Search

This contribution aims at an understanding, at a molecular scale, of the state of CO2 molecules absorbed in glassy poly(ether imide) (PEI). This issue has been challenged by combining different approaches that tackle the problem from both the experimental and the theoretical viewpoints and providing a comprehensive physicochemical picture. In situ FTIR spectroscopy and gravimetry were exploited to gather relevant experimental information, while molecular dynamics (MD), density functional theory (DFT), and statistical thermodynamics approaches were used to model the behavior of the binary system at different scales. Based on the findings of FTIR spectroscopy and on DFT and MD calculations, it was determined that, among the possible interaction configurations, some are prevailing. In particular, the carbon atom of carbon dioxide molecules establishes relatively weak interactions prevalently with the carbonyl groups of PEI. A quantitative estimate of such interaction has been provided by MD calculations. The system was also analyzed using a lattice fluid model, specifically developed to deal with sorption of low molecular weight compounds in glassy polymers, that is rooted on statistical thermodynamics, determining the values of the isosteric heat of sorption and carbon-dioxide-polymer interaction energy. Finally, experimental data of CO2-PEI mutual diffusivity have been interpreted using a semiempirical theoretical model accounting for the effects of the penetrant concentration, of energy barriers associated with the occurrence of an effective diffusive jump, and of a thermodynamic factor.

The spreading of respiratory diseases through deposited saliva droplets is strongly dependent on the water evaporation process that may determine the virus viability due to the increase in concentration of nonvolatile compounds that are harmful for the virus. The drying time of a virus-laden droplet is influenced by environmental conditions, such as relative humidity and temperature, physical properties, such as the features of the surface on which it is deposited, and the wetting regime. Under this perspective, we addressed the modeling of an evaporating sessile droplet resting on a flat smooth surface, extending a previous diffusion-based model from the same authors. The evaporation behavior of sessile droplets of aqueous sodium chloride solutions and the virus viability of a surrogate virus (MS2) have been simulated considering different wetting regimes to account for different types of surfaces, ranging from highly hydrophilic to highly hydrophobic, as well as different ambient conditions, in terms of temperature and relative humidity. The results of calculations were given in terms of time evolution of contact angle, contact radius, mixture volume, and salt concentration, unveiling the importance of different wetting regimes for evaporation behavior and drying time. Longer evaporation times have been observed as temperature decreases and relative humidity increases. With reference to a surrogate virus, we evaluated the virus viability at different selected conditions, observing the classical U-shape of relative viability as a function of relative humidity of the environment at a certain temperature.

Polymer matrices, when placed in contact with a fluid phase made of low molecular weight compounds, undergo a depression of their glass transition temperature ( T g ) determined by the absorption of these compounds and the associated plasticization phenomena. Frequently, this effect is coupled with the mechanical action of the compressive stress exerted by the pressure of the fluid phase that, in contrast, promotes an increase in the T g . This issue is relevant for technological and structural applications of composites with high-performance glassy polymer matrices, due to their significant impact on mechanical properties. We propose an approach to model and predict rubbery-glassy states maps of polymer–penetrant mixtures as a function of pressure and temperature based on the Gibbs–Di Marzio criterion. This criterion establishes that a ‘thermodynamic’ glass transition does occur when the configurational entropy of the system vanishes. Although questioned and criticized, this criterion constitutes a good practical approach to analyse changes of T g and, in some way, reflects the idea of an ‘entropy catastrophe’ occurring at the glass transition. Several polymer–penetrant systems have been analysed modelling configurational entropy by means of the Non-Random Hydrogen Bond lattice fluid theory, able to cope with possible non-random mixing and occurrence of strong interactions. This article is part of the theme issue 'Ageing and durability of composite materials'.

