Sorption of CO2, CH4 and Their Mixtures in Amorphous Poly(2,6-dimethyl-1,4-phenylene)oxide (PPO).

Sorption of pure CO2 and CH4 and CO2/CH4 binary gas mixtures in amorphous glassy Poly(2,6-dimethyl-1,4-phenylene) oxide (PPO) at 35 °C up to 1000 Torr was investigated. Sorption experiments were carried out using an approach that combines barometry with FTIR spectroscopy in the transmission mode to quantify the sorption of pure and mixed gases in polymers. The pressure range was chosen to prevent any variation of the glassy polymer density. The solubility within the polymer of the CO2 present in the gaseous binary mixtures was practically coincident with the solubility of pure gaseous CO2, up to a total pressure of the gaseous mixtures equal to 1000 Torr and for CO2 mole fractions of ~0.5 mol mol-1 and ~0.3 mol mol-1. The Non-Equilibrium Thermodynamics for Glassy Polymers (NET-GP) modelling approach has been applied to the Non-Random Hydrogen Bonding (NRHB) lattice fluid model to fit the solubility data of pure gases. We have assumed here that no specific interactions were occurring between the matrix and the absorbed gas. The same thermodynamic approach has been then used to predict the solubility of CO2/CH4 mixed gases in PPO, resulting in a deviation lower than 9.5% from the experimental results for CO2 solubility.

Modelling changes in glass transition temperature in polymer matrices exposed to low molecular weight penetrants.

Polymer matrices, when placed in contact with a fluid phase made of low molecular weight compounds, undergo a depression of their glass transition temperature (Tg) 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 Tg. 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 Tg 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'.

Survey on Adsorption of Low Molecular Weight Compounds in Cu-BTC Metal-Organic Framework: Experimental Results and Thermodynamic Modeling.

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.

Chemical Vapour Deposition Graphene-PMMA Nanolaminates for Flexible Gas Barrier.

Successful ways of fully exploiting the excellent structural and multifunctional performance of graphene and related materials are of great scientific and technological interest. New opportunities are provided by the fabrication of a novel class of nanocomposites with a nanolaminate architecture. In this work, by using the iterative lift-off/float-on process combined with wet depositions, we incorporated cm-size graphene monolayers produced via Chemical Vapour Deposition into a poly (methyl methacrylate) (PMMA) matrix with a controlled, alternate-layered structure. The produced nanolaminate shows a significant improvement in mechanical properties, with enhanced stiffness, strength and toughness, with the addition of only 0.06 vol% of graphene. Furthermore, oxygen and carbon dioxide permeability measurements performed at different relative humidity levels, reveal that the addition of graphene leads to significant reduction of permeability, compared to neat PMMA. Overall, we demonstrate that the produced graphene-PMMA nanolaminate surpasses, in terms of gas barrier properties, the traditional discontinuous graphene-particle composites with a similar filler content. Moreover, we found that the gas permeability through the nanocomposites departs from a monotonic decrease as a function of relative humidity, which is instead evident in the case of the pure PMMA nanolaminate. This work suggests the possible use of Chemical Vapour Deposition graphene-polymer nanolaminates as a flexible gas barrier, thus enlarging the spectrum of applications for this novel material.

Modelling Sorption Thermodynamics and Mass Transport of n-Hexane in a Propylene-Ethylene Elastomer.

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.

A Molecular Interpretation of the Dynamics of Diffusive Mass Transport of Water within a Glassy Polyetherimide.

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.

Sorption Thermodynamics of CO2, H2O, and CH3OH in a Glassy Polyetherimide: A Molecular Perspective.

