Thesis (Ph.D.)--Drexel University, 2008. / Includes abstract and vita. Includes bibliographical references (leaves 262-277).

Thesis (Ph.D.)--Drexel University, 2010. / Includes abstract and vita. Includes bibliographical references (leaves 151-159).

The effect of graft molecular architecture on the formation of self-assembling morphologies of strongly phase-separated, amorphous block copolymers have been systematically investigated. Three series of samples across a range of component volume fractions were characterized using transmission electron microscope and small angle x-ray and neutron for different model architectures of polystyrene-polyisoprene single graft and double graft copolymers. The single graft architecture was an "asymmetric single graft", ASG, formed by grafting a polystyrene block on a polyisoprene backbone. This architecture is considered the fundamental unit, or constituting block copolymer, from which all more complex graft architecture with multiple trifunctional junction points are constructed. The ASG architecture is found to shift the volume fraction windows in which specific strongly microphase separated morphologies are observed to higher volume fractions of the PS graft material than in the corresponding linear diblock copolymers with a same molecular weight and chemical composition. As the polystyrene is grafted from the center of polyisoprene backbone, the ASG architecture becomes an I$\sb2$S star architecture. The morphological behavior of the I$\sb2$S block copolymers was predicted by the morphological diagram of S. T. Milner for miktoarm stars (Macromolecules, 27, 2333 (1994)). However, it is found that the morphology diagram overestimates the amount of shift in the order-order transition lines produces by asymmetry in molecular architecture. This overestimation in the theory is attributed to an effect of chain crowding close the junction points. The effect of asymmetric grafting of the single PS chain on the PI backbone was investigated in a series of materials where the single PS graft is asymmetrically located along the PI backbone. Additionally the effect of multiple graft architecture was explored with S$\sb2$IS$\sb2$ (H-shaped) and (SI)I$\sp\prime$(SI) $\pi$-shaped) materials, each of which has two trifunctional branch points per molecule. It was found that to a good approximation the behavior of the double graft materials can be mapped onto the behavior of the single graft materials by considering the double graft molecules to be divided into component single graft parts.

In recent years, supercritical fluids (SCF), specifically carbon dioxide (CO2), have been tested and applied as alternative solvents for polymer processing and modification. The principal utility of CO2 in heterogeneous polymer systems lies in the sorption of significant mass fractions of CO2, which influence properties that are driven by free volume. The effects include depressed glass transition temperatures, enhanced transport within the dilated polymer and decreased viscosity. The exploitation of these effects in multicomponent systems requires an understanding of the influence of compressible fluid sorption on polymer-polymer compatibility that to date has been unexplored. In this dissertation, it is demonstrated for the first time, that sorption of SCF's can induce phase segregation in polymer systems exhibiting Lower Critical Solution Temperature-type (LCST) behavior at temperatures hundreds of degrees below the ambient pressure transition. For LCST systems, the relative compressibilities of the components play a dominant role, which can be exacerbated by sorption of SCF's. For example, fluorescence quenching experiments indicate that sorbed gas (CO2) depresses the LCST's of blends of polystyrene/poly (vinyl methyl ether) (PS/PVME) by over 100°C at modest pressures (around 20 bar) of the gas with negligible dependence on temperature and molecular weights of the polymer components. Absorbed CO2 has similar effects on blends of deutrated-polybutadiene/polyisoprene as studied by Small Angle Neutron Scattering. The phase behavior of PS/PVME in the presence of CO2 has been modeled using the Sanchez-Lacombe equation of state, which indicates that the polymer blend phase separation is driven primarily by the selective dilation of PVME by CO2 relative to PS. Ethane, with a weaker selectivity also induces phase separation in PS/PVME system, but is significantly different from the effect of CO2 with respect to temperature and polymer molecular weights, indicating the role of selectivity of poor solvents that is superimposed on compressibility effects. Finally, the design, development and application of neutron reflectivity to high-pressure systems for in situ measurements are discussed including results on swelling of thin homopolymer films and investigations of the phase behavior of a diblock copolymers of polystyrene and poly (n-butyl methacrylate) that exhibit Lower Disorder-Order Transition (LDOT).