The study of the reinforcement network in elastomer compounds is one of the most relevant issues for the application of these materials because their properties are strongly dependent on the obtained morphology. To this regard, the viscoelastic and dielectric behavior of vulcanized styrene butadiene rubber (SBR) reinforced with different amounts of carbon nanotubes (CNT) have been investigated and compared with the vulcanized unfilled SBR and the vulcanized SBR samples reinforced with a conventional amount of carbon black (40 phr). Differential scanning calorimetry (DSC) measurements have been carried out to highlight possible differences of the glass transition temperatures for all the reinforced compounds. The percolation threshold value of the nanocomposite samples has been estimated by dielectric analysis. Finally, dynamic mechanical analysis (DMA) measurements have been performed in tensile mode in the temperature range of −60 to 80 °C to obtain both E′ and E′′. From these experimental data, the master curve for each sample has been estimated by using the time–temperature superposition principle in combination with the vertical shift approach. From the analysis of this latter, the activation energy, associated to the thermal movement of the reinforcement network, has been calculated to better elucidate the reinforcement mechanism in the nanocomposites.

In the recent years a considerable effort has been devoted to foster the understanding of the basic mechanisms underlying the dynamical arrest that is involved in glass forming in supercooled liquids and in the sol-gel transition. The elucidation of the nature of such processes represents one of the most challenging unsolved problems in the field of material science. In this context, two important theories have contributed significantly to the interpretation of these phenomena: the Mode-Coupling theory (MCT) and the Percolation theory (PT). These theories are rooted on the two pillars of statistical physics, universality and scale laws, and their original formulations have been subsequently modified to account for the fundamental concepts of Energy Landscape (EL) and of the universality of the fragile to strong dynamical crossover (FSC). In this review, we discuss experimental and theoretical results, including Molecular Dynamics (MD) simulations, reported in the literature for colloidal and polymer systems displaying both glass and sol-gel transitions. Special focus is dedicated to the analysis of the interferences between these transitions and on the possible interplay between MCT and PT. By reviewing recent theoretical developments, we show that such interplay between sol-gel and glass transitions may be interpreted in terms of the extended F13 MCT model that describes these processes based on the presence of a glass-glass transition line terminating in an A3 cusp-like singularity (near which the logarithmic decay of the density correlator is observed). This transition line originates from the presence of two different amorphous structures, one generated by the inter-particle attraction and the other by the pure repulsion characteristic of hard spheres. We show here, combining literature results with some new results, that such a situation can be generated, and therefore experimentally studied, by considering colloidal-like particles interacting via a hard core plus an attractive square well potential. In the final part of this review, scaling laws associated both to MCT and PT are applied to describe, by means of these two theories, the specific viscoelastic properties of some systems.

The NETGP-NRHB lattice-fluid model describes the sorption thermodynamics of penetrants in amorphous polymer-penetrant mixtures locked in an out-of-equilibrium “glassy” state. It accounts for the nonrandom distribution of mean-field contacts and voids and for the possible occurrence of specific interactions. In this contribution, we assess the suitability of the NETGP-NRHB model to interpret the sorption thermodynamics of binary penetrant mixtures in glassy polymers. Moreover, adopting the gradients of NETGP-NRHB penetrant chemical potentials as driving forces for their diffusive fluxes, a self-consistent framework is proposed to predict the permeability of light gas binary mixtures in glassy polymer membranes once all of the model parameters are obtained by nonlinear regression of data of the corresponding binary and pure component subsystems. This approach is validated against solubility data of CO2/CH4 and CO2/C2H4 mixtures in glassy amorphous poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and poly(methyl methacrylate) (PMMA), respectively, and permeability data of CO2/CH4 mixtures in three glassy amorphous polymer membranes (PAr, PSf, PH); all of the data are taken from the literature. In particular, solubility data are investigated up to a CH4 partial pressure of 15 atm at a fixed CO2 partial pressure equal to 5.1 atm for the system CO2/CH4/PPO and up to a CO2 partial pressure of 4 atm at a fixed C2H4 partial pressure equal to 2.09 atm for the system CO2/C2H4/PMMA. Permeability data are investigated at a fixed upstream molar concentration of the binary gas mixture equal to 0.5 by changing the total pressure up to 18 atm for the systems CO2/CH4/PAr and CO2/CH4/PSf and up to 14 atm for the system CO2/CH4/PH; the downstream total pressure is equal to 0 atm for all of the systems. All of the experimental sets of data are obtained at 35 °C. Solubility and permeability predictions for the ternary systems compare very well with all experimental literature data without additional parameters besides those required for the corresponding binary subsystems.

