Eliminating the Enhanced Mobility at the Free Surface of Polystyrene: Fluorescence Studies of the Glass Transition Temperature in Thin Bilayer Films of Immiscible Polymers

Dramatic Increase in Polymer Glass Transition Temperature under Extreme Nanoconfinement in Weakly Interacting Nanoparticle Films

Properties of polymers in polymer nanocomposites and nanopores have been shown to deviate from their respective bulk properties due to physical confinement as well as polymer-particle interfacial interactions. However, separating the confinement effects from the interfacial effects under extreme nanoconfinement is experimentally challenging. Capillary rise infiltration enables polymer infiltration into nanoparticle (NP) packings, thereby confining polymers within extremely small pores and dramatically increasing the interfacial area, providing a good system to systematically distinguish the role of each effect on polymer properties. In this study, we investigate the effect of spatial confinement on the glass transition temperature ( T_g) of polystyrene (PS) infiltrated into SiO₂ NP films. The degree of confinement is tuned by varying the molecular weight of polymers, the size of NPs (diameters between 11 and 100 nm, producing 3-30 nm average pore sizes), and the fill-fraction of PS in the NP films. We show that in these dense NP packings the T_g of confined PS, which interacts weakly with SiO₂ NPs, significantly increases with decreasing pore size such that for the two molecular weights of PS studied the T_g increases by up to 50 K in 11 nm NP packings, while T_g is close to the bulk T_g in 100 nm NP packings. Interestingly, as the fill-fraction of PS is decreased, resulting in the accumulation of the polymer in the contacts between nanoparticles, hence an increased specific interfacial area, the T_g further increases relative to the fully filled films by another 5-8 K, indicating the strong role of geometrical confinement as opposed to the interfacial effects on the measured T_g values.

Exploring the broadening and the existence of two glass transitions due to competing interfacial effects in thin, supported polymer films

In this report, we use ellipsometry to characterize the glass transition in ultra-thin films of poly(2-vinyl pyridine) (P2VP) supported on a silicon substrate. P2VP is known to have attractive substrate interactions, which can increase the T_g of ultra-thin films compared to the bulk value. Here, we use an extended temperature range to show that the glass transition can be extremely broad, indicating that a large gradient of the dynamics exists through the film with slow dynamics near the substrate and enhanced dynamics at the free surface. To observe the effect of these two interfaces on the average thin film dynamics, cooling rate-dependent T_g (CR-T_g) measurements were used to indirectly probe the average relaxation times of the films. We demonstrate that ultra-thin films have lower fragility compared to bulk films, and, when cooled at slow cooling rates (<1 K/min), exhibit extreme broadening of the dynamics (<70 nm) and eventually complete decoupling between the free surface and substrate regions to produce films with two distinct T_g's (<16 nm). T_g,high increases with decreasing thickness in a similar manner to what has been observed in previous studies on P2VP, and T_g,low decreases with decreasing film thickness in a similar manner to what has been observed in polymer films with enhanced free surfaces and neutral substrate interactions. These observations indicate that the dynamics in thin films of P2VP can be strongly coupled over a length scale of ∼10-20 nm, resulting in two co-existing layers with two distinct glass transitions when the range of the dynamical gradients become too large to sustain (breadth of the transition > 50 K).

Aging near rough and smooth boundaries in colloidal glasses

We use a confocal microscope to study the aging of a bidisperse colloidal glass near rough and smooth boundaries. Near smooth boundaries, the particles form layers, and particle motion is dramatically slower near the boundary as compared to the bulk. Near rough boundaries, the layers nearly vanish, and particle motion is nearly identical to that of the bulk. The gradient in dynamics near the boundaries is demonstrated to be a function of the gradient in structure for both types of boundaries. Our observations show that wall-induced layer structures strongly influence aging.

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.

Role of "Hard" and "Soft" Confinement on Polymer Dynamics at the Nanoscale

We investigated the segmental dynamics of asymmetrically confined polymer films and report an unusual phenomenon in which the presence and thickness of a soft confining layer are responsible for significant changes in the segmental dynamics of the confined films. Specifically, the segmental dynamics of poly(vinyl alcohol) (PVA) thin films asymmetrically confined between hard aluminum (Al), and soft polystyrene (PS) films are shown to shift by as much as half an order of magnitude upon changes in the thicknesses of the confining PS layer. These effects are more significant than those due to symmetric confinement between hard Al substrates or exposure to a free surface. These observations, partially rationalized in terms of recent simulations and theory, implicate the role of the moduli of the confining layers.

How Do Organic Vapors Swell Ultrathin Films of Polymer of Intrinsic Microporosity PIM-1?

