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           COLLOQUIUM OF THE COMPUTATIONAL MATERIALS SCIENCE CENTER
                              College of Science
                       (CDS Department CSI 898-Sec 001)
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	Kinetics of Phase Transitions in block-copolymers
 			 Rama Bansil
    Department of Physics, Boston University, Boston, MA

Blck copolymers are known to form micelles of different shapes in selective 
solvents that preferentially dissolve one of the constituent blocks. These 
micellar fluids exhibit various ordered and disordered phases. To gain 
microscopic insight into the mechanisms involved in the transformation between 
different phases we use synchrotron-based time-resolved small angle x-ray 
scattering (SAXS) and coarse-grained discrete Brownian Dynamics simulations. 
In this talk I will focus on the kinetics of the transformation from the 
hexagonal packed cylinder (HEX) phase to cubic phases (FCC or BCC). SAXS 
measurements reveal an epitaxial transformation. The data was interpreted with 
a geometrical model for the transformation from cylinders to spheres. Brownian 
Dynamics simulation for an ABA triblock copolymer in a solvent selective for 
the A block will also be presented. In this system isolated micelles containing 
B in the core and A in corona are formed. The kinetics of the transformation 
was investigated by following the time evolution of the simulated system 
following an instantaneous jump in temperature or by changing the depth of the 
Lennard-Jones potential well. Detailed analysis showed that the transformation 
occurred via a rippling mechanism similar to the "pearling instability," with 
the ripple starting on one cylinder and then inducing ripples in neighboring 
cylinders.

                        Monday, May 4, 2009
                              4:30 pm
                     Room 301, Research I, Fairfax Campus

Refreshments will be served at 4:15 PM.
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Find the schedule at www.cmasc.gmu.edu/seminar/schedule.html
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