This contribution aims at providing a critical overview of experimental results for the sorption of low molecular weight compounds in the Cu-BTC Metal–Organic Framework (MOF) and of their interpretation using available and new, specifically developed, theoretical approaches. First, a literature review of experimental results for the sorption of gases and vapors is presented, with particular focus on the results obtained from vibrational spectroscopy techniques. Then, an overview of theoretical models available in the literature is presented starting from semiempirical theoretical approaches suitable to interpret the adsorption thermodynamics of gases and vapors in Cu-BTC. A more detailed description is provided of a recently proposed Lattice Fluid approach, the Rigid Adsorbent Lattice Fluid (RALF) model. In addition, to deal with the cases where specific self- and cross-interactions (e.g., H-bonding, Lewis acid/Lewis base interactions) play a role, a modification of the RALF model, i.e., the RALFHB model, is introduced here for the first time. An extension of both RALF and RALFHB is also presented to cope with the cases in which the heterogeneity of the rigid adsorbent displaying a different kind of adsorbent cages is of relevance, as it occurs for the adsorption of some low molecular weight substances in Cu-BTC MOF.

A dynamic method for the storage of calibration gas mixtures is presented using thermal mass flow controllers (MFCs) connected to a dead end volume. The approach relies on barometry to calibrate the unknown constant volume and two MFCs which regulate the flow rate of carbon dioxide and methane respectively. Binary gas mixtures of these components are prepared and stored within the calibrated volume tank up to 9 bar at ambient temperature. Thermodynamic data of pressure, temperature and density of the gas mixtures are retrieved. The expanded uncertainties at 95% confidence level of the molar fraction and the density of the mixture are evaluated. FTIR Spectroscopy is used in situ to analyse the gas mixtures.

Compressible lattice-fluid Equation of State (EoS) models have been widely used to interpret phase equilibria and related properties of pure fluids and their mixtures, including polymer solutions. One of the first models of this family has been proposed by Sanchez and Lacombe (SL), providing useful expressions for EoS and chemical potentials for both pure components and mixtures. The assessment of the thermodynamic consistency of the expressions provided by these models for the chemical potential of each component in a mixture is a relevant issue, to which has seldom been devoted the attention it deserves. In fact, it has been demonstrated in the literature that the equations provided by the SL theory for the equilibrium chemical potential of a component in a mixture exhibits a thermodynamic inconsistency emerging from the adopted mixing rules for the close packed volume. Consequently, these expressions do not converge to the proper form in the limit of ideal gas mixtures. Later, the Non-Random Hydrogen Bonding (NRHB) lattice-fluid model was introduced to overcome some limitations of the SL model, by accounting for possible presence of strong specific intermolecular interactions, such as hydrogen bonding, as well as for non-random distribution of intermolecular contacts. In the present work, using a thermodynamic framework endowed with internal state variables, it is shown that the expressions of the chemical potential provided by the NRHB model for a multicomponent mixture at equilibrium, converge consistently to the correct form in the limit of ideal gas mixtures, thus overcoming the inconsistency implicit in the most common corresponding formulations of the SL model. We demonstrate that this feature is essentially related to the assumption of a constant value of the segmental volume, v*, that takes the same ‘universal’ value (9.75/NA cm3/molecular segment, where NA is the Avogadro number) for all pure fluids as well as for their mixtures. In addition, we have re-examined also the inconsistency issue of the SL model proving that this model recovers the thermodynamic consistency if it is assumed again a constant value of v* or if a particular type of mixing rule is assumed for v*.

A hyphenated technique combining FTIR Spectroscopy and Barometry is implemented to study transport properties of pure and mixed gases in rubbery polymers. FTIR spectroscopy is operated in situ and in the transmission mode. The specific case of transport of pure CO2 and CH4 in polydimethylsiloxane (PDMS) was addressed by performing the experimental investigation at ambient temperature and pressure values up to 9 bar, analyzing quantitatively both the gas phase and the solid polymer phase. The IR signals of each species in the gaseous phase were first calibrated against density data of each pure gas. Then, sorption experiments from a unary gas phase were conducted increasing the pressure stepwise and the amount of gas sorbed at each pressure within the polymer was quantitatively determined by measuring the absorbance decay within the gas phase. From these measurements, equilibrium sorption isotherms and sorption kinetics of both pure gases in PDMS have been evaluated. At the same time, FTIR spectra of pure CO2 absorbed within the polymer phase were collected and calibrated. The spectroscopic contrast in the gas and the polymer phase allowed us to apply the same approaches to sorption of gas mixtures, a very difficult task with the techniques currently available. Preliminary results for the sorption of carbon dioxide from CO2/CH4 gas mixtures are presented.

