Jeffrey S. Wiggins

Synthesis of Bioabsorbable Networks From Methacrylate-Endcapped Polyesters

A low-molecular-weight poly(e-caprolactone) triol (Mn = 540) was reacted with an excess of methacryloyl chloride to give a 3-arm methacrylate-endcapped polyester. The loss of the hydroxyl endgroups, which are capable of hydrogen bonding, lowered the viscosity of the polymer, thus enabling the formation of networks without solvent processing. The prepolymer was free radically homopolymerized, and copolymerized with methyl methacrylate, styrene, and 2-methylene-1,3-dioxepane to give a series of twelve biodegradable networks. Extraction studies indicated substantial network formation for all of the networks except those in which 2-methylene-1,3-dioxepane was used as the reactive diluent. A series of three low-molecular-weight poly(d,l-lactide) triols (Mn < 10 000) was synthesized by the trimethylolpropane-initiated ring-opening polymerization of d,l-lactide. Number-average molecular weights of the triols were estimated to be 2300, 5100, and 8700, by using gel permeation chromatography (g.p.c.), calibrated with polystyrene standards. The triols were reacted with excess methacryloyl chloride to produce a series of low-molecular-weight 3-arm methacrylate-endcapped polyesters. These prepolymers were free radically homopolymerized, and copolymerized with methyl methacrylate and styrene to give a series of twenty-seven biodegradable networks. Extraction studies indicated substantial network formation for all of these systems. The Tg values of each of the three homopolymer networks were higher than the corresponding Tg values of either of their respective copolymer networks, thus illustrating substantial network formation in the absence of reactive diluents. The ultimate strengths and tensile modulii of the homopolymer networks synthesized from the higher-molecular-weight prepolymers (Mn = 5100 and 8700) were higher than those measured for corresponding copolymer networks.

Effects of POSS Addition on Bisphenol-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.

Novel POSS-Cerium Oxide Thermoset Nanocomposites For UV Degradation Mitigation

This project focused on developing ceria-POSS compounds for protecting epoxy-based composites against VUV radiation. Two types of ceria-POSS mixtures were created, introduced into an epoxy-amine system, and analyzed for physical and mechanical properties, and particulate morphology. The first combination was a novel attempted covalent bond between ceria and silanol groups on trisilanol phenyl POSS. The second mixture was utilizing intermolecular attractions between the epoxy functionality on glycidyl POSS and ceria. The covalent compound had issues with low reaction yield, but produced a thermally stable nanocomposite with potential for aggregation growth. The CeO2-glycidyl POSS mixture showed two distinct tan delta transitions, but did have significantly improved thermal degradation compared to the neat control. Microscopy showed good capture of the CeO2 by the glycidyl POSS, even with the lack of bonding. A noticeable layer of particulates formed at the lower mold-surface interface and AFM surface mapping revealing a homogenous surface of high modulus aggregates. This is promising for barrier coating applications. Selected coatings have been sent for VUV exposure testing, and will be analyzed for mass loss, surface morphology and any change in mechanical properties. Overall, this study will be an additional step in creating a cohesive, lightweight, barrier layer for protecting carbon fiber composites in the challenging low Earth orbit environment.