In this paper, the sorption thermodynamics of low-molecular-weight penetrants in a glassy polyetherimide, endowed with specific interactions, is addressed by combining an experimental approach based on vibrational spectroscopy with thermodynamics modeling. This modeling approach is based on the extension of equilibrium theories to the out-of-equilibrium glassy state. Specific interactions are accounted for in the framework of a compressible lattice fluid theory. In particular, the sorption of carbon dioxide, water, and methanol is illustrated, exploiting the wealth of information gathered at a molecular level from Fourier-transform infrared (FTIR) spectroscopy to tailor thermodynamics modeling. The investigated penetrants display a different interacting characteristic with respect to the polymer substrate, which reflects itself in the sorption thermodynamics. For the specific case of water, the outcomes from molecular dynamics simulations are compared with the results of the present analysis.

Modeling Retrograde Vitrification in the Polystyrene-Toluene System.

Atactic polystyrene, as reported in a recent contribution by our group, displays a marked change in glass transition when exposed to toluene vapor due to plasticization associated with vapor sorption within the polymer. The dependence of the glass transition temperature of the polymer-penetrant mixture on the pressure of toluene vapor is characterized by the so-called "retrograde vitrification" phenomenon, in that, at a constant pressure, a rubber to glass transition occurs by increasing the temperature. In this contribution, we have used a theoretical approach, based on the nonrandom lattice fluid thermodynamic model for the polymer-toluene mixture, to predict the state of this system, i.e., rubbery or glassy, as a function of fluid pressure and system temperature. The experimentally detectable glass transition is assumed to be a kinetically affected evidence of an underlying II order thermodynamic transition of the polymer mixture. On the basis of this hypothesis, the Gibbs-Di Marzio criterion, stating that equilibrium configurational entropy is zeroed at the glass transition, has been applied to locate the transition. The working set of equations consists of the expression of configurational entropy obtained from the adopted lattice fluid model equated to zero, coupled with the equation expressing the phase equilibrium between the polymer phase and the pure toluene vapor phase in contact and with the equations of state for the two phases. Theoretical predictions are in good qualitative and quantitative agreement with the experimental results previously obtained gravimetrically performing "dynamic" sorption experiments, which represent a neat example of the occurrence of so-called "type IV" glass transition temperature vs pressure behavior. The peculiar retrograde vitrification phenomenon and the glass transition temperature vs pressure envelope determined experimentally are well described by the proposed theoretical approach.

Analysis of a Polystyrene-Toluene System through "Dynamic" Sorption Tests: Glass Transitions and Retrograde Vitrification.

Exposing a glassy polymer to a fluid phase (in gaseous or liquid state) containing a low molecular weight compound results in the sorption of the latter within the polymer, inducing, among other effects, the plasticization of the material which also promotes a change in the glass transition temperature. The amount of sorbed penetrant is often related in a complex fashion to the temperature and pressure of the fluid, thus determining that the locus of glass transition, when represented in pressure-temperature coordinates, may display as well rather complex patterns. This is an issue of particular importance in several applications of glassy polymers. In particular, we investigated the behavior of polystyrene in contact with toluene vapor by performing several modes of dynamic sorption experiments, in which the rate of change of the temperature of the system and/or of the pressure of the vapor phase are controlled with high accuracy, with the aim of creating a map of rubbery and glassy states of the polymer as a function of temperature and pressure of the toluene vapor. Isothermal tests were performed by changing the pressure at a controlled rate, isobaric tests were performed by changing the temperature at a controlled rate, and isoactivity tests were performed by concurrently changing, in a proper way, both temperature and pressure. A relevant feature resulting from these experiments is the presence of a discontinuity in the slope of the mass of toluene sorbed within polystyrene reported as a function of temperature and/or pressure. This discontinuity has been interpreted as the indication of the occurrence of a glass transition. The elaboration of the experimental results allowed identification of the pressure/temperature conditions at which rubbery or glassy states of the polymer mixture are established. Quite interestingly, the system displays the so- called "retrograde vitrification" phenomenon, which consists of the occurrence of a rubbery-to-glassy state transition as the temperature increases at a fixed pressure. The whole set of results has been successfully interpreted on the basis of thermodynamics of II order transitions accounting for the fact that experimental evidence of such transitions is significantly affected by the kinetics of polymer relaxation.

