For polymer films that are prone to rupture, it has been long observed that by increasing the molecular weight (Mw) of the polymer much above ~100 kg/mol does not improve the film durability much even though the viscosity of the bulk polymer increases steeply with Mw, following ~Mw3.4. In a recent experiment, the PI’s group found that the viscosity of polystyrene (PS) coated on silicon oxide (SiOx) - a representative system - is independent of Mw for Mw > ~100kg/mol (Fig. 1), providing phenomenological explanation to the old observation. Importantly, the high-Mw viscosity was found to depend on the substrate surface but the low-Mw viscosity was not (Fig. 2). The latter can be understood from earlier findings that the viscosity at low-Mw is dominated by the high-mobility polymer chains present in the polymer-air interface. The data of Fig. 2 suggests that the high-Mw viscosity is dictated by a different mechanism that depends on the substrate surface. This renders the exciting possibility that the durability of unstable polymer films may be significantly improved by proper design of the substrate surface. Work is under way to explore this possibility.

Viscosity of Polymer Confined between a Free Surface and a Solid Substrate

Ophelia K. C. Tsui, Trustees of Boston University, DMR 1310536

104 105 106103

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Fig. 1. Viscosity versus Mw for PS films coated on SiOx. Different symbols denote different film thicknesses (see figure legend). The dotted line denotes the bulk viscosity.

Fig. 2. Viscosity of 3nm thick PS films coated on hydrogen- terminated silicon (HSi) divided by that of 3nm thick PS films on SiOx, plotted versus Mw.

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Viscosity of Polymer Confined between a Free Surface and a Solid Substrate

Ophelia K. C. Tsui, Trustees of Boston University, DMR 1310536

Publications:

1. “The Surface Mobility of Glasses”, F. Chen, C.-H. Lam, O. K. C. Tsui, Science, 343, 975 (2014).2. “Enhanced Water Flux in the Vertically Aligned Carbon Nanotube Array and the Polyethersulfone Composite

Membrane”, S. Li, G. Liao, Z. Liu, Y. Pan, Q. Wu, Y. Wng, X. Zhang, Z. Yang and O. K. C. Tsui, J. Mater. Chem. A, 2, 12171-6 (2014). (This work was showcased in the journal cover.)

3. “Viscosity of Polymer Confined between a Free Surface and a Solid Substrate”, D. Peng, F. Chen, C. -H. Lam and O. K. C. Tsui (submitted).

4.  "Molecular-weight Dependent Tg Depression of Silica-supported Poly( -a methyl styrene) Films”, K. Geng, F. Chen and O. K. C. Tsui (submitted).

Mr. Xuyang Wang, a Masters student of Soochow University in China, visited the PI’s lab. last fall to get trained on using atomic force microscopy for surface characterization.

Training of 1 postdoc, 1 graduate student, and 1 visiting student from China (see highlights below.)

Graduate student, Kun Geng presented a poster at the Gordon Research Conference on Polymer Physics this July.

Visiting student, Xuyang Wang, of Soochow University, China, received training on the use of atomic force microscopy in the PI’s lab. It was the first time he studied overseas.

Members of WISE at Warren, a group of Boston University freshmen women toured the PI’s lab. this April.

Four items were published1-2 or submitted3-4 as refereed journal articles.

Seven presentations were given at professional meetings.

Highlights of Broader Impact: