Polymer-substrate Interactions Research Articles

Substrate influence on the surface glass transition temperature of polymers

Enhanced molecular mobility near the free (polymer–air) surface is crucial for advancing organic electronic devices, yet understanding the substrate's impact on the surface glass transition temperature (Tgsurf) as film thickness decreases remains limited. This study explores how polymer films possessing attractive, neutral, and unfavorable polymer–substrate interactions affect Tgsurf. Results show that neutral interactions have no effect, while attractive or unfavorable interactions can increase or decrease Tgsurf by up to ∼37 °C. The onset thickness for this change is smaller for attractive interactions (up to 37 nm) than for unfavorable interactions (>100 nm), supporting the observed broadening of the surface glass transition with attractive interactions. We surmise that segment exchange between surface and subsurface regions introduces disparate dynamic components at the surface. Therefore, attractive interactions causing a sharper change in Tgsurf with film thickness lead to a broader surface glass transition.

Insights into the crystallization behavior of structurally and spatially doubly confined polymer systems studied by molecular simulation

The impact of grafting density and polymer-substrate interactions on the crystallization behavior of structurally and spatially doubly confined polymer systems was studied using Monte Carlo simulation methods. In the confined polymer system with low grafting density, as the polymer-substrate interactions increase, the nucleation mode shifts from homogeneous nucleation to heterogeneous nucleation. The melting temperature and undercooling increase, while the critical nucleation free energy barrier decreases, favoring the crystallization process. Nevertheless, significant polymer-substrate interactions can lead to a decrease in the mobility of polymer chains, thereby inhibiting the crystallization process. In highly grafted polymer systems, homogeneous nucleation controls the entire crystallization process and the crystallization rate and final degree of crystallinity are almost unaffected by polymer-substrate interactions. Grafting density and polymer-substrate interactions play a dual role, acting as a “double-edged sword” in the crystallization process of doubly confined polymer systems. An appropriate grafting density and polymer-substrate interactions facilitate the nucleation ability, thereby promoting the crystallization capability of polymer systems. Conversely, excessive interactions can weaken nucleation ability and suppress crystal growth.

New Insights into Spin Coating of Polymer Thin Films in Both Wetting and Nonwetting Regimes.

Spin coating is a common method for fabricating polymer thin films on flat substrates. The well-established Meyerhofer relationship between film thickness (h) and spin rate (ω), h ∝ ω-1/2, enables the preparation of thin films with desired thickness by adjusting the spin rate and other experimental parameters. The 1/2 exponent has been verified by previous studies involving organic thin films prepared on silicon wafers. In this study, 88% and >99% hydrolyzed poly(vinyl alcohol) (PVOH) polymers were adsorbed and spin-coated from an aqueous solution onto four different substrates. The substrates were prepared by covalently attaching poly(dimethylsiloxane) (PDMS) of different molecular weights onto silicon wafers (SiO2). Atomic force microscopy images indicate that the PVOH films transitioned from stable on SiO2, to metastable, and then to unstable as PDMS molecular weight was increased. Notably, none of the polymer-substrate systems studied here exhibited the thickness-spin rate profile predicted by the Meyerhofer model. Based on the experimental results, a more general adsorption-deposition model is proposed that decouples the total spin-coated thickness into two components─the adsorbed thickness (h1) and the spin-deposited thickness (h2). The former accounts for polymer-substrate interactions, and the latter depends on polymer concentration and spin rate. In unstable systems, the exponents were found to be ∼0 because slip takes place at the solution-substrate interface during spin and the spin-deposited thickness is 0. In metastable and stable systems, a universal relationship between spin-deposited thickness and spin rate emerged, independent of the substrate type and polymer concentration for each polymer examined. Our findings indicate the importance of film stability and polymer-substrate interactions in the application of spin coating.

Decoupling of Glassy Dynamics from Viscosity in Thin Supported Poly(n-butyl methacrylate) Films.

