Formation Mechanism of High-Density, Flattened Polymer Nanolayers Adsorbed on Planar Solids

Abstract

Thermal annealing is one of the most indispensable polymer fabrication processes and plays essential roles in controlling morphologies and properties of polymeric materials. We here report that thermal annealing also facilitates polymer adsorption from the melt on planar silicon (Si) substrates, resulting in the formation of a high-density polymer nanolayer with flattened chain confirmations. Three different homopolymers (polystyrene, poly(2-vinylpyridine), and poly(methyl methacrylate)), which have similar inherent stiffness and bulk glass transition temperature (T g ), but have different affinities with Si substrates, were chosen as models. Spin-cast films (∼50 nm in thickness) with the three polymers were prepared on cleaned Si substrates and then placed in a vacuum oven set at a temperature far above the bulk T g . In order to monitor the polymer adsorption process at the solid-polymer melt interface during thermal annealing, we used the protocol that combines vitrification of the annealed films (via rapid quench to room temperature) and subsequent intensive solvent leaching (to remove nonadsorbed chains). The detailed structures of the residual films (i.e., flattened layers with 2−3 nm in thickness) were characterized by using X-ray reflectivity and atomic force microscopy. As a result, we found that the film thicknesses of the flattened layers for the three different polymers increase as a power-law of annealing time before reaching the "quasiequilibrium" state where the film growth is saturated. We have also revealed that the final thickness of the flattened layer at the quasiequilibrium state increases with increasing the solid-segment interaction, while the kinetics becomes more sluggish. The observed formation kinetics corresponds to a "zipping-down" process of the transient flattened chains on planar solids in order to further increase the number of solid/segment points, which is the driving force for flattening so as to overcome the conformational entropy loss in the total free energy.

Stability of Polymer Ultrathin Films (<7 nm) Made by a Top-Down Approach

“…0 (R g ranging from 3 nm to 18 nm) was obtained, close to the one obtained by Fujii et al 25 for native oxide covered Si (h ads ≈ 0.47×R g ) and confirm that the longer the PS chains, the thicker the adsorbed layer as reported also by Housmans 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 in agreement with the work of Fujii. 26 For longer times, differences only arise when films are preannealed or not.…”

“…When films are annealed at 160 o C during 24 h before rinsing, ascribed as preannealed films, the decrement of thickness after the first rinsing is smaller as shown inFigure 2b. The adsorption of PS segments initiated during the spin-coating is thus favoured by annealing the samples above T g as reported by Napolitano10 and Jiang et al25 …”

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 surface (presumably no dewetting) and a film thickness close to 2.9 nm. According to the study of Koga's group, 24,25 we can assume that during this first rinsing, the loosely attached layer was partly removed. This rinsing process was repeated several times on the same sample as shown in Figure 2a.…”

“…This observation is in agreement with Jiang et al's report. 25 Hence a rinsing time (t rins ) below 100 min was chosen for exploring the stability of such smooth ultrathin residual films presented in the second part of this paper. Additionally, we observed that in pre-annealed samples the adsorbed layer thickness obtained after 10 min of toluene rinsing was found to increase almost linearly with the radius of gyration (e.g.…”

Thermoreversible gelation in poly(ethylene oxide)/carbon black hybrid melts

“…When polymer is adsorbed to solid substrates, the inner polymer layer is permanently attached to the substrate and resists detachment even under intensive chemical leaching, but the outer polymer layer is only loosely attached and displays contraction upon heating, in a thermoreversible manner (25,26). The combination of those factors might account for the behaviour observed here, e.g.…”