I highlight my research throughout my academic career as a Ph. D. student at the Pennsylvania State University as well as an M.S. student at Northwestern University. My research mainly focuses on surface modification via polymer thin films, especially polymer brushes which are chemically tethered on substrates.

In collaboration with Dr. Cyrille Boyer from the University of New South Wales (Sydney, Austrilia), I developed surface-initiated photoinduced electron transfer-reversible addition−fragmentation chain transfer polymerization (SI-PET-RAFT) as a novel chemistry platform. SI-PET-RAFT affords functionalization of surfaces with spatiotemporal control and provides oxygen tolerance under ambient conditions.

To improve the reproducibility of surface-initiated controlled radical polymerizations (SI-CRP), I compared different storage conditions and examined the stability of surface-initiator in those conditions. Furthermore, I leveraged the advantage of orthogonal light-mediated SI-CRPs to fabricate mixed polymer brushes (MPBs). MPBs are considered as “smart” materials due to the unique reversible switching ability of surface property upon exposure to external stimuli. My preliminary work has demonstrated the ability to prepare MPBs with limited switching properties. To improve the switching ability, I am currently working on exploring varied polymer combinations, attempting alternative photo-polymerization pathways, and understanding the nanostructure of mixed brushes via ellipsometry and resonance soft/tender X-ray reflectivity.

Research Interests:

My research focuses on surface modification via polymer brushes grafted on substrates. My interest in polymer thin films was sparked when working with Dr. John Torkelson as a M.S. student at Northwestern University. As I pursue my Ph.D. degree at the Pennsylvania State University, I get the opportunity to further study the polymer thin films which are chemically grafted on substrates (i.e., polymer brushes). During my Ph.D. studies, I leverage the advantage of light-mediated polymerizations to achieve surface modifications in a more flexible and eco-friendly fashion. My research work ranges from synthesizing small organic molecules, establishing novel chemistry platform for polymer brushes, all the way to characterizing nanostructures of complex polymer brushes. I also dedicated to bridging the gap between lab research and industrial application by introducing oxygen tolerance to surface-initiated controlled radical polymerizations (SI-CRPs).

T_g- and Fagility-Confinement Effects of Ultrathin Polymer Films on Substrates

Ultrathin films have been widely used in lithography, semi-conductor, and nano-devices etc. As ultrathin films on these devices may be required to be operated over a wide range of working temperatures, the glass transition temperature (T_g) is vital for a steady performance. T_g of a thin polymer film was found to be thickness-dependent once it reaches certain ultrathin region, i.e., T_g-confinement effect. The similar confinement effect in ultrathin polymer films was also observed on another physical parameter, fragility. Here, I contributed to establish fundamental understanding of T_g- and fragility-confinement effect by probing (i) the role of interactions between films and substrates, (ii) the role of molecular architectures, and (iii) the lateral distribution of confinement effect in a polymer film. My work also provided important data to establish the intrinsic correlation between T_g- and fragility-confinement effects.

SI-PET-RAFT: Surface-Initiated Photoinduced Electron Transfer-Reversible Addition-Fragmentation Chain Transfer Polymerization

In collaboration with Dr. Cyrille Boyer from the University of New South Wales (Sydney, Austrilia), for the first time, I introduced surface-initiated photoinduced electron transfer-reversible addition−fragmentation chain transfer polymerization (SI-PET-RAFT) as a novel chemistry platform. SI-PET-RAFT affords functionalization of surfaces with spatiotemporal control and provides oxygen tolerance under ambient conditions. The modularity and versatility of SI-PET-RAFT are highlighted through significant flexibility with respect to the choice of monomer, light source and wavelength, and photoredox catalyst. The ability to obtain complex 3-D patterns on surfaces in the presence of air is a significant contribution to help pave the way for CRP-based surface functionalization into commercial application.

Comparison of Long-Term Stability of Initiating Monolayers in Surface-Initiated Controlled Radical Polymerizations

The reproducibility of polymer brush synthesis is vital for consistent lab research as well as robust quality control in industry. I designed experiments to compare the stability of initiating monolayers for surface-initiated (SI) reversible addition-fragmentation chain transfer polymerization (SI-RAFT) and SI atom transfer radical polymerization (SI-ATRP). Initiator-functionalized substrates were stored under various conditions and grafting densities of the resulting polymer brush films were determined via in situ ellipsometry swelling experiments. Decomposition of one of the examined SI-RAFT initiators resulted in limited reproducibility for polymer brush surface modification. In contrast, initiators for SI-ATRP showed excellent stability and reproducibility. While both techniques bring inherent benefits and limitations, the described findings will help scientists choose the most efficient technique for their goals in chemical and topographical surface modification.

