Stability of Adsorbed Polystyrene Nanolayers on Silicon Substrates

Investigations of the adsorbed layer of polysulfone: Influence of the thickness of the adsorbed layer on the glass transition of thin films

This work studies the influence of the adsorbed layer on the glass transition of thin films of polysulfone. Therefore, the growth kinetics of the irreversibly adsorbed layer of polysulfone on silicon substrates was first investigated using the solvent leaching approach, and the thickness of the remaining layer was measured with atomic force microscopy. Annealing conditions before leaching were varied in temperature and time (0-336 h). The growth kinetics showed three distinct regions: a pre-growth step where it was assumed that phenyl rings align parallel to the substrate at the shortest annealing times, a linear growth region, and a crossover from linear to logarithmic growth observed at higher temperatures for the longest annealing times. No signs of desorption were observed, pointing to the formation of a strongly adsorbed layer. Second, the glass transition of thin polysulfone films was studied in dependence on the film thickness using spectroscopic ellipsometry. Three annealing conditions were compared: two with only a tightly bound layer formed in the linear growth regime and one with both tightly bound and loosely adsorbed layers formed in the logarithmic growth regime. The onset thickness and increase in the glass transition temperature increases with annealing time and temperature. These differences were attributed to the distinct conformations of the formed adsorbed layers.

Neutron reflectivity study on the nanostructure of PMMA chains near substrate interfaces based on contrast variation accompanied with small molecule sorption

In the case of poly(methyl methacrylate) (PMMA) thin films on a Si substrate, thermal annealing induces the formation of a layer of PMMA chains tightly adsorbed near the substrate interface, and the strongly adsorbed PMMA remains on the substrate, even after washing with toluene (hereinafter called adsorbed sample). Neutron reflectometry revealed that the concerned structure consists of three layers: an inner layer (tightly bound on the substrate), a middle layer (bulk-like), and an outer layer (surface) in the adsorbed sample. When an adsorbed sample was exposed to toluene vapor, it became clear that, between the solid adsorption layer (which does not swell) and bulk-like swollen layer, there was a "buffer layer" that could sorb more toluene molecules than the bulk-like layer. This buffer layer was found not only in the adsorbed sample but also in the standard spin-cast PMMA thin films on the substrate. When the polymer chains were firmly adsorbed and immobilized on the Si substrate, the freedom of the possible structure right next to the tightly bound layer was reduced, which restricted the relaxation of the conformation of the polymer chain strongly. The "buffer layer" was manifested by the sorption of toluene with different scattering length density contrasts.

Effect of Oligomer Segregation on the Aggregation State and Strength at the Polystyrene/Substrate Interface

The interfacial strength of polystyrene (PS) with and without PS oligomers in contact with a glass substrate was examined to determine the relationship between the interfacial aggregation state and adhesion. The shear bond strength and adsorbed layer thickness of neat PS exhibited a similar dependence on the thermal annealing time: they increased to constant values within almost the same time. This implies that the adhesion of the polymer is closely related to the formation of an adsorbed layer at the adhesion interface. Nevertheless, in the case of PS with a small amount of oligomer, the shear bond strength decreased, while the adsorbed layer thickness was almost the same as that of neat PS. Based on the results of interfacial analyses, we propose that the interfacial segregation of the oligomer reduced the entanglement between the interfacial free chains in the adsorbed layer and the bulk chains.

Adsorption Kinetics of Polystyrene and Poly(9-anthracenyl methyl methacrylate) onto SiO2 Surface Measured by Chip Nano-Calorimetry

The alternating current (AC) chip nano-calorimetry is a powerful tool to investigate the physical properties of polymer thin films. In this paper, we report on the adsorption kinetics of polymers in which an AC chip nano-calorimetry was used for the first time. This technique allows for the real-time measurement of the adsorption kinetics of polymer chains onto the SiO2 surface. We used polystyrene (PS) and poly(9-anthracenyl methyl methacrylate) (PAMMA), which have different chemical natures and side group sizes. It was confirmed that the observed adsorption kinetics for PS were consistent with previously reported results obtained by dielectric spectroscopy. For PAMMA, we found characteristic adsorption kinetics, which shows a clear kink at the crossover between the early and later stages, while PS exhibits a lesser tendency of showing the kink as demonstrated by previously reported results.

