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April 21, 2004 Time-Resolved Small-Angle X-ray Scattering Study of the Kinetics of Disorder-Order Transition in a Triblock Copolymer in a Selective Solvent for the Middle BlockH. Nie1, R. Bansil1, K. Ludwig1, M. Steinhart2, C. Konak3 and J. Bang3 Solutions of block copolymers in solvents that selectively dissolve one of the component polymers form micelles with the insoluble component in the cores. These complex fluid or gel states exhibit a rich phase diagram with many different ordered structures. Understanding the phase behavior and kinetics of these materials is of considerable importance to controlling their properties. We reported the first time-resolved small angle x-ray scattering (SAXS) study on the kinetics of the formation of a bcc ordered state from a disordered micellar fluid in solutions of Kraton, a commercial triblock copolymer. Our studies reveal a two-stage process that shows significant change of the micelle’s size at the onset of ordering.
Further SAXS studies on order-order transitions are underway. BEAMLINE FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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Solutions of multi-block copolymers exhibit rich phase diagrams
and novel morphologies, and can form clusters, networks, and gels.
These properties determine their numerous applications, such as
detergents, adhesives, elastomers, bio-mimetic vesicles, and
templates for nanomaterials. The phase behavior and properties of
these materials can be tuned by varying the number of component
polymers and blocks, the chemical composition and length of the
blocks, and the blocks’ interactions with solvents. For example, an
ABA triblock copolymer solution in a solvent that is good for the
outer (A) block forms isolated micelles with cores containing the B
block. On the other hand, in a solvent that is good for the middle B
block, loops or bridges will be formed and the middle block can
tether two adjoining micelles, eventually forming a network. The
micelles, which can be either spherical or cylindrical, either exist
as a complex fluid or form ordered structures, such as cubic or
hexagonal. Our focus is to understand the kinetics of the phase
transitions between the different disordered and ordered states of
the micellar phases of these solutions. Since the micellar cores are
on nanometer-length scales and the phase transitions occur on the
time scale of minutes, synchrotron based time-resolved small angle
x-ray scattering (SAXS) is an ideal tool for examining their
structure and kinetics. 
A few SAXS studies of the kinetics of order-order and
order-disorder transitions in melts of block copolymers have been
reported. To the best of our knowledge, our recent publication is
the first temperature jump study of isothermally evolving kinetics
in solutions of triblocks. We examined the disorder-order transition
kinetics of a styrene (S) ethylene-co-butylene (EB) triblock
copolymer, SEBS, in mineral oil, which preferentially dissolves the
middle EB block. This commercial thermoplastic elastomer from Shell,
known as Kraton®, is used in footwear, adhesive sealants and
coatings, roofing, and packaging. Using the high flux monochromator
at beamline X27C and a one-dimensional wire detector, we were able
to measure the scattered intensity I(q) as a function of the
wavenumber, q, with a time resolution of 10 seconds over the q-range
0.01 to 0.3 Ǻ-1, and thus follow the kinetics of the micellar
fluid’s ordering onto a bcc lattice. The transition was typically
followed either by ramping the temperature over a wide range (Fig.
1) or by rapidly quenching the sample temperature into the ordered
phase (Fig. 2). The temperature ramp experiment identifies the phase
transition temperature, while the temperature quench experiment
provides the isothermal kinetics. A model based on interacting hard
spheres was used to analyze the data and provide detailed
information about the time evolution of the micellar structural
parameters during the ordering process. For example, the results
showed that the core radius decreased sharply at the onset of the
ordering process (Fig. 3a). The kinetics exhibits two stages
corresponding to temperature equilibration and supercooling of the
micellar fluid, followed by the nucleation and growth of the ordered
state (Fig. 3b). The induction-time for the onset of the second
stage shows a minimum around the glass transition temperature of the
polystyrene cores.