Dynamic sorption of ethanol and toluene vapor into ultrathin supported films of polymer of intrinsic microporosity PIM-1 down to a thickness of 6 nm are studied with a combination of in situ spectroscopic ellipsometry and in situ X-ray reflectivity. Both ethanol and toluene significantly swell the PIM-1 matrix and, at the same time, induce persistent structural relaxations of the frozen-in glassy PIM-1 morphology. For ethanol below 20 nm, three effects were identified. First, the swelling magnitude at high vapor pressures is reduced by about 30% as compared to that of thicker films. Second, at low penetrant activities (below 0.3p/p₀), films below 20 nm are able to absorb slightly more penetrant as compared with thicker films despite a similar swelling magnitude. Third, for the ultrathin films, the onset of the dynamic penetrant-induced glass transition P_g has been found to shift to higher values, indicating higher resistance to plasticization. All of these effects are consistent with a view where immobilization of the superglassy PIM-1 at the substrate surface leads to an arrested, even more rigid, and plasticization-resistant, yet still very open, microporous structure. PIM-1 in contact with the larger and more condensable toluene shows very complex, heterogeneous swelling dynamics, and two distinct penetrant-induced relaxation phenomena, probably associated with the film outer surface and the bulk, are detected. Following the direction of the penetrant's diffusion, the surface seems to plasticize earlier than the bulk, and the two relaxations remain well separated down to 6 nm film thickness, where they remarkably merge to form just a single relaxation.

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 (T_g) 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 T_g 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 T_g (both as T_g,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 T_g are present for PS nanorods with M_n and M_w ≥ ∼175 kg/mol, with effects increasing in magnitude with increasing MW. For example, in 24-nm-diameter template-supported nanorods, T_{g, rod} - T_{g, bulk} = -2.0 to -2.5 °C for PS with M_n = 175 kg/mol and M_w = 182 kg/mol, and T_{g, rod} - T_{g, bulk} = ∼-8 °C for PS with M_n = 929 kg/mol and M_w = 1420 kg/mol. In general, reductions in T_g occur when d ≤ ∼2R_g, where R_g is the bulk polymer radius of gyration. Thus, intrinsic size effects are significant when the rod diameter is smaller than the diameter (2R_g) associated with the spherical volume pervaded by coils in bulk. We hypothesize that the T_g 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 T_g-confinement effect in supported PS films with one free surface exhibits little or no MW dependence.

Glass transition of polymers in bulk, confined geometries, and near interfaces

When cooled or pressurized, polymer melts exhibit a tremendous reduction in molecular mobility. If the process is performed at a constant rate, the structural relaxation time of the liquid eventually exceeds the time allowed for equilibration. This brings the system out of equilibrium, and the liquid is operationally defined as a glass-a solid lacking long-range order. Despite almost 100 years of research on the (liquid/)glass transition, it is not yet clear which molecular mechanisms are responsible for the unique slow-down in molecular dynamics. In this review, we first introduce the reader to experimental methodologies, theories, and simulations of glassy polymer dynamics and vitrification. We then analyse the impact of connectivity, structure, and chain environment on molecular motion at the length scale of a few monomers, as well as how macromolecular architecture affects the glass transition of non-linear polymers. We then discuss a revised picture of nanoconfinement, going beyond a simple picture based on interfacial interactions and surface/volume ratio. Analysis of a large body of experimental evidence, results from molecular simulations, and predictions from theory supports, instead, a more complex framework where other parameters are relevant. We focus discussion specifically on local order, free volume, irreversible chain adsorption, the Debye-Waller factor of confined and confining media, chain rigidity, and the absolute value of the vitrification temperature. We end by highlighting the molecular origin of distributions in relaxation times and glass transition temperatures which exceed, by far, the size of a chain. Fast relaxation modes, almost universally present at the free surface between polymer and air, are also remarked upon. These modes relax at rates far larger than those characteristic of glassy dynamics in bulk. We speculate on how these may be a signature of unique relaxation processes occurring in confined or heterogeneous polymeric systems.

Changes in the temperature-dependent specific volume of supported polystyrene films with film thickness

Recent studies have measured or predicted thickness-dependent shifts in density or specific volume of polymer films as a possible means of understanding changes in the glass transition temperature Tg(h) with decreasing film thickness with some experimental works claiming unrealistically large (25%-30%) increases in film density with decreasing thickness. Here we use ellipsometry to measure the temperature-dependent index of refraction of polystyrene (PS) films supported on silicon and investigate the validity of the commonly used Lorentz-Lorenz equation for inferring changes in density or specific volume from very thin films. We find that the density (specific volume) of these supported PS films does not vary by more than ±0.4% of the bulk value for film thicknesses above 30 nm, and that the small variations we do observe are uncorrelated with any free volume explanation for the Tg(h) decrease exhibited by these films. We conclude that the derivation of the Lorentz-Lorenz equation becomes invalid for very thin films as the film thickness approaches ∼20 nm, and that reports of large density changes greater than ±1% of bulk for films thinner than this likely suffer from breakdown in the validity of this equation or in the difficulties associated with accurately measuring the index of refraction of such thin films. For larger film thicknesses, we do observed small variations in the effective specific volume of the films of 0.4 ± 0.2%, outside of our experimental error. These shifts occur simultaneously in both the liquid and glassy regimes uniformly together starting at film thicknesses less than ∼120 nm but appear to be uncorrelated with Tg(h) decreases; possible causes for these variations are discussed.

