Ames Laboratory materials chemist Surya Mallapragada and her research team are focusing on pentablock polymers - polymers that form in strings of five chains. Each string is comprised of two cationic (positively charged) blocks, two hydrophilic (water loving) blocks, and one hydrophobic block. Because the hydrophobic block tries to avoid water, it forms the center of the string, with the hydrophilic next and the cationic blocks on the outside. In solution, these strings form in small clusters called micelles, again with the hydrophobic blocks at the center.
"The interesting thing about these polymers is that they respond to changes in temperature and pH," Mallapragada says. "As the temperature goes up, the micelles cluster together more, forming a polymer gel. A similar reaction takes place as pH rises - the hydrophobicity of the cationic blocks increases which also helps in gel formation."
As temperature and/or pH drops, the process reverses itself and the gels dissolve back into micelles and polymer strands. Using cryotransmission electron microscopy, Mallapragada's group is working to understand just how these micelles look and how fast the polymers respond to changes in temperature and pH.
"Samples are plunged into liquid ethane which freezes them so quickly that ice doesn't form and disrupt the crystal structure," she says. "We're able to then view the gel formation at various stages (temperature and pH) under very controlled conditions." She ad
'"/>
Contact: Surya Mallapragada
suryakm@iastate.edu
515-294-7407
DOE/Ames Laboratory
1-Apr-2004