Altering the Surface Properties of Metal Alloys Utilizing Facile and Ecological Methods.

The development of a superhydrophobic and, even, water-repellent metal alloy surface is reported utilizing a simple, fast, and economical way that requires minimum demands on the necessary equipment and/or methods used. The procedure involves an initial irradiation of the metallic specimen using a femtosecond laser, which results in a randomly roughened surface, that is subsequently followed by placing the item in an environment under moderate vacuum (pressure 10-2 mbar) and/or under low-temperature heating (at temperatures below 120 °C). The effects of both temperature and low pressure on the surface properties (water contact angle and contact angle hysteresis) are investigated and surfaces with similar superhydrophobicity are obtained in both cases; however, a significant difference concerning their water-repellent ability is obtained. The surfaces that remained under vacuum were water-repellent, exhibiting very high values of contact angle with a very low contact angle hysteresis, whereas the surfaces, which underwent thermal processing, exhibited superhydrophobicity with high water adhesion, where water droplets did not roll off even after a significant inclination of the surface. The kinetics of the development of superhydrophobic behavior was investigated as well. The findings were understood when the surface roughness characteristics were considered together with the chemical composition of the surface.

Impact of Graphene Derivatives as Artificial Extracellular Matrices on Mesenchymal Stem Cells.

Thanks to stem cells' capability to differentiate into multiple cell types, damaged human tissues and organs can be rapidly well-repaired. Therefore, their applicability in the emerging field of regenerative medicine can be further expanded, serving as a promising multifunctional tool for tissue engineering, treatments for various diseases, and other biomedical applications as well. However, the differentiation and survival of the stem cells into specific lineages is crucial to be exclusively controlled. In this frame, growth factors and chemical agents are utilized to stimulate and adjust proliferation and differentiation of the stem cells, although challenges related with degradation, side effects, and high cost should be overcome. Owing to their unique physicochemical and biological properties, graphene-based nanomaterials have been widely used as scaffolds to manipulate stem cell growth and differentiation potential. Herein, we provide the most recent research progress in mesenchymal stem cells (MSCs) growth, differentiation and function utilizing graphene derivatives as extracellular scaffolds. The interaction of graphene derivatives in human and rat MSCs has been also evaluated. Graphene-based nanomaterials are biocompatible, exhibiting a great potential applicability in stem-cell-mediated regenerative medicine as they may promote the behaviour control of the stem cells. Finally, the challenges, prospects and future trends in the field are discussed.

Film Properties and Antimicrobial Efficacy of Quaternized PDMAEMA Brushes: Short vs Long Alkyl Chain Length.

Quaternized poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes bearing quaternary ammonium groups of different alkyl chain lengths (ACLs) were prepared and assessed as biocidal coatings. For the synthesis of the antimicrobial brushes, first well-defined PDMAEMA chains were grown by surface-initiated atom transfer radical polymerization on glass and silicon substrates. Next, the tertiary amine groups of the polymer brushes were modified via a quaternization reaction, using alkyl halides, to obtain the cationic polymers. The polymer films were characterized by Fourier-transform infrared spectroscopy, ellipsometry, atomic force microscopy, and water contact angle measurements. The effect of the ACL of the quaternary ammonium groups on the physicochemical properties of the films as well as the contact killing efficiency of the surfaces against representative Gram-positive and Gram-negative bacteria was investigated. A hydrophilic to hydrophobic transition of the surfaces and a significant decrease of the degree of quaternization of the DMAEMA moieties was found upon increasing the ACL of the quaternization agent above six carbon atoms, allowing the wettability, the thickness, and the pH-response of the brushes to be tuned via a facile postpolymerization, quaternization reaction. At the same time, antimicrobial tests revealed that the hydrophilic polymer brushes exhibited enhanced bactericidal activity against Escherichia coli and Bacillus cereus, whereas the hydrophobic surfaces showed a significant deterioration of the in vitro bactericidal performance. Our results elucidate the antimicrobial action of quaternized polymer brushes, dictating the appropriate choice of the ACL of the quaternization agent for the development of coatings that effectively inhibit biofilm formation on surfaces.