Solid-state polymer adsorption for surface modification: The role of molecular weight

HYPOTHESIS

Solid-state polymer adsorption offers a distinct approach for surface modification. These ultrathin, so-called Guiselin layers can easily be obtained by placing a polymer melt in contact with an interface, followed by a removal of the non-adsorbed layer with a good solvent. While the mechanism of formation has been well established for Guiselin layers, their stability, crucial from the perspective of materials applications, is not. The stability is a trade-off in the entropic penalty between cooperative detachment of the number of segments directly adsorbed on the substrate and consecutively pinned monomers.

EXPERIMENTS

Experimental model systems of Guiselin layers of polystyrene (PS) on silicon wafers with native oxide layer on top were employed. The stability of the adsorbed layers was studied as a function of PS molecular weight and polydispersibility by various microscopic and spectroscopic tools as well as quasi-static contact angle measurements.

FINDINGS

Adsorbed layers from low molecular weight PS were disrupted with typical spinodal decomposition patterns whereas high molecular weight (>500 kDa) PS resulted in stable, continuous layers. Moreover, we show that Guiselin layers offer an enticing way to modify a surface, as demonstrated by adsorbed PS that imparts a hydrophobic character to initially hydrophilic silicon wafers.

Dynamics and Structure Formation of Confined Polymer Thin Films Supported on Solid Substrates

The stability/instability behavior of polystyrene (PS) films with tunable thickness ranging from higher as-cast to lower residual made on Si substrates with and without native oxide layer was studied in this paper. For further extraction of residual PS thin film (hresi) and to investigate the polymer-substrate interaction, Guiselin's method was used by decomposing the polymer thin films in different solvents. The solvents for removing loosely adsorbed chains and extracting the strongly adsorbed irreversible chains were selected based on their relative desorption energy difference with polymer. The PS thin films rinsed in chloroform with higher polarity than that of toluene showed a higher decrease in the residual film thickness but exhibited earlier growth of holes and dewetting in the film. The un-annealed samples with a higher oxide film thickness showed a higher decrease in the PS residual film thickness. The effective viscosity of PS thin films spin-coated on H-Si substrates increased because of more resistance to flow dynamics due to the stronger polymer-substrate interaction as compared to that of Si-SiOx substrates. By decreasing the film thickness, the overall effective mobility of the film increased and led to the decrease in the effective viscosity, with matching results of the film morphology from atomic force microscopy (AFM). The polymer film maintained low viscosity until a certain period of time, whereupon further annealing occurred, and the formation of holes in the film grew, which ultimately dewetted the film. The residual film decrement, growth of holes in the film, and dewetting of the polymer-confined thin film showed dependence on the effective viscosity, the strength of solvent used, and various involved interactions on the surface of substrates.

Review and reproducibility of forming adsorbed layers from solvent washing of melt annealed films

Recent studies suggest chain adsorption in the melt may be responsible for a number of property changes in thin films by making correlations between the residual adsorbed layer thickness h_ads(t) measured after a given solvent washing procedure as a function of annealing time t of the film at an elevated temperature prior to this solvent rinse. This procedure, frequently called "Guiselin's experiment", refers to the thought experiment proposed in a 1992 theoretical treatment by Guiselin that assumed chain segments in contact with the surface are irreversibly adsorbed whereby unadsorbed chains could be washed away by solvent without disturbing the adsorbed substrate contact points in the melt. In the present work, we review this recent literature, identifying and experimentally testing a common protocol for forming adsorbed layers h_ads(t) from solvent washing melt films. We find h_ads(t) curves to be far less reproducible and reliable than implied in the literature, strongly dependent on solvent washing and substrate cleaning conditions, and annealing at elevated temperatures is unnecessary as densification of films sitting at room temperature makes the glassy film harder to wash off, leaving behind h_ads of comparable thickness. This review also summarizes literature understanding developed over several decades of study on polymer adsorption in solution, which experimentally demonstrated that polymer chains in solution are highly mobile, diffusing and exchanging on the surface even in the limit of strong adsorption, contradicting Guiselin's assumption. Preformed adsorbed layers of different thicknesses h_ads are shown to not affect the average glass transition temperature or physical aging of 30 nm thick films. In summary, a number of open questions and implications are discussed related to thin films and polymer nanocomposites.

