Doctoral Dissertations

Date of Award

12-2006

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Mark Dadmun

Committee Members

Jimmy Mays, Charles Fiegerle, John Turner, Roberto Benson

Abstract

The work presented in this dissertation attempts to form an understanding of the reaction parameters that govern the reaction of telechelic polymers to form multiply bound polymer chains, resulting in polymer loop formation, at both soft and hard polymeric interfaces. Both theoretical and experimental studies offer evidence that blocky copolymers, as opposed to random or alternating morphologies, are the best compatibilizers of a given polymer blend system. This has been attributed to the fact that blocky copolymers spread across the interface, with each block lying in or near the homopolymer region in which it is miscible, effectively “stitching” the interface together. The “loops” formed at the interface by each block of the copolymer entangle with the homopolymer chains of its respective region and can lead to finer dispersion and less coalescence of the minor phase during melt processing. We believe that interfacial loop formation is not only important to the compatibilization of immiscible polymer/polymer blend systems, but can be applied to compatibilization efforts on filled polymeric nanocomposite systems as well.

The aim of the first portion of this study was to gain a fundamental understanding of the kinetics of the in-situ formation of blocky copolymer molecules at a soft, immiscible, biphasic polymer interface by the reaction of telechelic polymers across the interface, as well as gain an understanding of the parameters that govern this reaction. The changes in interfacial morphology that resulted from this reaction were also examined. Specular neutron reflectivity measurements were made on bilayer PS/PMMA samples, containing dPS telechelics and PMMA monochelics, with varying reactive groups, after various annealing times. Three reactive pair systems, epoxy-carboxy, epoxy-amine, and anhydride-amine, were studied. We investigated the effects of annealing time, telechelic functionality, and telechelic molecular weight on the kinetics and resultant interfacial morphology of the blend systems. The thickness of this interfacial modifier layer, as well as the roughness between the matrix layers, increased with annealing time for all systems studied, albeit, to varying degrees. Lower molecular weight telechelics led to greater interfacial widths and interfacial excess values for all reactive pairs investigated. When the molecular weight of the telechelic was low (<80K g/mol), the measured interfacial excess values at the longest anneal times were all above the theoretical value above which the interfacial tension should have been reduced to zero, and in all cases the interfacial widths were much greater than for the higher molecular weight telechelics. For the case of the epoxy-carboxy and epoxy-amine reactive pair systems, the reaction is shown not to be diffusion-limited, as most theories of the interfacial reaction of functionalized polymers predict. This was supported by the fact that the reaction kinetics were vastly different when the telechelic molecular weight was held constant and only the functional groups were varied. The question of whether the anhydride-amine reactive pair system follows diffusion-limited kinetics could not be unambiguously elucidated. The anhydride-amine reaction showed the fastest kinetics, followed by the carboxy-epoxy and epoxy-amine systems, respectively. While the total interfacial widths at the longest anneal times were similar for the epoxy-carboxy and anhydride-amine systems, the interfacial excesses were greater for the epoxy-carboxy system at a given telechelic molecular weight. This is attributed to the epoxy-carboxy system samples possibly displaying a microemulsion or micelle formation at the interface due to a greater reduction in interfacial tension.

We also believe that this idea of interfacial loops formation can be utilized in the compatibilization of filled polymeric nanocomposites. Many properties of a polymer system can often be improved with the addition of organic or inorganic fillers. However, many of the fillers used, such as layered silicates and carbon nanotubes, display unfavorable thermodynamic interactions with the polymer matrix and result in alloys that tend not to mix very well. By forming polymeric loops on filler particle surfaces, we believe that entanglements between the surface bound loops and the matrix polymer chains will lead to finer filler dispersion, improved interfacial strength, and ultimately the enhanced property improvements promised by polymeric nanocomposites. Towards this goal, we undertook work to attempt to form surface bound polymeric loops and to understand the reaction parameters that govern multiply bound polymer chain formation on hard surfaces. For this work, we have studied a model system consisting of an epoxy functionalized silicon substrate and dicarboxy terminated polystyrene to monitor the reaction conditions leading to the formation of surface bound molecular loops. Techniques including ellipsometry, goniometry, and X-ray photoelectron spectroscopy were used. We were able to prove that the reaction of telechelic polymers can, indeed, produce surface functionalization layers that are primarily composed of multiply bound polymer loops. Contact angle measurement analysis suggests that more than 80% of the surface area is covered with PS chains as opposed to unreacted chain ends, leading us to believe that we have many loops, as opposed to singly bound tails, present in the surface layers. We were also able to form very dense loop layers on our model surfaces as evidenced by the fact that bulk PS would not wet the loop modified surfaces even under pressure and heat. This led us to conclude that control over the density of surface reactive sites, i.e. control of grafting density, could allow the tailoring of loop layer wettability and other surface properties that could have applications in other fields, such as tribology.

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