Simone Napolitano

Glassy dynamics of soft matter under 1D confinement: How irreversible adsorption affects molecular packing, mobility gradients and orientational polarization in thin films

The structural dynamics of polymers and simple liquids confined at the nanometer scale has been intensively investigated in the last two decades in order to test the validity of theories on the glass transition predicting a characteristic length scale of a few nanometers. Although this goal has not yet been reached, the anomalous behavior displayed by some systems --e.g. thin films of polystyrene exhibit reductions of Tg exceeding 70K and a tremendous increase in the elastic modulus-- has attracted a broad community of researchers, and provided astonishing advancement of both theoretical and experimental soft matter physics. 1D confinement is achieved in thin films, which are commonly treated as systems at thermodynamic equilibrium where free surfaces and solid interfaces introduce monotonous mobility gradients, extending for several molecular sizes. Limiting the discussion to finite-size and interfacial effects implies that film thickness and surface interactions should be sufficient to univocally determine the deviation from bulk behavior. On the contrary, such an oversimplified picture, although intuitive, cannot explain phenomena like the enhancement of segmental mobility in proximity of an adsorbing interface, or the presence of long-lasting metastable states in the liquid state. Based on our recent work, we propose a new picture on the dynamics of soft matter confined in ultrathin films, focusing on non-equilibrium and on the impact of irreversibly chain adsorption on the structural relaxation. We describe the enhancement of dynamics in terms of the excess in interfacial free volume, originating from packing frustration in the adsorbed layer (Guiselin brush) at t* ≪ 1 , where t* is the ratio between the annealing time and the time scale of adsorption. Prolonged annealing at times exceeding the reptation time (usually t* ≫ 1 induces densification, and thus reduces the deviation from bulk behavior. In this Colloquium, after reviewing the experimental approaches permitting to investigate the structural relaxation of films with one, two or no free surfaces by means of dielectric spectroscopy, we propose several methods to determine gradients of mobility in thin films, and then discuss on the unexploited potential of analyses based on the time, temperature and thickness dependence of the orientational polarization via the dielectric strength.Graphical abstract

Unusual Deviations from Bulk Behavior in Ultrathin Films of Poly(tert-butylstyrene): Can Dead Layers Induce a Reduction of T g ?

The temperature and thickness dependence of the thermal expansivity of ultrathin thin films of poly(tert-butylstyrene) capped between aluminum layers revealed an unusual and intriguing confinement scenario. Below 50 nm, both the glass transition temperature and the thermal expansion coefficients decreased. Such a mixed behavior implies an enhancement of the molecular mobility, without the presence of any free surface, but dead layers. On the basis of a careful analysis of averaged quantities measured by capacitive dilatometry, we deduced a profile of thermal expansivity that explains our experimental data and previous observations in polymer nanocomposites. The effect of density-conformation coupling in proximity of a nonattractive interface allows the coexistence of an immobilized fraction in direct contact with the metal and an excess of thermal expansivity, arising from the long-range effects of packing frustration penetrating inside the bulk-like core of the film.

Adsorption Kinetics of Ultrathin Polymer Films in the Melt Probed by Dielectric Spectroscopy and Second-Harmonic Generation

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.

Supercooled liquids with enhanced orientational order

The nature of the glass transition, the transformation of a liquid into a disordered solid, still remains one of the most intriguing unsolved problems in materials science. Recent models rationalize crucial features of vitrification with the presence of medium-range ordered regions coexisting with the isotropic liquid. Here, in line with this prediction, we report an extraordinary enhancement in bond orientational order in ultrathin films of supercooled polyols, grown by physical vapour deposition. By varying the deposition conditions and the molecular size, we could tune the kinetic stability of the liquid phase enriched in bond orientational order towards conversion into the ordinary liquid phase. We observed a strong increase in the dielectric strength with respect to the ordinary supercooled liquid and slower structural dynamics, suggesting the existence of a metastable liquid phase with improved orientational correlations.

Is the Reduction in Tracer Diffusivity under Nanoscopic Confinement Related to a Frustrated Segmental Mobility

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.

Deviation from bulk behaviour in the cold crystallization kinetics of ultrathin films of poly(3-hydroxybutyrate)

The cold crystallization kinetics of ultrathin films of poly(3-hydroxybutyrate) (PHB) have been investigated by dielectric spectroscopy. Upon reduction of the film thickness, a lowering of the Avrami exponent accompanied by an increase of the crystallization time was observed. The experimental results are analysed in terms of reduction of the total number of nuclei involved in the crystallization process.

Unexpected impact of irreversible adsorption on thermal expansion: Adsorbed layers are not that dead

We investigated the impact of irreversible adsorption on the mechanisms of thermal expansion of 1D confined polymer layers. For spincoated films (polystyrene on aluminum) of constant thickness, the thermal expansion coefficient of the melt drops upon annealing following the kinetics of irreversible adsorption of the chains onto the supporting substrate, while the thermal expansion of the glass is annealing invariant. These perturbations are explained in terms of the reduction in free volume content, upon immobilization of monomers onto the substrate. To shed more light on this phenomenon, we performed an extensive investigation of the thermal expansion of irreversibly adsorbed layers of polystyrene on silicon oxide. We verified that, contrarily to recent speculations, these films cannot be modeled as dead layers - immobilized slabs lacking of segmental relaxation. On the contrary, thin adsorbed layers show an increase in thermal expansion with respect to the bulk, due to packing frustration. Immobilization plays a role only when the thickness of the adsorbed layers overcomes ∼10 nm. Finally, we show that for adsorbed layers the difference in thermal expansion between the melt and the glass is sufficiently high to investigate the glass transition down to 3 nm. Owing to this unique feature, not shared by spincoated films, adsorbed layers are the perfect candidate to study the properties of extremely thin polymer films.

Dynamics of the crystal to plastic crystal transition in the hydrogen bonded N-isopropylpropionamide

N-Isopropylpropionamide (NiPPA), which can self-associate via hydrogen bonds, was found to undergo a solid-solid transition as identified by DSC and X-ray diffraction. Below the melting temperature of 51 °C NIPPA adopts a plastic crystalline state with a tetragonal unit cell until it transforms into an ordered crystal with a monoclinic structure at temperatures ≤10 °C. Dielectric spectroscopy was used to characterize the dynamics of the system, determining the activation parameters for the plastic to crystalline phase transition. The activation enthalpy is relatively high, as expected for a system that involves hydrogen bonds. However, most of the activation energy as the plastic phase assumes a more crystalline state is due to the activation entropy, suggesting that the increased cooperativity observed in the relaxation processes is due to a steric locking of the molecules.

Irreversible Adsorption Controls Crystallization in Vapor-Deposited Polymer Thin Films

Matrix-assisted pulsed laser evaporation (MAPLE) provides a gentle means for the quasi-vapor deposition of macromolecules. It offers a unique opportunity for the bottom-up control of polymer crystallization as film growth and crystallization occur simultaneously. Surprisingly, with increasing deposition time, it has been shown that crystallization becomes prohibited despite the availability of polymer via continuous deposition. In this Letter, we investigate the molecular origins of suppressed crystallization in poly(ethylene oxide) films deposited by MAPLE atop silicon substrates. We find that suppressed crystallization results from the formation of an irreversibly adsorbed polymer nanolayer at the substrate that forms during deposition. Substrate temperature is shown to influence the stability of the irreversibly adsorbed nanolayer and, hence, polymer thin film crystallization. Our investigation offers new insight into how temperature and interfacial interactions can serve as a new toolbox to tune polymer film morphology in bottom-up deposition.

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.