Simulation Of Polymer Nanoparticle Formation

A computational chemistry project has been initiated to parallel the new experimental technique recently developed by us at ORNL for creating very fine polymer particles of arbitrary composition and size. We have used molecular dynamics (MD) simulations of nano-scale polymer particles to gain insight into the properties and behavior of ultra fine polymer powders. A computational procedure to model polymer particles has been developed to generate initial particles that are as similar as possible to the experimentally created polymer particles. Using an efficient computational method to create initial conditions, we have modeled the nano-sized particles generated with up to 300,000 atoms under solvent free and vacuum conditions. Figure 1 illustrates the formation of stable polymer nano-particle from the collapsing chains when the solvent evaporates. The solvent molecules are not shown as they escape from the liquid drop so that the condensation of polymer chains can be visualized. We have simulated a variety of different polymers with different chain lengths and functionalities to investigate effects of structure and various statistical properties. Thermodynamic analyses have been performed to obtain melting point, glass transition temperature, and heat capacity for the particles by calculating molecular volume and total energy as a function of temperature and particle size. These analyses show that there is a surface effect, dramatically dependent on the size of the particles, and leading to such effects as greatly increased reactivity and modified spectra. Our simulations also predict an interesting reduction of the melting point and glass-transition temperature in comparison with the bulk system.

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