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CATEGORIES:Extra Theoretical Chemistry Seminars
SUMMARY:Stabilizing ordered bicontinuous phases in diblock
  copolymer systems - Dr Francisco Martinez (CUC3)
DTSTART;TZID=Europe/London:20091106T120000
DTEND;TZID=Europe/London:20091106T130000
UID:TALK20027AThttp://talks.cam.ac.uk
URL:http://talks.cam.ac.uk/talk/index/20027
DESCRIPTION:A combination of coarse-grained particle-based mol
 ecular simulations and self-consistent field theor
 y (SCFT) were used to detect and circumvent the pa
 cking frustration responsible for the very limited
  stability of the bicontinuous phases in diblock c
 opolymer (DBC) systems. Such bicontinuous phases h
 ave promising applications in many technological a
 reas like: solar-cells and photonic crystals. Latt
 ice and continuum Monte Carlo simulations together
  with Molecular Dynamics  were used to map out the
  phase diagram of the pure DBC melt. The morpholog
 ies observed were found to be dependent upon the s
 ize of the simulation box because of severe finite
 -size effects caused by the long-range periodicity
  of this type of systems. Thus\, accurate free ene
 rgy calculations\, achievable only through Expande
 d Ensemble methods\, were needed to discern the st
 ability of the competing mesophases at each thermo
 dynamic condition. The gyroid (G) phase was found 
 to be stable in a narrow region of phase diagram. 
 Direct evidence of packing frustration in the form
  of chain-stretching was found in the G phase node
 s for all the models studied. Chain-length bidispe
 rsity was then investigated as a means to reduce s
 uch packing frustration\; it was found that the pr
 eferential segregation of the longer chains inside
  the G phase nodes caused an increase in the range
  of temperatures where the G phase was stable. As 
 an alternative strategy to diminish packing frustr
 ation\, addition of minority-component homopolymer
 \, was studied at the conditions were the G phase 
 was found stable in the pure DBC system. The progr
 ession of morphologies G  → C → Double Diamond (DD
 ) → Plumber’s Nightmare (P) was observed upon incr
 easing homopolymer content. The homopolymer concen
 trated in the nodes of the DD and P phases to redu
 ce the packing frustration. Though tentative phase
  boundaries were delineated via free energy calcul
 ations\, macrophase separation could not be satisf
 actorily assessed within the framework of particle
 -based simulations. Thus\, SCFT was used to explor
 e 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 bico
 ntinuous phases\, the DD and P phases can indeed b
 e stable in some regions of the phase diagram.
LOCATION:Pfizer Lecture Theatre\, Department of Chemistry
CONTACT:Dr Mark Miller
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