The architecture of the adsorbed layer at the substrate interface determines the glass transition of supported ultrathin polystyrene films

Absolute local conformation of poly(methyl methacrylate) chains adsorbed on a quartz surface

Polymer chains at a buried interface with an inorganic solid play a critical role in the performance of polymer nanocomposites and adhesives. Sum frequency generation (SFG) vibrational spectroscopy with a sub-nanometer depth resolution provides valuable information regarding the orientation angle of functional groups at interfaces. However, in the case of conventional SFG, since the signal intensity is proportional to the square of the second-order nonlinear optical susceptibility and thereby loses phase information, it cannot be unambiguously determined whether the functional groups face upward or downward. This problem can be solved by phase-sensitive SFG (ps-SFG). We here applied ps-SFG to poly(methyl methacrylate) (PMMA) chains in direct contact with a quartz surface, shedding light on the local conformation of chains adsorbed onto the solid surface. The measurements made it possible to determine the absolute orientation of the ester methyl groups of PMMA, which were oriented toward the quartz interface. Combining ps-SFG with all-atomistic molecular dynamics simulation, the distribution of the local conformation and the driving force are also discussed.

Flattened chains dominate the adsorption dynamics of loosely adsorbed chains on modified planar substrates

Herein, the adsorption of polystyrene (PS) on phenyl-modified SiO2-Si substrates was investigated. Different from those for PS adsorption on a neat SiO2-Si substrate, the growth rate (vads) in the linear regime and hads/Rg (hads, thickness of flattened and loosely adsorbed layers on the substrate; Rg, radius of gyration) declined with increasing molecular weight (Mw) of PS and the phenyl content on the modified substrates, while the thickness of the flattened layer (hflat) and its coverage increased with increasing phenyl content. The results indicated that the adsorption of loose chains was controlled by the adsorption of flattened chains, as it only occurred in the empty contact sites remaining after the adsorption of flattened chains. Before approaching quasi-equilibrium (t < tcross), the number of flattened chain contact sites increased due to an enthalpically favorable process and, correspondingly, their spatial positions dynamically changed, which perturbed the adsorption of loose chains. When the adsorption of flattened chains reached quasi-equilibrium (t > tcross), the adsorption of loose chains was determined by the empty contact sites. The coverage of flattened chains and time to reach quasi-equilibrium were increased with more phenyl groups on the substrate, enhancing π-π interfacial interactions and resulting in a decreased adsorption rate and fewer loosely adsorbed chains. Mw-dependent vads and hads/Rg differed on phenyl-modified substrates compared to the neat SiO2-Si substrate owing to fewer empty contact sites for loose chains. The study findings improve our understanding of the mechanism responsible for the formation and structure of the adsorbed layer on solid surfaces.

Exploring the Molecular Origin for the Long-Range Propagation of the Substrate Effect in Unentangled Poly(methyl methacrylate) Films

The polymer/substrate interface plays a significant role in the dynamics of nanoconfined polymers because of its suppression on polymer mobility and its long-range propagation feature, while the molecular origin of the long-range substrate effect in unentangled polymer material is still ambiguous. Herein, we investigated the propagation distances of the substrate effect (h*) by a fluorinated tracer-labeled method of two unentangled polymer films supported on silicon substrates: linear and ring poly(methyl methacrylate) films with relatively low molecular weights. The results indicate that the value of h* has a molecular weight dependence of h*∝N (N is the degree of polymerization) in the unentangled polymer films, while h*∝N1/2 was presented as previously reported in the entangled films. A theoretical model, depending on the polymer/polymer intermolecular interaction, was proposed to describe the above long-range propagation behavior of the substrate effect and agrees with our experiment results very well. From the model, it revealed that the intermolecular friction determines the long-range propagation of the substrate effect in the unentangled system, but the intermolecular entanglement is the dominant role in entangled system. These results give us a deeper understanding of the long-range substrate effect.

Langevin dynamics simulation on optimal conditions for large and stable loops of adsorbed homopolymers on substrates

Adsorption of polymer chains on a solid surface is a universal interfacial behavior. Loops in the adsorbed chains are considered to exert a significant effect on the overall properties of a substrate-supported polymer film via entanglement with non-adsorbed chains in the film. In this work, the size and stability of loops formed by adsorbed homopolymer chains on an attractive substrate were studied by Langevin dynamics simulations. The size of loops decreases while the stability increases with increasing attraction strength of the substrate. In contrast, with an increase in the polymer concentration, the size of loops increases but the stability decreases. However, both the size and stability of loops increase with increasing chain length. Simulation results show that the optimal conditions for forming large and stable loops are long homopolymer chains, substrates with moderate attraction strength, and moderate polymer concentration.