Responsive Surface with Switchable Surface Properties via Mixed Polymer Brushes

Mixed polymer brushes are important “smart” materials due to the unique reversible switching ability of surface property. To leverage the advantage of light-mediated polymerizations in our group, orthogonal chemistry can be performed by carefully choosing the reaction wavelength for each individual step. Combining the SI-PET-RAFT described above and light-mediated SI-ATRP, the orthogonal manufacturing of binary polymer brushes can be achieved. My preliminary work confirms the feasibility of combining SI-RAFT and SI-ATRP to make binary polymer brushes. The switching of hydrophilicity after treatment with different solvent was concluded as a result of phase separation to minimize the free energy. Note that although responsiveness of binary polymer brushes has been observed, the change of surface chemical composition after solvent treatment is expected to be more distinctive. To improve the switching ability, I am currently working on exploring varied polymer combinations, attempting alternative photo-polymerization pathways (i.e., orthogonal dual SI-PET-RAFTs), and understanding the nanostructure of mixed brushes via ellipsometry and resonance soft/tender X-ray reflectivity.

Surface Characterization via Variable Angle Spectroscopic Ellipsometry (Multiple Collaborations with Other Groups)

Variable Angle Spectroscopic Ellipsometry (VASE) is a potent tool for surface characterizations on not only film thicknesses and grafting density mentioned above, but also optical properties, roughness, surface anisotropy, etc. I measured poly(N,N-dimethyl acrylamide) (PDMA) brushes grafted on glass slides prepared by Boyer Group (the University of New South Wales, Sydney, Australia), and established a model to fit their glass substrates and PDMA brush films. I also assisted in the study of corroded glass by Kim Group (the Penn State University) where anisotropic model was used to fit corroded layer with presence of potassium phosphate-based buffer solutions.

Future Direction

Polymer surface coatings find various applications ranging from household products to nanotechnology devices. As my past and current work has secured collaboration with 3M and Corning, I view my research as a good complement to industrial manufacturing. Thus, I would like to push forward my research into industrial development. By dedicating my knowledge into industrial applications, I believe I can convert my professional skills into products that will benefit broader communities.

Publications:

1. Mingxiao Li, Michele Fromel, Dhanesh Ranaweera, and Christian Pester*, Comparison of Long-Term Stability of Initiating Monolayers in Surface-Initiated Controlled Radical Polymerizations, Rapid Commun., 2020, 41, 2000337, DOI: 10.1002/marc.202000337
2. Mingxiao Li and Christian Pester*, Mixed Polymer Brushes for “Smart” Surfaces, Polymers, 2020, 12, 1553, DOI: 10.3390/polym12071553
3. Mingxiao Li, Michele Fromel, Dhanesh Ranaweera, Sergio Rocha, Cyrille Boyer* and Christian Pester*, SI-PET-RAFT: Surface-Initiated Photoinduced Electron Transfer-Reversible Addition–Fragmentation Chain Transfer Polymerization, ACS Macro Lett. 2019, 8(4), 374-380, DOI: 10.1021/acsmacrolett.9b00089
4. Michele Fromel, Mingxiao Li, and Christian Pester*, Surface Engineering with Polymer Brush Photolithography, Rapid Commun., 2020, 41, 2000177, DOI: 10.1002/marc.202000177
5. Gervase Ng, Mingxiao Li, Jonathan Yeow, Kenward Jung, Christian W. Pester,* and Cyrille Boyer*, Benchtop Preparation of Polymer Brushes by SI-PET-RAFT: The Effect of the Polymer Composition and Structure on Inhibition of a Pseudomonas Biofilm, ACS Appl. Mater. Interfaces, 2020, 12, 55243-55254, DOI: 10.1021/acsami.0c15221
6. Huseyin Kaya, Dien Ngo, Seung Ho Hahn, Mingxiao Li, Hongtu He, Beyza Yedikardeş, İlkay Sökmen, Christian W. Pester, Nikolas J. Podraza, Stephane Gin, and Seong H. Kim, Estimating Internal Stress of Alteration Layer Formed on Corroded Glass Through Spectroscopic Ellipsometry Analysis, Submitted to ACS Applied Materials & Interfaces