Molecular Dynamics of Water Embedded Carbon Nanocones: Surface Waves Observation

We employed molecular dynamics simulations on the water solvation of conically shaped carbon nanoparticles. We explored the hydrophobic behaviour of the nanoparticles and investigated microscopically the cavitation of water in a conical confinement with different angles. We performed additional molecular dynamics simulations in which the carbon structures do not interact with water as if they were in vacuum. We detected a waving on the surface of the cones that resembles the shape agitations of artificial water channels and biological porins. The surface waves were induced by the pentagonal carbon rings (in an otherwise hexagonal network of carbon rings) concentrated near the apex of the cones. The waves were affected by the curvature gradients on the surface. They were almost undetected for the case of an armchair nanotube. Understanding such nanoscale phenomena is the key to better designed molecular models for membrane systems and nanodevices for energy applications and separation

A Study of the Reinforcement Effect of MWCNTs onto Polyimide Flat Sheet Membranes

Polyimides rank among the most heat-resistant polymers and find application in a variety of fields, including transportation, electronics, and membrane technology. The aim of this work is to study the structural, thermal, mechanical, and gas permeation properties of polyimide based nanocomposite membranes in flat sheet configuration. For this purpose, numerous advanced techniques such as atomic force microscopy (AFM), SEM, TEM, TGA, FT-IR, tensile strength, elongation test, and gas permeability measurements were carried out. In particular, BTDA–TDI/MDI (P84) co-polyimide was used as the matrix of the studied membranes, whereas multi-wall carbon nanotubes were employed as filler material at concentrations of up to 5 wt.% All studied films were prepared by the dry-cast process resulting in non-porous films of about 30–50 μm of thickness. An optimum filler concentration of 2 wt.% was estimated. At this concentration, both thermal and mechanical properties of the prepared membranes were improved, and the highest gas permeability values were also obtained. Finally, gas permeability experiments were carried out at 25, 50, and 100 ◦C with seven different pure gases. The results revealed that the uniform carbon nanotubes dispersion lead to enhanced gas permeation properties

New Porous Heterostructures Based on Organo-Modified Graphene Oxide for CO(2)Capture

In this work, we report on a facile and rapid synthetic procedure to create highly porous heterostructures with tailored properties through the silylation of organically modified graphene oxide. Three silica precursors with various structural characteristics (comprising alkyl or phenyl groups) were employed to create high-yield silica networks as pillars between the organo-modified graphene oxide layers. The removal of organic molecules through the thermal decomposition generates porous heterostructures with very high surface areas (>= 500 m(2)/g), which are very attractive for potential use in diverse applications such as catalysis, adsorption and as fillers in polymer nanocomposites. The final hybrid products were characterized by X-ray diffraction, Fourier transform infrared and X-ray photoelectron spectroscopies, thermogravimetric analysis, scanning electron microscopy and porosity measurements. As proof of principle, the porous heterostructure with the maximum surface area was chosen for investigating its CO(2)adsorption properties