Influence of grafting on the glass transition temperature of PS thin films

We present an investigation of the effect of the interaction between a thin polystyrene film and its supporting substrate on its glass transition temperature ([Formula: see text]). We modulate this interaction by depositing the film on end-tethered polystyrene grafted layers of controlled molecular parameters. By comparing [Formula: see text] measurements versus film thickness for films deposited on different grafted layers and films deposited directly on a silicon substrate, we can conclude that there is no important effect of the film-subtrate interaction. Our interpretation of these results is that local orientation and dynamic effects substantial enough to influence [Formula: see text] cannot readily be obtained by grafting prepolymerized chains to a surface, due to intrinsic limitation of the surface grafting density.

Single-Step Nanoporation of Water-Immersed Polystyrene Film by Gaseous Nanobubbles

Nanoporation of planar polystyrene (PS) film by gaseous nanobubbles is described. Ambient gas (air) surface nanobubbles are used to create rounded nanopinholes in supported ultrathin PS films immersed in water. Nanoporation proceeds under biocompatible conditions-in deionized water at room temperature (20 °C) in the absence of any high-energy-demanding processes, in a single step triggered by short (5 s) moderate pressure drop (Δp ≈ -10 kPa) applied on an aqueous phase. Atomic force microscopy (AFM) analysis shows a relatively narrow dimensional distribution, with the prevailing nanopinhole radius of 10 nm and uniform spread with an appearance density of ∼600/μm². The nanoporation mechanism is discussed. "Nanobubble/tip-assisted" nanoindentation phenomenon is observed based on the compounded interaction of the surface nanobubble with the AFM tip upon prolonged in situ nanobubble scanning.

Effect of tethering on the surface dynamics of a thin polymer melt layer

The surface height fluctuations of a layer of low molecular weight (2.2k) untethered perdeuterated polystyrene (dPS) chains adjacent to a densely grafted polystyrene brush are slowed dramatically. Due to the interpenetration of the brush with the layer of "untethered chains" a hydrodynamic continuum theory can only describe the fluctuations when the effective thickness of the film is taken to be that which remains above the swollen brush. The portion of the film of initially untethered chains that interpenetrates with the brush becomes so viscous as to effectively play the role of a rigid substrate. Since these hybrid samples containing a covalently tethered layer at the bottom do not readily dewet, and are more robust than thin layers of untethered short chains on rigid substrates, they provide a route for tailoring polymer layer surface properties such as wetting, adhesion and friction.

Calorimetric evidence for a mobile surface layer in ultrathin polymeric films: poly(2-vinyl pyridine)

Specific heat spectroscopy was used to study the dynamic glass transition of ultrathin poly(2-vinyl pyridine) films (thicknesses: 405-10 nm). The amplitude and the phase angle of the differential voltage were obtained as a measure of the complex heat capacity. In a traditional data analysis, the dynamic glass transition temperature Tg is estimated from the phase angle. These data showed no thickness dependency on Tg down to 22 nm (error of the measurement of ±3 K). A derivative-based method was established, evidencing a decrease in Tg with decreasing thickness up to 7 K, which can be explained by a surface layer. For ultrathin films, data showed broadening at the lower temperature side of the spectra, supporting the existence of a surface layer. Finally, temperature dependence of the heat capacity in the glassy and liquid states changes with film thickness, which can be considered as a confinement effect.

Enhanced diffusion and mobile fronts in a simple lattice model of glass-forming liquids

The diffusion of mobility in bulk and thin film fluids near their glass transition is examined with a kinetic lattice model, and compared to recent experiments on bulk liquids and vapor-deposited thin film glasses. The "limited mobility" (LM) lattice model exhibits dynamic heterogeneity of mobility when the fluid is near its kinetic arrest transition; a finite-parameter second-order critical point in the LM model bearing strong resemblance to the glass transition in real fluids. The spatial heterogeneity of mobility near kinetic arrest leads to dynamics that violate the Stokes-Einstein relation. To make connections with experiment, LM model simulations of self-diffusion constants in fluids near kinetic arrest are compared to those in two organic glass-formers. In addition, simulations of mobility in films that have been temperature-jumped above kinetic arrest (starting from an arrested state) are carried out. The films develop a "front" of mobility at their free surface that progresses into the film interior at a constant rate, thereby mobilising the entire film to fluidity. The velocity of the front scales with the self-diffusion constant for analogous bulk systems-an observation consistent with experiments on vapor-deposited molecular thin films.

