SURFACE AND INTERFACE MECHANICS OF POLYMERIC MATERIALS
Jing Zhou1,*, Andrew Minor2, Lisa A. Pruitt1, Kyriakos Komvopoulos1
1 Mechanical Engineering, University of California, Berkeley
2 National Center of Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California
Over the last century, a wide variety of polymers have emerged as invaluable engineering materials for biomedicine, functional coatings and microelectronics. In almost all cases, the surfaces and interfaces of polymeric materials are crucial to their performance. When the polymers have at least one dimension comparable to the molecular size, their molecular chains are perturbed in the vicinity of surfaces or interfaces, resulting in different properties from the bulk. It is therefore crucial to investigate surface and interface properties to improve the performance of polymeric materials for their applications. This research is aimed at understanding the surface and interface nanomechanics of polymer thin films that have been widely used in biomedical engineering and microelectronics.
Ultra high molecular weight polyethylene (UHMWPE) is the primary polymer used in total joint replacements. An in-situ TEM study was conducted on thin UHMWPE specimens to investigate nanoscale viscoelastic-plastic deformation and crack growth processes during a full nanoindentation cycle. The indentation-induced development of a deformation zone during loading and the residual plastic strain zone after unloading were quantitatively measured. A pre-existing surface nanodefect was found to introduce two-step crack growth: parallel propagation and lateral crack deflection, leading to ultimate material failure. These direct observations in real time are in good agreement with earlier numerical predictions using finite element simulations.
Quantitative nanoindentation was conducted to study surface and interface nanomechanics of polymer thin films. The effects of loading rate, thickness, and molecular weight on surface and interface viscoelastic behavior of a model system, poly (methyl methacrylate) (PMMA), thin films was studied. The through-thickness variation of the stiffness for film thickness greater than the radius of gyration revealed the existence of three regimes of distinctly different viscoelastic behavior – a surface layer, a mid-layer and an interface layer. Loading rate dependence indicates different chain relaxation rates at both surfaces and interfaces.
Dynamic nanoindentation was also performed on PMMA thin films. This study demonstrates that the polymer thin film exhibits similar behavior as its bulk counterpart, but is distinctive at the surface and the interface where it shows relatively lower viscosity at low frequency (10 Hz < f <105 Hz). There is no distinguishable difference in the viscoelastic response at high frequencies (105 Hz < f < 200 Hz).
These studies expand the understanding of the fundamental physics of polymer surfaces and interfaces at nanoscales, and will find important use in a variety of technological applications, including biomedicine, tissue engineering and micro/nanoelectroncis.
*Present address: BLDG 224, Room A 323, Polymers Division
National Institute of Standard and Technology
100 Bureau Drive
Gaithersburg, MD 20899-8541
Tel: (301) 975-4599
Fax: (301) 975-3928
Advisor: Christopher Soles