Prof. Ophelia K. C. Tsui      

Department of Physics, Boston University

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Dynamics of Polymer Thin 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. 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 polymer thin films. Experiments were carried out to understand the factors affecting the dynamic behaviors.

I. Viscosity of Polymer Thin Films

Experimental and theoretical findings suggest that the segmental mobility of polymers is enhanced at the polymer-air interface, but is usually unaffected or slowed down at the polymer-substrate interface. We have developed a technique, based on the dynamics of surface capillary waves, that has enabled us to measure the viscosity of polymer films, either unentangled (viscous) or entangled (viscoelastic) with thickness down to 2 nm.  Our results show that when the film thickness is decreased below ~10 nm, the thin film viscosity often vary significantly with the film thickness. By embracing surface effects, we have been able to account for our observations. Specifically, in systems where the substrate effect is relatively insignificant, as in polystyrene films supported by oxide-coated silicon, a two-layer model assuming hydrodynamic coupling between an upper surface mobile layer and a bulk-like lower layer is able to reproduce the observed viscosity reduction. But in systems such as poly(methyl methacrylate) films supported by silica, where simultaneous viscosity reduction and enlargement were observed, a three-layer model including an additional, slow substrate layer is required.

Key Publications

1.   "Glass Transition Dynamics and Surface Layer Mobility in Unentangled Polystyrene Films”, Z. Yang, Y. Fujii, F. K. Lee, C. -H. Lam, and O. K. C. Tsui, Science, 328, 1676-1679 (2010).

2. "Surface Dynamics of Noisy Viscoelastic Films by Adiabatic Approximation", Chi -Hang Lam, Ophelia K. C. Tsui, Dongdong Peng, Langmuir, 28, 10217-10222 (2012).

3. “Viscosity of PMMA on Silica: Epitome of Systems with Strong Polymer-Substrate Interactions", Ranxing N. Li, Fei Chen, Chi-Hang Lam, O. K. C. Tsui, Macromolecules, 46, 7889-93 (2013).

4.  “Crossover to Surface Flow in Supercooled Unentangled Polymer Films, C. -H. Lam and O. K. C. Tsui, Phys. Rev. E, 88, 042604 (2013).

5. “Viscosity and Surface-Promoted Slippage of Thin Polymer Films Supported by a Solid Substrate”, F. Chen, D. Peng, C. -H. Lam and O. K. C. Tsui, Macromolecules, 48, pp. 5034-9 (2015)”.

6. “"Effect of Confinement on the Effective Viscosity of Plasticized Polymer Films", F. Chen, D. Peng, Y. Ogata, K. Tanaka, Z. Yang, Y. Fujii, N. L. Yamada, C. -H. Lam, and O. K. C. Tsui, Macromolecules, 48, pp. 7719-26 (2015)”.

 

 

II. Effect of a high-mobility surface layer at the polymer/air interface on the Tg of the films

In this series of experiments, we investigate how the thin film Tg may be altered upon imposition of factors that can potentially affect the chain mobility at the free surface. These include variations in the polymer molecular weight, polymer end groups and substrate surfaces with various interfacial energy with the polymer.

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.  “Molecular-weight Dependent Tg Depression of Silica-Supported Poly(-methyl styrene) Films”, K. Geng, F. Chen, O. K. C. Tsui, J. Non-Cryst. Solids, 407, 296-301 (2015)”.

6. “Effects of Polymer Tacticity and Molecular Weight on the Glass Transition Temperature of Poly(methyl methacrylate) Films on Silica”, Kung Geng and Ophelia K. C. Tsui (submitted).

 

 

III. 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).

                                                                                                                                                                                                                                                                     

Last revised on January 21, 2016.