Number, Size and Lifetime of Dynamic Clusters Near the Gel Point (Jian Qin, Stanford)

Abstract:

The population statistics, physical dimension, and relaxation dynamics of polymer clusters formed by random crosslinking are highly susceptible to the degree of gelation, which are normally characterized by a set of power-law exponents. Away from the gel point, the mean-field exponents predicted by the Flory-Stockmayer theory are expected to apply. In the close vicinity of the gel point, however, the classical scaling fails because the clusters overfill space, and the critical exponents set in. This crossover has been employed to interpret the rheological responses of physical gels, but has not been directly probed, partially because the gelation window is narrow. Here, we present the data from a hybrid MC/MD simulation model for dynamic polymer networks, and report the distributions of cluster number, size and lifetime. The results demonstrate a clear crossover from classical to critical regimes. The dependence of the threshold on chain stiffness and monomer bulkiness can be rationalized by examining the excluded volume interactions among clusters following the power-law scaling.

Biography:

Jian Qin is an Associate Professor in the Department of Chemical Engineering at Stanford University. He received B.S. and M.S. degrees in Materials Science from Tsinghua University, and his Ph.D. from University of Minnesota under the supervision of Profs. David Morse and Frank Bates. Following postdoctoral fellowships at Pennsylvania State University, with Prof. Scott Milner, and The University of Chicago, with Prof. Juan de Pablo, he joined Stanford University as a Terman Faculty Fellow in 2016. His research focuses on theoretical study of morphological and rheological behavior of polymeric fluids, electrostatic interactions in structured electrolytes, and surface charge polarization. He has held the Kadanoff-Rice Fellowship and has been recognized by the 3M Non-Tenured Faculty Award, the Hellman Faculty Award, the NSF CAREER Award, the ACS PMSE Arthur Doolittle Award, the ACS PMSE Young Investigator Award, the Tau Beta Pi (Stanford) Teaching Honor Roll, and the APS Dillon Medal.