Interactions between nanoparticles (NPs) determine their self-organization and dynamic processes. In these systems, a quantitative description of the interparticle forces is complicated by the presence of the hydrophobic effect (HE), treatable only qualitatively, and due to the competition between the hydrophobic and hydrophilic forces. Recently, instead, a sort of crossover of HE from hydrophilic to hydrophobic has been experimentally observed on a local scale, by increasing the temperature, in pure confined water and studying the occurrence of this crossover in different water–methanol solutions. Starting from these results, we then considered the idea of studying this process in different nanoparticle solutions. By using photon correlation spectroscopy (PCS) experiments on dendrimer with OH terminal groups (dissolved in water and methanol, respectively), we show the existence of this hydrophobic–hydrophilic crossover with a well defined temperature and nanoparticle volume fraction dependence. In this frame, we have used the mode coupling theory extended model to evaluate the measured time-dependent density correlation functions (ISFs). In this context we will, therefore, show how the measured spectra are strongly dependent on the specificity of the interactions between the particles in solution. The observed transition demonstrates that just the HE, depending sensitively on the system thermodynamics, determines the hydrophobic and hydrophilic interaction properties of the studied nanostructures surface.

This work aims at obtaining a systematic description and prediction of the multicomponent gas separation behavior of membranes, using the Non-Equilibrium Lattice Fluid (NELF) model for gas sorption and the Standard Transport (ST) model for gas permeation. The scheme is applied to a comprehensive analysis of CO2/CH4 separation with cellulose acetate membranes. A dedicated experimental campaign (pressure-volume-temperature (pVT), differential scanning calorimetry (DSC) and pure gas sorption tests) was performed to obtain reliable model parameters, accounting also for crystallinity. The approach was validated against a complete set of literature data, including mixed gas sorption and permeation.The parameters obtained were used to perform predictive simulations of mixed CO2/CH4 sorption and permeation in a wide mixture composition range. The NELF model accurately predicts the effect of temperature on sorption, as well as the strong competitive exclusion of CH4 when CO2 is present, that enhances the solubility-selectivity. The ST model correctly estimates the experimentally observed lower-than-ideal CO2/CH4 perm-selectivity. The model shows that low perm-selectivity is due to mixed gas diffusivity-selectivity (which is not available experimentally): even though solubility-selectivity increases in the mixed gas case, diffusivity-selectivity suffers a larger departure from pure gas conditions, ultimately leading to a lower perm-selectivity under multicomponent conditions.

Sorption of benzene vapor in amorphous poly(2,6-dimethyl-1,4-phenylene) oxide (PPO) has been investigated experimentally at 40 °C by gravimetric analysis and in situ FTIR spectroscopy. This analysis indicates the occurrence of polymer crystallization during sorption when experiments are performed at elevated benzene activities. In fact, absorbed benzene determines a depression of the glass transition temperature of the system and, in turn, an increase of macromolecular mobility that promotes a solvent induced crystallization. The event of crystallization is evidenced by the presence of an overshoot in the sorption curve, both in gravimetric and spectroscopic sorption experiments performed at a relative pressure of benzene vapor close to 0.80. A theoretical approach, based on the combination of a nonrandom compressible lattice fluid theory and of the Gibbs-Di Marzio criterion, has been used to predict the glassy-rubbery state domain map of the PPO-benzene system, in temperature-benzene vapor pressure coordinates, showing that the system should display a retrograde vitrification phenomenon. This modeling allowed the identification of values of benzene vapor pressure favoring the occurrence of solvent induced crystallization that are consistent with the indications emerging from the gravimetric sorption isotherm. In addition, relevant insight into the crystallization process occurring during sorption has been obtained from vibrational spectroscopy, evidencing a rather peculiar behavior that points to the formation of cocrystalline benzene-PPO structures.

