Date of Award

Spring 5-2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Polymers and High Performance Materials

Committee Chair

Sarah Morgan

Committee Chair Department

Polymers and High Performance Materials

Committee Member 2

Jeffrey S. Wiggins

Committee Member 2 Department

Polymers and High Performance Materials

Committee Member 3

Charles McCormick

Committee Member 3 Department

Polymers and High Performance Materials

Committee Member 4

Kenneth Mauritz

Committee Member 4 Department

Polymers and High Performance Materials

Committee Member 5

John Flock

Abstract

As the constituent phases in a polymer composite approach the molecular level, specific phenomena occur that can lead to significant changes in material properties when only minimal quantities of the additive are incorporated into the polymer matrix. Molecular composite and nanocomposites are state-of-the-art polymeric materials that contain nanostructured additives effectively dispersed within polymer matrices. The properties of molecular composites and nanocomposites are directly related to the interactions of the nanostructured additive and the polymer matrix. Subtle changes to the nanostructured additive can have profound effects on the ultimate properties of the composite material. Therefore, understanding the structure-property relationships in these systems represents a fundamental step in the realization of these advanced materials.

A molecular dispersion of rigid-rod and flexible coil macromolecules is known as a molecular composite. Similar to carbon or glass fiber composites, strain in a molecular composite is transferred to a stiff reinforcing agent with a high aspect ratio. However, in a molecular composite the reinforcing agent is a rigid macromolecule, and these materials are inherently homogeneous, transparent, possess a single coefficient of thermal expansion and are potentially recyclable. The degree of mechanical reinforcement in a molecular composite is directly related to the modulus and aspect ratio of the rigid-rod macromolecule as well as its state of dispersion within the flexible coil matrix. In the first portion of this dissertation, semi rigid-rod macromolecules having phenylketone substituted para-phenylene and unsubstituted meta-phenylene recurring units (i.e. SRPs) at two different ratios are blended by rapid coagulation from solution with polyphenylsulfone (PPSU), and the resulting effects on miscibility, morphology and nanomechanical properties are assessed. Initially, the nanomechanical behavior of an SRP having a completely sp2 hybridized backbone was demonstrated in comparison to conventional high performance engineering thermoplastics as a function of polymer rigidity via nanoprobe instrumentation techniques. Next, various light scattering techniques were employed to obtain key molecular and structural parameters of the SRPs and PPSU in dilute solution, which were related to polymer conformation, theoretical entropic and enthalpic contributions, and predicted blend compatibility. Miscibility was investigated using thermal analysis techniques to monitor the glass transition as a function of blend composition. The bulk and surface morphologies of these blends were analyzed via atomic force microscopy (AFM) to confirm a homogeneous morphology or determine the mechanism of phase separation, and the mechanical properties of these blends were evaluated using nanoindentation. Finally, an understanding of the relationship between the ratio of substituted para and unsubstituted meta recurring units in the SRP copolymer backbone to miscibility, morphology and nanomechanical properties in blends (or molecular composites) with PPSU was developed.

A polymer nanocomposite is broadly defined as a polymeric composite material in which one of the phases has dimensions less than 100 nm. These materials are not new since polymer blends often have dimensions much less than 100 nm. However, polymer iv composites containing nanofillers have experienced a recently renewed interest from the scientific community due to the potential for these materials to exhibit not only superior mechanical properties, but also elevated thermal and dimensional stability and an array of other property improvements at relatively low additions of nanofiller. A special class of nanofillers is polyhedral oligomeric silsesquioxane (POSS®) nanostructured chemicals. POSS molecules with their hybrid organic/inorganic structure, well defined threedimensional architecture and mono-disperse particle size have been the subject of a great deal of both academic and scientific interest for their potential to increase the strength and modulus of a polymer matrix without the negative side effects to processing observed with many traditional fillers. In fact, significant enhancements in the rheological and melt flow behavior of amorphous polymers have been observed with only minimal additions of POSS. These enhancements depend upon the interactions of POSS with the amorphous matrix based on the chemical structure of POSS. However, few detailed studies of these relationships have been performed, and the mechanism of this behavior has not been clearly defined. In the second portion of this dissertation improvement in the melt processing and rheological behavior of an amorphous polymer, PPSU, and the resulting thermomechanical properties of the nanocomposite by the addition of different types of POSS at various loading levels is discussed. The relationship of POSS chemical structure to the final properties of the nanocomposite materials was defined in terms of the difference in solubility parameters of POSS and PPSU, the dispersion of POSS within the PPSU matrix and the phase transformations POSS undergoes as a function of temperature.

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