Kramer Group Research Spotlight

"Hairy" Polymer Interfaces Become Unstable

 
 
Steve Medd, 
Graduate Student, 
Chemical Engineering, 
UCSB
Santa Barbara, CA 93106
 
 
We are investigating the grafting kinetics and interface stability in bilayer thin films produced by spin-casting a thin bottom layer of poly(styrene-r-maleic anhydride) [PSMA] on a silicon wafer and then transferring a polysyrene [PS] film about 500 nm thick from a glass slide to the PSMA-coated wafer via a water surface. The PS film contains a volume fraction of amino-terminated deuterated PS molecules (dPS-NH2). The amine of the dPS-NH2 chains reacts with the anhydride units of the PSMA to form a graft copolymer at the interface between the two films at the annealing temperature of 463K.
After the reaction has been allowed to proceed for some time, the PS layer is washed off in cyclohexane which is a non-solvent for the PSMA. The important grafting parameter turns out to be the normalized grafting density z*/Rg where z* is the interfacial excess of the dPS-grafted chains (This is measured by either FRES or SIMS) and Rg the radius of gyration of the dPS chains. If z*/Rg is less than one the grafted chains are not stretched, whereas if z*/Rg is much greater than one, the chains are stretched. Such stretching decreases the interfacial tension (An increase in the area of the interface will reduce the stretching) and thus eventually if z*/Rg continues to increase due to the grafting reaction, the interfacial tension will be driven negative. A consequence of the negative interfacial tension is that the "hairy" interface becomes unstable and roughens spontaneously, eventually generating a highly convoluted interface and even small graft copolymer coated particles of the two different polymers. The roughening can be detected using scanning force microscopy (SFM). The interface remains relatively flat until a critical z*/Rg is reached and then becomes quite rough as shown in the figure to the right. The micrograph (a) corresponds to a interface grafted with dPS-NH2 chains with N = 55 that has reached z*/Rg = 0.5 whereas micrograph (b) corresponds to an interface grafted with the same chains to z*/Rg = 4. The lateral size of the images is 1 micrometer while the vertical scale bar is 100 nm.

This project was started at Cornell by the former Kramer Group member Jinbao Jiao (PhD'97) as part of a collaboration with Prof. Martin Möller at the University of Ulm, Germany, his student Sicco de Vos (PhD'97) and Dr. Cor Koning of DSM in Geleen, The Netherlands. FRES measurements were done at the Ion Beam Facility of the Cornell Center for Materials Research. Funding was provided in part by the Cornell Polymer Outreach program and Steve Medd's support is provided at UCSB by funding through the MRL from NSF Grant DMR-9632716.