|Severin, , Nikolai: Molecular Dynamics Simulations of Polymers and Micelles at Interfaces |
The key-parts of the work are:
(1) The starting structures of the interfaces were chosen close to those suggested in the experiments on the epitaxial crystallisation of PE on iPP and several reference interfaces. While it is possible in the experiment to establish the orientation of the crystallographic axes of PE and iPP at the interface, it is not possible to establish whether PE crystallises preferably on the iPP surface with high or low density of methyl groups. In comparing adhesion energies computed from MD simulation of different interfaces it was found that the lowest adhesion energy corresponds to the interface with the low concentration of methyl groups on the iPP surface. Hence it was suggested that PE crystallises on the iPP surface with low density of methyl groups.
It was also found that the entire layer of orthorhombic crystalline PE in the simulated system with the strongest adhesion has transformed to the monoclinic form. This phenomena was critically discussed. It was argued that while the transformation from the monoclinic to the orthorhombic form could be due to some general shortcoming of the MD simulation, it is also possible that indeed the thin layer of PE close to the iPP surface transformed to monoclinic form, while bulk PE is still in the orthorhombic crystalline form.
(2) The purpose of the simulation of the cylindrical micelle in water was first to check the reliability of the simulation because there are many experimental results for the cylindrical micelles. Second, up to now there was not any detailed computer simulation of large spherical or cylindrical micelles. The advantage of the simulation of the cylindrical micelle over the simulation of the spherical micelle is that applying periodic boundary conditions allows to simulate virtually infinite cylindrical micelles by simulation of a rather small part of the cylindrical micelle, while for a spherical micelle the entire micelle should be simulated.
A good agreement between experimental results and the MD simulated cylindrical micelle was found. Nevertheless the MD simulated cylindrical micelle does not exhibit a homogeneous micelle core density, while the commonly accepted point of view is a homogeneous liquid like micelle core. It was argued that the micelle core could be considered just in a first approach as homogeneous. The order in the micelle core is induced by the fact that the head groups sit on the micelle surface. The non-uniform distribution of the terminal methyl groups produces the non-homogeneous density of the micelle core.
(3) Half cylindrical micelles formed by TAB molecules on the gold and paraffin substrates were simulated. It was found that the gold substrate induces much higher order in the half cylindrical micelle than the paraffin substrate.
Also a cylindrical micelle formed by TAB molecules on a gold substrate was simulated. It was found that the cylindrical micelle started to flatten on the gold substrate and the alkyl chains tended to orient perpendicular to the substrate plane. The flattening and the reorientation of the alkyl chains was compared favourably to experimentally established flattening of the structures formed by TAB molecules on silica.
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