Browsing by Author "Webster, H. Francis"

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    Characterization of the molecular structure at modified polymer surfaces and polyphenylene sulfide/copper interphases
    Webster, H. Francis (Virginia Tech, 1992)
    The interphase region at modified polymer surfaces or at polymer\ metal interfaces may be critical in determining the strength and durability in adhesive applications. Methods to investigate these regions are limited however and this research has focused on the use of infrared reflection absorption spectroscopy (IRRAS) and x-ray photoelectron spectroscopy (XPS) to investigate the molecular structure of both modified and unmodified thin films. The optical constants of polyphenylene sulfide (PPS) were determined and exact optical theory was utilized to simulate spectra for a variety of reflectance techniques. This method was also utilized to confirm the ordered state of thin spin coated PPS films. The surface modification of polystyrene, polyphenylene sulfide, and poly(arylene ether) phosphine oxides was also examined by these techniques and optical theory used to optimize experimental conditions. Results after plasma treatment indicated a very thin modified surface layer (< 10 nm) where the thickness and specific surface chemistry depended on the polymer and plasma gas used. The interaction of an epoxy resin with a surface modified PPS film showed that while most of the modified surface layer is removed after this treatment, a remaining amount can serve to cross-link a thin adsorbed epoxy film. Results for the oxygen plasma treatment of poly(arylene ether) phosphine oxides showed the formation of a surface phosphate layer that inhibited further plasma etching. The kinetics of formation and the particular chemistry involved were examined in detail. A new technique, variable temperature reflection absorption spectroscopy (VTRAS) was developed as a method to investigate the reorganization of thin PPS films on a variety of substrates. Both the crystallization and melting temperatures could be determined for quenched coatings on a variety of substrates. While degradation under vacuum was not observed on chromium and aluminum surfaces, PPS films on copper surfaces showed a loss in crystallizability, and did not return to the original ordered state after exposure to temperatures near 300°C. Loss of cuprous oxide was also observed, and chain scission mechanisms were postulated. Additional measurements on thin sputtered cuprous oxide films showed less degradation for the same temperature treatments, emphasizing the role of the underlying metal in the degradation process. Spin coated films of polyetherimide were shown to be oriented after spin coating, and the relaxation to a more random state could also be observed by the VTRAS technique. Degradation of PPS films in air was examined and the diffusion of copper species into the bulk of the film with the formation of copper carboxylates was observed. The use of the VTRAS technique in air also was useful in determining the temperature needed for the onset of degradation. Bonded PPS/copper laminates were investigated and results showed that the particular surface chemistry was crucial in determining the peel strength observed. After a simple thermal! oxidation pretreatment for copper foil, an increase in the peel strength of almost one order of magnitude was observed over non-oxidized foils. Chemical oxidation with alkaline persulfate solutions resulted in a needle-like surface oxide morphology, and bond strengths were also increased by this pretreatment method. Failure surface analysis and model interaction studies with PPS tetramer showed that the formation of excess cuprous sulfide at the interface was the most probable cause of poor adhesion in these systems. Foil pretreatment by thermal oxidation gave the highest peel strength, and also exhibited the lowest amount of interfacial cuprous sulfide.
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    Characterization of thin silicone films formed by migration across defined polymer substrates
    Webster, H. Francis (Virginia Polytechnic Institute and State University, 1985)
    While theoretical treatments of liquid spreading on solid surfaces can be found in abundance in the literature, relatively few experimental studies have been performed. This study focused on the experimental detection of polydimethylsiloxane spreading on polymer surfaces in the spreading regime where gravitational and inertial forces are negligible. The techniques of x-ray photoelectron spectroscopy (XPS), reflection absorption spectroscopy (RAS), and contact angle analysis were used to investigate this problem. XPS analysis indicated that the surface spreading or "creeping" was very slow, moving only several centimeters per several hundred hours in some cases. An interesting observation was that the spreading film seemed to reach a "steady state" concentration when covering an area. Similar results were seen for both horizontal and vertical substrate surfaces with XPS, and there seemed to be a substrate dependence. Angular-dependent XPS analysis revealed that the film was very thin (<20A) and not a complete layer. Contact angle analysis was used to monitor the siloxane movement, although there was a contact angle dependence on x-ray exposure time, indicating cross-linking of the surface film. Contact angle variations were also used to show that the film characteristics may be dependent on the nature of the polymer substrate. Reflection absorption spectroscopy was used to investigate thin substrate polymer films, and it was shown that theoretical predictions of absorption band intensity correlated reasonably well with the experiment. This technique was also used to investigate the surface siloxane layer and indicated that under some conditions, the orientation of the surface silicone layer may be occurring.