Thesis (Ph. D.)--University of Michigan, 1947. / Caption title. Reprint: Chemical engineering progress. Vol. 44, no. 1 (Jan. 1948).

Th. 3e cycle--Chim. struct.--Besançon, 1979. N°: 324.

Three experiments are discussed. Each experiment uses surface plasmon resonance (SPR) spectroscopy to examine the adsorption of organic molecules from solution onto a solid substrate. A description of experimental techniques and materials is included. An extensive background discussion precedes the description of each experiment. For the first experiment, SPR and x-ray photoelectron spectroscopies are used to determine the amount of a polyelectrolyte adsorbed to a surface with a pre-selected charge density (1). The observed behavior indicates that the Coulomb attraction between the surface charges and the polymer charges is the chief interaction controlling the adsorption for high surface charge densities, i.e., charge densities which correspond to average charge spacings less than the charge separation between the polyelectrolyte segments. When the average distance between adjacent surface charges exceeds the length of a fully extended polymer segment, the polymer layer appears to maintain a constant density. A clear shift in the adsorption occurs when the average charge separation distance on the substrate equals the intercharge separation for the polyelectrolyte. The second experiment uses SPR spectroscopy to monitor the in situ adsorption kinetics of an antibody protein from aqueous solution onto an antigen-coated surface (2). We attribute the shape of the adsorption isotherm as indicative of an evolution in the orientation of the adsorbed antibodies. The kinetics data are interpreted in terms of random sequential adsorption. The third experiment describes efforts to obtain an image of domain growth during a phase transition in a monolayer of amphiphilic molecules using surface plasmon resonance microscopy. The feasibility of using SPR microscopy is demonstrated. It is shown that the polarizability of the molecules in the monolayer may be higher than the polarizability of those in solution, providing evidence that the transition is occurring. The results suggest that the phase segregation, if it occurs, possesses length scales shorter than the resolution afforded by SPR microscopy.

Modern theories of polymer solutions are highly successful in describing the physical properties of polymers. However, they encounter enormous difficulty when the system involves long range interactions, such as Coulomb and hydrodynamic interactions. Complexity usually arises from coupling between these interactions and the long polymer chain. In this thesis, I illustrate aspects of these complicated systems by using computer modeling techniques. For charged systems, I have been mainly interested in two physical aspects, the so-called counterion condensation and the conformational properties of polyions. Specifically, we studied the distribution of counterions around the polyions and the conformation as a function of Coulomb strength, chain length, monomer density and salt concentration. We found that condensed ions are arranged in such a way that neighboring dipoles are attractive. The counterion condensation rates are deviated from the Manning's condensation theory. The total charge fraction of polyions decrease as salt concentrations increase. We simulated polyion size and compared values to those of modern conformation theories, in particular, Muthukumar's theories. While we found, that theory can describe polyion size for most of cases, it fails when multivalent salts are added to the system. The failure of theory follows the neglect of the role of individual ions by mean field theory and a need to describe the effect of multivalent ions. We propose a simple modification of the original theory and obtain satisfactory results. For the systems with hydrodynamic interactions, we are particularly interested in the simple shear and extensional flow. We simulated in Ottinger's hydrodynamics approximation a bead-rod chain under the extensional flow and observed the possible existence of a first order transition. The critical strain rate for the transition scales as a power law in chain length with an exponent of −1.4, in good agreement with experimental results.

