The rheological behavior of polymers at the gel point (critical gels) is characterized by a power law distribution of relaxation times, large normal stress differences, and physical rupture of molecular network strands. This is demonstrated in a series of well-defined experiments conducted on a model network polymer system consisting of a linear, telechelic, vinyl-terminated polydimethylsiloxane (PDMS) and a four-functional siloxane crosslinker. Stable samples of this model polymer can be prepared at different extents of reaction in the vicinity of the gel point (GP), thereby separating the effects of reaction and flow. The linear viscoelastic constitutive equation (Gel Equation) has been extended to include the effect of finite strains, and has been found to describe the rheological behavior of critical gels extremely well. Large strains have been found to disrupt the network structure of the crosslinking polymer, and introduce a mechanical delay to gelation. As a result, a stable sample can be reduced from a viscoelastic solid to a critical gel, or even to a viscoelastic liquid, depending on the magnitude of the shear strain. This mechanical effect also shifts the value of the relaxation exponent, n, at the gel point towards lower values. The material properties can therefore be controlled by both the chemistry and the processing, to suit the application.

The flow-induced birefringence of dilute polymer solutions in periodically constricted tubes and converging-diverging channels has been employed to study the dynamics of polymer chains under transient stretching stresses. Under these conditions, flexible and semi-flexible systems exhibit memories on time scales much greater than normally displayed at equilibrium. The dynamic behavior of polymer chains determines the rheological properties of polymeric solutions. A correlation has been found between molecular deformation and the excess pressure drop effect observed in porous media flows. In the case of flexible chains, the presence of centerline birefringence (which is associated with purely elongational flow) coincides with the sudden increase in pressure drop of the polymeric solution. The influence of flow on the adsorption and desorption of high molecular weight poly(styrene) on chromium has also been studied, by ellipsometry, for two solution conditions: in decalin at 22$\sp\circ$C (a slightly good solvent), and in cyclohexane at 35$\sp\circ$C (a theta solvent). Measurements of layer thickness after collapse induced by solvent removal provide adsorption levels (mass/area) in both shear (0 $<$ $\dot\gamma$ $<$ 1000 s$\sp{-1}$) and biaxial elongation (0 $<$ $\dot\varepsilon$ $<$ 25,000 s$\sp{-1}$). With decalin, the different flow environments have little influence on either adsorption or desorption processes; with cyclohexane, a greater impact is noted, although only in adsorption. It appears that flow only affects the adsorption level of a weakly attached subfraction of the bound chain population.

Block copolymers comprised of incompatible blocks form ordered microphase separated domains. The domains orient within a microscopic granular structure. But, the director of the grains varies randomly from grain to grain, giving a macroscopically isotropic structure. However, application of flow orients the domains, producing a macroscopically anisotropic structure that dictates the final properties. The effects of flow on domain orientation and the resulting anisotropic properties of triblock copolymers were investigated. Domain orientation effects were best seen in the most ideal structure where all the domains are aligned uniformly throughout the sample, known as 'single crystal' structure. Appropriate deformation conditions to produce single crystal in microphase separated triblock copolymers were developed. The flow induced morphologies were studied by transmission electron microscopy (TEM) and small angle x-ray scattering (SAXS). The linear viscoelastic properties of the anisotropic morphologies were investigated as a function of domain orientation with small amplitude oscillatory shear flow. Two triblock copolymer materials were studied: polystyrene-polybutadiene-polystyrene (SBS81) and polystyrene-polyisoprene-polystyrene (SIS56). Both materials have an equilibrium morphology of hexagonally packed cylinders of polystyrene in a rubbery matrix. However, the phase behavior of these two materials is quite different. SBS81 remains phase separated at all accessible temperatures while SIS56 has an accessible homogeneous phase. SBS81 samples were subjected to planar extension to produce single crystal structure. Optimum extension criteria were defined by following domain orientation as a function of extension conditions. Development of a novel sample preparation technique allowed linear viscoelastic property measurements with respect to the domain orientation. Dynamic shear moduli revealed the flow mechanism for domain orientation is dependent on the domain orientation direction. Large strain behavior of single crystal structure in combination with SAXS provided information about the flow processes during domain orientation. The second material studied has an accessible homogeneous phase, where the transition to the homogeneous phase was characterized with rheology and SAXS. These two experimental techniques were in good agreement. The ordering kinetics of SIS56 were exploited to produce single crystal structure with large amplitude oscillatory shear during phase separation. The linear viscoelastic properties of SIS56 single crystal were found to be influenced by domain orientation.

The goal of this thesis is to quantitatively study the relationship between interchange reaction and blend phase behavior in polyester/bisphenol-A-polycarbonate (PC) blends. Before transreaction studies are conducted, equilibrium phase behavior prior to reaction is identified and used as a basis to judge the reacting blends. To facilitate all of these studies, poly(2-ethyl-2-methyl-1,3-propylene terephthalate) (PEMPT) has been selected as a model polyester for blends with PC. PEMPTs having M$\sb{\rm n}$s from 4,100-37,500 g/mol were synthesized and fully characterized by a variety of elemental, spectroscopic and thermal analysis techniques. In addition, PEMPT was end capped to produce samples having heptafluorobutyrate (HFB) and benzylate (BNZ) end groups. These end capped samples exhibit different thermal characteristics compared to the hydroxyl (OH) terminated PEMPTs. Blends of PEMPT with PCs having M$\sb{\rm n}$s of 11,000 and 21,000 g/mol were prepared by solution casting techniques. Equilibrium phase behavior studies were conducted as a function of blend composition, molecular weight and end group type. In most cases, PEMPT/PC blends exhibited partial miscibility, which was monitored by the shifting of glass transition temperatures as measured by DSC. Blend composition had little effect on the phase behavior. Molecular weight reduction lead to improved intermixing between components for the OH and BNZ terminated PEMPTs. No improvement in the degree of intermixing was observed with decreasing M$\sb{\rm n}$ in the HFB terminated PEMPT/PC blends. Detailed $\sp1$H NMR spectroscopy enabled the identification of separate resonances corresponding to direct midchain and alcoholysis interchange reactions in PEMPT/PC non-stabilized blends. It was found that transreaction of $\sim$4% of the terephthalate groups was required to shift the phase behavior from two phase to single phase in a PEMPT/PC 50/50 wt. % blend of high molecular weight. This reaction extent represented 2.8% alcoholysis and 1.2% direct midchain reaction. These results were in agreement with calculations based on simplified models of the reacting blend. Alcoholysis exchange reaction was also observed at varying degrees in stabilized blends. Complete alcoholysis led to the formation of single phase blends at low PEMPT molecular weights.