Engineering topics

Meng, Siqi

Publications and source records attributed to Meng, Siqi.

3 records

DOE OSTI2020

Spatiotemporal Formation and Growth Kinetics of Polyelectrolyte Complex Micelles with Millisecond Resolution

We have directly observed the in situ self-assembly kinetics of polyelectrolyte complex (PEC) micelles by synchrotron time-resolved small-angle X-ray scattering, equipped with a stopped-flow device that provides millisecond temporal resolution. A synthesized neutral-charged diblock polycation and homopolyanion that we have previously investigated as a model charge-matched, core-shell micelle system were selected for this work. The initial micellization of the oppositely charged polyelectrolytes was completed within the dead time of mixing of 100 ms, followed by micelle growth and equilibration up to several seconds. By combining the structural evolution of the radius of gyration (R-g) with complementary molecular dynamics simulations, we show how the self-assemblies evolve incrementally in size over time through a two-step kinetic process: first, oppositely charged polyelectrolyte chains pair to form nascent aggregates that immediately assemble into spherical micelles, and second, these PEC micelles grow into larger micellar entities. Furthermore, this work has determined one possible kinetic pathway for the initial formation of PEC micelles, which provides useful physical insights for increasing fundamental understanding self-assembly dynamics, driven by polyelectrolyte complexation that occurs on ultrafast time scales.

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DOE OSTI2020

Effects of Non-Electrostatic Intermolecular Interactions on the Phase Behavior of pH-Sensitive Polyelectrolyte Complexes

Polyelectrolyte complexes (PECs) offer enormous material tunability and desirable functionalities, and consequently have found broad utility in biomedical and material industries. While poly(acrylic acid) (PAA) and poly( allylamine hydrochloride) (PAH) are a commonly used pairing, various aspects of the phase behavior of PAA-PAH complexes have not been sufficiently quantified. In this work, we present a comprehensive experimental study depicting the binodal phase boundaries for the PAA-PAH complexes prepared under acidic, neutral, and basic conditions using thermogravimetric analysis, turbidimetry, and optical microscopy. Under neutral and basic conditions, phase behaviors of the complexes were largely similar to one another and followed general expectations of PEC phase behavior, except for unusually high resistance to disruption of the complex with added salt. Stable complexes are observed up to 4 M NaCl concentrations. Under acidic conditions, strikingly different phase behaviors of the PAA-PAH complexes were observed. The polymer content in the complex phase increased initially, followed by an expected decrease as salt was added to the complexes. This behavior may result from a combination of associative phase separation of PAA and PAH chains, influenced by electrostatic interactions, and segregative phase separation, which can be ascribed to the influence of a combination of the hydrophobic interactions of the aliphatic polymer backbone and the interpolymer hydrogen bonding of unionized acrylic monomer units. Our systematic investigations over a range of pH detailing these discrepancies in the PAA-PAH phase behavior are expected to clarify the inconsistencies among the reports in the literature and provide the material design strategies for practical use of the PAA-PAH complexes and multilayer assemblies.

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DOE OSTI2019

Integrating Systems Thinking into Teaching Emerging Technologies.

Collaborations and partnerships across disciplines are becoming increasingly recognized as valuable endeavors toward solving emerging global challenges in our rapidly changing world. Thus, it is crucial to create STEM educational strategies that infuse diverse and interconnected perspectives among scientists, policymakers, and the general public. Systems thinking represents one such pedagogical approach, in which a holistic framework empowers both teachers and students to recognize how fundamental concepts taught in the classroom can be used as resourceful tools to better address complex, multicomponent modern challenges. For chemical educators, topics on technology (the "T" in STEM education) represent a particularly meaningful avenue to introduce and study the effectiveness of systems thinking, but there have been few examples reported to date. Here, we show that a molecular engineering course introduced at the University of Chicago 5 years ago ("Introduction to Emerging Technologies") provides a model course to this end, where explicitly designing in elements of systems thinking to the course could further strengthen students' understanding of the already well suited, holistic course topics. Integrating systems thinking would illuminate how tissue engineering, nanomedicine, batteries, quantum information, and meat alternatives are developed from multiple, interrelated fields as well as from basic science to a practical reality. For each technology, we outline the direct connections between the current course topics and future opportunities to incorporate systems thinking strategies, challenging students to identify the interdependence of components in a successful technology. Finally, to offer further insight for instructors interested in utilizing systems thinking, we broaden our discussion to reflect on the challenges of redesigning any STEM course to utilize systems thinking, and we present corresponding teaching strategies to overcome them.

Interdisciplinary/Multidisciplinary↗