Non-equilibrium polymer dynamics
Polymer materials, including plastics, fibers, thin films, and membranes, are often in a non-equilibrium state. This is due to deviations in chain conformations from Gaussian statistics (which cause entropic stress), distortions of chemical bonds (which cause enthalpic stress) because of the non-equilibrium processing conditions, and sluggish dynamics induced by vitrification that prevent structure recovery during storage and use. We studied the behavior of polymers in non-equilibrium states, examining temporal and spatial extents ranging from local rearrangement during physical aging to entropic recoiling of polymer chains. Our goal was to explore the molecular-level mechanisms underlying various dynamical processes and non-equilibrium phenomena, including glass transition, structural relaxation, physical aging, conformational recovery, and orientational relaxation of polymer melts and glasses.
The topics under investigation are:
1. Glass transition of long chain polymers
2. Physical aging and structural relaxation of polymers and polymer nanocomposites
3. Relaxation of the entropically stressed polymer glasses
Featured publications:
(1) J. Luo, X. Wang, B. Tong, Z. Li, L. A. Rocchi, V. Di Lisio, D. Cangialosi*, B. Zuo*. Length Scale of Molecular Motions Governing Glass Equilibration in Hyperquenched and Slow-Cooled Polystyrene. J. Phys. Chem. Lett. 2024, 15, 357−363.
(2) Y. Yang, Q. Xu, T. Jin, X. Wang, S. Napolitano*, B. Zuo*. Relaxations of Entropically Stressed Polymer Glasses. Macromolecules 2023,56, 5924–5931.
Polymers under confinement
The rapid development of nanotechnology in recent years has created a demand for polymer-based nano-sized devices or materials that exhibit superior performance and stability. As material size decreases to the nanoscale, approaching the typical chain coil size, molecular dynamics and associated physical properties such as glass transition, molecular diffusion, and viscosity deviate greatly from bulk behavior. This phenomenon is known as the confinement effect and severely hampers practical usage. Our research focuses on developing new methods to measure the confined polymer behavior, especially the viscoelasticity, rheology, and molecular dynamics, in the nanopolymer systems, e.g., nanoscale thin films, and exploring the mechanisms underpinning confinement induced variation in polymer behaviors. Results of the studies could help produce polymer nanodevices in a more controlled manner.
The topics under investigation are:
1. Glass transition and equilibrium recovery of nanoscale thin polymer films
2. Nano-rheology and linear viscoelasticity of thin polymer films
3. Crack formation and failure of thin polymer films
4. Rheology and chain dynamics of polymer nanocomposites
Featured publications:
(1) Q. Xu, N. Zhu, H. Fang, X. Wang, R. D. Priestley, B. Zuo*. Decoupling Role of Film Thickness and Interfacial Effect on Polymer Thin Film Dynamics. ACS Macro Lett. 2020, 1, 1-8.
(2) H. Zha, Q. Wang, X. Wang, D. Cangialosi, B. Zuo*. Enhanced Free Surface Mobility Facilitates the Release of Free-Volume Holes in Thin-Film Polymer Glasses. Macromolecules 2021, 54, 2022-2028.
Polymers at surface and Interface
Polymeric materials display distinctive structures and dynamics at their surfaces and interfaces due to effects such as the modification of neighboring interactions, symmetry breaking, and chain reflection from the boundary. Polymers located near the air surface experience enhanced mobility, reduced glass transition temperature, a gradient in segmental mobility, reduced entanglement, and an increment of local orientation. On the other hand, polymers located near a substrate interface experience suppressed segmental dynamics and hindered diffusion due to physical adsorption, as well as slowed polymer crystallization from glasses. We studied how chains behave near the air surface and substrate interface of polymer materials. This helps us understand the mechanisms behind fundamental behaviors of polymer materials at surface and interfaces, such as interfacial adhesion, tribology, and wetting of polymer materials.
The topics under investigation are:
1. Viscoelasticity and chain dynamics at surface of polymer glasses
2. Pseudoentanglments and rubbery behaviors at surface of polymer materials
3. Chain adsorption and dynamics at solid interface
4. Interface engineering of polymer based photoelectric devices
Featured publications:
(1) Z. Hao, A. Ghanekarade, N. Zhu, K. Randazzo, D. Kawaguchi, K. Tanaka, X. Wang, D. S. Simmons*, R. D. Priestley*, B. Zuo*. Mobility gradients yield rubbery surfaces on top of polymer glasses. Nature 2021, 596, 372-376.
(2) Houkuan Tian, Jintian Luo, Qiyun Tang*, Hao Zha, Rodney D. Priestley*, Wenbing Hu, Biao Zuo*. Intramolecular Dynamic Coupling Slows Surface Relaxation of Polymer Glasses. Nature Communications 2024, 15, 6082.
(3) H. Tian, C. Bi, Z. Li, C. Wang, B. Zuo*. Metastable polymer adsorption dictates the dynamical gradients at interfaces. Macromolecules 2023, 56, 4346-4353.
(4) Y. Yang, H. Tian, S. Napolitano*, B. Zuo*. Crystallization in thin films of polymer glasses: The role of free surfaces, solid interfaces and their competition. Prog. Polym. Sci. 2023, 144, 101725.
(5) H. Tian, Q. Xu, H. Zhang, R. D. Priestley, B. Zuo*. Surface dynamics of glasses.Appl. Phys. Rev.2022, 9, 011316.