Non-Liquidlike Behaviors in Molten Polymer FilmsNon-Liquidlike Behaviors in Molten Polymer Films

Ophelia Tsui, Boston University, DMR 0706096

Log-log plot of qlc vs. film thickness for PS films (Mw = 13.7 kg/mol) spin-coated on silicon upon annealing at 120 oC for 1 h (open triangles) and 16 h (solid triangles). The dash line has slope = -2. Both solid lines have slope = -3/4.

Several experiments suggest that polymers that are normally a liquid above the melting (or glass transition) temperature can behave like a solid under confinement in nanometers. The elastic modulus, G, found, namely about 1 kPa, though small, is expected to significantly lengthen the rupture time of the films if they are unstable. For example, a rupture time of 1 day for a liquid film (G = 0) should lengthen to several years if the film had been elastic with G = 1 kPa. On the other hand, no extraordinary stability has ever been found in these films, leading to a mystery.

In this work, we demonstrate that insufficient thermal annealing of the films can produce the same non-liquidlike behavior observed in those early experiments, namely exhibition of a slope of ~ -3/4 in a log-log plot of qlc vs. film thickness, h, where qlc is related to the characteristic length scale, lc, in the surface structure of the films by qlc 2/lc. Liquid films, on the other hand, should show a slope of -2.

As the figure at right shows, all the films we studied exhibit the non-liquidlike behavior when the annealing time is short (= 1 h); but many of them (i.e., those with h < 13 nm) display the liquid behavior when the annealing time is increased to 16 h. Persistence of the nonliquidlike behavior in the thicker films can be explained by the longer length-scales involved in their surface structure (or smaller qlc) that take longer times to evolve. Our result provides an explanation for the above mystery. This work has been published in Macromolecules, 41, 8785 (2008).

10 100

1

10

slope = -3/4

slope = -2

Nonliquid-like

16 h 1 h

q lc

(m

-1)

h (nm)

Liquidbehavior

0 10 20 30 40 50 60 70-10

-8

-6

-4

-2

0

Y A

xis T

itle

X Axis Title

B

0 10 20 30 40 50 60 70-10

-8

-6

-4

-2

0

Y A

xis T

itle

X Axis Title

B

Freshly preparedfilm:Intermediacy:(nonliquidlike behavior)

Equilibrium:(liquid behavior)

Education: Ophelia Tsui, Boston University, DMR 0706096

Training of 2 post-doctoral fellows, 2 graduate students, 2 undergraduate students and 1 high-school senior.

Postdoctoral fellow, Dr. Yoshihisa Fujii from Japan gave his first oral presentation in the U.S. at the 10th Annual Greater Boston Area Statistical Mechanics Meeting. Physics junior Jessica Leach gave her first oral presentation at the 10th Annual Greater Boston Area Statistical Mechanics Meeting and poster presentation at the Conference for Undergraduate Women in Physics at Yale. She also co-authored a paper with the group members that has been submitted to a Journal. High-school intern Rebecca Masel gave an oral presentation to the group towards the end of her summer internship in 2008. She also wrote a report on the work she did.

Outreach: Academic advisor to Boston University’s Women in Physics, a university student organization aimed at fostering an environment that promotes female physicists. Coedited the book, Polymer Thin Films with Prof. Tom Russell of UMass Amhert, embracing chapters contributed by chemists, engineers and physicists with expertise in the subject from around the world. 5 refereed journal articles and 1 book chapter were published1-4 or submitted5-6. One of the published articles3 was selected for the July 6, 2009 issue of Virtual Journal of Nanoscale Science and Technology. 8 presentations were given in professional conferences or academic institutions.

BU Physics junior Jessica Leach (class of ’10) is organizing her experimental data by the group’s ellipsometer.

Rebecca Masel of The Tatnall School, Wilmington, Delaware is preparing polymer thin film samples.

1 Z. H. Yang, Y. J. Wang, L. Todorova and O. K. C. Tsui, “Examination of Non-Liquidlike Behaviors in Molten Polymer Films”, Macromolecules, 41, 8785 (2008).2 X. Zhang, F. K. Lee, O. K. C. Tsui, "Wettability of End-Grafted Polymer Brush by Chemically Identical Polymer Films", Macromolecules, 41, 8148 (2008).3Z. H. Yang, C. -H. Lam, N. Buoet, Elaine DiMasi, J. Jordan-Sweet, O. K. C. Tsui, “Method to Measure the Viscosity of Nanometer Liquid Films from the Surface Fluctuations”, App. Phys. Lett., 94, 251906 (2009).

4O. K. C. Tsui, “Anomalous Dynamics of Polymer Films” in Polymer Thin Films, Ch. 11, eds. O. K. C. Tsui, T. P. Russell, World Scientific, 2009.5Y. Fujii, Z. H. Yang, J. Leach, O. K. C. Tsui (submitted).6Z. Yang, Y. Fujii, F. K. Lee, C. -H. Lam, and O. K. C. Tsui (submitted).