Simple Model to Predict the Adsorption Rate of Polymer Melts.

We determine the adsorption rate of polymer melts by means of measurements of molecular mobility. We show that the complex set of molecular rearrangements involved in the adsorption of polymers on flat surfaces can be modeled as an equilibration kinetics driven by the slow Arrhenius process (SAP), a recently discovered molecular mechanism. Our predictive model is based on the single hypothesis that the number of chains adsorbed per unit surface within the timescale of spontaneous fluctuations associated to the SAP is a temperature-invariant constant, not depending on the chemical structure of the polymer. Going beyond the qualitative arguments setting a correlation between equilibrium and nonequilibrium properties, we demonstrate that the rate at which an adsorbed layer grows does not depend on interfacial interactions. By considering simple physical arguments, we demonstrate that this quantity can be straightforwardly determined using the energy barrier of molecular motion as only input.

The slow Arrhenius process in small organic molecules.

Equilibration, the complex set of molecular rearrangements leading to more stable states, is usually dominated by density fluctuations, occurring through the structural (α-)relaxation, whose timescale quickly increases upon cooling. Growing evidence shows, however, that equilibration can be reached also through an alternative pathway provided by the Slow Arrhenius process (SAP), a molecular mode slower than the structural processes in the liquid state and faster in glass. The SAP, widely observed in polymers, has not yet been reported in small molecules, probably because of the larger experimental difficulties in handling these systems. Here, we report the presence of the SAP in three different molecular glassformers, by investigating these systems in the thin film geometry via dielectric spectroscopy. These results reinforce the idea that the SAP is a universal feature of liquid and glassy dynamics.

Tracing the slow Arrhenius process deep in the glassy state-quantitative evaluation of the dielectric relaxation of bulk samples and thin polymer films in the temperature domain.

The slow Arrhenius process (SAP) is a dielectric mode connected to thermally activated equilibration mechanisms, allowing for a fast reduction in free energy in liquids and glasses. The SAP, however, is still poorly understood, and so far, this process has mainly been investigated at temperatures above the glass transition. By employing a combination of methods to analyze dielectric measurements under both isochronal and isothermal conditions, we were able to quantitatively reproduce the dielectric response of the SAP of different polymers and to expand the experimental regime over which this process can be observed down to lower temperatures, up to 70 K below the glass transition. Employing thin films of thicknesses varying between 10 and 800 nm, we further verified that the peak shape and activation energy of the SAP of poly(4-bromostyrene) are not sensitive to temperature, nor do they vary upon confinement at the nanoscale level. These observations confirm the preliminary trends reported for other polymers. We find that one single set of parameters-meaning the activation barrier and the pre-exponential factor, respectively, linked to the enthalpic and entropic components of the process-can describe the dynamics of the SAP in both the supercooled liquid and glassy states, in bulk and thin films. These results are discussed in terms of possible molecular origins of the slow Arrhenius process in polymers.

Physical Aging of Polystyrene Confined in Anodic Aluminum Oxide Nanopores.

We investigated the glassy dynamics of polystyrene (PS) confined in anodic aluminum oxide (AAO) nanopores by differential scanning calorimetry. Based on the outcome of our experiments, we show that the cooling rate applied to process the 2D confined PS melt has a significant impact on both the glass transition and the structural relaxation in the glassy state. A single glass transition temperature (Tg) is observed in quenched samples, while slow-cooled PS chains show two Tgs corresponding to a core-shell structure. The former phenomenon resembles what is observed in freestanding structures, while the latter is imputed to the adsorption of PS onto AAO walls. A more complex picture was drawn for physical aging. In the case of quenched samples, we observed a non-monotonic trend of the apparent aging rate that in 400 nm pores, reaches a value almost twice as larger than what is measured in bulk and decreases upon further confinement in smaller nanopores. For slow-cooled samples, by adequately varying the aging conditions, we were able to control the equilibration kinetics and either separate the two aging processes or induce an intermediate aging regime. We propose a possible explanation of these findings in terms of distribution in free volume and the presence of different aging mechanisms.

Enthalpy-entropy compensation in the slow Arrhenius process.