We utilized fast scanning calorimetry to characterize the glass transition temperature (Tg) and intrinsic molecular mobility of low-molecular-weight poly(n-butyl methacrylate) thin films of varying thicknesses. We found that the Tg and intrinsic molecular mobility were coupled, showing no film thickness-dependent variation. We further employed a unique noncontact capillary nanoshearing technique to directly probe layer-resolved gradients in the rheological response of these films. We found that layer-resolved shear mobility was enhanced with a reduction in film thickness, whereas the effective viscosity decreased. Our results highlight the importance of polymer–substrate attractive interactions and free surface-promoted enhanced mobility, establishing a competitive nanoconfinement effect in poly(n-butyl methacrylate) thin films. Moreover, the findings indicate a decoupling in the thickness-dependent variation of Tg and intrinsic molecular mobility with the mechanical responses (shear mobility and effective viscosity).

Origin of Rebound Suppression for Dilute Polymer Solution Droplets on Superhydrophobic Substrate.

Controlling droplet deposition with a minute amount of polymer additives is of profound practical importance in a wild range of applications. Previous work ascribed the relevant mechanisms to extensional viscosity, normal stress, wetting properties, etc., but the mechanism remains controversial. In this paper, we employ molecular dynamics simulations systematically for the first time to investigate the origin of rebound suppression for dilute polymer solution droplets on a flat superhydrophobic substrate. The results demonstrate that polymer-substrate interactions and impact velocities dominate the antirebound phenomenon. For low impact velocities, the dynamic characteristics of droplets are insensitive to polymer additives. However, large impact velocities will enhance the stretch behavior of polymer chains and make the chains closer to the substrate, increasing the probability of polymer molecules contacting the bottom substrate. With the cooperation of strong polymer-substrate interactions, polymer molecules can be absorbed easily by the bottom substrate, resisting the retraction process and leading to the onset of the antirebound behavior.

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.

Dynamics and glass transition temperature of polystyrene films supported on hydroxyl terminated substrates

All atom molecular simulations are employed to systematically study the properties of polystyrene (PS) films supported on hydroxyl (–OH) terminated Si substrates. We modulate the –OH grafting density ϕG to probe its effect on polymer diffusion, local segmental relaxation, and glass transition temperature Tg. First, our results indicate a monotonic facilitation of polymer dynamics with increasing ϕG. The underlying mechanism is investigated in terms of energy and PS free volume change in detail, and a consistent scenario is obtained. The facilitated diffusion dynamics can be attributed to the –OH induced reduction of interaction energy between PS and the substrate. In addition, –OH grafting leads to the augment of the polymer free volume, which is responsible for the faster local segmental relaxation process. Second, we uncover a very intriguing increase–decrease non-monotonic tendency of glass transition temperature with increasing grafting density, where the maximum Tg occurs in a moderate grafting case. A wetting-friction transition is proposed to rationalize such an unusual behavior. As ϕG increases from a native case to a half grafting situation, polymer films undergo a sharply enhanced interpenetration (or wetting) into the substrate, resulting in the pronounced increment of Tg. As ϕG further increases, the polymer–substrate interaction energy experiences a heavy drop, which implies a greatly reduced friction effect, leading to the decreasing trend of Tg as observed in the range from half grafting to full grafting.

Revealing Interfacial Interactions in Random Copolymer Adsorbed Layers by Solvent Leaching.

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.

Thermal stability and dynamics of soft nanoparticle membranes: role of entropy, enthalpy and membrane compressibility.

Nanoparticle based ultra-thin membranes have been shown to have remarkable mechanical properties while also possessing novel electrical, optical or magnetic properties, which could be controlled by tailoring properties at the level of individual nanoparticles. Since in most cases the ultra-thin membranes are coupled to some substrates, the role of membrane-substrate interactions, apart from nanoparticle-nanoparticle interactions become very crucial in understanding their mechanical and thermal stability, as well as their plethora of applications. However, systematic studies in this direction have been conspicuously absent. Here we report thermal stability and the corresponding microscopic dynamics of polymer supported ultra-thin membranes comprising of self-assembled, ordered grains of polymer grafted nanoparticles having tunable mechanical properties. The initially ordered membranes show distinct pathways for temperature induced disordering depending on membrane flexibility as well as on interfacial entropic and enthalpic interactions with the underlying polymer thin film. We also observe contrasting temperature dependence of microscopic dynamics of these membranes depending on whether the graft polymer-substrate polymer interactions are predominantly entropic or enthalpic in nature. Our results suggest that apart from their varied applications, the soft nanoparticle-polymer hybrid membranes are a playground for rich physics involving subtle entropic and enthalpic effects along with the nanoparticles softness, which eventually determine their thermo-mechanical stability.