Preparation and characterization og polymer nanocomposites

In this study a series of poly(vinyl alcohol) – clay nanocomposite materials has been prepared by common solvent method. The PVA used was Mowiol 8-88 supplied by Sigma Aldrich. The clay used was a very abundant mineral, Bentonite which is consisted in majority from Calcium Montmorillonite. The samples were prepared by mixing the Bentonite suspension with the polymer solution in quantities that gave 5, 10 and 20 wt% clay loading content on the produced films and left to dry slowly. The nanocomposites exhibited high levels of clay organization inside the polymer matrix, due to favorable polymer-particle interactions, leading to unique properties. The produced materials were thoroughly characterized by wide angle X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Atomic Forces Microscopy (AFM), UV- visible transmission spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), mechanical strength, oxygen and water permeability and water sorption study. Emphasis was given to the implementation of neutron diffraction and in particular the contrast matching technique which is not used for the characterization of polymer nanocomposites. AFM images of the nanocomposite’s structure reveal that the clay particles oriented on the surface of the sample. The particles sized 70 nm is clay which orients in plane with the sample’s surface while the lines observed with an approximate width of 20 nm is polymer chains aggregated with way due to space confinement The wide angle XRD curves of the produced PVAB20 sample has a peak appeared at 2FS = 3.2SOH, corresponding to a basal spacing of around 27Å. This increase in basal spacing is due to the polymer intercalation in the clay galleries. From the obtained patterns it is concluded that the clay particles are well organized inside the matrix. These observations are in agreement with the results obtained from microscopy techniques. In the same spectra the characteristic peak of the clay basal spacing (2FS= 7SOH) is also present. This indicates that a small part of the clay is aggregated during film formation. For the samples PVAB05 and PVAB10 the XRD patterns indicate a very good dispersion of the clay in the polymer matrix, since there is no significant peak evident around the expected 2FS angle (3-14SOH). In these cases almost the whole amount of clay was completely exfoliated. In order to investigate the presence of certain orientation in the PVAB20 sample, the film was fragmentated into powder. The diffraction patterns of PVAB20 sample in powder and film form revealed that the 001 peak of the clay is much more pronounced in the case of the film, giving evidence of well orientation of clay particles, parallel to the surface of the nanocomposite film The UV-Vis transmission spectra of the nanocomposites samples show that they retain the poly(vinyl alcohol)’s high optical clarity (above 95% in comparison with the pure PVA film) for the visible region (400-700 nm), due to well nanoscale dispersion of clay particles into the polymer matrix. From the oxygen permeability results it is clear that the permeability decreases as the volume fraction of clay in the composites increases, indicating that the clay particles are well dispersed and oriented in the polymeric matrix. In DSC the existence of new polymer crystalline phase is evident, induced by the presence of the clays. These findings are in agreement with the results obtained