The Meyer-Neldel compensation law, observed in a wide variety of chemical reactions and other thermally activated processes, provides a proportionality between the entropic and the enthalpic components of an energy barrier. By analyzing 31 different polymer systems, we show that such an intriguing behavior is encountered also in the slow Arrhenius process, a recently discovered microscopic relaxation mode, responsible for several equilibration mechanisms both in the liquid and the glassy state. We interpret this behavior in terms of the multiexcitation entropy model, indicating that overcoming large energy barriers can require a high number of low-energy local excitations, providing a multiphonon relaxation process.

Substrate Roughness Speeds Up Segmental Dynamics of Thin Polymer Films.

We show that the segmental mobility of thin films of poly(4-chlorostyrene) prepared under nonequilibrium conditions gets enhanced in the proximity of rough substrates. This trend is in contrast to existing treatments of roughness which conclude it is a source of slower dynamics, and to measurements of thin films of poly(2-vinylpiridine), whose dynamics is roughness invariant. Our experimental evidence indicates the faster interfacial dynamics originate from a reduction in interfacial density, due to the noncomplete filling of substrate asperities. Importantly, our results agree with the same scaling that describes the density dependence of bulk materials, correlating segmental mobility to a term exponential in the specific volume, and with empirical relations linking an increase in glass transition temperature to larger interfacial energy.

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.

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.

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.

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.

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.

Are polymers glassier upon confinement?

Glass forming systems are characterized by a stability against crystallization upon heating and by the easiness with which their liquid phase can be transformed into a solid lacking of long-range order upon cooling (glass forming ability). Here, we report the thickness dependence of the thermal phase transition temperatures of poly(l-lactide acid) thin films supported onto solid substrates. The determination of the glass transition, cold crystallization and melting temperatures down to a thickness of 6 nm, permitted us to build up parameters describing glass stability and glass forming ability. We observed a strong influence of the film thickness on the latter, while the former is not affected by 1D confinement. Further experiments permitted us to highlight key structural morphology features giving insights to our ellipsometric results via a physical picture based on the changes in the free volume content in proximity of the supporting interfaces.

Slowing down of accelerated structural relaxation in ultrathin polymer films.

We demonstrate with molecular simulation that the acceleration of structural relaxation, also known as physical aging, commonly experimentally observed in thin polymer films slows down at extremely small thicknesses. This phenomenon can be attributed to an inversed free volume diffusion process caused by the sliding motion of chain molecules. Our findings provide direct evidence of the relationship between the sliding motion of short chain fragments and the structural relaxation of ultrathin polymer films, and also verify the existence of a new confinement effect at the nanoscale.

Fast Processing Affects the Relaxation of Polymers: The Slow Arrhenius Process Can Probe Stress at the Molecular Level.

Polymer materials are commonly processed at rates higher than those at which their molecules spontaneously reach equilibrium conditions. The resulting nonequilibrium conformations might significantly affect the mechanical behavior and the shelf time of the final products. To understand how processing-properties relations work, we investigated the impact of spin coating, an archetypical method to fabricate thin polymer layers. By using a geometry in which nonequilibrium conformations are frozen over sufficiently long experimental times, we could identify how molecular relaxation is affected by fast preparation methods. We find that while the (α-)segmental relaxation is not affected by the rate at which films are processed, the intensity of the slow Arrhenius process (SAP), a relaxation mechanism active both above and below the glass transition, can be used as a probe of the degree of mechanical stress experienced by the material.

Quantifying and Modeling the Crystallinity of Polymers Confined in Nanopores.

We propose a methodology to characterize the crystalline content of interfacial polymer layers in systems confined at the nanoscale level in a 2D geometry. Based on the crystallinity data of a set of polymers, we introduce a simple model to describe the gradient in crystallinity introduced by confining polymer chains in nanopores. Our model underscores the pivotal role that interfaces play in crystallization and unequivocally contradicts the existence of interfacial "dead" layers where crystallization cannot take place. Further, we verified that the organization of crystals near the pore walls resembles the macromolecular architecture of adsorbed layers, hinting at a strong interplay between crystallization and adsorption.

