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

Abstract

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 (∼120 days), are reversibly densified with increasing temperature up to 150 °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.

Unexpected Molecular Weight Effect in Polymer Nanocomposites

“…The revealed molecular weight dependence of the interfacial layer thickness is in stark contrast to what has been anticipated from theories [8][9][10][11][12] and to what can be expected from supported thin polymer film studies [20]. We see two possible reasons for this unexpected behavior: (1) PNCs require long annealing time to reach equilibrium [43,44]; or (2) the polymer chains experience a strong frustration in chain packing in the interfacial region due to the formation of large loops at higher MW.…”

“…Recently, Koga et al [20,29] explained the observed interfacial slowing down in supported thin films in terms of a bound loop layer (BLL) made of the loops of the adsorbed chains. According to them [20,29], the average height of the BLL provides a good estimate for the thickness of the DIL that experiences slower dynamics.…”

“…According to them [20,29], the average height of the BLL provides a good estimate for the thickness of the DIL that experiences slower dynamics. Considering the analogy between polymer thin films and PNCs [7,30], it is reasonable to anticipate a similar interfacial chain packing-dynamics relationship in PNCs [31,32].…”

Glass Transition of Ultrathin Sugar Films Probed by X-Ray Reflectivity

“…It has also been observed in ultrathin polymer films [16,17,20]. Mukherjee et al reported NTE below the T g in polystyrene (PS) films with thicknesses of several dozen nanometers, together with zero thermal expansion (ZTE) behavior [17].…”

“…The importance of these findings in the pharmaceutical field cannot be overstated. Specular X-ray reflectivity (XRR) is a unique and powerful analytical method (Figure 2) that is frequently applied to condensed soft-matter films, including glass transition studies of ultrathin polymer films [16][17][18][19][20], but which has not been applied in amorphous sugar studies. XRR can evaluate the layered structure of a material, such as the film thickness, electron density, surface roughness, and interfacial width [21,22].…”