Tg Confinement Effect of Random Copolymers of 4-tert-Butylstyrene and 4-Acetoxystyrene with Different Compositions.

We observe that the Tg confinement effect of polymer films can saturate with polymer-substrate interaction. Thickness dependences of the glass transition temperature, Tg(h0), of random copolymer films of 4-tert-butylstyrene (TBS) and 4-acetoxystyrene (AS) supported by silica (SiOx) were measured for different TBS concentrations, XTBS. For 0 ≤ XTBS ≤ 0.47, Tg(h0) displays identical enhancements, independent of XTBS. For XTBS > ∼0.66; however, Tg(h0) decreases steadily with XTBS. The XTBS > 0.66 result is in keeping with expectations that TBS interacts less strongly with SiOx than AS does, and weaker polymer-substrate interaction renders greater dominance of the air surface over substrate surface on Tg, and thereby Tg reduction. We propose that saturation in Tg(h0) found for XTBS ≤ 0.47 is caused by the maximization in polymer-substrate-specific bond formation. Further experiments and a calculation support this proposition.

Interesting Facets of Surface, Interfacial, and Bulk Characteristics of Perfluorinated Ionomer Films.

Ion-containing perfluorinated polymers possess unique viscoelastic properties, excellent proton conductivity, and nanophase-segregated structure all arising from the clustering of hydrophilic sulfonic acid groups within a matrix of hydrophobic fluorocarbons. When these ionomers are confined to nanothin films, a broad swathe of structural organization imparting a rich variety of surface, interfacial, and bulk characteristics can be expected. However, our understanding of perfluorinated ionomer thin film behavior is still in a rudimentary stage, and much of the research focus to date has been on its hydration-related structure and properties pertinent to electrochemical applications. Thus, many hidden gems-their interesting surface and interfacial properties-have been overlooked. In this Invited Feature Article, which is a summary of the key contributions by the author's group, including several collaborative publications on ionomer thin films, we unravel many of these facets. In addition, the article attempts to integrate knowledge acquired from a variety of investigations of different aspects of the ionomer thin films to refine and develop a consistent picture of their structure and behavior. First, we focus on the self-assembly of ionomers and show that dispersion media and hydrophobicity/hydrophilicity of the substrate can result in partial or even no coverage of substrates, shedding light on the complexity of polymer-substrate, polymer-solvent, and polymer-polymer interactions, an insight completely obscured when the spin-coating method is adopted for film creation. We demonstrate that the same ionomer can be used to create a variety of surfaces ranging from superhydrophilic to highly hydrophobic by controlling the film thickness or through the choice of substrate material. The ultrathin, hydrophilic surfaces of self-assembled Nafion ionomer films exhibit wettability switching behavior which opens the door to creating stimuli-responsive smart surfaces. The thermoresponsive behavior of the films is discussed in the context of surface (wettability) and bulk (thermal expansion) characteristics as well as a newly discovered vibrational mode. The substrate- and film thickness-dependent thermal expansion coefficients reinforce the importance of interfacial interactions and confinement on the structure/properties of these films. They also open up the potential of tuning ionomer bulk properties via substrate chemistry. The discovery of a vibrational mode that becomes thermally activated at high temperature has provided new insights into the origins of the molecular motions responsible for the α-relaxation of the Nafion ionomer as well as the underlying reason for wettability switching. Our recent neutron reflectometry study of different ionomers varying in side-chain composition/length on a platinum substrate shows that the interfacial hydration level is correlated to the side-chain length, which opens up the possibility of the controlling the interfacial electrochemistry. Finally, a systematic analysis of factors affecting proton conduction is presented to elucidate the yet-unresolved origins of the suppressed conduction of nanothin ionomer films compared to that of the bulk membrane. By revealing these interesting yet poorly understood facets of ionomer thin films, the article aims to stimulate further scientific pursuit on this topic.