Change in local conformation of polymer chains at film surface attached to solid surface

Adhesion is a molecular event where polymer chains contact with a material surface to form an interfacial layer. To obtain a better understanding of the adhesion on a molecular scale, we herein examined the conformational change of polystyrene (PS) chains at the film surface after contacting with hydrophobic or hydrophilic surfaces using sum-frequency generation (SFG) spectroscopy. Chains altered their local conformations with a quartz surface more quickly than a hydrophobic alkyl-functionalized one. A full-atomistic molecular dynamics simulation showed that these results, which were coupled with the contact process of PS chains with the solid surface, could be explained in terms of the Coulomb interaction between them.

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.

The role of the molecular weight of the adsorbed layer on a substrate in the suppressed dynamics of supported thin polystyrene films

The adsorbed layer on a solid surface plays a crucial role in the dynamics of nanoconfinement polymer materials. However, the influence of the adsorbed layer is complex, and clarifying this influence on the dynamics of confined polymers remains a major challenge. In this paper, SiO₂-Si substrates with various thicknesses and adsorbed layers of PS with various molecular weights were used to reveal the effect of the adsorbed layer on the corresponding segmental dynamics of the supported thin PS films. Strongly suppressed segmental dynamics of thin PS films were observed for the films supported on thicker adsorbed layers or prepared using higher molecular weight. Neutron reflectivity revealed that the overlap region thickness between the adsorbed layer and the top overlayer increased with increasing thickness and molecular weight of the adsorbed layer, both of which correlate well with the distance over which the polystyrene dynamics were depressed by the adsorbed layer. The results show that the influencing distance of the adsorbed layer is related to the overlap zone formed between the adsorption layer and the upper thin film. The effect of the adsorbed layer molecular weight can be ascribed to the fact that large loops and long tails in the adsorbed layer result in stronger interpenetrations and entanglements between polymer chains in the adsorbed layer and in the overlayer, causing a stronger substrate effect and suppression of the segment dynamics of the supported thin PS films.

Enhanced Glass Transition Temperature of Thin Polystyrene Films Having an Underneath Cross-Linked Layer

Due to the importance of the interface in the segmental dynamics of supported macromolecule ultrathin films, the glass transition temperature (T_g) of polystyrene (PS) ultrathin films upon solid substrates modified with a cross-linked PS (CLPS) layer has been investigated. The results showed that the T_g of the thin PS films on a silica surface with a ∼5 nm cross-linked layer increased with reducing film thickness. Meanwhile, the increase in T_g of the thin PS films became more pronounced with increasing the cross-linking density of the layer. For example, a 20 nm thick PS film supported on CLPS with 1.8 kDa of cross-linking degree exhibited a ∼35 and ∼50 K increase in T_g compared to its bulk and that on neat SiO₂ substrate, respectively. Such a large T_g elevation for the ultrathin PS films was attributed to the interfacial aggregation states in which chains diffused through nanolevel voids formed in the cross-linked layer to the SiO₂-Si surface. In such a situation, the chains were topologically constrained in the cross-linked layer with less mobility. These results offer us the opportunity to tailor interfacial effects by changing the degree of cross-linking, which has great potential application in many polymer nanocomposites.

Can we define a unique microscopic pressure in inhomogeneous fluids?

The estimation of a microscopic pressure tensor in an adsorbed thin film on a planar surface remains a challenge in both experiment and theory. While the normal pressure is well-defined for a planar surface, the tangential pressure at a point is not uniquely defined at the nanoscale. We report a new method that allows us to calculate the local pressure tensor and its spatial integral using an arbitrary contour definition of the "virial-route" local pressure tensor. We show that by integrating the local tangential pressure over a small region of space, roughly the range of the intermolecular forces, it is possible to define a coarse-grained tangential pressure that appears to be unique and free from ambiguities in the definition of the local pressure tensor. We support our argument by presenting the results for more than ten types of contour definitions of the local pressure tensor. By defining the coarse-grained tangential pressure, we can also find the effective thickness of the adsorbed layer and, in the case of a porous material, the statistical pore width. The coarse-grained in-layer and in-pore tangential pressures are determined for Lennard-Jones argon adsorbed in realistic carbon slit pores, providing a better understanding of the pressure enhancement for strongly wetting systems.

