Kramer Group Research Spotlight

Spinodal Decomposition in 2D Layers of Binary Polymer Mixtures

 
 
Jakob Heier, 
Graduate Student, 
Materials Science & Engineering, 
Cornell University,  Ithaca, NY 14853, USA 
Current address:
Materials Department
UCSB
Santa Barbara, CA 93106
 
 
Phase separation phenomena have been extensively studied in the past two decades both theoretically and experimentally. Only recently it became clear that the kinetics and morphology of phase separation are altered in the vicinity of surfaces. When a binary polymer mixture is quenched into the metastable region of its phase diagram, nucleation and growth of the minority phase occurs slowly in the bulk of the initially mixed but metastable state. If the surface energy of the majority phase is lower than the minority phase, the majority phase is attracted by, and forms a wetting layer against this surface. In our experiments we found an enrichment layer of the minority component in the near-surface region where the composition of mixture is driven inside the spinodal. In this layer, and this layer only, phase separation takes place by a fast 2-dimensional spinodal decomposition.
We observe this effect in a mixture of poly(ethylene propylene) (PEP) and its perdeuterated counterpart (dPEP). We show a 2-dimensional ToF-energy FRES spectrum of a off-critical mixture with a volume fraction of dPEP of 64% which has been annealed at 294 K for 14 days.
The deuterium depth profile displayed in the insert shows the formation of a dPEP-rich wetting layer at the surface and a dPEP poor layer just beneath it, where spinodal decomposition into a layer of bicontinuous PEP- and dPEP-rich domains takes place. The spinodal decomposition in this layer gives rise to a surface roughness pattern that can be seen on the accompanying AFM image. Please click here to see it.

This project is a collaboration with Prof. Frank Bates at the University of Minnesota. The TOF-FRES measurements were done at the Ion Beam Facility of the Cornell Center for Materials Research using its state-of-the-art high resolution time-of-flight. energy spectrometer.   Funding was provided in part by NSF Grant DMR-9632275 and NSF-DMR Polymers Program DMR 9803738. The 2D-ToF energy spectrometer is sponsored by ARO DURIP P-33751-MS-RIP, DAAH04-95-1-0019