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Thermodynamics and dynamics ofpolymers at fluid interfaces
[Thesis]. Manchester, UK: The University of Manchester; 2016.
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Abstract
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.