[Todos] Coloquios DF - Jueves 27/10 SUSPENDIDO

[Augusto Roncaglia]

Estimados, les comunicamos que se suspende el coloquio del Jueves 27/10.

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COLOQUIOS DEL DEPARTAMENTO DE F�SICA FCEYN - UBA


              En el Aula Seminario, 2do piso, Pab. I,

              Jueves 27/10, 14hs:

              MARCUS MULLER

              University of Göttingen


*Defect motion and annihilation in DSA of lamella-forming copolymers*

Applications of DSA require defect densities of less than one defect per
100 cm2. Since the excess free energy of a defect exceeds the thermal
energy scale by far, the equilibrium defect density is vanishingly low, and
defects observed in experiments and simulation kinetically arise in the
course of structure formation. The kinetics of structure formation from an
initially disordered structure can be divided into two stages: First, the
topographic or chemical guiding patterns direct the spontaneous, spinodal
microphase separation into structures that are orientated and registered
with respect to the guiding pattern but riddled with defects. Subsequently,
these defects move in response to long-range strain fields, collide, and
may annihilate upon collision. The thermodynamics of dislocations in thin
films of lamella-forming diblock copolymers, their glide and climb motions,
and the mechanisms of defect annihilation are investigated using
Single-Chain-in-Mean-Field (SCMF) simulations and self-consistent field
theory (SCFT) in conjunction with the string method. The glide motion of a
prototypical dislocation defect perpendicular to the stripe pattern is
characterized by large free-energy barriers. These barriers not only stem
from altering the domain topology; an additional barrier arises from a
small-amplitude but long-range domain displacement. In contrast, the climb
motion along the stripes does not involve any free-energy barrier. Thus the
perpendicular distance (“impact parameter�) between a pair of defects is
approximately conserved. Dislocation pairs with opposite Burgers vectors
attract each other and move towards each other (“collide�) via climb
motion. We find that the forces between apposing defects significantly
depend on the dimension of the guiding patterns. Moreover, we observe in
SCMF simulations that the defect annihilation time qualitatively and
non-monotonously depends on the defect’s perpendicular distance and
rationalize this finding by the collective kinetics along the minimum free
energy path (MFEP) and the single-chain dynamics in an inhomogeneous
environment. Importantly, at intermediate segregation, χN < χN*,
dislocation defects are not even metastable but spontaneously annihilate
upon collision. This opens opportunities for process-directed self-assembly
where by varying external thermodynamic control parameters – like
temperature or solvent content – one tailors the kinetics of structure
formation.
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