This thesis investigates the macroscopic yield behavior and microscopic energy absorption mechanisms in single and multiphase polymers. One unique aspect is the evaluation of polymers under multiaxial loading conditions. This is important because in many applications polymers are subjected to complex loading conditions and hence optimal design requires experimental evaluation and modeling of behavior under multiaxial stress states. This work has resulted in a more quantitative understanding of yield and energy absorption in the different polymers considered. Multiaxial stress states are achieved using thin-walled hollow cylinder specimens. The hollow tubes are simultaneously subjected to internal pressure and axial load, leading to biaxial stress states. Stress states ranging from uniaxial compression to equibiaxial tension are interrogated using the same specimen geometry, a procedure uncovering true material behavior. In the first part of this study, a generalized model for the yield behavior of single-phase polymers is evaluated for a polycarbonate system. The generalized model accounts not only accounts for viscoelasticity (i.e., rate and temperature dependence) but also the effect of pressure on yield behavior. The effects of physical aging on the behavior of amorphous polycarbonate are also highlighted. For rubber-modified polymers, existing models for both macroscopic yield behavior and the onset of microscopic damage (e.g., cavitation) are evaluated under multiaxial conditions (chapter 3). Serious discrepancies are found for both cases, prompting an investigation into the nature of energy absorption mechanisms in the materials. Apart from the chosen rubber-modified systems, a toughening mechanism in the form of overlapping parallel cracks is identified to be generic to a range of polymers (chapter 4). The last part of the thesis (chapter 5) involves a quantitative investigation of interactions in overlapping crack patterns. This effort is vital, because for better design of materials, the interaction has to be related to intrinsic material properties. The interactions in an infinite 2D array of parallel and overlapping cracks are analyzed using a complex stress function method. The size and number density of cracks in the array are related to intrinsic material properties and conditions for damage stability are identified.

Liquid crystalline polymers (LCPs) are a new class of high strength materials. Their rheological behavior is different from that of ordinary isotropic polymers because they are inherently anisotropic in the melt state. Crystallization in rigid-rod mesogenic systems in the nematic melt and super-cooled states has been studied by small amplitude oscillatory shear, which has been found to be in some ways more sensitive than conventional techniques like DSC or X-ray scattering (in that changes are measured by orders of magnitude instead of merely on a linear scale). A scheme of preheating is suggested for unsubstituted, rigid-rod polymers whereby a metastable nematic melt can be achieved, effectively suppressing crystallization and enabling a thorough rheological characterization. For example, an excellent agreement has been obtained for the stress relaxation modulus between experimental values and those calculated from dynamic oscillatory measurements, thereby confirming the existence of a linear viscoelastic range for the LCP. Non-linear creep studies illustrate how rheological properties are strongly affected by structural changes upon deformation. Structure in unsubstituted rigid-rod nematic systems is hypothesized to exist at two levels--Non-Periodic Layer (NPL) Crystallites and Domains/Disclination Network. Another aspect of research concerns a comparative study of the phase behavior of liquid crystalline components in closely related blends and copolymers.

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Thesis (M.App.Sc.) - Carleton University, 2007. / Includes bibliographical references (p. 107-111). Also available in electronic format on the Internet.

Thesis (M.S.)--California Polytechnic State University, 2009. / Mode of access: Internet. Title from PDF title page; viewed on November 19, 2009. Major professor: Dr. Kristen O'Halloran Cardinal. "Presented to the faculty of California Polytechnic State University, San Luis Obispo." "In partial fulfillment of the requirements for the degree [of] Master of Science in Biomedical Engineering." "August 2009." Includes bibliographical references (p. 143-158).