Interfacial Morphology and Effects on Device Performance of Organic Bilayer Heterojunction Solar Cells

The effects of interface roughness between donor and acceptor in a bilayer heterojunction solar cell were investigated on a polymer-polymer system based on poly(3-hexylthiophene) (P3HT) and poly(dioctylfluorene-alt-benzothiadiazole) (F8BT). Both polymers are known to reorganize into semicrystalline structures when heated above their glass-transition temperature. Here, the bilayers were thermally annealed below glass transition of the bulk polymers (≈140 °C) at temperatures of 90, 100, and 110 °C for time periods from 2 min up to 250 min. No change of crystallinity could be observed at those temperatures. However, X-ray reflectivity and device characteristics reveal a coherent trend upon heat treatment. In X-ray reflectivity investigations, an increasing interface roughness between the two polymers is observed as a function of temperature and annealing time, up to a value of 1 nm. Simultaneously, according bilayer devices show an up to 80% increase of power conversion efficiency (PCE) for short annealing periods at any of the mentioned temperatures. Together, this is in agreement with the expectations for enlargement of the interfacial area. However, for longer annealing times, a decrease of PCE is observed, despite the ongoing increase of interface roughness. The onset of decreasing PCE shifts to shorter durations the higher the annealing temperature. Both, X-ray reflectivity and device characteristics display a significant change at temperatures below the glass transition temperatures of P3HT and F8BT.

Facilitation of interfacial dynamics in entangled polymer films

In this article, we use cooling-rate dependent Tg measurements (CR-Tg) to indirectly probe the relaxation dynamics of supported polystyrene thin films of various molecular weights, all chosen to be above the entanglement molecular weight. We show that the dynamics in these films deviate from bulk dynamics below a temperature T(*) = Tg + 6 K = 380  K ± 1  K. We show that T(*) for films of all thicknesses and molecular weights is the same as the temperature at which the free surface dynamics deviate from the bulk dynamics. The apparent activation barrier of the glass transition in thin films decreases towards that of the free surface as the film thickness decreases. This provides strong evidence that thin film dynamics are facilitated by the enhanced mobility at the free surface. The observation of T(*) can help resolve some seemingly contradictory data by suggesting that studies performed at higher temperatures (T > T(*)), or which probe shorter relaxation times (τ < τ(*) ∼ 1 s) would not observe properties that deviate from bulk values. We also demonstrate that the relaxation dynamics of supported entangled polystyrene films slow down slightly as the molecular weight of polystyrene increases. An eight nanometer film of Mw =2240 kg/mol polystyrene shows a Tg reduction of 27 K at a cooling rate of 1 K/min, while a film of the same thickness made of Mw =45.8 kg/mol polystyrene has a 36 K reduction of Tg compared to the bulk film at the same cooling rate. We hypothesize this is either due to the density of a dynamically "dead" layer near the substrate increasing with molecular weight, or partial anchoring of long chains, which could hinder segmental diffusion near the free surface.

Manipulating the glass transition behavior of sulfonated polystyrene by functionalized nanoparticle inclusion

Nanoscale interfaces can modify the phase transition behaviors of polymeric materials. Here, we report the double glass transition temperature (Tg) behavior of sulfonated polystyrene (sPS) by the inclusion of 14 nm amine-functionalized silica (NH2-SiO2) nanoparticles, which is different from the single Tg behaviors of neat sPS and silica (SiO2)-filled sPS. The inclusion of 20 wt% NH2-SiO2 nanoparticles results in an increase of Tg by 9.3 °C as well as revealing a second Tg reduced by 44.7 °C compared to the Tg of neat sPS. By contrast, when SiO2 nanoparticles with an identical concentration and size to NH2-SiO2 are dispersed, sPS composites possess a single Tg of 7.3 °C higher than that of the neat sPS. While a nanoscale dispersion is observed for SiO2 nanoparticles, as confirmed by microscopic and X-ray scattering analyses, NH2-SiO2 nanoparticles show the coexistence of micron-scale clustering along with a nanoscale dispersion of the individual nanoparticles. The micro-phase separation contributes to the free volume induced Tg reduction by the plasticization effect, whereas the Tg increase originates from the polymer segment mobility constrained by nanoconfinement and the rigid amorphous fractions deriving from strong polymer-particle interactions.