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The behavior of polymers confined in ultrathin films (thickness < 200 nm) can sensitively differ from that observed in macroscopic samples. Based on the simple arguments of finite size and interfacial effects, film thickness, and surface interactions should be sufficient to univocally determine the deviation from bulk behavior. However, recent models suggest that a third key parameter, namely, the interfacial free volume, should also be considered. We describe a novel methodology that quantifies the volume available for structural relaxation at the interface between a thin polymer layer and its supporting substrate. Experiments performed at different annealing conditions verified that the shift in glass transition temperature, measured in thin films upon confinement, is proportional to the degree of adsorption and, thus, to the interfacial free volume.

We study the leakage of fluids (liquids or gases) in syringes with glass barrel, steel plunger and rubber O-ring stopper. The leakrate depends on the interfacial surface roughness and on the viscoelastic properties of the rubber. Random surface roughness is produced by sandblasting the rubber O-rings. We present a very simple theory for gas flow which takes into account both the diffusive and ballistic flow. The theory shows that the interfacial fluid flow (leakage) channels are so narrow that the gas flow is mainly ballistic (the so called Knudsen limit). We compare the leakrate obtained using air and helium. For barrels filled with water we observe no leakage even if leakage occurs for gases. We interpret this as resulting from capillary (Laplace pressure or surface energy) effects., 18 pages, 28 figures

Cocontinuous nanostructured blends of semicrystalline polymers tend to align along a preferential direction when subjected to steady flow. We investigate the flow induced changes experienced by cocontinuous blends using transmission electron microscopy and small-angle X-ray scattering and we demonstrate that shear flow induces a preferential direction while preserving a cocontinuous structure, which is oriented along the flow direction. In parallel, we study the viscoelastic properties of the aligned morphology by linear oscillatory shear above the melting temperature of the blend components and observe a liquid-like behavior at short times. Upon annealing at high temperature, the aligned morphology relaxes back to the original isotropic structure with a solid-like viscoelastic behavior.

We studied the adsorption kinetics of supported ultrathin films of dye-labeled polystyrene (l-PS) by combining dielectric spectroscopy (DS) and the interface-specific nonlinear optical second harmonic generation (SHG) technique. While DS is sensitive to the fraction of mobile dye moieties (chromophores), the SHG signal probes their anisotropic orientation. Time-resolved measurements were performed above the glass transition temperature on two different sample geometries. In one configuration, the l-PS layer is placed in contact with the aluminum surface, while in the other one, the deposition is done on a strongly adsorbed layer of neat PS. From the time dependence of the dielectric strength and SHG signal of the l-PS layer in contact with the metal, we detected two different kinetics regimes. We interpret these regimes in terms of the interplay between adsorption and orientation of the adsorbing labeling moieties. At early times, dye moieties get adsorbed adopting an orientation parallel to the surface. When adsorption proceeds to completeness, the kinetics slows down and the dye moieties progressively orient normal to the surface. Conversely, when the layer of l-PS layer is deposited on the strongly adsorbed layer of neat PS, both the dielectric strength and the SHG signal do not show any variation with time. This means that no adsorption takes place.

We investigated by dielectric relaxation spectroscopy the distribution of glass transition temperatures and dielectric relaxation strength inside ultrathin polymer films capped between metallic layers. Measurements of the local dielectric properties were achieved by selectively placing layers of dye-labeled polystyrene at different depth inside films of neat polystyrene of different thickness. We show experimental evidence for an interfacial nature of the deviations from bulk behavior; in particular, the value of the dielectric strength and the glass transition temperature strongly depend on the distance from the solid interface. These peculiar profiles of static and dynamic dielectric properties are discussed in terms of a physical picture based on competition between chain adsorption and packing frustration at different annealing conditions. Such a picture was able to rationalize common features observed in properties of ultrathin films like reduction of the relaxation strength, broadening of the dynamic glass transition process, and finally a shift of the structural relaxation time. © 2010 American Chemical Society. ispartof: Macromolecules vol:43 issue:20 pages:8686-8691 status: published