Decoupling Role of Film Thickness and Interfacial Effect on Polymer Thin Film Dynamics

The film thickness and substrate interface are the two most common parameters to tune the dynamics of supported thin films. Here, we investigated the glass transition temperature (T_g) and thermal expansion of thin poly(methyl methacrylate) (PMMA) films with various thicknesses and different interfacial effects. We showed that, although the T_g of the thin films can be modulated equivalently by the two factors, their ability to change the expansivity (β) is quite different; that is, β increases notably with a reduction in the thickness, while it is insensitive to perturbations at the interface. We attribute the deviation in modulating β by the thickness and the interfacial effect to the disparate abilities to change the free volume content in the film by a free surface and substrate interface. This leads to a situation where thin films with dissimilar thicknesses and interfacial properties can have the same T_g but very different β values, suggesting that T_g alone cannot unequivocally quantify thin film dynamics.

Glass transition behaviour of thin polymer films coated on the 3D networks of porous CNT sponges

The glass transition behaviors of thin polymer films on the sidewalls of carbon nanotubes (CNTs) in CNT sponges (CNTSs) were studied. Due to the extremely large surface area of CNTS, the glass transition temperatures (Tg) of thin polystyrene (PS) and poly(methyl methacrylate) (PMMA) films were measured using a routine experimental method, differential scanning calorimetry (DSC). We thus provide a direct Tg comparison between the thin film and the bulk sample using the same DSC technique. For thin polymer films on the CNT sidewalls, free surface and polymer-substrate interfacial interactions co-exist. It is well-known that polymer chains at the liquid-like free surface tend to have a relatively high mobility, but the mobility in the interfacial layer near the substrate depends strongly on the polymer-substrate interaction strength. Accordingly, we tuned the polymer-substrate interaction strength by introducing an amphiphilic sodiumdodecylsulfate (SDS) molecule layer on the CNT sidewalls. The value and sign of Tg deviation were influenced by the competition between the free surface effect and the interfacial interactions. Strong polymer-substrate interactions led to a decrease in the mobility of polymer chains near the substrate and weak polymer-substrate interactions have little influence on the mobility of polymer chains near the substrate. When the polymer-substrate interactions are strong, both the free surface effect and the polymer-substrate interaction are key factors influencing the glass transition temperature. For thin polymer films having weak interactions with substrates, the free surface effect dominates the glass transition behavior and Tgs shows a large reduction. We also observed a double Tg behavior in the thin PS film and found the thickness of the PS film on the substrate was a deciding factor for controlling the spatial variation of Tg.

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.

Irreversible adsorption of polymer melts and nanoconfinement effects

For almost a decade, growing experimental evidence has revealed a strong correlation between the properties of nanoconfined polymers and the number of chains irreversibly adsorbed onto nonrepulsive interfaces, e.g. the supporting substrate of thin polymer coatings, or nanofillers dispersed in polymer melts. Based on such a correlation, it has already been possible to tailor structural and dynamics properties - such as the glass transition temperature, the crystallization rate, the thermal expansion coefficients, the viscosity and the wettability - of nanomaterials by controlling the adsorption kinetics. This evidence indicates that irreversible adsorption affects nanoconfinement effects. More recently, also the opposite phenomenon was experimentally observed: nanoconfinement alters interfacial interactions and, consequently, also the number of chains adsorbed in equilibrium conditions. In this review we discuss this intriguing interplay between irreversible adsorption and nanoconfinement effects in ultrathin polymer films. After introducing the methods currently used to prepare adsorbed layers and to measure the number of irreversibly adsorbed chains, we analyze the models employed to describe the kinetics of adsorption in polymer melts. We then discuss the structure of adsorbed polymer layers, focusing on the complex macromolecular architecture of interfacial chains and on their thermal expansion; we examine the way in which the structure of the adsorbed layer affects the thermal glass transition temperature, vitrification, and crystallization. By analyzing segmental dynamics of 1D confined systems, we describe experiments to track the changes in density during adsorption. We conclude this review with an analysis of the impact of nanoconfinement on adsorption, and a perspective on future work where we also address the key ideas of irreversibility, equilibration and long-range interactions.

Alkaline Stability of Anion-Conductive Ionomer Coated on a Carbon Surface

Anion-exchange membrane fuel cells (AEMFCs) are promising technologies that allow the use of nonprecious metals as catalysts because the oxidation reduction reaction at the cathode occurs readily at the high pH of AEMFCs. However, the insufficient chemical stability of the anion-conductive materials in AEMFCs currently limits their development. We studied the chemical stability of the electrolyte in the catalyst layer of AEMFCs containing cationic dimethyl polybenzimidazole (mPBI). Although degradation was observed in an mPBI membrane under alkaline conditions, mPBI coated on a carbon support showed excellent alkaline stability. Because no glass transition temperature was observed for mPBI after coating on the support, the increase of chemical stability was probably associated with the decrease of polymer flexibility.