Probing interfacial interactions of nafion ionomer: Thermal expansion of nafion thin films on substrates of different hydrophilicity/hydrophobicity

ABSTRACTInterfacial interactions of Nafion ionomer with superhydrophilic (Pt, Au), hydrophilic (SiO2), and hydrophobic (graphene, octyltrichlorosilane [OTS]‐modified SiO2) is investigated, using in situ thermal ellipsometry, by quantification of substrate‐ and thickness‐dependent thermal properties of the ultrathin Nafion films of nominal thickness ranging 25–135 nm. For sub‐50 nm thin Nafion films, the thermal expansion coefficient of films decreased in the order of most hydrophobic to most hydrophilic substrate: OTS > graphene > SiO2 > Au > Pt, implying weaker interpolymer and polymer–substrate interactions for films on hydrophobic substrates. Expansion coefficient of films on SiO2, graphene, and OTS‐modified SiO2 decreased with thickness whereas that of films on Au and Pt substrates increased with thickness. Above ~100 nm of thickness, films on all substrates converged toward a common value representative of bulk Nafion. Thermal transition temperature was found to be higher for films on hydrophilic SiO2 than that for films on hydrophobic graphene and OTS‐modified SiO2 but was not discernible for films on Au and Pt substrates. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 343–352

Why we need to look beyond the glass transition temperature to characterize the dynamics of thin supported polymer films

There is significant variation in the reported magnitude and even the sign of [Formula: see text] shifts in thin polymer films with nominally the same chemistry, film thickness, and supporting substrate. The implicit assumption is that methods used to estimate [Formula: see text] in bulk materials are relevant for inferring dynamic changes in thin films. To test the validity of this assumption, we perform molecular simulations of a coarse-grained polymer melt supported on an attractive substrate. As observed in many experiments, we find that [Formula: see text] based on thermodynamic criteria (temperature dependence of film height or enthalpy) decreases with decreasing film thickness, regardless of the polymer-substrate interaction strength ε. In contrast, we find that [Formula: see text] based on a dynamic criterion (relaxation of the dynamic structure factor) also decreases with decreasing thickness when ε is relatively weak, but [Formula: see text] increases when ε exceeds the polymer-polymer interaction strength. We show that these qualitatively different trends in [Formula: see text] reflect differing sensitivities to the mobility gradient across the film. Apparently, the slowly relaxing polymer segments in the substrate region make the largest contribution to the shift of [Formula: see text] in the dynamic measurement, but this part of the film contributes less to the thermodynamic estimate of [Formula: see text] Our results emphasize the limitations of using [Formula: see text] to infer changes in the dynamics of polymer thin films. However, we show that the thermodynamic and dynamic estimates of [Formula: see text] can be combined to predict local changes in [Formula: see text] near the substrate, providing a simple method to infer information about the mobility gradient.

Thermal Properties and Segmental Dynamics of Polymer Melt Chains Adsorbed on Solid Surfaces.

The glass transition of supported polystyrene (PS) and poly(2-vinylpyridine) (P2VP) thin films in the vicinity of the substrate interface was studied by using a nanoplasmonic sensing (NPS) method. This "nanocalorimetric" approach utilizes localized surface plasmon resonance from two-dimensional arrangements of sensor nanoparticles deposited on SiO2-coated glass substrates. The NPS results demonstrated the existence of a high glass transition temperature ( Tg,high) along with the bulk glass transition temperature ( Tg,bulk ≈ 100 °C for PS and P2VP) within the thin films: Tg,high ≈ 160 °C for PS and Tg,high ≈ 200 °C for P2VP. To understand the origin of the Tg,high, we also studied the thermal transitions of lone polymer chains strongly adsorbed onto the substrate surface using solvent rinsing. Interestingly, the NPS data indicated that the Tg,high is attributed to the adsorbed polymer chains. To provide a better understanding of the mechanism of the Tg,high, molecular dynamics simulations were performed on a PS film adsorbed on hydrophobic and hydrophilic substrates. The simulation results illuminated the presence of a higher density region closest to the substrate surface regardless of the magnitude of the polymer-solid interactions. We postulate that the highly packed chain conformation reduces the free volume at the substrate interface, resulting in the Tg,high. Moreover, the simulation results revealed that the deviation of the Tg,high from the bulk Tg,bulk becomes larger as the polymer-substrate interaction increases, which is in line with the experimental findings.