Facile monomer synthesis provided routes towards novel step-growth polymers for emerging applications. Adjustment of reaction conditions enabled green synthetic strategies, and promising scalability studies offered impetus for industrial funding. Engineering thermoplastics, such as linear polyetherimides (PEIs), had carefully targeted molecular weights for analysis of the effect of molecular weight and regiochemistry on the thermomechanical and rheological properties of PEIs. The design of linear, high performance PEIs comprising 3,3'- and 4,4'-bisphenol-A dianhydride (bis-DA) and m-phenylene diamine (mPD) provided an opportunity to elucidate the influence of dianhydride regiochemistry on thermomechanical and rheological properties. This unique pair of regioisomers allowed the tuning of the thermal and rheological properties for high glass transition temperature polyimides for engineering applications. The selection of the dianhydride regioisomer influenced the weight loss profile, entanglement molecular weight, glass transition temperature (Tg), tensile strain-at-break, zero-shear melt viscosity, average hole-size free volume, and the plateau modulus prior to viscous flow during dynamic mechanical analysis (DMA). The 3,3'-PEI composition interestingly exhibited a ~20 °C higher Tg than the corresponding 4,4'-PEI analog. Moreover, melt rheological analysis revealed a two-fold increase in Me for 3,3'-PEI, which pointed to the origin of the differences in mechanical and rheological properties as a function of PEI backbone geometry. The frequently studied 4,4'-PEI exhibited exceptional thermal, mechanical, and rheological properties, yet the 3,3'-PEI regioisomer lacked significant study in the industrial and academic sectors due to its 'inferior' properties, namely poor mechanical properties. Introduction of long-chain branching (LCB) into PEIs provided a unique comparison between a commercially relevant PEI (Ultem® 1000) and a regioisomer infrequently found in the literature. Thermal stability remained consistent for each regioisomer, and Tgs for the 3,3'- and 4,4'-LCB-PEIs agreed well with prior literature. Rheological analysis demonstrated typical shear thinning and low-shear viscosity trends for LCB systems. The targeted molecular weights for the 3,3'-LCB-PEIs were well below the Me cutoff for "high molecular weight," and for this reason the rheological properties demonstrated inconsistent trends. Further study of PEIs led to the incorporation of ionic endgroups. These provided physical crosslinks, which enhanced mechanical and rheological properties of branched PEIs compared to their non-ionic analogs. The Tgs decreased with an increase in branching concentration for the phenyl-terminated PEI, while it remained unchanged for the ionically-endcapped PEIs. The divalent salts demonstrated higher mechanical strength and melt viscosities compared to the monovalent salt and the non-ionic PEIs. Interestingly, the zinc-endcapped PEI series exhibited decreased high-shear viscosities compared to the other PEIs, lending to promising industrial applications for the zinc-endcapped branched and linear PEIs for high temperature applications. Additional engineering thermoplastics in the form of bio-based polyureas exhibited mechanical properties similar to those of non-bio-based polyureas. The isocyanate-free synthetic route incorporated an essential urea degradation mechanism at elevated temperatures to produce isocyanic acid, which then reacted with amines to produce linear polyurea thermoplastics. Urea provided a sustainable and bio-friendly reagent for high molecular weight, isocyanate-free polyureas. Poly(propylene glycol) triamine enabled the long-chain branching of thermoplastic polyureas. Differential scanning calorimetry (DSC) showed no change in Tg for the series; however, melting peaks decreased in intensity as the branching concentration increased, indicating a reduction in crystallinity. Tensile testing eluded to a decrease in ultimate stress values for higher branching concentrations, while melt rheology showed significant differences in melt viscosities. Viscosities increased markedly with an increase in branching concentration, signifying greater entanglement and stronger physical crosslinks for the branched polyureas. Further analysis of possible isocyanate-free routes led to the use of 1,1'-carbonyldiimidazole (CDI) to generate polyureas and polyurethanes. CDI, known in the literature for its use in amidation and functionalization reactions, enabled the production of well-defined and stable polyurethane monomers. The functionalization of butanediol with CDI yielded an electrophilic biscarbamate monomer, bis-carbonylimidazolide (BCI), suitable for further step-growth polymerization in the presence of amines. The reaction of this novel monomer with aliphatic diamines produced thermoplastic polyurethanes with high thermal stability, tunable glass transition temperatures based on incorporation of flexible polyether segments, and creasable thin films. It is envisioned that CDI functionalized diols will afford access to various polymeric backbones without the use of toxic isocyanate-containing strategies. Additionally, non-isocyanate polyurethane (NIPU) foams were produced from BCI monomers without the need of blowing agents, catalysts, or solvents. These materials offered an alternative to existing foaming technology, which typically employed isocyanates. Polyurethanes were foamed through a CO2 thermal decomposition mechanism involving the BCI monomers. We investigated two series of polyurethane foams with a tunable Tg range from ~0 °C to ~110 °C. We found that the incorporation of aromatic amines vastly altered the foam thermomechanical properties, and the resulting foams were closed-cell in nature. / Doctor of Philosophy / Step-growth polymers play a significant role in commercial and industrial applications. On-going work in this field focuses on sustainability, biodegradability, and improved processability. This dissertation encompasses the improvement and innovation of current and novel engineering thermoplastics and foams. The careful purification and step-growth synthetic strategies herein, afforded targeted molecular weights for analysis of linear and long-chain branched (LCB) polyetherimides (PEIs). Further analysis of LCB-PEIs, with monovalent and divalent ionic endgroups, provided an opportunity to study the effect of ionic interactions and physical crosslinks at high temperatures (>300 °C). The long branches improved the melt processability compared to linear analogues at equivalent molecular weights. The challenge to investigate polyurethanes using non-isocyanate methodologies offered an opportunity to apply fundamental small-molecule, organic synthesis to macromolecular science. 1,1'-Carbonyldiimidazole (CDI) provided a platform to generate polymeric chains from industrially relevant monomers. Additional testing serendipitously discovered the generation of CO2 upon thermal degradation of the novel monomers. Harnessing the release of CO2, during the gelation of polyurethanes, provided an isocyanate-, catalyst-, and solvent-free synthetic route towards polyurethane foams that boasts scalability and industrial relevance.