Solution filtering affects the glassy dynamics of spincoated thin films of poly(4-chlorostyrene)

We investigated the impact of sample preparation on the glassy dynamics of thin films of poly(4-chlorostyrene), a polymer whose molecular mobility is particularly sensitive to changes in the specific volume. Samples were obtained by spincoating, the technique most commonly used to prepare thin organic layers, which consists of pouring dilute polymer solutions onto a plate rotating at a high rate. Our experimental results demonstrate that filtering the solutions before spincoating affects the value of the segmental relaxation time of the as-prepared films. Thin polymer layers obtained via filtered solutions show accelerated segmental dynamics upon confinement at the nanoscale level, once below 100nm, while the samples obtained via unfiltered solutions exhibit bulk-like dynamics down to 15-20nm. We analyzed these results by means of the cooperative free volume rate model, considering a larger free volume content in thin films obtained via filtered solutions. The validity of the model predictions was finally verified by measurements of irreversible adsorption, confirming a larger adsorbed amount, corresponding to a higher specific volume, in the case of samples obtained via unfiltered solutions. Our results prove that filtering is a crucial step in the preparation of thin films, and it could be used to switch on and off nanoconfinement effects.

Effect of Local Chain Conformation in Adsorbed Nanolayers on Confined Polymer Molecular Mobility

Interfaces play an important role in modifying the dynamics of polymers confined to the nanoscale. We demonstrate that the distance over which an interface suppresses molecular mobility in poly(styrene) thin films can be systematically increased by tens of nanometers by controlling the chain of conformation, i.e., the height of the loops in irreversibly adsorbed nanolayers. These effects arise from topological interaction between adsorbed and neighboring unadsorbed chains, respectively, which increase their motional coupling to facilitate the propagation of suppressed dynamics originating at the interface, thus highlighting the ability to manipulate interfacial effects by local conformation of chains in adsorbed nanolayers.

Connecting 1D and 2D Confined Polymer Dynamics to Its Bulk Behavior via Density Scaling

Under confinement, the properties of polymers can be much different from the bulk. Because of the potential applications in technology and hope to reveal fundamental problems related to the glass-transition, it is important to realize whether the nanoscale and macroscopic behavior of polymer glass-formers are related to each other in any simple way. In this work, we have addressed this issue by studying the segmental dynamics of poly(4-chlorostyrene) (P4ClS) in the bulk and upon geometrical confinement at the nanoscale level, in either one- (thin films on Al substrate) or two- (within alumina nanopores) dimensions. The results demonstrate that the segmental relaxation time, irrespective of the confinement size or its dimensionality, can be scaled onto a single curve when plotted versus ρ^γ/T with the same single scaling exponent, γ = 3.1, obtained via measurements at high pressures in bulk. The implication is that the macro- and nanoscale confined polymer dynamics are intrinsically connected and governed by the same underlying rules.

Competing polymer-substrate interactions mitigate random copolymer adsorption

Annealing a supported polymer film in the melt state, a common practice to relieve residual stresses and erase thermal history, can result in the development of an irreversibly adsorbed nanolayer. This layer of polymer chains physically adsorbed to the substrate interface has been shown to influence thin film properties such as viscosity and glass transition temperature. Its growth is attributed to many simultaneous interactions between individual monomer units and the substrate stabilizing chains against desorption. A better understanding of how these specific polymer-substrate interactions influence the growth of the adsorbed layer is needed, particularly given how strongly the properties of geometrically-confined polymeric systems are impacted by interfaces. Here, we use homopolymers and random copolymers of styrene and methyl methacrylate to form adsorbed layers and examine the influence of chemical composition and the resulting polymer-substrate interactions on adsorbed layer growth and structure. Ellipsometric measurements reveal a non-monotonic trend between composition and thickness of the adsorbed layers that is inconsistent with the behavior normally exhibited by random copolymers, being intermediate to their respective homopolymers. We examine this trend in terms of plateau thickness and growth kinetics at two different annealing temperatures and propose a mechanism for how different polymer-substrate interactions combine to influence adsorption when copolymer films are annealed. By introducing compositional heterogeneity, this mechanism extends the study of irreversible adsorption to complex chemistries and provides for a more general understanding of how annealing should be accounted for in the proper selection and processing of polymer thin films for applications in nanotechnology.

