Alexander I. Norman1, João T. Cabral1, Derek L. Ho2, Alamgir Karim1 and Eric J. Amis1
1 Polymers Division, Materials Science and Engineering Laboratory.
2 NIST Center for Neutron Research
Scattering methods have been employed to study the aqueous phase behavior of two low molecular weight diblock copolymers, EO(6)BO(11) and EO(18)BO(9), where EO denotes an ethyleneoxide unit and BO denotes a butyleneoxide unit. Depending on the lengths of these blocks, either micelles or vesicles form. High throughput methods have been used to produce discrete composition gradients using a library substrate, either glass or PDMS (Poly(dimethylsiloxane)), and a home built liquid dispenser. Such polymers were chosen due to their miscibility with water and their relatively low viscosity. Small angle scattering techniques, using x-rays, light and neutrons have studied these gradient libraries.
The x-ray scattering measurements were performed on a 5 ´ 5 PDMS array with polyimide windows. 25 simultaneous SAXS/WAXS (small angle and wide angle x-ray scattering) data sets were obtained in less than 1 h. The concentration regime investigated was 0 % (by mass fraction) up to 20 % (by mass fraction), which was believed to be the range in which vesicles form. The SAXS data showed an increase in intensity as the concentration of polymer increased. In addition, the primary SAXS peak moved to higher q values with increasing concentration, indicating that the vesicle shell becomes thinner. WAXS measurements showed only amorphous scattering. The absence of sharp Bragg peaks indicated no crystallinity within the structure.
SANS (small angle neutron scattering) measurements on the polymers were carried out at elevated temperatures to study the micelle to liquid crystal transition. Initial results indicated that EO(18)BO(9) form a wide range of structures such as spherical and worm-like micelles, hexagonal packed rod-like micelles, and lamellar sheets. Modeling of the SANS data concluded that the vesicles exist with a once folded PEO (Poly(ethyleneoxide)) chain unlike the micelles that form.
Small angle light scattering is currently being used in house to obtain
the phase behavior of these polymers on a larger scale. In addition SALS
(small angle light scattering) measurements will be performed on binary
and ternary blends of polystyrene and polybutadiene using a 10 ´
10-glass substrate held in a copper heating stage. This will provide a
SALS image for every 1 % increment in volume fraction and every 1 °C
temperature rise. Such measurements provide much more accurate phase diagrams
than previous measurements.
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