Spatio-temporal heterogeneities in nanosegregated single-molecule polymeric nanoparticles.

The study of the coupling between structural and dynamical heterogeneities in nanostructured systems is essential for the design of hybrid materials with the desired properties. Here, we use atomistic molecular dynamics simulations to closely examine the dynamical heterogeneities in nanostructured single-molecule nanoparticles consisting of mikto-arm star copolymers with poly(ethylene oxide), PEO, and polystyrene, PS, arms. The particles exhibit an internally nanostructured morphology, resembling either "Janus-like" or "patchy-like" morphology when the functionality of the stars varies. The differences in the local environment result in strong intramolecular dynamical heterogeneities. In the proximity of the star core, geometric constraints promote unfavorable PEO:PS contacts that lead to a behavior similar to dynamically asymmetric miscible polymer blends or disordered copolymers. In contrast, further away from the core, the nanosegregation induces segmental dynamics very similar to the one found in the homopolymer star analogues.

Structure and Dynamics of Biobased Polyester Nanocomposites.

The structure and the dynamics of two bio-based polyester polyols are investigated in the bulk and close to surfaces in polymer/layered silicate nanocomposites. The morphology of the neat polymers as well as the structure of the nanohybrids are investigated with X-ray diffraction and their thermal properties are studied by differential scanning calorimetry. One of the investigated polyesters is amorphous, whereas the second one is a semicrystalline polymer with intriguing thermal behavior. Hybrids have been synthesized over a broad range of compositions and intercalated structures are always obtained. The thermal transitions in the nanocomposites are observed only when the polymers are in excess outside the completely filled galleries. The glass transition, whenever it can be resolved, appears insensitive to the presence of the inorganic material, whereas the way the crystallization takes place depends on the composition of the nanohybrid. Dielectric relaxation spectroscopy was utilized to study the polymer dynamics. It revealed multiple relaxation processes for the neat polymers both below and above their glass transition temperatures, whereas in the nanocomposites, similarities and differences are observed depending on the specific mode of the dynamic process.

Formation of Oriented Polar Crystals in Bulk Poly(vinylidene fluoride)/High-Aspect-Ratio Organoclay Nanocomposites.

We have investigated the formation of lamellar crystals of poly(vinylidene fluoride) (PVDF) in the presence of oriented clay particles with different aspect ratios (ARs) and surface properties. Hot-melt screw extrusion of PVDF with 5 wt % of montmorillonite (AR ≈ 12) or fluoromica (AR ≈ 27) resulted in formation of phase-separated blends. Replacing the clays with their organoclay derivatives, organomontmorillonite or organofluoromica, resulted in the corresponding intercalated nanocomposites. The organoclays induced formation of polar ß- and γ-polymorphs of PVDF in contrast to the α-polymorph, which dominates in the pure PVDF and the PVDF/clay blends. Solid-state nuclear magnetic resonance revealed that the content of the α-phase in the nanocomposites was never higher than 7% of the total crystalline phase, whereas the ß/γ mass ratio was close to 1:2, irrespective of the AR or crystallization conditions. X-ray diffraction showed that the oriented particles with a larger AR caused orientation of the polar lamellar crystals of PVDF. In the presence of the organofluoromica, PVDF formed a chevron-like lamellar nanostructure, where the polymer chains are extended along the extrusion direction, whereas the lamellar crystals were slanted from normal to the extrusion direction. Time-resolved X-ray diffraction experiments allowed the identification of the formation mechanism of the chevron-like nanostructure.

Effect of macromolecular architecture on the self-assembly behavior of copolymers in a selective polymer host.

