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Attempts on Accessing Larger Spatial and Temporal Scales in Polymer Simulations

Author: ZhuYouLiang
Tutor: LvZhongYuan
School: Jilin University
Course: Physical and chemical
Keywords: Molecular dynamics GALAMOST Coarse-graining Harmonic potential
CLC: O631
Type: PhD thesis
Year: 2013
Downloads: 58
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Abstract


In the field of polymer simulation, an outstanding issue is that regular moleculardynamics simulation can not reach the length and time scales at which somephenomena happen. Thereby, we are in an effort to increase the length and time scalesof molecular simulation in two aspects: by developing an efficient molecularsimulation package GALAMOST which fully takes the computational power of GPUto increase the high performance computational capability in simulation; meanwhiledeveloping coarse-graining method, even a highly coarse-grained model in which achain is taken as a particle, to enlarge the simulated length and time scales. The maincontents are as follow:(1) GALAMOST is a molecular simulation package designed to utilize thecomputational power of GPUs. To accelerate simulations, GALAMOST contains ahybrid particle-field MD technique where particle–particle interactions are replacedby interactions of particles with density fields. Moreover, the numerical potentialobtained by bottom-up coarse-graining methods can be implemented in simulationswith GALAMOST. In addition, GALAMOST encompasses two specific models, thatis, a soft anisotropic particle model and a chain-growth polymerization model, bywhich the hierarchical self-assembly of soft anisotropic particles and the problemsrelated to polymerization can be studied, respectively.(2) We introduce a highly coarse-grained model to simulate the entangled polymer melts. In this model, a polymer chain is taken as a single coarse-grainedparticle, and the creation and annihilation of entanglements are regarded as stochasticevents in proper time intervals according to certain rules and possibilities. We buildthe relationship between the probability of appearance of an entanglement betweenany pair of neighboring chains at a given time interval and the rate of variation ofentanglements which describes the concurrence of birth and death of entanglements.The probability of disappearance of entanglements is tuned to keep the totalentanglement number around the target value. This useful model can reflect manycharacteristics of entanglements and macroscopic properties of polymer melts. As anillustration, we apply this model to simulate the polyethylene melt of C1000H2002at450K and further validate this model by comparing to experimental data and othersimulation results.(3) We construct the phase diagram of spherical particles interacted withharmonic repulsions, which are ultrasoft and bounded at fully overlapping. Thissimple potential form can be used for describing the thermodynamic properties anddynamic behavior of interpenetrable globular micelles, microgels, starlike polymersolutions, and so on. Using dissipative particle dynamics simulations combined withthermodynamic integration, we compute chemical potentials of fluid phase and anumber of crystal structures. In addition to the face-centered cubic and body-centeredcubic structures, we also find the tetragonal, hexagonal, orthorhombic, and diamondcrystal structures stable for this system. In the phase diagram, we identify multiplere-entrant melting regions and polymorphic transitions between the crystals.

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CLC: > Mathematical sciences and chemical > Chemistry > Polymer chemistry ( polymer ) > Polymer physics and physical chemistry of polymers
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