Revealing Interfacial Interactions in Random Copolymer Adsorbed Layers by Solvent Leaching

Abstract

Annealing a supported polymer film in the melt state results in the growth of an irreversibly adsorbed layer, which has been shown to influence thin film properties such as diffusion and glass transition temperature. Adsorbed layer growth is attributed to many simultaneous interactions between individual monomer units and the substrate, stabilizing chains against desorption. In this study, adsorbed layers of polystyrene (PS), poly(methyl methacrylate) (PMMA), and their random copolymers are isolated by select solvents. While PS adsorbed layer thickness is largely unaffected by the choice of washing solvent, the PMMA adsorbed layer completely desorbs when washed with tetrahydrofuran and chloroform, as opposed to toluene. Scaling relationships between adsorbed layer thickness and degree of chain adsorption at the substrate enable the use of adsorbed layer thickness to probe specific polymer‐substrate interactions. Composition‐dependent desorption trends indicate non‐preferential adsorption between styrene and methyl methacrylate repeat units at the substrate, despite differences in substrate interaction strength. This insight contributes to the developing mechanism for the adsorption of random copolymers during melt‐state annealing, further extending the ability to predict processing‐inducted changes to the properties of polymer thin films to heterogeneous systems.

Direct Visualization and Characterization of Interfacially Adsorbed Polymer atop Nanoparticles and within Nanocomposites

“…The growth curve reflects the rapid coverage of the nanoparticle surface at short annealing times, followed by little to no additional growth as the previously adsorbed polymer physically impedes the adsorption of subsequent chains. Observations of rapid initial growth of PS layers on the NPs are consistent with the proposed kinetic model accounting for surface coverage, , as well as growth trends reported for irreversibly adsorbed layers of PS in analogous thin-film experiments. ,, The equilibrium thickness ( t ann ≥ 10 h) of the adsorbed layer was measured to be 9.7 ± 1.1 nm, consistent with the predicted equilibrium thickness of 9.7 nm estimated on the basis of PS R g in THF (a good solvent for PS, comparable to DMF) and the linear scaling relationship h ∞ = 0.47 R g for PS adsorption upon a SiO x surface …”

“…The nanocomposite fabrication process begins with the dispersion of silica nanoparticles (SiO x NPs) into a pyrene-labeled PS matrix followed by high-temperature annealing (∼ T g + 50 K) for a select time to induce the formation of adsorbed nanolayers at the NP/matrix interface. Subsequent dissolving of the nanocomposite in toluenea good solvent for PS which does not interfere with PS/SiO x interactions to induce desorption , frees NPs of unadsorbed matrix chains but leaves irreversibly adsorbed layers intact. This approach is corroborated by several reports that sustained washing of adsorbed thin films yields a plateau layer thickness ,− ,,− and that exchange between adsorbed polymer chains and overlaying unadsorbed chains is negligible under typical working conditions of polymer/substrate systems. , Adsorbed nanolayers of pyrene-labeled PS atop isolated NPs can be directly characterized via TEM imaging and fluorescence to measure layer thickness and T g , respectively.…”

“…Subsequent dissolving of the nanocomposite in toluenea good solvent for PS which does not interfere with PS/SiO x interactions to induce desorption , frees NPs of unadsorbed matrix chains but leaves irreversibly adsorbed layers intact. This approach is corroborated by several reports that sustained washing of adsorbed thin films yields a plateau layer thickness ,− ,,− and that exchange between adsorbed polymer chains and overlaying unadsorbed chains is negligible under typical working conditions of polymer/substrate systems. , Adsorbed nanolayers of pyrene-labeled PS atop isolated NPs can be directly characterized via TEM imaging and fluorescence to measure layer thickness and T g , respectively. Furthermore, the isolated NPs can also be redispersed into a matrix of neat PS to yield nanocomposites wherein pyrene-labeled PS is isolated within the irreversibly adsorbed layer at the NP/matrix interface.…”

“…Adsorbed layer formation is strongly dependent upon processing conditions. Due to governing chain kinetics during a typical experimental timescale, adsorption occurs more readily at elevated temperatures, coinciding for practical purposes with standard processing parameters. − Moreover, longer annealing times elicit adsorbed layers of greater thickness. ,, The processing-dependent nature of irreversibly adsorbed layers has important implications in polymer nanocomposites, whose preparation typically involves a sustained high-temperature annealing step. − Taken together with the large specific surface area of polymer nanocomposites, the temperature dependence of adsorption suggests that adsorbed layers may significantly contribute to modifying nanocomposite properties. However, direct investigations of irreversible adsorptionits evolution and its consequenceshave not yet extended beyond polymer thin films , to consider polymer nanocomposites.…”

“…15−17 Moreover, longer annealing times elicit adsorbed layers of greater thickness. 6,9,18 The processing-dependent nature of irreversibly adsorbed layers has important implications in polymer nanocomposites, whose preparation typically involves a sustained high-temperature annealing step. 19−24 Taken together with the large specific surface area of polymer nanocomposites, the temperature dependence of adsorption suggests that adsorbed layers may significantly contribute to modifying nanocomposite properties.…”