We present a detailed simulation study of the structural and dynamical behavior of star-shaped mikto-arm (polystyrene)8(poly(ethylene oxide))8, (PS)8(PEO)8, copolymers with eight arms of each type, versus that of a linear polystyrene-block-poly(ethylene oxide), PS-b-PEO, diblock, in a selective homopolymer host. Both copolymers are blended at the same weight fraction 33% with an oligomeric PEO host. We use atomistic molecular dynamics simulations to account for the molecular interactions present in the blends and to study quantitatively the dynamical and structural properties of these systems. The presence of the selective oligomeric PEO host leads to the formation of complex self-assembled structures. While cylindrical structures are formed in the case of linear diblock copolymers, mikto-arm star copolymers form percolated interconnected assemblies within the PEO host. The cylindrical objects formed by the linear diblock copolymers exhibit a higher degree of compactness and a weaker temperature dependence than the percolated network formed by their star-shaped analogues. The dynamics is governed primarily by the local structural heterogeneity, i.e., the environment around a segment, which is determined by the interaction between the different components, the macromolecular architecture of the copolymer as well as the associated geometrical constrains. Our data further stress the fact that the structural and dynamical properties in these blends may be controlled/tuned by the macromolecular architecture of the copolymer and/or by adjusting the temperature.

Temperature- and/or pH-Responsive Surfaces with Controllable Wettability: From Parahydrophobicity to Superhydrophilicity.

Multifunctional surfaces with reversible wetting characteristics are fabricated utilizing end-anchored polymer chains on hierarchically roughened surfaces. Temperature- and/or pH-responsive surfaces are developed that exhibit reversible and controllable wettability, from the "parahydrophobic" behavior of natural plant leaves all the way to superhydrophilic properties in response to the external stimuli. For this purpose, dual scale micro/nanoroughened surfaces were prepared by laser irradiation of inorganic surfaces (Si wafers) utilizing ultrafast (femtosecond) laser pulses under a reactive gas atmosphere. End-functionalized polymer chains were anchored onto those surfaces utilizing the "grafting to" method; poly(N-isopropylacrylamide), PNIPAM, and poly(2-vinylpyridine), P2VP, were used for the formation of monofunctional as well as mixed brushes. The surfaces exhibit "parahydrophobic" behavior in the hydrophobic state (high temperature and/or high pH), with high static contact angles (∼120°) and high water adhesion (∼30° contact angle hysteresis), whereas they show superhydrophilic behavior in the hydrophilic state (low temperature and/or low pH). The surfaces were tested for their wettability under repetitive cycles and found to be stable and reproducible.

A facile route towards PDMAEMA homopolymer amphiphiles.

Well-defined poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) has been modified at low degrees of quaternization of the tertiary amine groups using alkyl halides with long alkyl chains as the quaternization agents. The resulting PDMAEMA-co-PQDMAEMA copolymers were studied in aqueous solution using potentiometric titrations, turbidimetry, surface tensiometry, dynamic light scattering and zeta potential measurements. An increase of the hydrophilicity of the precursor polymer, leading to an increase or even elimination of the lower critical solution temperature (LCST) for the quaternized copolymers was found; this extended the temperature range of the stable polymer solution. At the same time, it was shown that the hydrophobic character of the polymer increases upon quaternization, leading to higher surface activity compared to the precursor PDMAEMA homopolymer, and, thus, to more effective polymeric surfactants. This contradiction in the copolymer behavior was attributed to the interplay between the polymer self-assembly in the aqueous medium and the polymer adsorption at the air/water interface, which dominate the cloud point and the surface properties, respectively.

Effects of nanoscopic-confinement on polymer dynamics.

The static and dynamic behavior of polymers in confinement close to interfaces can be very different from that in the bulk. Among the various geometries, intercalated nanocomposites, in which polymer films of ∼1 nm thickness reside between the parallel inorganic surfaces of layered silicates in a well-ordered multilayer, offer a unique avenue for the investigation of the effects of nanoconfinement on polymer structure and dynamics by utilizing conventional analytical techniques and macroscopic specimens. In this article, we provide a review of research activities mainly in our laboratory on polymer dynamics under severe confinement utilizing different polymer systems: polar and non-polar polymers were mixed with hydrophilic or organophilic silicates, respectively, whereas hyperbranched polymers were studied in an attempt to probe the effect of polymer-surface interactions by altering the number and the kinds of functional groups in the periphery of the branched polymers. The polymer dynamics was probed by quasielastic neutron scattering and dielectric relaxation spectroscopy and was compared with that of the polymers in the bulk. In all cases, very local sub-Tg processes related to the motion of side and/or end groups as well as the segmental α-relaxation were identified with distinct differences recorded between the bulk and the confined systems. Confinement was found not to affect the very local motion in the case of the linear chains whereas it made it easier for hyperbranched polymers due to modifications of the hydrogen bond network. The segmental relaxation in confinement becomes faster than that in the bulk, exhibits Arrhenius temperature dependence and is observed even below the bulk Tg due to reduced cooperativity in the confined systems.

