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.