(14aa) Structure-Property of Polymer and Its Composites: Multiscale Experimental and Computational Studies

Polymers play vital roles in nearly every application in our current era, which range from mundane food packaging and personal care to advanced energy devices and protective gears. Such versatility of polymer use makes it an attractive platform to incorporate inorganic fillers with novel functionalities, which either enhance the material property of an existing platform or process as an entirely new polymer composite material. Recent studies have found that a strategic placement of nanofillers within a polymer can dictate the composite material properties, whether electrical, mechanical, or thermal. Thus, predictions of the nanostructure and its structure-property relationship are vital to tailor the nanocomposites to advanced applications such as in batteries, molecular sensors, and protective gear.

As with other classes of materials, investigating soft materials by theory and simulation are crucial to contributing towards expedited discovery of new materials. However, computational studies of polymers present more challenges than other classes of materials. Unlike other crystalline materials with unit cells of 10 nm or less, polymers are often amorphous or semi-crystalline entities that can span over microns to millimeters^[2]in length scale. These macromolecules also span over a much larger time scale to respond to an external force, which results in higher computational cost. Although predictive equilibrium assemblies of polymer and its composites, or â??nanocompositesâ?, are still valuable studies, polymeric materials are almost always processed to become a final product. Thus, investigating the structural dynamics of the nanocomposites with nonequilibrium dynamics such as flow and crystallization are crucial to accurately predict its end-use material properties.

To this end, I have sought answers to the following two questions:

First, how does the polymer and its composite structure change under nonequilibrium dynamics? Second, what is the structure-property relationship of the nanocomposites and what are the applicable advanced material applications?

Using coarse-grained molecular dynamics, multiple combinations of polymer(homopolymer or block copolymers) and nanofillers(nanoparticles, nanorods, and nanotubes) composite structures under A) shear and extensional flow and B) cylindrical nanoscale confinement were investigated. Our study reveals that depending on the control variables of the aforementioned dynamics, we can induce novel functional structures such as highly oriented polymer and nanomaterials, evenly placed nanomaterials within polymer matrix, and interconnected nanorods.

As a comparative experiment, electrospinning was used to fabricate polymer/nanofiller nanofibers which undergoes both extensional deformation and nanoscale cylindrical confinement. The structural agreement between the simulation and the experiment result confirms the validity of the model. As such, some of the novel structures observed in simulation were also observed in experiment, such as well-dispersed and well-aligned nanorods within the nano-scale polymer fibers. The novel functional properties that arise from nanostructures of electrospun nanofiber are then readily utilized in various applications such as molecular sensors and high performance battery electrodes.

I utilized structure-property relationship of polymer configuration and its molecular orientation to significantly alter the mechanical properties of polymeric fibers. For instance, by improving the molecular orientation and degree of crystallinity from gel-electrospinning ultra high molecular weight polyethylene, I successfully fabricated ultrathin fibers with high stiffness that rivals the commercial protective vest such as Spectra. By defining a fundamental connection between the predicted nanostructure and its functionality, we can expand much further on the capabilities of electrospun nanofibers to significantly impact the polymer processing and its application as a whole.

Teaching Interests:

From my extensive teaching and research experiences, I believe I have the credentials to make significant contributions to teaching in the Chemical Engineering or Material Science Department. My life-long training as a chemical engineer prepared me to analyze open-ended ill-defined processes in a more well-defined and systematic manner. I believe my research and teaching expertise has prepared me to teach transport phenomena (heat, mass, momentum) as well as thermodynamics. Of course, I can also teach statistical mechanics, which is a basis for many computational studies. I believe effective teaching empowers the studentsâ?? ability to critically think and create their own knowledge, and this ultimately benefits the research community.