Prof. Ophelia K. C. Tsui
Department of Physics, Boston University
Dynamics of Polymer Ultrathin Films
Under the effect of confinement and specific interactions with the interfaces, mechanical and dynamical properties of a polymer may be perturbed notably. These changes are expected to occur to thin films with thickness comparable to the size of the polymer molecules, which is typically of tens of nanometers. We refer to these films as ultrathin films. Experimentally, it was observed that the diffusion coefficient of polymers in a thin film decreased precipitously as the film thickness was decreased below about the size of the macromolecules; the glass transition temperature, Tg was also found to be a strong function of the film thickness. We are interested in the dynamical regime below and near the Tg . A number of new techniques were developed to probe the dynamic behaviors of the ultrathin polymer film systems. Experiments were carried out to understand the factors affecting the dynamic behaviors.
I. Dynamics revealed by Atomic Force Microscopic Adhesion Measurements (AFMAM)
By using AFMAM, we were able to directly probe the ac mechanical response of the polymer thin film over a wide dynamical range near the Tg, which at present is only possible with AFM Lateral Force Microscopy. Effort is now being put into extending the present understanding of the technique into studying polymer films of different thicknesses and correlating the results with the measured Tg of the films from other techniques such as X-ray Reflectivity and Ellipsometry.
During measurement, force-distance (FD) curves were acquired with the AFM. Adhesion force, Fad is determined from the pull off leg of the force-distance curves as the pull off force required to cause detachment between the AFM tip and the sample surface.
Figure 1 shows the FD curves obtained as a poly (tert-butyl acrylate) (PtBuA) film (Tg ~ 50oC) evolves from the glassy to the rubbery state (top to bottom). Development of curvatures in the FD curves can be attributed to softening in the polymer as the rubbery state is approached, either by increasing the temperature, T, or lowering the probe rate, f, (i.e. the rate at which the tip probes into the sample and being pulled out). However, what is immediately evident from the data is the time-temperature equivalence of the dynamics revealed by the FD curves, wherein FD curves that are acquired at equivalent time-temperature conditions (solid and dashed lines) are essentially the same.
Fig. 1 Fig. 2
A plot of Fad vs. f at different T near the Tg (Fig. 2a) further confirms the time-temperature equivalence characteristics of the dynamics being probed. Upon rescaling the abscissa of each Fad (f) curve by a temperature-dependent shift factor, aT(T), an adhesion master curve is obtained (Fig. 2b). The temperature dependence of the shift factors obtained can then be compared to the bulk data to check for any deviation.
Detailed analysis of the mechanical problem reveal that the shape of the adhesion master curve in the high-frequency regime (above the low- frequency plateau region) is proportional to the reciprocal of the time-dependent elastic modulus of the polymer, E(t). The plateau region, however, is where the sample-tip bond fracture occurs cohesively so should be considered separately from the behavior in the high-frequency regime.
Key Publications
1. "Surface Viscoelasticity Studies of Ultrathin Polymer Films Using Atomic Force Microscopic Adhesion Measurements", X. P. Wang, Xudong Xiao, O. K. C. Tsui, Macromolecules 34, 4180-4185 (2001).
2. "Studying Surface Glass-to-Rubber Transition Using Atomic Force Microscopic Adhesion Measurements", O.K.C. Tsui, X.P. Wang, Jacob Y.L. Ho, T.K. Ng, Xudong Xiao, Macromolecules, 33, 4198-4204 (2000).
Experimental and theoretical findings suggest that the segmental mobility of polymers with low molecular weight and/or with low surface-energy end groups is enhanced at the polymer-air interface. We have undertaken systematic studies on the glass transition temperature of polymer ultrathin films with different surface mobility to investigate if the surface layer has any influence on the overall dynamics of the polymer in thin films.
Key Publications
1. "Effect of Chain Ends and Chain Entanglement on Glass Transition Temperature of Polymer Thin Films", O. K. C. Tsui, H. F. Zhang, Macromolecules, 34, 9139-9142 (2001).
2. "Effect of Interfacial Interactions on the Glass Transition of Polymer Thin Films", O. K. C. Tsui, T.P. Russell, C.J. Hawker, Macromolecules, 34, 5535-5539 (2001).
3. "Dynamics of Polymers Confined in Thin Films", O. K. C. Tsui, Binyang Du, In Recent Advances of Polymer Science Overseas, eds. Tianbai He and Hangjie Hu, Chemical Industry Publishing Co., Beijing, Chapter 16, 246-263 (2001).
4. "Effect of Low Surface Energy Chain Ends on the Glass Transition Temperature of Polymer Thin Films", Fengchao Xie, H. F. Zhang, Fuk Kay Lee, Binyang Du, Ophelia K. C. Tsui, Y. Yokoe, K. Tanaka, A. Takahara, T. Kajiyama, Tianbai He, Macromolecules, 35, 1491-1492 (2002).
5. "Dynamic Study of Polymer Films by Friction Force Microscopy With Continuously Varying Load", Xiaoping Wang, O.K.C. Tsui, Xudong Xiao, Langmuir, 18(18), 7066-7072 (2002).
6. "Effect of Interfacial Interactions on the Glass Transition of Polymer Thin Films", O. K. C. Tsui, T. P. Russell, C. J. Hawker, Slow Dynamics in Complex System: 3rd International Symposium, Sendai, Japan, Nov. 2003., AIP Conference Proceedings Vol. 708, M. Tokuyama and I. Oppenheim, eds., pp. 598-600 (2004).
III. Reflectance Difference Spectroscopy (RDS)
This method detects the in-plane optical anisotropy of a sample as a function of photon energy from 1 to 6 eV, and has been widely employed in the study of semiconductor and metal surfaces. By making use of the phase-sensitive detection technique, RDS can detect optical anisotropy, ∆n*t as small as 0.2 Angstrom, where ∆n is the difference between the two orthogonal in-plane refractive indices and t the thickness of the polymer film. Because of the high sensitivity, RDS can be applied to study polymer thin film samples, which typically has very small optical anisotropy, if any. In the setup we currently have, the measurement time can be as small as ~1 s so that temporal relaxation of polymer samples near the Tg can be monitored. We had demonstrated the use of RDS in the study of rigid-rod polymer films.
Key Publications
1. "Rubbing Induced Molecular Alignment and Its Relaxation in Polystyrene Thin Films", O. C. Tsang, Fengchao Xie, O. K. C. Tsui, Z. Yang, Jianmin Zhang, Deyan Shen, Xiaozhen Yang, J. Polym. Sci. Part B: Polym. Phys. 39(22), 2906-2914 (2001).
2. "Temporal Evolution of Relaxation in Rubbed Polystyrene Thin Films", O. C. Tsang, O. K. C. Tsui, Z. Yang, Phys. Rev. E, 63, 061603 (2001).
3. "Observation of In-plane Optical Anisotropy of Spin-cast Rigid-rod Electroluminescent Polymer Films", C.W. Law, K.S. Wong, Z. Yang, L.E. Horsburgh, and A.P. Monkman, Appl. Phys. Lett., 76, 1416 (2000).
Last revised on 28 May 2007.