within the composite.

In situ oxygen atom erosion study of a polyhedral oligomeric silsesquioxane-polyurethane copolymer

The surface of a polyhedral oligomeric silsesquioxane-polyurethane copolymer has been characterized in situ using X-ray photoelectron spectroscopy before and after exposure to incremental fluences of oxygen atoms produced by a hyperthermal oxygen atom source. The data indicate that the atomic oxygen initially attacks the cyclopentyl groups that surround the polyhedral oligomeric silsesquioxane cage most likely resulting in the formation and desorption of CO and/or CO2 and H2O from the surface. The carbon concentration in the near-surface region is reduced from 72.5 at.% for the as-entered surface to 37.8 at.% following 63 h of O-atom exposure at a flux of 2.0 × 1013 O atom/cm2-s. The oxygen and silicon concentrations are increased with incremental exposures to the O-atom flux. The oxygen concentration increases from 18.5 at.% for the as-entered sample to 32.6 at.% following the 63-h exposure, and the silicon concentration increases from 8.1 to 11.1 at.% after 63 h. The data reveal the formation of a silica layer on the surface, which serves as a protective barrier preventing further degradation of the polymer underneath with increased exposure to the O-atom flux.

Properties and improved space survivability of POSS (polyhedral oligomeric silsesquioxane) polyimides

Kapton polyimide (PI) is widely used on the exterior of spacecraft as a thermal insulator. Atomic oxygen (AO) in lower earth orbit (LEO) causes severe degradation in Kapton resulting in reduced spacecraft lifetimes. One solution is to coat the polymer surface with SiO 2 since this coating is known to impart remarkable oxidation resistance. Imperfections in the SiO 2 application process and micrometeoroid / debris impact in orbit damage the SiO 2 coating, leading to erosion of Kapton. A self passivating, self healing silica layer protecting underlying Kapton upon exposure to AO may result from the nanodispersion of silicon and oxygen within the polymer matrix. Polyhedral oligomeric silsesquioxane (POSS) is composed of an inorganic cage structure with a 2:3 Si:O ratio surrounded by tailorable organic groups and is a possible delivery system for nanodispersed silica. A POSS dianiline was copolymerized with pyromellitic dianhydride and 4, 4′-oxydianiline resulting in POSS Kapton Polyimide. The glass transition temperature (Tg) of 5 to 25 weight % POSS Polyimide was determined to be slightly lower, 5 – 10 %, than that of unmodified polyimides (414 °C). Furthermore the room temperature modulus of polyimide is unaffected by POSS, and the modulus at temperatures greater than the Tg of the polyimide is doubled by the incorporation of 20 wt % POSS. To simulate LEO conditions, POSS PI films underwent exposure to a hyperthermal O-atom beam. Surface analysis of exposed and unexposed films conducted with X-ray photoelectron spectroscopy, atomic force microscopy, and surface profilometry support the formation of a SiO 2 self healing passivation layer upon AO exposure. This is exemplified by erosion rates of 10 and 20 weight % POSS PI samples which were 3.7 and 0.98 percent, respectively, of the erosion rate for Kapton H at a fluence of 8.5 × 10 20 O atoms cm -2 . This data corresponds to an erosion yield for 10 wt % POSS PI of 4.8 % of Kapton H. In a separate exposure, at a fluence of 7.33 × 10 20 O atoms cm -2 , 25 wt % POSS Polyimide showed the erosion yield of about 1.1 % of that of Kapton H. Also, recently at a lower fluence of 2.03 × 10 20 O atoms cm -2 , in going from 20 to 25 wt % POSS PI the erosion was decreased by a factor of 2 with an erosion yield too minor to be measured for 25 wt % POSS PI.