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.

Effects of a “bound” substrate layer on the dynamics of supported polymer films

It is widely appreciated that an attractive polymer-substrate interaction can slow relaxation in thin supported polymer films and polymer nanocomposites. Recent measurements and simulations on nancomposites have indicated that this slowing of polymer dynamics occurs more strongly near a highly attractive particle surface where a "bound" layer having a much lower mobility can form, strongly influencing the thermodynamics and dynamics of the film. Here we use molecular simulations to show that a bound interfacial layer having a very similar nature arises in thin supported polymer films when the polymer-polymer attraction is stronger than the polymer-polymer interaction strength. This bound polymer layer effectively insulates the remainder of the film from the strong interfacial interactions, and the resulting thermodynamically determined Tg is relatively insensitive to the polymer-substrate interaction strength when it exceeds that of the polymer-polymer interactions. The presence of this layer gives rise to an additional relaxation process in the self-intermediate scattering function that is not observed in the bulk material and leads to a slowing down of the average relaxation time of the film as a whole. On the other hand, the average relaxation time of the film outside the bound layer does not grow in proportion to the strength of the substrate attraction due to the weak coupling of the substrate relaxation to the relaxation in the interior of the film. At large substrate attraction, the bound layer effectively "cloaks" the substrate, reducing the effect of the polymer-surface interaction on Tg.

Effect of the polymer-substrate interactions on crystal nucleation of polymers grafted on a flat solid substrate as studied by molecular simulations

The crystallization behaviors of three types of polymer systems grafted on a flat solid substrate with different grafting densities were investigated by dynamic Monte Carlo simulations. For both of the systems with low and medium grafting densities, nucleation induction period becomes shorter with the increasing polymer-substrate interactions. However, the nucleation mechanisms are different. For the systems with low grafting density, the increase of the attractive interactions results in the enhancement of heterogeneous nucleation process of grafted polymers. For the systems with medium grafting density, the attractive interactions can compensate for the conformational entropy loss induced by the restriction of the substrate, resulting in the increase of local segment density near substrate surface and the improvement of nucleation ability. Meanwhile, nucleation mode changes from intermolecular fringed-micelle nucleation to intramolecular chain-folding nucleation with the increasing interactions. For the systems with high grafting density, crystallization behaviors are almost not affected by polymer-substrate interactions. These findings are helpful to reveal the microscopic mechanism of crystallization behaviors of polymer nanocomposites and the corresponding reinforcement mechanism.

Tg-confinement effects in strongly miscible blends of poly(2,6-dimethyl-1,4-phenylene oxide) and polystyrene: Roles of bulk fragility and chain segregation

The glass transition temperature (Tg)-confinement effects in single-layer, supported films of strongly miscible poly(2,6-dimethyl-1,4-phenylene oxide)/polystyrene (PPO/PS) blends are characterized using ellipsometry and fluorescence, both of which report a single, reduced blend Tg upon confinement. These results indicate that PPO/PS blends exhibit strong miscibility under conditions of nanoscale confinement and that there is a lack of strong attractive polymer-substrate interactions in PPO/PS films supported on Si/SiOx. The strength of the Tg-confinement effect obtained via ellipsometry scales with bulk blend fragility in PPO/PS blends and exhibits a weaker dependence on bulk fragility than that observed previously [Evans et al. Macromolecules 2013, 46, 6091] in supported films of linear homopolymers lacking attractive polymer-substrate interactions. We further compare Tg results obtained from fluorescence of both pyrenyl labels (covalently attached to PS) and dopants (freely dispersed in PPO/PS blends) along with those from ellipsometry. In the confined state, pyrenyl dopant fluorescence reports blend Tg values that agree relatively well with Tg values reported by ellipsometry; in contrast, pyrenyl label fluorescence reports blend Tg values that are significantly lower than those reported by both ellipsometry and pyrenyl dopant fluorescence. We attribute this difference to the combined effects of the distribution of pyrenyl dye across the film and underlying surface segregation. Our results indicate that the combination of ellipsometry and fluorescence provides a powerful methodology to study interface-induced chain segregation in nanoconfined miscible polymer blends, whether in film geometry or in nanocomposites.

