(6hx) Functional Materials Design Guided By Polymer Physics | AIChE

(6hx) Functional Materials Design Guided By Polymer Physics

Authors 

Xie, R. - Presenter, The Pennsylvania State University
Chabinyc, M. L., University of California, Santa Barbara
Colby, R. H., Pennsylvania State University
Gomez, E. D., The Pennsylvania State University
Research Interests:

The functions of polymers have been expanded tremendously by the emergence of new classes of polymers, such as electron (or ion) conducting polymers (i.e., conjugated polymer or ionomer), entanglement-free bottlebrush polymers, and bond-exchanging vitrimers. Nevertheless, the semiflexible backbone of conjugated polymers and the crowded side chains of bottlebrush polymers are distinctly different from linear chain coil-like polymers. This difference from conventional polymers results in completely different chain conformations and new behaviors in glass transition, entanglements, liquid crystalline ordering, and crystallization. The lack of physically intuitive models that predicts the structure-property relationship for these new functional polymers hinders the ability to control their properties.

My research goal is thus to build upon the classic theory of polymer physics to quantitatively control or predict the properties from structure for functional polymers, thus guiding the materials design for specific applications. In the meantime, rheology and mechanical behavior can provide a powerful experimental window into the underlying polymer dynamics and verify the proposed model.

During my Ph.D. work co-advised by Professors Ralph Colby and Enrique Gomez at Penn State University, my research is centered on characterization of the fundamental properties of conjugated polymers and building their interrelationships with structure and mechanical performance. Driven by the application in stretchable electronics, complex repeat units are needed to varying the electronic structure of the conjugated backbone, but it is poorly understood how such designs impact the behavior of polymer chains in solids. We have first developed a rheological method to unambiguously determine the glass transition temperature (Tg) of conjugated polymers. Then, simplified from the group contribution theory of Tg, we have experimentally verified a new model that quantitatively predicts the Tg for various conjugated polymers with alkyl side chain. Further rheological study on the viscoelasticy of a conjugated polymer melt has led to a new phenomenon of reduced entanglements via local chain alignment in liquid crystalline phase. Both glass transition temperature and chain entanglements dictate the stretchability of polymeric material, so our work have made essential contributions to guide the design of conjugated polymer for desired mechanical performance.

My ongoing postdoctoral work, co-advised by Professors Michael Chabinyc and Chris Bates at University of California, Santa Barbara, is focused on bottlebrush polymers. Currently, we have demonstrated a versatile strategy to photo-crosslink bottlebrush polymers into a super-soft elastomer. These soft elastomers have the modulus of wet polymer gels, but are 100% solids which allows them to be used in pressure sensors. The correlation between the network modulus, which is inversely related to the pressure sensitivity, and the photo-crosslinker concentration is well described by a novel network model. This proposed model is adapted from the classic Phantom network model and has been tested across various classes of bottlebrush polymers with different backbone and side chain lengths. As a result, we can quantitatively control the network modulus of a photo-crosslinked bottlebrush elastomer.

Recent synthetic advancements have enabled facile and accurate control on the chain architecture of a bottlebrush polymer in terms of the backbone length, side chain length and grafting density, thus allowing exploration of new physical behaviors caused by this unconventional chain architecture. For instance, the glass transition temperature of a bottlebrush polymer can still approach that of a linear polymer in the high molar mass limit, despite of a lot more chain ends per unit volume. Forming liquid crystalline phase from the stretched backbone by crowded side chains in a non-mesogenic bottlebrush polymer is indeed possible but hardly controllable or predictable. On the other hand, apart from the stiffness of a conjugated backbone, the role of π-π stacking interaction in the formation of liquid crystalline phase is mostly unexplored in the field of conjugated polymer. Therefore, understanding the impact of chain architecture and molecular anisotropy on polymer dynamics could allow development of polymers with new functionality to address challenges in various fields, including stretchable electronics.

There is a clear trade-off between stretchability (or softness) and charge mobility of a conjugated polymer, since charges are known to transport faster along the conjugated and stiff backbone rather than through the electrically insulating and soft side chains. Adding more soft segments usually impedes the macroscopic charge mobility. By intelligently combining bottlebrush polymer and conjugated polymer, it’s possible to break this trade-off and obtain a super-soft and semiconducting material. Nevertheless, the choices of conjugated polymer repeat unit and bottlebrush polymer architecture rely on the prior knowledge of the structure-property relationship of these emerging functional materials. Therefore, I have summarized the following questions to be addressed in my future research:

  1. Can we understand how the side chain grafting density affects the glass transition temperature of a bottlebrush polymer? Do we know the role of bottlebrush backbone in the crystallization process of side chain? Can we understand how entanglement scales with chain stiffness for semiflexible conjugated polymers?
  2. Can we predict the formation of liquid crystalline phase of a bottlebrush polymer from its molecular anisotropy? Can we decouple the contribution of the molecular anisotropy from that of the π-π stacking interaction for driving the liquid crystalline ordering in a conjugated polymer?
  3. How do we control the phase separation between the semiconducting conjugated component and the super-soft bottlebrush component?

Teaching Interests:

Teaching is not merely passing the knowledge but to endow students with self-motivated interest and passion. A good teacher, advisor or mentor can strongly change a student’s future trajectory in a positive manner or vice versa. As a Ph.D. student, I was lucky enough to have mentored two outstanding undergraduate students and one graduate student for about two years. Each had an individual project and has contributed to the published works. Among them, one undergraduate student is even motivated enough to pursue her Ph.D. in chemical engineering at University of Massachusetts, Amherst this year. In addition, teaching in the classroom, such as introduction to polymer science, rheology and polymer processing, also brings me great joy as an instructor whenever I see students’ eyes light up. Also, teaching itself is of course an incredible opportunity for the lecturer to review and refresh the fundamental knowledge, which may illuminate the current research with new ideas.