The proposed dissertation research project will examine the behavior of polyelectrolytes in the presence of oppositely charged surfactants. When polyelectrolytes and surfactants of opposite charge are mixed at a stoichiometric charge ratio, they self-assemble into highly ordered complexes. These types of assembly processes have been well documented in the past, but a great deal still remains to be understood. The proposed research will specifically be related to the examination of the thermodynamics associated with polymer/surfactant interactions, the morphological sensitivity to surfactant concentration, and the overall effect of ionic strength on aggregation. We have chosen to study a number of polyanions, of varying hydrophobic character and charge density to determine how they will interact with the cationic surfactant, cetyltrimethylammonium chloride (CTAC). Structural evaluation of complexes will be accomplished using small-angle neutron scattering (SANS) to examine soluble complexes, while insoluble precipitates will be examined through small-angle X-ray scattering (SAXS). Thermodynamic investigations will be carried out through the use of isothermal titration microcalorimetry in conjunction with potentiometry. The enthalpic data is of much greater value when combined with measured changes in surfactant chemical potential during the binding process. The change in chemical potential can be used to calculate the total change in free energy for the surfactant during the binding process. Combining this information with the free enthalpy change will provide the means necessary to describe the full thermodynamic profile of polyelectrolyte/surfactant binding. We will achieve these measurements through the use of a surfactant selective electrode that directly allows for measurement of the free surfactant concentration. Both the thermodynamics of binding and the structural evolution will be greatly influenced by the concentration of small-molecule electrolytes present in the solutions. By varying the ionic strength during these experiments, we would also like to be able to quantify the contribution made by counter-ion release to the binding process. In a similar fashion, the effect of polyelectrolyte charge density will also be examined, to address the effect of localized charge density on the driving force for surfactant binding.

The kinetics of phase separation of sodium Poly(styrene sulfonate) (NaPSS) in water with barium chloride (BaCl2) was studied by static and dynamic light scattering. This polyelectrolyte system shows an upper critical solution temperature behavior, such that the solutions are homogeneious at temperatures above the phase boundary. Using dynamic light scattering, two diffusive modes were detected, and unaggregated polyelectrolyte chains and aggregates coexist in the homogeneous phase. The hydrodynamic radius (RH) of the unaggregated chains is of the order of 1 to 10 nm depending on the molecular weight of NaPSS, while RH of aggregates is of the order of 100 nm independent of the molecular weight of NaPSS. Unaggregated chains follow good solution behavior with a fractal dimension of 5/3. On the other hand, the fractal dimension of aggregates is larger than 3.5. The value of the fractal dimension suggests that the structure of aggregate is similar to branched polymers, such as star polymers. Upon rapidly cooling the NaPSS sample below the phase boundary, phase separation takes place. During the initial stage, unaggregated chains were converted to aggregates. Newly generated aggregates have a similar fractal dimension and size to aggregates found above the phase boundary. The size of aggregates remained constant while the number of aggregates increased. During the initial stage, nucleation time is sensitive to quench depth and salt concentration. Either deepening a quench depth or increasing BaCl 2 concentration shortened the nucleation time. After the nucleation time, the size of aggregates grew linearly with time. This marks the growth period, at which time the growth rate is higher for deeper quench depths and higher BaCl2 concentrations. The mechanism of phase separation of NaPSS in water and BaCl2 is similar to the nucleation and growth mechanism where the aggregate serves as a nucleus. However, it is different from conventional nucleation and growth theory in that the nucleus already exists above the phase boundary.

