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SUMMARY:Stabilizing ordered bicontinuous phases in diblock copolymer syste
 ms - Dr Francisco Martinez (CUC3)
DTSTART:20091106T120000Z
DTEND:20091106T130000Z
UID:TALK20027@talks.cam.ac.uk
CONTACT:Dr Mark Miller
DESCRIPTION:A combination of coarse-grained particle-based molecular simul
 ations and self-consistent field theory (SCFT) were used to detect and cir
 cumvent the packing frustration responsible for the very limited stability
  of the bicontinuous phases in diblock copolymer (DBC) systems. Such bicon
 tinuous phases have promising applications in many technological areas lik
 e: solar-cells and photonic crystals. Lattice and continuum Monte Carlo si
 mulations together with Molecular Dynamics  were used to map out the phase
  diagram of the pure DBC melt. The morphologies observed were found to be 
 dependent upon the size of the simulation box because of severe finite-siz
 e effects caused by the long-range periodicity of this type of systems. Th
 us\, accurate free energy calculations\, achievable only through Expanded 
 Ensemble methods\, were needed to discern the stability of the competing m
 esophases at each thermodynamic condition. The gyroid (G) phase was found 
 to be stable in a narrow region of phase diagram. Direct evidence of packi
 ng frustration in the form of chain-stretching was found in the G phase no
 des for all the models studied. Chain-length bidispersity was then investi
 gated as a means to reduce such packing frustration\; it was found that th
 e preferential segregation of the longer chains inside the G phase nodes c
 aused an increase in the range of temperatures where the G phase was stabl
 e. As an alternative strategy to diminish packing frustration\, addition o
 f minority-component homopolymer\, was studied at the conditions were the 
 G phase was found stable in the pure DBC system. The progression of morpho
 logies G  → C → Double Diamond (DD) → Plumber’s Nightmare (P) was 
 observed upon increasing homopolymer content. The homopolymer concentrated
  in the nodes of the DD and P phases to reduce the packing frustration. Th
 ough tentative phase boundaries were delineated via free energy calculatio
 ns\, macrophase separation could not be satisfactorily assessed within the
  framework of particle-based simulations. Thus\, SCFT was used to explore 
 in more detail the DBC/homopolymer phase diagram\, showing that\, although
  in many cases two-phase coexistence of a DBC-rich phase and a homopolymer
 -rich phase precedes the stability of complex bicontinuous phases\, the DD
  and P phases can indeed be stable in some regions of the phase diagram.
LOCATION:Pfizer Lecture Theatre\, Department of Chemistry
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