Viscosity of PMMA on Silicon: Evidence of Simultaneous Influences of the Free Surface and Substrate Interface

Ophelia Tsui, Trustees of Boston University, DMR 1004648

In the last reporting year, the PI’s group found that the viscosity - a measure of the flow property - of polymer nanometer films can be significantly different from the bulk. Contingent to whether the free surface or substrate interface dominates, it can decrease or increase as a function of the film thickness, respectively. This year, they study in detail the viscosity of poly(methyl methacrylate) (PMMA) supported by silicon, a system well-known to be dominated by the substrate interface. The data, displayed at left, shows unexpectedly that different interfaces can in fact dominate in different temperature regions, demonstrating the difficulty to infer the flow property of the films from anything less than a full study as accomplished here. By applying a three-layer model illustrated by the drawing at left – which is an extension of the two-layer model the PI and coworkers developed under the NSF award DMR-0706096 – this group was able to use the bulk data and the data in the low temperature region alone to predict the data in the high temperature region, and vice-versa. This work has been submitted to Macromolecules for publication.4

60 80 100 120 140 160102

103

104

105

106

107

108

109

1010

Vis

cosi

ty (

Pa

s)

Temperature (oC)

Bulk 86nm 41nm 24nm 14nm 7.8nm 5.3nm 3.2nm

Mobile

No flowPMMA

Silicon

Bulklike

Surface dominant

Substrate dominant

Ophelia Tsui, Trustees of Boston University, DMR 1004648

Education: Training of two Ph.D. students and one exchange Ph.D. student from China.

Materials & Engineering Ph.D., Nancy Ranxing Li graduated this May. She is currently an engineer in the Shell Oil Co.

Physics Ph.D. student, Dongdong Peng is graduating this September. He has accepted a job offer from CGGVeritas.

Nancy and Dong have both given oral presentations at professional meetings. Nancy gave a poster presentation at the Materials Research Society (MRS) meeting last fall.

Exchange student, Fei Chen gave an oral presentation at the 14th Annual Greater Boston Statistical Mechanics Meeting last fall. It was the first time he gave a talk in English.

Outreach: Four items were published1-3 or submitted4,5 as refereed journal articles or Ph.D. thesis.

Eleven presentations were given in professional meetings or academic institutions.Publications:

1. “Power Spectral Density of Free-standing Viscoelastic Films by Adiabatic Approximation”, Haiyao Teng, Nancy Ranxing Li, Haitao Huang, Ophelia K. C. Tsui, Chi-Hang Lam, Langmuir, 29, 4283-4289 (2013).

2. “Glass Transition Temperature of Polymer Nanocomposite Films with Tunable Particle-Polymer Interfacial Interactions”, Fei Chen, B. M. Reinhard, M. W. Grinstaff, N. Jiang, T. Koga, O. K. C. Tsui, Macromolecules, 46, 4663-4669 (2013).

3. “The Influence of Interface and Solvent Quality on the Dynamics of Polymer Films”, Ph.D. Dissertation for the School of Engineering, Boston University, Boston, MA (May 2013).

4. “Viscosity of PMMA on Silica: Epitome of Systems with Strong Polymer-Substrate Interactions”, R. N. Li, F. Chen, C. H. Lam, and O. K. C. Tsui (submitted).

5. “Crossover to Surface Flow in Supercooled Unentangled Polymer Films, C. -H. Lam and O. K. C. Tsui (submitted).

Members of WISE at Warren, a Boston University student group comprised of freshmen women in science toured the PI’s lab. on April 19, 2013. The equipment being demonstrated is a new atomic force microscope installed in the lab. a year ago.