Adhesion Control of Elastomer Sheet on the Basis of Interfacial Segregation of Hyperbranched Polymer

Adhesion of a model rubbery material, cross-linked poly(dimethylsiloxane) (PDMS), onto a solid surface was studied by sum-frequency generation spectroscopy and X-ray photoelectron spectroscopy. To do so, here, we have focused on the adhesive deposit and insoluble layer. The former and latter were defined as the residual amount on the substrate after the peeling and residual layer after washing with a good solvent, respectively. The peel strength of a PDMS sheet adhered onto a glass plate increased with the contact time. Both adhesive deposit and insoluble layer also exhibited comparable contact time dependence. Once a hyperbranched polymer (HBP), which was segregated to the adhesive interface, was incorporated into PDMS, the peel strength and adhesive deposit decreased, although the thickness of the insoluble layer remained almost unchanged. These results suggest that the formation of loosely adsorbed chains on the solid surface, which possess not only trains but also many loop portions and tail parts, plays an important role in the macroscopic adhesion behavior of the PDMS sheet and the interfacial segregation of HBP can prevent it.

Self-Organization of Triblock Copolymer Melt Chains Physisorbed on Non-neutral Surfaces

We here report the self-organization process of poly(styrene-b-ethylene/butadiene-b-styrene) (SEBS) triblock copolymer chains physically adsorbed on a non-neutral surface. Spin-cast SEBS thin films were prepared on silicon (Si) substrates and then annealed at a high temperature far above the bulk glass transition temperatures of the two constituent blocks. To reveal the buried interfacial structure, we utilized solvent rinsing processes and a suite of surface-sensitive techniques including ellipsometry, X-ray reflectivity, atomic force microscopy, and grazing incidence small angle X-ray scattering. We revealed that the SEBS chains form two different chain structures on the substrate simultaneously: (i) "flattened chains" with the average height of 2.5 nm but without forming microdomain structures; (ii) "loosely adsorbed chains" with the average height of 11.0 nm and the formation of perpendicularly oriented cylindrical microdomains to the substrate surface. In addition, the kinetics to form the perpendicular-oriented cylinder was sluggish (∼200 h) and proceeded via multistep processes toward the equilibrium state. We also found that the lateral microdomain structures were distorted, and the characteristic lengths of the microdomains were slightly different from the bulk even after reaching "quasiequilibrium" state within the observed time window. Furthermore, we highlight the vital role of the adsorbed chains in the self-assembling process of the entire SEBS thin film: a long-range perturbation associated with the adsorbed chains propagates into the film interior, overwhelming the free surface effect associated with surface segregation of the lower surface tension of polystyrene blocks.

Driving Coordination Polymer Monolayer Formation by Competitive Reactions at the Air/Water Interface

We have developed a novel approach enabling us to follow and facilitate the formation of two-dimensional coordination polymer monolayers directly at the air/water interface without the need of complex instrumentation. The method is based on the use of a surface active ligand that, when spread at the air/water interface, progressively undergoes hydrolysis with consequent gradual decrease in surface pressure. Notably, if the aqueous subphase contains metal ions capable of coordinating the ligand, coordination competes with hydrolysis, resulting in a lower surface pressure decrease. As a consequence, the formation of the coordination polymer monolayer can be verified simply by surface pressure measurements. Competition between hydrolysis and coordination was investigated as a function of the main experimental parameters affecting the two reactions, enabling the formation of stable coordination polymer monolayers with controlled density. Finally, the formation of continuous rigid 2D layers was confirmed by compression isotherms and ex situ morphological characterization. This work will simplify the verification of coordination polymer monolayer formation; thus, it will boost the synthesis of novel and innovative 2D materials.