Kramer Group Research Spotlight

Structure and Self-Diffusion in Asymmetric Block Copolymer Films

 
 
Hideaki Yokoyama, 
Graduate Student, 
Materials Science & Engineering, 
Cornell University,  Ithaca, NY 14853, USA 
Current address:
Materials Department
UCSB
Santa Barbara, CA 93106
 
 
We have measured the self-diffusion coefficient D of asymmetric PS-b-PVP diblock copolymers where the PVP block fraction f is about 0.10. These block copolymers have an cubic (bcc) arrangement of spherical domains. Secondary ion mass spectrometry (SIMS) of thin films of these diblock copolymers reveals that the spheres arrange themselves in regularly spaced layers aligned parallel to the surface and that these layers are the (110) planes of the bcc structure. Despite the good long range ordering into planes of spheres parallel to the surface, the order in the plane of the surface is short range only, as shown in the figure on the right. The spacing between spheres is roughly 30 nm.

Keeping f fixed we increase the degree of polymerization NPVP of the PVP block, and thus the entire block copolymer and measure D by fitting the experimental concentration profile determined by FRES to the solution to the diffusion equation. If we normalize D by Do, the diffusion coefficient at the annealing temperature of a PS with a degree of polymerization equal to that of the block copolymer, we find that all the data for different NPVP and annealing temperatures T fall on a single line described by D/Do ~ exp(-NPVP) where = (63/T) - 0.033. These results are consistent with an "activated hopping" diffusion mechanism in which individual block copolymer chains "hop" from one plane of spheres to the next, each time surrmounting a free energy barrier kBTNPVP arising when the PVP block is completely surrounded by the PS matrix.

The FRES measurements were done at the Ion Beam Facility of the Cornell Center for Materials Research. Funding was provided in part by NSF Grant DMR-9632275 and NSF-DMR Polymers Program DMR 9803738.