The paper proposes a mixed strain- and stress-based topology optimization method for designing the ideal geometry of carbon fibers in composite laminates subjected to either applied tractions or prescribed displacements. On the basis of standard micromechanical approaches, analytical elastic solutions for a single cell, assumed to be a Representative Volume Element (RVE), are ad hoc constructed by involving anisotropy induced by fiber orientation and volume fraction, also taking into account inter-laminar stresses and strains. The analytical solutions are then implemented in a Finite Element (FE) custom-made topology optimization-based procedure rewritten to have as output the best curves the reinforcing fibers have to draw in any composite laminate layer to maximize the overall panel stiffness or to minimize the elastic energy. To verify the effectiveness of the proposed strategy, different structures undergoing either in-plane or out-plane boundary conditions have been selected and theoretically investigated, determining the optimal fibers' maps and showing the related results in comparison to standard sequences of alternate fibers disposition for the same composites. Two optimized panels were at the end actually produced using an innovative Automated Fiber Placement (AFP) machine and consolidating the materials by means of autoclave curing processes, in this way replicating the fiber paths obtained from theoretical outcomes. As a control, two corresponding composite structures were also built without employing the fiber optimization strategy. The panels have been tested in laboratory and the theoretical results have been compared with the experimental findings, showing a very good agreement with our predictions and confirming the capability of the proposed algorithm in suggesting the arrangement of the fibers to obtain enhanced mechanical performances. It is felt that the hybrid analytical-FE topology optimization strategy, in conjunction with the possibilities offered by AFP devices, could pave the way for a new generation of ultra-lightweight composites for aerospace, automotive and many industrial applications.

Optimization of post polymerization processes of polyolefin elastomers (POE) involving solvents is of considerable industrial interest. To this aim, experimental determination and theoretical interpretation of the thermodynamics and mass transport properties of POE-solvent mixtures is relevant. Sorption behavior of n-hexane vapor in a commercial propylene-ethylene elastomer (V8880 VistamaxxTM from ExxonMobil, Machelen, Belgium) is addressed here, determining experimentally the sorption isotherms at temperatures ranging from 115 to 140 °C and pressure values of n-hexane vapor up to 1 atm. Sorption isotherms have been interpreted using a Non Random Lattice Fluid (NRLF) Equation of State model retrieving, from data fitting, the value of the binary interaction parameter for the n-hexane/V8880 system. Both the cases of temperature-independent and of temperature-dependent binary interaction parameter have been considered. Sorption kinetics was also investigated at different pressures and has been interpreted using a Fick’s model determining values of the mutual diffusivity as a function of temperature and of n-hexane/V8880 mixture composition. From these values, n-hexane intra-diffusion coefficient has been calculated interpreting its dependence on mixture concentration and temperature by a semi-empiric model based on free volume arguments.

The diffusion process of water molecules within a polyetherimide (PEI) glassy matrix has been analyzed by combining the experimental analysis of water sorption kinetics performed by FTIR spectroscopy with theoretical information gathered from Molecular Dynamics simulations and with the expression of water chemical potential provided by a non-equilibrium lattice fluid model able to describe the thermodynamics of glassy polymers. This approach allowed us to construct a convincing description of the diffusion mechanism of water in PEI providing molecular details of the process related to the effects of the cross- and self-hydrogen bonding established in the system on the dynamics of water mass transport.

Polymeric foams are low-density key components for many lightweight structures used in construction and means of transportation, and for applications where porosity determines specific functionalities for insulation, packaging, cushioning, safety, and biomedical uses. The aims of this chapter are (i) to illustrate the evolution of the foam concept and its functions from the beginning to the current applications, (ii) to provide an overview on the main properties for conventional and more advanced materials, and (iii) to introduce some of the most important current developments.

