Phase separation of polymeric gels
Consisting of a crosslinked network of both hydrophobic and hydrophilic groups, a temperature-sensitive gel may collapse by expelling a large amount of solvent when heated over the lower critical solution temperature. For smaller samples which undergo a slow temperature change, this phase transition is homogeneous and the resultant deformation is uniform. When superheated, a relatively large gel may develop heterogeneous structures by nucleating solvent-rich domains inside. A Ginzburg-Landau-type phase-field model is formulated to study the microstructure evolution in a phase-separating gel. Numerical calculations show that the phase-separated domains coarsen non-self-similarly and form a spongy structure with thin walls of the dryer phase. Theoretical model further confirms that such a structure the deformation in each phase is more uniform, as favored by the energy of mixing. Furthermore, it is found that the coarsening will give way to a domain dilation mechanism if the system is brought to a far-from-equilibrium state. The thermodynamic and kinetic processes revealed by the models may help in designing stimuli-responsive soft machines, and in understanding similar phenomena in natural and biological systems.
Biography: Trained as a solid mechanician, Wei Hong’s expertise is in the theoretical modeling on the physics of materials and structures and his current research interest is in the behaviors of smart and soft materials under multiphysics fields, such as the instability and fracture of soft solids, phase-transition-induced toughening, and dielectric breakdown. Dr. Hong got his B.S. (2000) and M.S. degrees (2002) both from Tsinghua University (Beijing, China). Following his Ph.D. in Engineering Sciences (2006) at Harvard University, he was a postdoctoral fellow in the School of Engineering and Applied Sciences at Harvard, before joining the faculty of Iowa State. Since 2008, Dr. Hong has been an assistant professor in the Department of Aerospace Engineering at Iowa State University, where he also holds courtesy appointments from the departments of Materials Science and Engineering and Mechanical Engineering.