Extensive investigation of polymer layers confined in nanometer-sized geometries revealed that the presence of an absorbing substrate or a free surface alters properties of polymers such as biaxial creep behavior, flow and intermolecular entanglements, diffusion of small molecules inside the matrix, crystallization kinetics, physical aging, the glass transition temperature (Tg), 7 and thus local chain (segmental) mobility. In bulk, the local chain mobility can be investigated by means of several experimental approaches. On the contrary, due to obvious instrumental difficulties, a technique probing the local chain dynamics of freely standing ultrathin polymer films was not available so far. In this Communication, we introduce a novel experimental method taking advantage of the sensitivity of dielectric spectroscopy (DS) and being able to probe the segmental dynamics of freely standing ultrathin polymer layers over a broad frequency range (1 Hz-1 MHz) without altering or eliminating their two free surfaces. The glass transition temperatures assigned by the approach described in this Communication are in excellent agreement with data from the literature. In our approach, polymer films are suspended over interdigitated comb electrodes, IDE. The electric signal is measured by applying an ac voltage to elevated metallic electrodes made up by the two interdigitated comb structures deposited on a highly insulating substrate (see Figure 1a). For films of thickness D much smaller than the separation between two neighbored electrodes (∼10 μm), the electric field lines penetrate inside the layer with a direction parallel to the surface. Consequently, the individual complex electric capacitances of all sublayers (of thickness Di) constituting the polymer membrane add up to the total capacitance according to CTOT * (ω,T,D) ) ∑iCi(ω,T,Di), an expression that holds for any angular frequency ω and temperature T. Under these conditions, the different contributions to the relaxation spectra are linearly superimposed. The comb electrode geometry, in fact, avoids ambiguities arising from spectra of ultrathin films probed by an electric field orthogonal to the surface, as in the parallel plates geometry where capacitances enter in a series model, CTOT * (ω,T,D)-1 ) ∑iCi(ω,T,Di). To prove the feasibility of our technique, we investigated the structural relaxation dynamics of atactic * To whom correspondence should be addressed. E-mail: simone.napolitano@fys.kuleuven.be (S.N.) or wubbenhorst@fys.kuleuven.be (M.W.). Figure 1. (a) Schematic representation of the IDE chip; the red and blue fingers are at opposite potential. The height of the structures is 0.8 μm, the width of each finger is 5 μm, and the mean distance between two neighbored fingers is 8 μm. The black arrows represent the direction of the E-field inside the film. (b) AFM image 20 μm × 20 μm (topography) of a 65 nm thick film of PS after annealing at 70 °C above its Tg (see text). The white dashed lines reproduce the structure of the empty chip. The peaks at the edge of the electrodes metal buildups already present in the empty chip. Volume 42, Number 5 March 10, 2009

We developed an experimental method for the determination of the tracer diffusivity D(tr) in ultrathin polymer films, and the changes in the segmental mobility of tracer molecules while they diffuse through matrices of different thickness and get adsorbed onto a target substrate. D(tr) starts decreasing already at 120-150 nm and drops to 1% of its bulk value in films as thin as 7.5 nm. We discuss the results highlighting a strong decoupling between the reduction in mass transport at the nanoscale and the increase in the glass transition temperature determined via capacitive dilatometry together with a breakdown of the Stokes-Einstein relation between orientational and translational degrees of freedom.

The cold crystallization process in poly(ethylene terephthalate) (PET) spin-coated ultrathin films was studied by infrared spectroscopy. The conformational change associated to the formation of crystal phase during annealing at 107 °C was measured in real time, by monitoring both intensity and frequency shift of trans and gauche conformer bands of the PET glycol segment. Enhancement of crystallization kinetics was observed in thin films deposited on amorphous silicon, with respect to a 20 μm thick free standing film used as reference, where the fastest kinetics was observed for the thinnest (35 nm) film. Experimental findings were interpreted in terms of scarce interaction between PET films and silicon substrate, which does not provide slowing down of crystallization kinetics as observed on different substrates. This results in a dominant effect of the polymer/air interface, where faster kinetics is observed, as also confirmed by atomic force microscopy imaging, particularly on the thinnest film. Additionally, Avrami and Avramov analyses evidence a decrease of both the Avrami exponent, related to growth dimensionality, and induction time, related to delay of nucleation start, when decreasing film thickness. Therefore, the reported results enrich the description of confinement and substrate interaction effects on the cold crystallization process taking place in PET ultrathin films.

Dielectric measurements of ultrathin polymer layers capped between metallic electrodes revealed, besides controversial deviations in the dielectric glass transition temperature, a universal and continuous decrease of the dielectric strength which scales with the surface/volume ratio. In an attempt to describe the thickness dependence of this last quantity, proportional to the number of segments relaxing on the time and length scale of the dynamic glass transition and being a unique probe of the deviations from bulk behavior, we propose a model based on the impact of adsorption on the segmental dynamics and on the assumption of a smooth distribution of mobility inside the film. Differently to simplified bilayer and trilayer-models already described in the literature, our approach is able to reproduce the behavior of all the polymer systems so far investigated. Moreover, our calculations allow determining the penetration depth of the surface induced perturbations along with the thickness of the dead (or reduced) mobility layer where motions of polymer segments are highly inhibited (or reduced) on the time scale of the glassy dynamics. Analysis of the temperature dependence of this quantity confirmed that the gradient of mobility is limited by the gyration radius at temperatures much larger than Tg while in deeply supercooled melts the penetration of the interfacial interactions exceeds the dimension of the single polymer chain.