Collapse transitions in thermosensitive multi-block copolymers: a Monte Carlo study.

Monte Carlo simulations are performed on a simple cubic lattice to investigate the behavior of a single linear multiblock copolymer chain of various lengths N. The chain of type (AnBn)m consists of alternating A and B blocks, where A are solvophilic and B are solvophobic and N = 2nm. The conformations are classified in five cases of globule formation by the solvophobic blocks of the chain. The dependence of globule characteristics on the molecular weight and on the number of blocks, which participate in their formation, is examined. The focus is on relative high molecular weight blocks (i.e., N in the range of 500-5000 units) and very differing energetic conditions for the two blocks (very good-almost athermal solvent for A and bad solvent for B). A rich phase behavior is observed as a result of the alternating architecture of the multiblock copolymer chain. We trust that thermodynamic equilibrium has been reached for chains of N up to 2000 units; however, for longer chains kinetic entrapments are observed. The comparison among equivalent globules consisting of different number of B-blocks shows that the more the solvophobic blocks constituting the globule the bigger its radius of gyration and the looser its structure. Comparisons between globules formed by the solvophobic blocks of the multiblock copolymer chain and their homopolymer analogs highlight the important role of the solvophilic A-blocks.

Development of functional polymer surfaces with controlled wettability.

There is a demand for surfaces with new functional properties in almost all industrial branches. During the next few years, research input will be required for the development of coatings exhibiting an easy-to-clean or self-cleaning ability, switchability so that they can act as sensors/actuators, and defined tribological/mechanical properties and long-term stability. To achieve such behavior, the development of new advanced functional coatings that exhibit the proper chemistry and surface structure is necessary. In this Feature Article, we provide a review of the research activities in our laboratory on the development of functional and, especially, reversibly switchable polymer surfaces where the emphasis is on controlling their wettability. We will first discuss the fabrication of superhydrophobic surfaces by hierarchically micro- and nanostructuring a substrate surface with an ultrafast laser followed by appropriate hydrophobization. Then, we will summarize the development of surfaces that can alter their wetting behavior in response to changes in external stimuli such as humidity and light illumination. Finally, we will present our investigations on utilizing responsive (organic) coatings on hierarchically roughened substrates for the development of surfaces, which would be able to switch reversibly from superhydrophilic to superhydrophobic and water-repellent in response to an external stimulus (in this case, pH).

Special Issue "State-of-the-Art Polymer Science and Technology in Greece".

Polymer science and technology is an active and continuously developing field of research and innovation in Greece [...].

Modifying flexible polymer films towards superhydrophobicity and superoleophobicity by utilizing water-based nanohybrid coatings.

The development of superhydrophobic and/or superoleophobic materials has been attracting the attention of the scientific community due to their wide range of applications. In this work, waterborne nanocomposite coatings were developed to be deposited onto flexible polyethylene films in order to modify them into superhydrophobic and even superoleophobic. The coatings consisted of either a low surface energy mixture of silanes/siloxanes or a fluoropolymer in conjunction with the appropriate inorganic nanoparticles that provide the necessary roughness; the effects of nanoparticle type and content on the behaviour was investigated. In both cases, the surface properties were investigated, and the polymer films were found to be superhydrophobic. Depending on the system utilized, the final material exhibited either low water adhesion, thus, being water repellent, or high water adhesion. The use of the fluoropolymer has led to coatings that exhibited superoleophobic behaviour for various organic compounds, as well. The application of the coatings did not influence either the optical transparency or the thermal properties of the polyethylene films. Moreover, the coated surfaces show similar or even better mechanical properties, scratch resistance and chemical durability in comparison to the neat LDPE film.