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 70 K 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.

Intra-arterial thrombectomy versus standard intravenous thrombolysis in patients with anterior circulation stroke caused by intracranial arterial occlusions: a single-center experience.

BACKGROUND: Severely impaired patients with persisting intracranial occlusion despite standard treatment with intravenous (IV) administration of recombinant tissue plasminogen activator (rtPA) or presenting beyond the therapeutic window for IV rtPA may be candidates for interventional neurothrombectomy (NT). The safety and efficacy of NT by the Penumbra System (PS) were compared with standard IV rtPA treatment in patients with severe acute ischemic stroke (AIS) caused by large intracranial vessel occlusion in the anterior circulation. METHODS: Consecutive AIS patients underwent a predefined treatment algorithm based on arrival time, stroke severity as measured by the National Institutes of Health Stroke Scale (NIHSS) score, and site of arterial occlusion on computed tomographic angiography (CTA). NT was performed either after a standard dose of IV rtPA (bridging therapy [BT]) or as single treatment (stand-alone NT [SAT]). Rates of recanalization, symptomatic intracranial bleeding (SIB), mortality, and functional outcome in NT patients were compared with a historical cohort of IV rtPA treated patients (i.e., controls). Three-month favourable outcome was defined as a modified Rankin Scale (mRS) score ≤2. RESULTS: Forty-six AIS patients were treated with NT and 51 with IV rtPA. The 2 groups did not differ with regard to demographics, onset NIHSS score (18.5±4 v 17±5; P=.06), or site of intracranial occlusion. Onset-to-treatment time in the NT and IV rtPA groups was 230 minutes (±78) and 176.5 (±44) minutes, respectively (P=.001). NT patients had significantly higher percentages of major improvement (≥8 points NIHSS score change at 24 hours; 26% v 10%; P=.03) and partial/complete recanalization (93.5% v 45%; P<.0001) compared to controls. Treatment by either SAT or BT similarly improved the chance of early recanalization and early clinical improvement. No significant differences were observed in the rate of SIB (11% v 6%), 3-month mortality (24% v 25%), or favorable outcome (40% v 35%) between NT and IV rtPA patients. CONCLUSIONS: Despite significantly delayed time of intervention, NT patients had higher rates of recanalization and early major improvement, with no differences in symptomatic intracranial hemorrhages. Early NIHSS score improvement did not translate into better 3-month mortality or outcome. NT seems a safe and effective adjuvant treatment strategy for selected patients with severe AIS secondary to large intracranial vessel occlusion in the anterior circulation.

Molecular Mobility of Polymers at the Melting Transition.

Melting of crystals is an archetypical first order phase transition. Albeit extensive efforts, the molecular origin of this process in polymers is still not clear. Experiments are complicated by the tremendous change in mechanical properties and the occurrence of parasitic phenomena masking the genuine material response. Here, we present an experimental procedure permitting to circumvent these issues by investigating the dielectric response of thin polymer films. Extensive measurements on several commercially available semicrystalline polymers allowed us to identify a genuine molecular process associated with the newly formed liquid phase. In line with recent observations of amorphous polymer melts, we show this mechanism─known as the slow Arrhenius process (SAP)─involves time scales longer than those characteristics of segmental mobility and has the same energy barrier of the flow of the melt.

Ordered rippling of polymer surfaces by nanolithography: influence of scan pattern and boundary effects.

We demonstrate how AFM nanolithography, with a proper choice of scan pattern, can induce an exceptionally ordered alignment of ripples on the surface of polymer films on the first scan. By analogy with the manipulation of nanoparticles, the orientation of the ripples is determined by the material flow, which is ultimately fixed by the direction of motion of the probing tip. This makes a raster scan pattern the best choice for orienting the ripples, as opposed to the zigzag scan pattern commonly adopted by most AFM setups. Our hypothesis is substantiated by a series of measurements on a solvent-enriched ultrathin film of PET, which allowed ripple formation on the first scan. We also show how the ripple orientation is significantly modified by the boundary conditions appearing when nanolithography is performed on circular, triangular and L-shaped areas on the polymer surface.