The use of electric fields to control the orientation of copolymer microstructures in a thin film geometry is demonstrated. Copolymers of polystyrene-block-poly(methyl methacrylate), P(S-b-MMA), and polystyrene-block polyisoprene, P(S-b-I), were prepared by living anionic techniques for use in this study. An emphasis was given on scattering and reflectivity techniques to elucidate information regarding the mechanism of alignment. Alignment is understood, for all copolymer systems, in terms of a simple dielectric body argument whose energy is dependent on a dielectric mismatch parameter. The observed pathway along which the alignment is realized depends on the initial state of the sample. Vastly different behavior is observed for thick films, where the effect of the substrate can be effectively ignored, and thin films, where the surface interactions dominate. In thick films, the alignment mechanism was followed via in-situ small angle scattering (SAXS) experiments and proceeds via disruption of the grains of the copolymer microdomains with a subsequent rotation of the grains along the field direction. Observed similarities in alignment of diblock and triblock copolymers, further strengthen the grain rotation argument. If alignment occurred by an enhancement of interfacial fluctuations or a mechanism requiring a molecular response, distinct differences in the alignment mechanism between the di- and triblock copolymers would be evident. Additionally, it was observed that if a copolymer system is allowed to relax between applied fields, the resulting final alignment was found to be better than the alignment achieved under a constant applied field. This indicates that, under a constant applied field, the copolymer is trapped in a non-equilibrium state that can be overcome by allowing a partial relaxation of the system. In thin films, a direct competition between the interfacial interactions and the electric field force is observed. Complete reorientation occurs only when the applied field overcomes the interfacial interactions. This occurred at a well-defined threshold electric field strength, Et, and was observed to be independent of the film thickness over a range of thicknesses. In these thin films, the mechanism of alignment must proceed through a fluctuation pathway where the applied electric field amplifies the fluctuations until alignment is achieved. Preliminary neutron reflectivity data shows a large disruption of the surface-induced parallel orientation by the electric field that is directly related to the electric field strength, the interfacial interactions, as well as the copolymer initial state. This is consistent with an electric-field induced fluctuation disrupting the surface-induced lamellar stacking.

The effect of hydrostatic pressure on the lower disorder-to-order transition (LDOT) in poly(d-styrene-block-n-butyl methacrylate) having symmetric and asymmetric block lengths was investigated by in situ small-angle neutron scattering (SANS). Currently, linear diblock copolymers having styrenic and methacrylic monomers are the only systems that display a thermally accessible phase transition from the disordered homogeneous melt to the ordered microphase-separated state upon heating. The location of this phase transition was mapped as a function of temperature and pressure by analyzing one dimensional SANS intensities, where discontinuities in the width and height of the scattering peak indicated the traversal of the transition isothermally or isobarically. The T-P phase diagram of p(d-S-b-nBMA) built using this method shows an expansion of the disordered, homogenous region with increasing pressure. For the diblock copolymer with a lamellar morphology and Mw = 8.5 × 104, the slope of the phase transition line was ∼150 K/kbar, and was approximately linear over a range of 1 kbar. Increasing the molecular weight of p(d-S- b-nBMA) resulted in a vertical shift of the phase transition line in the phase diagram and no detectable change in slope. Similar effects were observed for diblock copolymers with a cylindrical morphology. The bulk enthalpy and volume changes at the phase transition, which dictate the pressure coefficient for a one component system, were either at the limit of experimental resolution or unobservable due to kinetic factors. In-situ X-ray reflectivity experiments, however, showed a significantly reduced thermal expansion coefficient in the disordered phase and a discontinuous increase in film thickness as a function of temperature at the bulk LDOT. The pressure coefficient, dTLDOT/dP, for these materials is greater by a factor of five than currently observed in diblock copolymers with conventional UODT phase behavior, i.e. ordering upon cooling . This dramatic phase behavior allows rapid and convenient access to the order-disorder phase transition, isothermally, and suggests that LDOT block copolymers could be employed in blend and multi-component systems as minor components to impart pressure-induced compatibilization, surface activity, or flow properties.

We present the speedup from a novel parallel implementation of the multicanonical method on the example of a lattice gas in two and three dimensions. In this approach, all cores perform independent equilibrium runs with identical weights, collecting their sampled histograms after each iteration in order to estimate consecutive weights. The weights are then redistributed to all cores. These steps are repeated until the weights are converged. This procedure benefits from a minimum of communication while distributing the necessary amount of statistics efficiently. Using this method allows us to study a broad temperature range for a variety of large and complex systems. Here, a gas is modeled as particles on the lattice, which interact only with their nearest neighbors. For a fixed density this model is equivalent to the Ising model with fixed magnetization. We compare our results to an analytic prediction for equilibrium droplet formation, confirming that a single macroscopic droplet forms only above a critical density.

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