Local Structure and Dynamics of Water Absorbed in Poly(ether imide): A Hydrogen Bonding Anatomy.

Hydrogen bonding (HB) interactions play a major role in determining the behavior of macromolecular systems absorbing water. In fact, functional and structural properties of polymer-water mixtures are affected by the amount and type of these interactions. This contribution aims at a molecular level understanding of the interactional scenario for the technologically relevant case of the poly(ether imide)-water system. The problem has been tackled by combining different experimental and theoretical approaches which, taken together, provide a comprehensive physical picture. Relevant experimental data were gathered by in situ FTIR spectroscopy, while molecular dynamics (MD) and statistical thermodynamics approaches were used as modeling theoretical tools. It was found that, among the possible configurations, some are strongly prevailing. In particular, water molecules preferentially establish water bridges with two carbonyl groups of the same PEI repeating unit. Water self-interactions were also detected, giving rise to a "second shell" species in the prevalent form of dimers. The population of the different water species was evaluated spectroscopically, and a remarkable agreement with theoretical predictions was found.

Raman Line Imaging of Poly(ε-caprolactone)/Carbon Dioxide Solutions at High Pressures: A Combined Experimental and Computational Study for Interpreting Intermolecular Interactions and Free-Volume Effects.

In the present study, a Raman line-imaging setup was employed to monitor in situ the CO2 sorption at elevated pressures (from 0.62 to 7.10 MPa) in molten PCL. The method allowed the quantitative measurement of gas concentration in both the time-resolved and the space-resolved modes. The combined experimental and theoretical approach allowed a molecular level characterization of the system. The dissolved CO2 was found to occupy a volume essentially coincident with its van der Waals volume and the estimated partial molar volume of the probe did not change with pressure. Lewis acid-Lewis base interactions with the PCL carbonyls was confirmed to be the main interaction mechanism. The geometry of the supramolecular complex and the preferential interaction site were controlled more by steric than electronic effects. On the basis of the indications emerging from Raman spectroscopy, an equation of state thermodynamic model for the PCL-CO2 system, based upon a compressible lattice fluid theory endowed with specific interactions, has been tailored to account for the interaction types detected spectroscopically. The predictions of the thermodynamic model in terms of molar volume of solution have been compared with available volumetric measurements while predictions for CO2 partial molar volume have been compared with the values estimated on the basis of Raman spectroscopy.

Depth profiles in confocal optical microscopy: a simulation approach based on the second Rayleigh-Sommerfeld diffraction integral.

A method is presented for recovering the intensity depth profile, by confocal optical microscopy, in transparent and amorphous samples with low scattering. The response function of a confocal Raman microscope has been determined by using the second Rayleigh-Sommerfeld diffraction integral and scalar wave optics within paraxial approximation, taking into account the refractive index mismatch between the sample and the medium surrounding the objective lens. An iterative multi-fitting-scheme, based on the conjugate gradient method and Brent algorithm, allowed to fit several depth profile curves simultaneously and retrieve the beam waist, the signal amplitude and the position of the sample surface. The reliability and accuracy of the theoretical procedure has been investigated through comparison with experimental measurements of the Raman depth profiles for different pinhole diameters. The model is shown to provide accurate description of the effect of the mismatch of the refractive index and of the dependence of the Raman signal on the depth with discrepancies lower than 3%. This procedure constitutes a first step towards the development of a manageable theoretical framework, amenable to a relatively simple numerical implementation, for the solution of the 'inverse' problem of finding the correct reconstruction of unknown profiles of chemical species within the sample, starting from experimental information gathered from micro-Raman depth profiling.

Time-resolved Fourier transform infrared spectroscopy, gravimetry, and thermodynamic modeling for a molecular level description of water sorption in poly(ε-caprolactone).

