Revealed Architectures of Adsorbed Polymer Chains at Solid-Polymer Melt Interfaces

We report the chain conformations of polymer molecules accommodated at the solid-polymer melt interfaces in equilibrium. Polystyrene “Guiselin” brushes (adsorbed layers) with different molecular weights were prepared on Si substrates and characterized by using x-ray and neutron reflectivity. The results are intriguing to show that the adsorbed layers are composed of the two different nanoarchitectures: flattened chains that constitute the inner higher density region of the adsorbed layers and loosely adsorbed polymer chains that form the outer bulklike density region. In addition, we found that the lone flattened chains, which are uncovered by the additional prolonged solvent leaching ($\sim 120$ days), are reversibly densified with increasing temperature up to $150°\mathrm{C}$. By generalizing the chain conformations of bulks, we postulate that the change in probabilities of the local chain conformations (i.e., trans and gauche states) of polymer molecules is the origin of this densification process.

Figure 1

Time evolution of the adsorbed PS ($M_w=170\mathrm{kDa}$) layer thickness during annealing at $150°\mathrm{C}$. The solid line corresponds to the proposed function described in the text. In the inset, the temperature dependence of the thickness of the equilibrium adsorbed PS ($M_w=170\mathrm{kDa}$) layer is shown.

Figure 2

Observed XR profiles for the equilibrium adsorbed PS ($M_w=123\mathrm{kDa}$) layer (circles). The solid line corresponds to the best fit based on the density profile against the distance ($z$) from the ${\mathrm{SiO}}_2$ surface shown in the inset. The dispersion values were converted into the density of the film relative to the bulk by using the measured dispersion value of the bulk.

Figure 3

Desorption kinetics of the equilibrium adsorbed PS ($M_w=170\mathrm{kDa}$) layer during the toluene leaching. The inset shows the schematic view of the two different chain conformations.

Figure 4

(a) Representative XR profiles for the flattened layer ($M_w=290\mathrm{kDa}$) at the three temperatures [the data correspond (from the top) to 50, 100, and 150 °C, respectively] and (b) temperature dependence of $h_{\mathrm{flat}}$ along with the calculated values of the $[(1+⟨\cos \varphi ⟩)/(1-⟨\cos \varphi ⟩){]}^{1/2}$ term in Eq. (1) (the dotted line) for PS. The inset shows the schematic conformation of the flattened chains.