[CCoE Notice] Dissertation Announcement: Farshad Safi Samghabadi, "Charge-Modulated Dynamics and Stimulus Response in Polyelectrolyte Solutions and Brushes"

[Greenwell, Stephen J]

[Dissertation Defense Announcement at the Cullen College of Engineering]
Charge-Modulated Dynamics and Stimulus Response in Polyelectrolyte Solutions and Brushes

Farshad Safi Samghabadi

April 10, 2025; 10 a.m. to 12 p.m.
Location: AERB, Room #126

Committee Chairs:
Jacinta Conrad, Ph.D.
Committee Members:
Alamgir Karim, Ph.D. | Jeremy Palmer, Ph.D. | Kamran Alba, Ph.D. |
Behrouz Ferdowsi, Ph.D.
Abstract
Polyelectrolytes offer enhanced properties compared to their neutral counterparts due to the intra-and interchain interactions between charged monomers. These interactions can be tuned through the charged monomer fraction and by modulating environmental conditions such as solution pH and ionic strength.  However, how these factors influence the dynamics of polyelectrolyte solutions and brushes remains incompletely understood.

In solutions, microrheology has become an indispensable tool for measuring the dynamics of macromolecular systems. Yet, its ability to characterize polymer dynamics across spatiotemporal scales, which vary among polymers and concentration regimes, is limited by the selection of probe morphologies and sizes.  Here, we introduce semiflexible M13 phage as a powerful microrheological probe able to circumvent these constraints to robustly capture the dynamics of polymeric solutions across decades of concentrations, sizes, and ionic conditions. We show that phage mobility is directly coupled to the relaxation dynamics of polystyrene sulfonate, DNA, and their composites, spanning from semidilute to entangled regimes and across four orders of magnitude in ionic strength. Using this approach, we uncover key differences in the dynamics of synthetic and natural polyelectrolytes through multiple quantitative metrics. Our results open the door to the use of phage probes to elucidate the complex dynamics of systems exhibiting a spectrum of thermal and active relaxation processes.

Extending our focus to weakly charged polyelectrolyte brushes, we systematically varied the fraction of ionizable monomers and studied their swelling behavior in response to changes in pH and ionic strength. Using in situ ellipsometry, streaming zeta potential, and atomic force microscopy, we reveal a strong correlation between the fraction of ionizable monomers and the pH-responsive swelling of the brushes, with significant hysteretic behavior observed during pH cycling in the intermediate pH range, which we attribute to a hydrophobic periphery mechanism. Moreover, we uncovered a salt-dependent extent of hysteresis, highlighting the interplay between electrostatic interactions and ionic screening in determining the conformation of brushes. Together, these studies provide new insight into the charge-dependent dynamics and stimulus-responsive behavior of polyelectrolyte solutions and brushes, offering a foundation for the design of smart, tunable materials for advanced applications.
[Engineered For What's Next]


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