How the Physicochemical Properties of Manufactured Nanomaterials Affect Their Performance in Dispersion and Their Applications in Biomedicine: A Review.

The growth in novel synthesis methods and in the range of possible applications has led to the development of a large variety of manufactured nanomaterials (MNMs), which can, in principle, come into close contact with humans and be dispersed in the environment. The nanomaterials interact with the surrounding environment, this being either the proteins and/or cells in a biological medium or the matrix constituent in a dispersion or composite, and an interface is formed whose properties depend on the physicochemical interactions and on colloidal forces. The development of predictive relationships between the characteristics of individual MNMs and their potential practical use critically depends on how the key parameters of MNMs, such as the size, shape, surface chemistry, surface charge, surface coating, etc., affect the behavior in a test medium. This relationship between the biophysicochemical properties of the MNMs and their practical use is defined as their functionality; understanding this relationship is very important for the safe use of these nanomaterials. In this mini review, we attempt to identify the key parameters of nanomaterials and establish a relationship between these and the main MNM functionalities, which would play an important role in the safe design of MNMs; thus, reducing the possible health and environmental risks early on in the innovation process, when the functionality of a nanomaterial and its toxicity/safety will be taken into account in an integrated way. This review aims to contribute to a decision tree strategy for the optimum design of safe nanomaterials, by going beyond the compromise between functionality and safety.

Dynamics of Polymer Chains in Poly(ethylene oxide)/Silica Nanocomposites via a Combined Computational and Experimental Approach.

The dynamics of polymer chains in poly(ethylene oxide)/silica (PEO/SiO2) nanoparticle nanohybrids have been investigated via a combined computational and experimental approach involving atomistic molecular dynamics simulations and dielectric relaxation spectroscopy (DRS) measurements. The complementarity of the approaches allows us to study systems with different polymer molecular weights, nanoparticle radii, and compositions across a broad range of temperatures. We study the effects of spatial confinement, which is induced by the nanoparticles, and chain adsorption on the polymer's structure and dynamics. The investigation of the static properties of the nanocomposites via detailed atomistic simulations revealed a heterogeneous polymer density layer at the vicinity of the PEO/SiO2 interface that exhibited an intense maximum close to the inorganic surface, whereas the bulk density was reached for distances ∼1-1.2 nm away from the nanoparticle. For small volume fractions of nanoparticles, the polymer dynamics, probed by the atomistic simulations of low-molecular-weight chains at high temperatures, are consistent with the presence of a thin adsorbed layer that exhibits slow dynamics, with the dynamics far away from the nanoparticle being similar to those in the bulk. However, for high volume fractions of nanoparticles (strong confinement), the dynamics of all polymer chains were predicted slower than that in the bulk. On the other hand, similar dynamics were found experimentally for both the local ß-process and the segmental dynamics for high-molecular-weight systems measured at temperatures below the melting temperature of the polymer, which were probed by DRS. These differences can be attributed to various parameters, including systems of different molecular weights and nanoparticle states of dispersion, the different temperature range studied by the different methods, the potential presence of a reduced-mobility PEO/SiO2 interfacial layer that does not contribute to the dielectric spectrum, and the presence of amorphous-crystalline interfaces in the experimental samples that may lead to a different dynamical behaviors of the PEO chains.

Static and Dynamic Behavior of Polymer/Graphite Oxide Nanocomposites before and after Thermal Reduction.

Nanocomposites of hyperbranched polymers with graphitic materials are investigated with respect to their structure and thermal properties as well as the dynamics of the polymer probing the effect of the different intercalated or exfoliated structure. Three generations of hyperbranched polyester polyols are mixed with graphite oxide (GO) and the favorable interactions between the polymers and the solid surfaces lead to intercalated structure. The thermal transitions of the confined chains are suppressed, whereas their dynamics show similarities and differences with the dynamics of the neat polymers. The three relaxation processes observed for the neat polymers are observed in the nanohybrids as well, but with different temperature dependencies. Thermal reduction of the graphite oxide in the presence of the polymer to produce reduced graphite oxide (rGO) reveals an increase in the reduction temperature, which is accompanied by decreased thermal stability of the polymer. The de-oxygenation of the graphite oxide leads to the destruction of the intercalated structure and to the dispersion of the rGO layers within the polymeric matrix because of the modification of the interactions between the polymer chains and the surfaces. A significant increase in the conductivity of the resulting nanocomposites, in comparison to both the polymers and the intercalated nanohybrids, indicates the formation of a percolated rGO network.

