Multiscale Methods: Polymeric and Polyelectrolytic Materials

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Multiscale Methods Polymeric and
Polyelectrolytic Materials

• Erik Luijten
• University of Illinois at Urbana-Champaign

Materials Computation Center External Advisory
Board Meeting November 7, 2005
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Overview

• Goal Advance understanding of polymeric
  materials and biomaterials by synergy between
  experiment simulationdevelop new methods
  where necessary
• Techniques
• Classical MD of coarse-grained models
• Classical MC, where helpful to overcome slow
  dynamics(cluster methods for size-asymmetric
  mixtures GCMC for coexistence)
• Main projects pursued within framework of MCC
• Single-molecule translocation through membranes
• Dynamics of polymers near surfaces
• Self assembly and gelation of block copolymers
• Cross-interactions with other projects
• Electrostatically-driven self assembly
• Polyelectrolyteprotein complexation
• Counterion-mediated attraction between
  polyelectrolytes
• Colloidal self assembly

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Illustration Gelation

• Self-assembly methods are widely advertised and
  used to create new materials
• Employ simulations to better understand dynamics
  in self-assembly process as well as resulting
  structures
• Example triblock copolymers

Petka et al., Science.
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Structure
Cluster formation coincides with
percolation transition
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Dynamics

• Gelation shows a strong increase in viscosity,
  but not the corresponding decrease in the
  diffusion coefficient? Breakdown of SE
• This is common in systems with dynamic
  heterogeneities,confirming the strong
  resemblance between gelation and glass formers
• Importance Studies of self-assembly processes
  frequently rely on single-particle tracking
  methods!

Viscosity follows from stress autocorrelation
function
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Polymer translocation
Polymer translocation through a nanopore studied
viaLangevin dynamics. We employ same techniques
for surface diffusion of adsorbed polymers

• Instantaneous velocity
• Various theories
• No simulations
• Not accessible in expt.
• Probe entropic barrier as
• a function of chain length

Chain departs fromequilibrium shape,in
contradiction withassumptions in mosttheories
fornonequilibriumtranslocation

• Explore effects of
• Polymerpore interactions
• Chain stiffness
• Ion flow within pore
• Heterogeneity(e.g., block copolymers)
• Hydrodynamics

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Polarization charge calculation
Combine with molecular dynamics calculation
within LAMMPS In principle applicable to any
sharp interface between dielectric regions
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Summary Impact Relevance

• Gelation
• Understand creation of hydrogels through
  self-assembly
• Understand effect of dynamic heterogeneities in
  self-assembly processes on tracer diffusion ?
  directly affects experiments!
• Polymer translocation
• Understand driven diffusion through
  membranes(biological processes, DNA sequencing)
• In progress Self-consistent calculation of
  polarization charge at interfaces for systems
  with a discontinuous dielectric
  constant(currently being incorporated into
  LAMMPS by L. Guo)? impact far beyond
  translocation of polyelectrolytes
• Cross-interactions with electrostatic
  self-assembly
• Biological problems (DNA condensation),
  biomaterials (hierarchical structures bound by
  multivalent ions)