Sorption of water in poly(ε-caprolactone) (PCL), with specific focus on the hydrogen-bonding interactions, has been analyzed by combining ab initio calculations, macroscopic thermodynamics modeling, and relevant features emerging from spectroscopic and gravimetric measurements. Fourier transform infrared (FTIR) data, analyzed by difference spectroscopy, two-dimensional correlation spectroscopy, and least-squares curve-fitting analysis associated with gravimetric determination of water sorption isotherm provided information on the system's behavior and on the molecular interactions established between the polymer and the penetrant. A consistent physical picture emerged pointing to the presence of two spectroscopically discernible water species (first-shell and second-shell layers) that have been quantified. Water molecules are present in the form of dimers within the polymer equilibrated with water vapor up to a relative humidity of 0.65. At higher humidities, clustering of water sorbed molecules starts to take place. The multicomponent ν(OH) band representative of absorbed water has been interpreted with the aid of ab initio calculations performed on suitably chosen model systems. The outcomes of spectroscopic analyses were interpreted at a macroscopic level by modeling the thermodynamics of water sorption in PCL based on a nonrandom compressible lattice theory accounting for hydrogen-bonding (HB) interactions. Starting from the fitting of the gravimetric sorption isotherm, the model provided quantitative estimates for the amount of self- and cross-HBs which compare favorably with the FTIR results.

Thermodynamics of water sorption in high performance glassy thermoplastic polymers.

Sorption thermodynamics of water in two glassy polymers, polyetherimide (PEI) and polyetheretherketone (PEEK), is investigated by coupling gravimetry and on line FTIR spectroscopy in order to gather information on the total amount of sorbed water as well as on the different species of water molecules absorbed within the polymers, addressing the issue of cross- and self-interactions occurring in the polymer/water systems. Water sorption isotherms have been determined at temperatures ranging from 30 to 70°C while FTIR spectroscopy has been performed only at 30°C. The experimental analysis provided information on the groups present on the polymer backbones involved in hydrogen bonding interactions with absorbed water molecules. Moreover, it also supplied qualitative indications about the different "populations" of water molecules present within the PEEK and a quantitative assessment of these "populations" in the case of PEI. The results of the experimental analysis have been interpreted using an equation of state theory based on a compressible lattice fluid model for the Gibbs energy of the polymer-water mixture, developed by extending to the case of out of equilibrium glassy polymers a previous model intended for equilibrium rubbery polymers. The model accounts for the non-equilibrium nature of glassy polymers as well as for mean field and for hydrogen bonding interactions, providing a satisfactory quantitative interpretation of the experimental data.

Tailoring assembly of reduced graphene oxide nanosheets to control gas barrier properties of natural rubber nanocomposites.

Self-assembling of reduced graphene oxide platelets, as a tailored interconnected network within a natural rubber matrix, is proposed as a mean for obtaining nanocomposites with improved gas barrier, as compared to neat natural rubber. Interestingly, this nanocomposite structure results to be much more effective than homogeneous dispersion of graphene platelike particles, even at low graphene loadings. Such behavior is interpreted on the grounds of a theoretical model describing permeability of heterogeneous systems specifically accounting for self-segregated graphene morphology.

Combining gravimetric and vibrational spectroscopy measurements to quantify first- and second-shell hydration layers in polyimides with different molecular architectures.

In-situ Fourier transform infrared (FTIR) measurements have been carried out at different relative pressures of water vapor to study the H(2)O diffusion in three polyimides differing in their molecular structure and fluorine substitution. Spectral data have been analyzed by difference spectroscopy, least-squares curve fitting, and two-dimensional (2D) correlation spectroscopy, which provided molecular level information on the diffusion mechanism. In particular, two distinct water species were identified corresponding, respectively, to the first and second-shell hydration layers. The spectroscopic analysis demonstrated that the relative population of these species is a function of the total water content in the system. A method has been devised to quantify the water concentration in the two hydration layers, based on a combination of spectroscopic and gravimetric data. The results have been compared with those from an earlier spectroscopic approach reported in the literature and based on the analysis of the carbonyl region.