Reorientation Kinetics of Local Conformation of Polyisoprene at Substrate Interface

The performance of a polymer composite material, in which inorganic fillers are dispersed, is closely related to the aggregation states and dynamics of polymer chains at the interface with the filler. In this study, the local conformation of polyisoprene (PI) at a quartz substrate interface was studied as a model system for the rubber/filler composite material. PI films were prepared from a toluene solution onto quartz substrates by a spin-coating method. Sum-frequency generation spectroscopy revealed that the local conformation of PI chains at the quartz interface depended on the spinning rate. The tilt angle of methyl groups increased with the rotational speed, probably due to the centrifugal force applied to chains and probably also the evaporation rate of the solvent during the solidification process. This result indicates that the interfacial orientation of PI chains can remain even at room temperature, which is 87 K higher than the bulk glass transition temperature (T_g^b). The interfacial orientation disappeared at a temperature approximately 120 K higher than T_g^b.

Altering surface fluctuations by blending tethered and untethered chains

"Partially tethering" a thin film of a polymer melt by covalently attaching to the substrate a fraction of the chains in an unentangled melt dramatically increases the relaxation time of the surface height fluctuations. This phenomenon is observed even when the film thickness, h, is 20 times the unperturbed chain radius, R_g,tethered, of the tethered chains, indicating that partial tethering is more influential than any physical attraction with the substrate. Furthermore, a partially tethered layer of a low average molecular weight of 5k showed much slower surface fluctuations than did a reference layer of pure untethered chains of much greater molecular weight (48k), so the partial tethering effect is stronger than the effects of entanglement and increase in glass transition temperature, T_g, with molecular weight. Partial tethering offers a means of tailoring these fluctuations which influence wetting, adhesion, and tribology of the surface.

Irreversible Adsorption Governs the Equilibration of Thin Polymer Films

We demonstrate that the enhanced segmental motion commonly observed in spin cast thin polymer films is a nonequilibrium phenomenon. In the presence of nonrepulsive interfaces, prolonged annealing in the liquid state allows, in fact, recovering bulk segmental mobility. Our measurements prove that, while the fraction of unrelaxed chains increases upon nanoconfinement, the dynamics of equilibration is almost unaffected by the film thickness. We show that the rate of equilibration of nanoconfined chains does not depend on the structural relaxation process but on the feasibility to form an adsorbed layer. We propose that the equilibration of the thin polymer melts is driven by the slow relaxation of interfacial chains upon irreversible adsorption on the confining walls.

How irreversible adsorption affects interfacial properties of polymers

Growing experimental evidence shows that the behavior of polymer chains confined at the nanoscale level strongly depends on the degree of adsorption correlated to the number density of monomers pinned onto the supporting substrate. In this contribution, after introducing the physics behind the mechanisms of irreversible adsorption, we review recent experimental observations on how adsorption affects properties of polymer melts confined in 1D, focusing on those related to the thermal glass transition, maximum water uptake, viscosity and crystallization. These findings strongly support a new physical framework of confined soft matter, not trivially limited to finite size effects and interfacial interactions, but also enriched by non-equilibrium phenomena.

Irreversible Adsorption Erases the Free Surface Effect on the Tg of Supported Films of Poly(4-tert-butylstyrene)

When cooled at constant rate, a 25 nm thin film of poly(4-tert-butylstyrene) vitrifies 50 K lower than in bulk. This record sets the largest depression in thermal glass transition temperature (T_g) ever observed upon confinement at the nanoscale level. Same as for other supported polymer layers, this reduction in T_g has been attributed to the presence of a free surface, the ensemble of molecules at the interface with air remaining in the liquid state also at temperatures well below bulk T_g. Here, we verify that such tremendous shifts can be erased upon prolonged annealing in the liquid state, hinting at a metastable nature of confinement effects. We demonstrate that the recovery of bulk behavior and the manifestation of the free surface are enslaved to the kinetics of irreversible adsorption of chains on the supporting substrate.

Grafting density dominant glass transition of dry polystyrene brushes

The effects of the grafting densities (σ_p), molecular weights (M_n) and thicknesses of dry polystyrene (PS) brushes on their glass transition temperature (T) were investigated by ellipsometry. The results show that T strongly depends on the grafting density of the PS brushes. The T of the PS brushes with σ_p > 0.30 increases with decreasing M_n (or brush thickness) and is mainly dominated by entropic effects, in which the grafted chains are highly extended along the film thickness direction resulting in a sharp reduction in configurational entropy. The T of PS brushes with σ_p < 0.30 decreases with decreasing M_n (or brush thickness) which is mainly dominated by surface effects. For intermediate-density brushes (σ_p = 0.30), T becomes independent of M_n or brush thickness. The reason for this grafting density dependence of T is attributed to the transition of the PS brush conformation from mushroom-to-brush.