from the study of similar polymer-clay systems For 45% RH the Young’s modulus of the samples increased with increasing clay content and it was around 60% more on 5 wt% clay addition and 260% more with 20 wt%. The ultimate tensile strength of the samples was less influenced by the clay content reaching a maximum increase of 60% for the PVAB20 sample. On the measurements for the 70 %RH pre-equilibrated samples the Young’s modulus was a lot more influenced on increasing clay concentration. The diffraction patterns obtained from lamellar and perpendicular sample position revealed that there is a specific orientation of the bentonite platelets parallel to the film surface giving unique properties to the sample.Στην παρούσα διδακτορική διατριβή παρασκευάστηκαν και χαρακτηρίστηκαν ενδελεχώς, νανοσύνθετες πολυμερικές μεμβράνες πολυβινυλικής αλκοόλης – μπεντονίτη σε αναλογία 5, 10 και 20% κατά μάζα, με την τεχνική ανάμιξης σε κοινό διαλύτη. Η πολυβινυλική αλκοόλη αποτελεί το πλέον χρησιμοποιούμενο υδατοδιαλυτό πολυμερές παγκοσμίως έχοντας αποκτήσει ιδίως τα τελευταία χρόνια πολλές βιομηχανικές εφαρμογές. Από την άλλη μεριά ο Μπεντονίτης είναι ένα ορυκτό που βρίσκεται σε μεγάλη αφθονία στον ελληνικό χώρο με πολλές εφαρμογές και χαμηλή τιμή. Τα νανοσύνθετα υλικά που παρασκευάστηκαν χαρακτηρίστηκαν με πλήθος πειραματικών τεχνικών όπως μικροσκοπία ατομικών δυνάμεων AFM, ηλεκτρονιακή μικροσκοπία σάρωσης και διέλευσης, φασματοσκοπία ορατού υπεριώδους καθώς και υπερύθρου, περίθλαση ακτινών Χ, μηχανικές ιδιότητες διαπερατότητα οξυγόνου, υγρασίας καθώς και προσροφήσεις νερού. Ιδιαίτερη βαρύτητα δόθηκε στην τεχνική της περίθλασης νετρονίων και ειδικά στην εφαρμογή της μεθόδου εξομάλυνσης αντιθέσεων σκέδασης. Εξαιτίας των μικρών διαφορών στις πυκνότητες σκέδασης του πολυμερούς και του πηλού δεν είναι δυνατή η ανίχνευση ανακλάσεων χωρίς την χρήση βοηθητικών σκεδαστών όπως το Η2Ο και το D2O. Οι μέθοδοι μικροσκοπίας απέδειξαν την ύπαρξη οργανωμένων δομών μπεντονίτη εμφανίζοντας παράλληλα εξαιρετική διασπορά και διαφάνεια, ιδιότητα πολύ σημαντική για εμπορική χρήση του υλικού. Από τις μετρήσεις διαπερατότητας οξυγόνου και νερού συμπεραίνεται ότι τα φυλλιδία μπεντονίτη δρουν ως εμπόδια στη διάχυση των μορίων μειώνοντας τις τιμές διαπερατότητας τόσο του οξυγόνου όσο και του νερού, η οποία επηρεάζεται αναλογικά με την ισόθερμη προσρόφησης του. Από την πλευρά των μηχανικών ιδιοτήτων τα νανοσύνθετα παρουσιάζουν αυξημένες ιδιότητες εξαιτίας των αλληλεπιδράσεων πολυμερούς-πηλού ιδιαίτερα για το δείγμα με 20 %w/w μπεντονίτη. Με την τεχνική της περίθλασης νετρονιών αποδείχθηκε ότι οι δομές παρεμβολής που σχηματίζονται στο παραπάνω δείγμα είναι προσανατολισμένες παράλληλα με την επιφάνεια της μεμβράνης. Το μέγεθος των δομών αυτών προσδιορίστηκε στα 40Å. Παράλληλα με χρήση της τεχνικής εξομάλυνσης αντίθεσης προσδιορίστηκε το μέγεθος της μοναδιαίας κυψελίδας της πολυβινυλικής αλκοόλης το οποίο διαφοροποιείται από την ύπαρξη του πηλού στο δείγμα. Το αποτέλεσμα αυτό έρχεται σε συμφωνία με τις μετρήσεις διαφορικής θερμιδομετρίας σάρωσης που υποδείκνυαν την ύπαρξη νέας, μεγαλύτερου μεγέθους, κρυσταλλικής φάσης πολυβινυλικής αλκοόλης. Τέλος αποδείχθηκε ότι εξαιτίας των μεταξύ τους ισχυρών αλληλεπιδράσεων, ο πηλός λειτουργεί προστατευτικά προς την κρυσταλλική δομή της πολυβινυλικής αλκοόλης, μη επιτρέποντας στη τελευταία να καταρρεύσει. Η παρατήρηση αυτή προσδίδει ιδιαίτερα πλεονεκτήματα σε ιδιότητες των πολυμερών όπως η μηχανική και θερμική σταθερότητα