Molecular weight dependence of the intrinsic size effect on Tg in AAO template-supported polymer nanorods: A DSC study.

Many studies have established a major effect of nanoscale confinement on the glass transition temperature (Tg) of polystyrene (PS), most commonly in thin films with one or two free surfaces. Here, we characterize smaller yet significant intrinsic size effects (in the absence of free surfaces or significant attractive polymer-substrate interactions) on the Tg and fragility of PS. Melt infiltration of various molecular weights (MWs) of PS into anodic aluminum oxide (AAO) templates is used to create nanorods supported on AAO with rod diameter (d) ranging from 24 to 210 nm. The Tg (both as Tg,onset and fictive temperature) and fragility values are characterized by differential scanning calorimetry. No intrinsic size effect is observed for 30 kg/mol PS in template-supported nanorods with d = 24 nm. However, effects on Tg are present for PS nanorods with Mn and Mw ≥ ∼175 kg/mol, with effects increasing in magnitude with increasing MW. For example, in 24-nm-diameter template-supported nanorods, Tg, rod - Tg, bulk = -2.0 to -2.5 °C for PS with Mn = 175 kg/mol and Mw = 182 kg/mol, and Tg, rod - Tg, bulk = ∼-8 °C for PS with Mn = 929 kg/mol and Mw = 1420 kg/mol. In general, reductions in Tg occur when d ≤ ∼2Rg, where Rg is the bulk polymer radius of gyration. Thus, intrinsic size effects are significant when the rod diameter is smaller than the diameter (2Rg) associated with the spherical volume pervaded by coils in bulk. We hypothesize that the Tg reduction occurs when chain segment packing frustration is sufficiently perturbed by confinement in the nanorods. This explanation is supported by observed reductions in fragility with the increasing extent of confinement. We also explain why these small intrinsic size effects do not contradict reports that the Tg-confinement effect in supported PS films with one free surface exhibits little or no MW dependence.

Adhesion and friction in polymer films on solid substrates: conformal sites analysis and corresponding surface measurements.

In this work, we present a statistical mechanical analysis to elucidate the molecular-level factors responsible for the static and dynamic properties of polymer films. This analysis, which we term conformal sites theory, establishes that three dimensionless parameters play important roles in determining differences from bulk behavior for thin polymer films near to surfaces: a microscopic wetting parameter, αwx, defined as the ratio of polymer-substrate interaction to polymer-polymer interaction; a dimensionless film thickness, H*; and dimensionless temperature, T*. The parameter αwx introduced here provides a more fundamental measure of wetting than previous metrics, since it is defined in terms of intermolecular forces and the atomic structure of the substrate, and so is valid at the nanoscale for gas, liquid or solid films. To test this theoretical analysis, we also report atomic force microscopy measurements of the friction coefficient (μ), adhesion force (FA) and glass transition temperature (Tg) for thin films of two polymers, poly(methyl methacrylate) (PMMA) and polystyrene (PS), on two planar substrates, graphite and silica. Both the friction coefficient and the glass transition temperature are found to increase as the film thickness decreases, and this increase is more pronounced for the graphite than for the silica surface. The adhesion force is also greater for the graphite surface. The larger effects encountered for the graphite surface are attributed to the fact that the microscopic wetting parameter, αwx, is larger for graphite than for silica, indicating stronger attraction of polymer chains to the graphite surface.