Nanoscale adsorbed structures as a robust approach for tailoring polymer film stability

Abstract

The stability or wettability of thin polymer films on solids is of vital interest in traditional technologies as well as in new emerging nanotechnologies. We report here that nanoscale structures of polymer chains adsorbed onto a solid surface play a crucial role in the thermal stability of the film. In this study, polystyrene (PS) spin-cast films (20 nm in thickness) with eight different molecular weights prepared on silicon (Si) substrates were used as a model. When low molecular weight (Mw≤ 50 kDa) PS films were subjected to thermal annealing at temperatures far above the bulk glass transition temperature, dewetting occurred promptly, while high molecular weight (Mw≥ 123 kDa) PS films were stable for at least 6 weeks at 150 °C. We reveal a strong correlation between the film stability and the two different interfacial structures of the adsorbed polymer chains: their opposing wettability against chemically identical free polymer chains results in a wetting-dewetting transition at the adsorbed polymer-free polymer interface. This is a unique aspect of the stability of polymer thin films and may be generalizable to other polymer systems regardless of the magnitude of solid-polymer attractive interactions.

Stability of Adsorbed Polystyrene Nanolayers on Silicon Substrates

“…The raw Si (100) wafers (purchased from University Wafer Inc.) were precleaned using a hot piranha solution (i.e., a mixture of H 2 SO 4 and H 2 O 2 , caution: a piranha solution is highly corrosive upon contact with skin or eyes and is an explosion hazard when mixed with organic chemicals/materials; extreme care should be taken when handing it) for 30 min, and were subsequently rinsed with deionized water thoroughly. Previous X‐ray reflectivity (XR) study proved that a native oxide (SiO x ) layer of about 2 nm in thickness was formed at the substrate surface with a surface roughness of less than 0.5 nm after the piranha solution cleaning . Hereafter, this substrate was assigned as “native oxide Si.” The water contact angle of the native oxide Si (just before spin‐coating of polymer films) was estimated to be less than 10°.…”

“…Note that all the native oxide Si substrates were treated with exactly the same protocol to avoid possible differences in the surface chemistry of the oxide layer that affects the stability of PS thin films . The “interfacial sublayer” (composed of the loosely adsorbed chains and flattened chains, as illustrated in the top part of Figure c) and “flattened layer” (composed of the lone flattened chains, as illustrated in the bottom part of Figure c) on the native oxide Si were prepared independently by the established protocol: first, PS thin films (≈6 R g in thickness) were annealed at T >> T g for given times (up to 144 h) in an oil‐free vacuum oven (below 10 −3 Torr). To extract the interfacial sublayer, the annealed PS films were then solvent leached in baths of fresh toluene at room temperature until no obvious change in thickness was observed (typically a total of 3 cycles with 30 min per cycle).…”

“…Since irreversible polymer adsorption takes place even when a polymer–solid interaction is weak, the formation of an adsorbed layer at the interface is almost unavoidable when a thin polymer film is annealed at temperatures far above the bulk glass transition temperature ( T >> T g ). Recent studies have shown that the irreversible adsorption of polymer chains at the polymer–solid interface leads to significant deviations in the physical and mechanical properties of thin polymer films, including glass transition, thermal expansivity, local viscosity, interdiffusion, thermal stability, and crystallization processes, relative to their bulk counterparts. Moreover, although the adsorbed layer is typically much thinner than the unperturbed radius of polymer gyration ( R g ), the effect can propagate several tens of nanometers into the film interior …”

“…The well‐established Guiselin's approach (based on solvent leaching) has been utilized to investigate the detailed structures and dynamics of the irreversibly adsorbed polymer layers. With the approach, we recently revealed that when the molecular weight ( M w ) of a polymer is beyond a critical value ( M w, c , e.g., 50 kDa < M w, c < 123 kDa for polystyrene (PS)), the adsorbed layer consists of two different chain conformations: flattened chains (indicated in blue in Figure c) that constitute the inner higher density region of the adsorbed layer and loosely adsorbed polymer chains (indicated in red in Figure c) that form the outer bulk‐like density region . It is evidenced that this two‐layer formation is attributed to piecemeal deposition with differential spreading dictated by the still‐uncovered surface area .…”

“…We further advanced the leaching conditions (type of a solvent, leaching time, leaching temperature, etc. ), allowing us to remove the outer loosely adsorbed chains preferentially and to uncover the inner flattened chains. This is possible by utilizing the large difference in desorption energy between the two layers which is proportional to the number of segment–surface contacts .…”