How Does the Number of Arms Affect the Properties of Mikto-Arm Stars in a Selective Oligomeric Matrix? Insights from Atomistic Simulations.

We present a simulation study of amphiphilic mikto-arm star copolymers in a selective polymer host. By means of atomistic molecular dynamics simulations, we examine the structural and dynamical properties of mikto-arm stars with varying number, n, of poly(ethylene oxide) (PEO) and polystyrene (PS) arms, (PEO) n (PS) n in a 33% wt blend with an oligomeric PEO host (o-PEO). As the number of arms increases, the stars resemble more spherical particles with less separated PEO and PS intramolecular domains. As a result of their internal morphology and associated geometrical constraints, the mikto-arm stars self-assemble either into cylindrical-like objects or a percolated network with increasing n, within the o-PEO matrix. The segmental dynamics is mostly governed by the star architecture and the heterogeneous local environment, formed by the intra- and intermolecular nanosegregation. We discuss the role of each factor and compare the results with previously published studies on mikto-arm stars.

The Micellization of Well-Defined Single Graft Copolymers in Block Copolymer/Homopolymer Blends.

A series of well-defined (polyisoprene)2(polystyrene), I2S, single graft copolymers with similar total molecular weights but different compositions, fPS, were blended with a low molecular weight polyisoprene homopolymer matrix at a constant concentration 2 wt%, and the micellar characteristics were studied by small-angle x-ray scattering. To investigate the effect of macromolecular architecture on the formation and characteristics of micelles, the results on the single graft copolymers were compared with those of the corresponding linear polystyrene-b-polyisoprene diblock copolymers, SI. The comparison reveals that the polystyrene core chains are more stretched in the case of graft copolymer micelles. Stretching turned out to be purely a result of the architecture due to the second polyisoprene block in the corona. The micellization of a (polystyrene)2(polyisoprene), S2I, graft copolymer was also studied, and the comparison with the results of the corresponding I2S and SI copolymers emphasizes the need for a critical core volume rather than a critical length of the core-forming block, in order to have stable micelles. Finally, the absence of micellization in the case of the I2S copolymer with the highest polystyrene volume fraction is discussed. For this sample, macrophase separation occurs, with polyisoprene cylinders formed in the copolymer-rich domains of the phase-separated blends.

Tribological Performance Investigation of a Commercial Engine Oil Incorporating Reduced Graphene Oxide as Additive.

We investigated the tribological behavior of commercialized, fully synthetic engine oil upon the incorporation of reduced graphene oxide in seven different concentrations between 0.01 and 0.2 wt %. Stability of the prepared samples was assessed by turbidimetry and dynamic light scattering measurements, and their tribological properties through a reciprocating tribometer, using a steel ball on special cut steel blocks. The addition of 0.02 wt % of reduced graphene oxide led to an improvement of the tribological behavior compared to the pristine engine oil, by significantly lowering the friction coefficient by 5% in the boundary lubrication regime. Both the surfaces and the reduced graphene oxide additive were thoroughly characterized by microscopic and optical spectroscopy techniques. We also verified that a protective layer was formed between the worn surfaces, due to the presence of reduced graphene oxide. Carbon accumulation and various additive elements such as Ca, Zn, S and P were detected on the rubbing surfaces of both the ball and the block through energy-dispersive X-ray spectroscopy. Finally, it was shown that the wear scar diameter on the surface of the steel ball was lower by 3%, upon testing the engine oil sample containing reduced graphene oxide at concentration 0.02 wt %, compared to the control sample.