Javier Sacristan

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Duration in group: 2005 - 2007

Research Summary

The segmental dynamics on polymer blends
During my stay we studied segmental dynamics on polymer blends as a function of effective concentration leaving all other variables constant. To accomplish this objective,we investigated the relationship between the effective concentration and dynamics in poly (ethylene oxide) [PEO]/poly (methyl methacrylate) [PMMA] homopolymer and diblock copolymer blends. Molecular dynamics simulation allowed us to measure both effective concentration and self concentration, neither of which is accessible experimentally, and to monitor the resulting dynamics as a function of local volume size.

The copolymer self-assemblies
Amphiphilic molecules are characterized by the feature that they contain both water "loving" (hydrophilic) and water "hating" (hydrophobic) structural units. Familiar examples are alcohols or lipids. Amphiphiles are important for many applications in technology and nature. They are very effective at helping to dissolve different substances in water, which makes them very useful, e. g., as detergents, or as coating materials to stabilize colloidal systems. Because of the hydrophobicity of one of the blocks the amphiphiles self-assemble in such a way as to minimize the contact between the tails (hydrophobic) and the solvent. The nature of the self-assemblies, of course, depends on a number of factors such as the chemical composition of the block copolymers, the polarity of the solvent and the relative solubility of the blocks in the solvent. Super-molecular aggregates, such as micelles, vesicles, etc. are typical example of such self-assemblies.

These structures may then order on an even higher level and form superstructures. Particular interesting from an application point of view are those phases where the material is filled with bilayers. Such bilayer interfaces can serve as barriers against the diffusion of particles, and help to divide the space into compartments. Indeed, lipid bilayers are the structural basis of all biological membranes, which in turn play a central role for the function of all cells and cell organelles. It is obvious that investigation of the aggregation behavior of the diblock copolymers has both theoretical and practical importance because of their conventional and potential applications in bio-materials, optics, microelectronics, thermoplastic elastomers, pressure-sensitive adhesives, colloidal dispersants, compatibilizers of polymer blends, foams, and surface modification. From an experimental point of view, studying the properties of biomembranes on a molecular scale in situ is not an easy task. So computer simulations result very helpful to study membranes and can contribute to an improved understanding of physical phenomena on that scale.


In this project we investigate through computer molecular dynamic simulations by means of different models, united atom (UA) and coarse grain (CG) models, diblock copolymer solutions in water. Molecular dynamics (MD) simulations on fully atomistic models are not possible because the timescale for spontaneous self-assembly is too long (microseconds) and because the system sizes are too large (many hundreds of thousands of atoms) to be routinely studied using currently available computers. So, UA models are used such an intermediate step in order to obtain the parameters needed for the CG simulation. Those (CG) models are particularly suited to study generic properties of amphiphiles. In general they can be regarded as simple, minimal systems that provide general insight into the mechanisms that drive the self-assembly and the phase behavior of amphiphiles. But our method of coarse graining retains connection to atomistic detail, this is one of its main advantages. The nature of the self-assemblies, of course, depends on the temperature, concentration, etc. Super-molecular aggregates, such as micelles, vesicles, etc. are typical example of such self-assemblies.


The goal of our study is not only to understand the stability and structure of the resulting self assemblies, but also to develop methods to study other similar systems and make the connection between the characteristics of the starting macromolecules and the initial conditions on one side, and the properties of the final assembly on the other.