Pulling Simulations and Hydrogen Sorption Modelling on Carbon Nanotube Bundles

Recent progress in molecular simulation technology has developed an interest in modernizing the usual computational methods and approaches. For instance, most of the theoretical work on hydrogen adsorption on carbon nanotubes was conducted a decade ago. It should be insightful to reinvestigate the field and take advantage of code improvements and features implemented in contemporary software. One example of such features is the pulling simulation modules now available in many molecular dynamics programs. We conduct pulling simulations on pairs of carbon nanotubes and measure the inter-tube distance before they dissociate in water. We use this distance to set the interval size between adjacent nanotubes as we arrange them in bundle configurations. We consider bundles with triangular, intermediate and honeycomb patterns, and armchair nanotubes with a chiral index from n = 5 to n = 10. Then, we simulate low pressure hydrogen adsorption isotherms at 77 K, using the grand canonical Monte Carlo method. The different bundle configurations adsorb great hydrogen amounts that may exceed 2% wt at ambient pressures. The computed hydrogen capacities are considered large for physisorption on carbon nanostructures and attributed to the ultra-microporous network and extraordinary high surface area of the configured models

On the Consistency of the Exfoliation Free Energy of Graphenes by Molecular Simulations

Monolayer graphene is now produced at significant yields, by liquid phase exfoliation of graphites in solvents. This has increased the interest in molecular simulation studies to give new insights in the field. We use decoupling simulations to compute the exfoliation free energy of graphenes in a liquid environment. Starting from a bilayer graphene configuration, we decouple the Van der Waals interactions of a graphene monolayer in the presence of saline water. Then, we introduce the monolayer back into water by coupling its interactions with water molecules and ions. A different approach to compute the graphene exfoliation free energy is to use umbrella sampling. We apply umbrella sampling after pulling the graphene monolayer on the shear direction up to a distance from a bilayer. We show that the decoupling and umbrella methods give highly consistent free energy results for three bilayer graphene samples with different size. This strongly suggests that the systems in both methods remain closely in equilibrium as we move between the states before and after the exfoliation. Therefore, the amount of nonequilibrium work needed to peel the two layers apart is minimized efficiently

Molecular Dynamics of Water Embedded Carbon Nanocones: Surface Waves Observation

We employed molecular dynamics simulations on the water solvation of conically shaped carbon nanoparticles. We explored the hydrophobic behaviour of the nanoparticles and investigated microscopically the cavitation of water in a conical confinement with different angles. We performed additional molecular dynamics simulations in which the carbon structures do not interact with water as if they were in vacuum. We detected a waving on the surface of the cones that resembles the shape agitations of artificial water channels and biological porins. The surface waves were induced by the pentagonal carbon rings (in an otherwise hexagonal network of carbon rings) concentrated near the apex of the cones. The waves were affected by the curvature gradients on the surface. They were almost undetected for the case of an armchair nanotube. Understanding such nanoscale phenomena is the key to better designed molecular models for membrane systems and nanodevices for energy applications and separation

Polymeric, metallic and carbon membranes for hydrogen separation: A review

Hydrogen (H2) as an energy carrier can deliver or/and store a huge amount of energy. Hydrogen can be used in internal combustion engines or in fuel cells to generate electricity, with only byproducts water and heat. It is no coincidence that it has been characterized as the energy carrier of the future. Nevertheless, irrespectively of its production method, raw hydrogen must first be separated/purified from other co-produced compounds. Among other separation methods membrane separation processes present numerous advantages and for this reason membrane technology has attracted the interest of many research groups worldwide.In this review article the main characteristics of three major membrane categories, namely polymeric, metallic and carbon membranes, are summarized. For each membrane material category, both their strengths and limitations are mentioned, discussed and highlighted. In addition, selected articles, mainly those which have been published recently, are reviewed and their highlighted evidences are presented and discussed

Ceramic-Supported Alginate Adsorbent for the Removal of Heavy Metal Ions

Hybrid alginate/ceramic support sorbents have been developed by the physical imbibing of an alginic acid solution into silica pores and γ-alumina microspheres. The metal ion-binding capacity of the prepared hybrids was examined by means of batch Cd 2+ ion adsorption experiments. In addition, since the porous γ-alumina microspheres proved capable of retaining higher quantities of alginic acid than silica, they were chosen as a more appropriate substrate for the application of a chemical modification procedure. In this context, the aim was to develop hybrid sorbents with an enhanced stability and adsorption capacity obtained by grafting the bio-molecule onto the substrate. Such chemical modification included grafting two different types of silanes onto the external surface and pores of γ-alumina, followed by chemical bonding of the alginate with the characteristic groups of the silane molecules involved. Such chemically-prepared sorbents exhibited almost twice the Cd 2+ ion adsorption capacity of sorbents prepared by physical imbibing methods. The best uptake achieved in the present work was 1.44 mg Cd 2+ ion/g substrate. Moreover, the adsorption capacity per bonded alginate mass exceeded the capacity often reported in the literature for alginate beads