Knowledge of the thermodynamics of sorption of low molecular weight compounds in rubbery and glassy polymers is of great relevance to understanding the physics and dynamics of polymer foaming and for the technological implementation of foaming processes. This chapter is devoted to the illustration of the recent advances in statistical thermodynamics approaches toward the elaboration of a theoretical framework aimed at the description of sorption thermodynamics of small molecules in polymers, addressing also systems that involve specific intra- and intermolecular interactions. In particular, the attention is focused on sorption models rooted on Equation of State theories. Notably, the models discussed here are structured to specifically account for nonrandom distribution of molecular species and for dealing with several kinds of specific interactions, addressing both for the case of rubbery polymers and of out-of-equilibrium glassy polymers. Moreover, a description of the experimental approaches devoted to the measurement of sorption isotherms is provided. Several kinds of experimental methodologies aimed at the description of sorption in polymers are described with attention also to the molecular-level characterization of the polymer-penetrant systems provided by vibrational spectroscopy (FTIR, Raman), which allows for the development of in situ, time-resolved measurements. Sorption thermodynamics in polymers is examined here considering the contact of polymer with a penetrant in a state of a subcritical vapor, of a supercritical fluid, or of a gas.

A new hyphenated technique based on simultaneous in situ FT-NIR spectroscopy and pressure-decay measurements has been implemented to study sorption of low-molecular-weight compounds in polymeric membranes and the induced swelling of the matrix. The FT-NIR measurements are performed in the transmission mode and, besides sorption equilibrium and kinetics, allow also the straightforward measurement of polymer swelling. The pressure decay method is used to provide quantitative information on the concentration of penetrant sorbed in the polymer. This measurement, once combined with the photometric data, allows an accurate estimation of the molar absorptivity of the analytical peaks as well. To validate the new experimental approach, sorption of CO₂ in polydimethylsiloxane at 35 °C and at pressures up to 9 bar has been investigated and the results are compared with available literature data.

Although a significant amount of research has already been conducted on the various aspects of foaming of a large variety of polymeric materials, much work still needs to be done in certain areas. The main scientific open questions and technological challenges, together with the current research trends and developments, are analyzed in this chapter with the aim of examining the perspectives for further advances in the field.

Quantitative modeling for the design of polymer foaming processes requires the knowledge of thermal properties of the mixtures of polymer and blowing agents. In fact, the presence of a penetrant within a polymer significantly affects the glass transition and crystallization/melting temperature of the starting neat polymer. Moreover, also energy transport parameters are affected, with particular reference to thermal conductivity. In this chapter, the phenomenology of glass transition in polymer-penetrant systems is first discussed, illustrating several theoretical approaches aimed at its interpretation. The effect of the presence of penetrant on the crystallization/melting behavior of the mixtures is then presented, introducing the recent advancements in their theoretical interpretation. Further discussion of these issues, in the perspective of the foaming process, is provided in Chapter 13 . Finally, experimental and modeling results for thermal conductivity of polymer-penetrant mixtures are discussed in some detail. All these aspects have been considered including the case of contact of a polymer with a physical blowing agent in the supercritical fluid state.

NMR spectroscopic literature data are used, in a wide temperature-pressure range (180–350 K and 0.1–400 MPa), to study the water polymorphism and the validity of the liquid–liquid transition (LLT) hypothesis. We have considered the self-diffusion coefficient DS and the reorientational correlation time τθ (obtained from spin-lattice T1 relaxation times), measured, respectively, in bulk and emulsion liquid water from the stable to well inside the metastable supercooled region. As an effect of the hydrogen bond (HB) networking, the isobars of both these transport functions evolve with T by changing by several orders of magnitude, whereas their pressure dependence become more and more pronounced at lower temperatures. Both these transport functions were then studied according to the Adam–Gibbs model, typical of glass forming liquids, obtaining the water configurational entropy and the corresponding specific heat contribution. The comparison of the evaluated CP,conf isobars with the experimentally measured water specific heat reveals the full consistency of this analysis. In particular, the observed CP,conf maxima and its diverging behaviors clearly reveals the presence of the LLT and with a reasonable approximation the liquid–liquid critical point (LLCP) locus in the phase diagram.

The gas foaming, which makes use of a physical blowing agent (e.g., carbon dioxide, nitrogen, hydrocarbons, etc.) to form bubbles in a softened polymer, is the most used method for the making of polymeric foams, mainly because of its high productivity. In this chapter, an overview of the state of the art of the gas foaming method and the proposed modeling approaches will be provided.