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archives@tulane.edu / 1 / Yang Wang

Compared to the solid and gaseous phases, liquids are more closely related to biological processes and the life sciences. In fact, it is generally believed that abiogenesis occurred in the liquid environment of the primordial sea which, itself, was formed only when appropriate conditions came to prevail on the young Earth, providing a striking illustration of the marginal character of the liquid state, in contrast with the solid and gaseous phases of the same substances, which exist over much wider ranges of temperature and pressure: the liquid state arises from a delicate balance between packing of molecules and cohesive forces or, more formally, between entropy and energy. The importance of a full quantitative understanding of liquids is only obvious. Following research in simple liquids, a new area of complex liquids emerged for the study of systems which exhibit ow, but whose liquid-like behaviour cannot be explained by the standard one-body picture used in simple liquids, as interatomic forces are significantly different from the hard-sphere type. The term complex liquid can be interchangeably used with complex fluid or soft matter - following P.G. de Gennes. Many examples of complex liquids involve the mixing of different phases, be they fluid or not in their own right, such as solid and liquid (to make up gel or sol), liquid and gas (to make up foam or liquid aerosol), and solid and gas (to make up solid foam or solid aerosol). Under appropriate conditions, these complex liquid examples, known as colloids, will exhibit fluid-like behaviour on the macroscopic scale. Further examples of complex fluids include polymers and liquid crystals. This Thesis focuses on polymers. Specifically, it researches polymers from a theoretical and a computer simulation perspective, in particular their interaction with surfaces in such a way that they become adsorbed. Setting out with a definition, examples of polymers, and a brief discussion of the practical applications of this work, the General Introduction gives an overview of the theoretical progress in the area of polymer adsorption. This sets the context for a subsequent description of the objectives of this Thesis. The General Introduction closes with an outline of the remainder of the chapters that follow.

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Self-avoiding walks on a d-dimensional hypercubic lattice are used to model a polymer interacting with a surface. One can choose to weight the walk by the number of vertices or the number of edges on the surface and define the free energy of the polymer using equilibrium statistical mechanics. We look at the behaviour of the free energy in the limit that temperature goes to zero and also derive inequalities relating the critical points of the two weighting schemes. A combined model with weights associated with both the number of vertices and the number of edges on the surface is investigated and the properties of its phase diagram are explored. Finally, we look at Motzkin paths and partially-directed walks in the combined edge and vertex model and compare their results to the self-avoiding walk’s.

Self-avoiding walks on a d-dimensional hypercubic lattice are used to model a polymer interacting with a surface. One can choose to weight the walk by the number of vertices or the number of edges on the surface and define the free energy of the polymer using equilibrium statistical mechanics. We look at the behaviour of the free energy in the limit that temperature goes to zero and also derive inequalities relating the critical points of the two weighting schemes. A combined model with weights associated with both the number of vertices and the number of edges on the surface is investigated and the properties of its phase diagram are explored. Finally, we look at Motzkin paths and partially-directed walks in the combined edge and vertex model and compare their results to the self-avoiding walk’s.

The aim of this thesis is to study the structural and thermodynamical properties of polymers at liquid/liquid interfaces by means of multiscale molecular dynamics simulations. This thesis is presented in alternative format, and the results, consisting of three journal articles, are divided into two main parts. The first part of the thesis looks at the structural and dynamical changes as well as the thermodynamic stability of polymers of varying topology (linear and star-shaped) at interfaces by performing molecular dynamics simulations on model systems. It was found that homopolymers are attracted to the interface in both good and poor solvent conditions making them a surface active molecule, despite not being amphiphilic. In most cases changing polymer topology had only a minor effect on the desorption free energy. A noticeable dependence on polymer topology is only seen for relatively high molecular weight polymers at the interface. Examining separately the enthalpic and entropic components of the desorption free energy suggests that its largest contribution is the decrease in the interfacial free energy caused by the adsorption of the polymer at the interface. Furthermore, we propose a simple method to qualitatively predict the trend of the interfacial free energy as a function of the polymer molecular weight. In terms of the dynamics of a linear polymer, the scaling behaviour of the polymer confined between two liquids did not follow that predicted for polymers adsorbed onsolid or soft surfaces such as lipid bilayers. Additionally, the results show that in the diffusive regime the polymer behaves like in bulk solution following the Zimm model and with the hydrodynamic interactions dominating its dynamics. Further simulations carried out when the liquid interface is sandwiched between two solid walls show that when the confinement is a few times larger than the blob size the Rouse dynamics is recovered. The second part of the thesis focuses on optimizing the MARTINI coarse-grained (CG) Model, which retains certain chemical properties of molecules, to reproduce solubility of polymers, in specific polyethylene oxide (PEO), in both polar and non-polar solvents. Performing molecular dynamics simulations using this CG model will then enable us to study the properties PEO in octanol/water and hexane/water systems with increased length and timescales not accessible by atomistic simulations. The MARTINI CG method (Marrink et al., J. Phys. Chem. B, 2007, 111, 7812) is based on developing the optimal Lennard-Jones parameters to reproduce the partition free energy between water (polar solvent) and octanol (apolar solvent). Here we test the MARTINI CG method when modelling the partitioning properties of PEO, with increasing molecular weight between solvents of different polarity by comparing the results with atomistic simulation. We show that using simply the free energy of transfer from water to octanol to obtain the force parameters does not guarantee the transferability of the model to other solvents. Instead one needs to match the solvation (or hydration) free energies to ensure that the polymer has the correct polarity. We propose a simple method to select the Lennard-Jones parameter to match the solvation free energies for different beads. We also show that, even when the partition coefficient of the monomer is correct, even for modestly high molecular weight of the polymer the predicted partitioning properties could be wrong.

In order to optimize the synthesis of metal nanoparticle-polymer systems, there are certain processes which must be understood. Perhaps the most important one is the selective interfacial interaction between the block copolymer and the growing metal nanoparticles. To investigate this interaction, four different approaches were taken. The first approach looked at the strength of interaction between the competing blocks of the copolymer and the metal nanoparticles surface. The second approach looked at the effect of polymer architecture on the metal nanoclusters. The third approach looked at the polymer composition and solvent effects on the phase behavior of the metal nanocluster-block copolymer nanocomposite. Finally, the influence of the metal precursor on the rate of the decomposition was examined. It was found that adsorbed layers of PS on the cobalt nanoparticles are completely displaced by PMMA when the solvent is a common good solvent. An adsorbed layer of only PMMA is also obtained through competitive adsorption from a common good solvent. However, in a selective solvent that is poor for PS, sequential adsorption leads to the formation of mixed layers. In homopolymer solutions, the cluster size reaches a minimum at a finite chain MW. In the case of diblock copolymers, the only parameter (for a fixed copolymer concentration) controlling the cluster size in suspensions of di-block copolymers is the molecular weight of one block, in this case PMMA, and is indifferent to other parameters including the molecular weight of the other block (PS) or the solvent quality. It was also found that the spatial distribution of the metal clusters synthesized in-situ coincided with the morphology dictated by thermodynamically-driven microdomain structure of the block copolymer. Moreover, the overall final morphology of the nanocomposite is locked into place while in solution, and fast solvent evaporation does not cause this morphology to change. Finally, results showed that the rate of nanocomposite synthesis occurred faster in the PS suspensions compared to PMMA, indicating that chemical bonds between PMMA and the cobalt nanoclusters slowed the thermal decomposition of the metal precursor. So the PMMA chains provided sites for nucleation, but did not necessarily aid particle growth.

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