two-layer model.

1. Correlating Anomalous Viscosity of Polymer Films and Their Interfacial Dynamics by the Two-layer Model Ophelia Tsui, Trustees of Boston University, DMR 1004648 Nanometer polymer films have many important applications in nano-technology. One property essential to these applications is the films’ viscosity or degree of fluidity. In this study, her group finds that the viscosity of a polymer film, h, can either decrease or increase as a function of the film thickness, h, as seen from the figures shown at right. By exploiting the two-layer model she and coworkers developed under the support of the NSF award #0706096, they deduce that the anomalous viscosity of the films is attributable to the differing dynamics within several nanometers of either surface of the film from that in the bulk polymer, hbulk. This finding suggests that the rheological properties of nanometer polymer films can be systematically tailored by exploiting their interfacial properties. Film viscosity normalized by bulk viscosity, h/hbulk plotted versus film thickness , h for polystyrene films with three different molecular weights, Mw supported by silicon oxide. The legend indicates the Mw’s in Da and measurement temperatures. The smooth lines are the best fits to the two-layer model assuming a mobile layer existing at the free surface of the films. Normalized film viscosity, h/hbulkplotted versus h for polymethylmethacrylate films with Mw = 2.7kDa supported by silicon oxide at 140oC. The solid and dotted lines are the least square fits to the two-layer model assuming a slow layer existing at the substrate and free surface, respectively. The poor fit of the latter shows that the location of the interfacial layer can be discerned by fitting the data to the two-layer model.

2. Ophelia Tsui, Trustees of Boston University, DMR 1004648 Education • Training of 1 postdoc, 2 graduate students, and 1 high school intern. Some highlights include: • Materials Science & Engineering graduate student, Ranxing Nancy Li passed the prospectus oral examination this April. In addition, she gave three talks, including one at the 13th Annual Greater Boston Area Statistical Mechanics (GBASM) Meeting, one at the 2012 APS March Meeting and one for her prospectus. • Physics graduate student, Dongdong Peng also gave talks at the GBASM and APS meetings this year. In addition, he gave a departmental seminar this June. • Katherine Jiang of Lynbrook High School, San Jose, CA took an internship with the PI last summer. She gave a poster presentation on her work in a local workshop toward the end. She is going to Caltech this fall to pursue an undergraduate degree in the STEM field. High-school intern, Katherine Jiang is using the group’s atomic force microscope (AFM) to study hole growth in a polymer film. The photo was taken in a temporary space where the AFM was relocated due to a major renovation. Outreach • The PI was the academic advisor of the Boston University Women in Physics, a student organization aimed at fostering a work environment that promotes female physicists. • Four refereed journal articles were published or submitted (please see below). • Five presentations were given in professional meetings or academic institutions. Publications • “Method to Measure the Viscoelastic Properties of Nanometer Entangled Polymer Films”, D. Peng, Z. Yang, and O. K. C. Tsui, Macromolecules, 44, 7460-7464 (2011). • “Glass Transition Dynamics and Surface Mobility of Entangled Polystyrene Films at Equilibrium” Z. Yang, A. Clough, C. -H. Lam, and O. K. C. Tsui, Macromolecules, 44, 8294-8300 (2011). • “Modeling the Dynamics of Viscoelastic Polymer Films by Adiabatic Approximation”, C. –H. Lam, D. Peng, O. K. C. Tsui, Langmuir, 28, 10217-10222 (2012). • “Two-layer Model Description of Polymer Thin Film Dynamics”, D. Peng, R. N. Li, C. –H. Lam, O. K. C. Tsui (submitted).