(742a) Self-Assembly of Pluronic Molecules in Water: A Simulation Study with Dissipative Particle Dynamics

In this work we simulate the self-assembly of Pluronic molecules in water. Pluronic is a tri-block co-polymer constituted by a central block of polypropilene oxide and two later blocks of polyethylene oxide. Simulations are performed with Dissipative Particle Dynamics (DPD), a coarse-grained technique based on the simple principle of grouping together atoms of a molecules, or entire molecules, into beads. These beads follow Langevin dynamics due to three main forces: dissipative, conservative and stochastic. Dissipative and stochastic forces are reconciled, thanks to the fluctuation dissipation theorem, to guarantee momentum conservation. The LAMMPS implementation of DPD is employed in this work. Two different types of Pluronic in water are considered: L64 and P103.

DPD is used to identify the peculiar self-assembled micro-structures that form when the concentration of Pluronic in water varies from dilute to dense. Experimental phase diagrams (measured with SAXS and rheology experiments) are used to verify that DPD simulations are capable of detecting the self-assembly of spherical and rod-like micelles and low Pluronic concentrations in water, of hexagonal and lamellar phases at intermediate concentrations and of reverse micelles at high concentrations.

A novel clustering algorithm is also used to identify the micro-structures formed and to characterize them in terms of radius of gyration, aggregation number and cluster mass distributions. The clustering algorithm is also used to determine the cluster mass distributions and to calculate the resulting chemical potentials associated with the different microstructures. The chemical potentials are in turn used to extract the critical micellar concentration and important shape factors. Comparison of model predictions with experimental data from the literature results in decent agreement.

DPD simulations are performed also when shear is applied in order to predict how the self-assembled micro-structures are affected by shear and how self-assembly is affected by shear. The analysis is conducted both from the qualitative and quantitative point of views. Besides understanding the effect of shear on the micro-structures, DPD is also used to estimate the rheological behavior of the resulting structured fluid (constituted by the mixture of Pluronic and water molecules). Results on this second part of the work show that non-Newtonian behaviors can be predicted by DPD and associated with variations of the observed microstructures. Current work is focusing on the fine tuning of the DPD model in order to predict the actual measured values of the predicted apparent viscosity.