Measurement of viscosity of confined polymer melt using capillary kinetics

We present a simple method to measure viscosity of thin polymer melt that is confined between a substrate and a permeable plate using a modified Poiseuille equation. When a patterned polydimethylsiloxane (PDMS) mold is placed on a styrene-butadiene-styrene (SBS) block copolymer film spin-coated onto a substrate and heated above the polymer's glass transition temperature, capillarity forces the polymer melt into the void of the channel. To calculate the viscosity of the confined polymer melt, the height of capillary rise was measured as a function of time for a number of film thicknesses (220, 350, 540, 1000, and 1300 nm) and at two different temperatures (70 and 100 degrees C). It was found that the viscosity increases with decreasing film thickness at 70 degrees C, whereas it decreases with decreasing film thickness at 100 degrees C. This discrepancy might be related to the confinement-induced solid-like behavior of the polymer melt and wall slip at the polymer/solid interface. Furthermore, the viscosity turned out to be nearly equal to the bulk value for relatively large film thickness (> similar to 500 nm) regardless of the temperature, which corresponds to earlier findings.