1,720,977 research outputs found
It's all about topology: The evolution of polymer brushes and their performance
No full textThe application of distinctive polymer topologies, beyond the simple linear chain, to yield cyclic and loopsforming
surface-grafted assemblies, enables a broad modulation of highly relevant, interfacial physico-chemical
properties. This is especially valid on flat surfaces, where the ultra-dense and highly compact character of cyclic
polymer “brushes” provide an enhanced steric stabilization of the interface and a superlubricious behavior.
Alternatively, when cyclic brushes form shells on inorganic nanoparticles (NPs), their extraordinary structural
properties make them impenetrable and long-lasting shields, which extend the stability of NP dispersions and hinder
any interaction with serum proteins.
Polymer topological effects, typically observed in bulk or in solution are amplified by adding an additional boundary
such as a grafting surface. Their precise tuning translates into materials with unprecedented properties and extremely
high applicability
Eutectic Nucleation in 7xxx Series Aluminum Alloys from a Non-classical Viewpoint
No full textThe early stages of eutectic solidification in a copper-containing 7xxx series aluminum alloy (AA 7068 or AMS 4331) were studied using the two-thermocouple computer-aided thermal analysis (CATA) technique. A feature was detected on the cooling rate curve at the equilibrium solidus temperature of the alloy which persists until the peak of the subsequent final eutectic solidification. Detailed analysis of the temperature difference between the wall and the center of the thermal analysis sample, together with examination of the eutectic solidified on the walls of porosities and a study of the eutectic nucleation on the basis of the non-classical theory of adsorption heterogeneous nucleation, indicated how the feature can be related to the faceting of the atomic structure of the solid/liquid (S/L) interface. The solidification of the remnant liquid after the faceting transition at the equilibrium solidus point depends on the interfacial undercooling and proceeds via either primary phase re-nucleation or secondary phase nucleation by adsorption. The eutectic solidification is affected by the presence of the primary phase which acts like an adsorbent
Increasing the Liquidus Temperature by Employing the Controlled Diffusion Solidification (CDS) Process: A Potential Route to Improved Castings
No full textRecent theories suggest the existence of an incubation time, over which a liquid alloy prepares for nucleation by decomposing into compositional fluctuations. Accordingly, in a recent work by the present authors, the solidification path of a Controlled Diffusion Solidification (CDS) mixture was calculated. The calculated CDS path begins at a higher liquidus temperature comparing to conventional solidification and the fraction solid values are achieved at a relatively higher temperature. To provide information on the CDS mechanism and physical structure of the CDS mixture in the mushy zone, Al-7.8Zn-2.6Mg-2Cu alloy was solidified, in this study, via conventional and CDS process in the presence and absence of recalescence. Typical grain structures obtained via the two solidification conditions is characterized using Electron Back Scattered Diffraction. Results showed that the nucleation continues to occur in the presence of recalescence, while it is suppressed in its absence. According to the two step nucleation theory, the increase in the nucleation temperature causes sufficient recalescence in the mixture, allowing the unnucleated liquid phase to decompose into chemical fluctuations and prepares for further nucleation. As a result, in the presence of recalescence, nucleation in a CDS mixture is not as readily halted as during the conventional solidification, which is in contradiction with the recent theories developed based on the classical theory of nucleation
Surface-grafted assemblies of cyclic polymers: Shifting between high friction and extreme lubricity
No full textThe interfacial physicochemical properties of “brushes” constituted by cyclic polymers strongly depend on their surface density, and they can be modulated within a broader range with respect to those displayed by linear brush analogues of identical composition. This is especially valid for the nanotribological characteristics of poly(2-ethyl-2-oxazoline) (PEOXA) brushes, assembled on titanium oxide surfaces by grafting-to technique. At low surface coverage, cyclic PEOXA (C-PEOXA) grafts flatten down towards the grafting surface and provide high friction when sheared against an identical countersurface. In contrast, densely grafted C-PEOXA assemblies stretch vertically forming a molecularly smooth surface that hinders interpenetration with the opposing brush, and dramatically reduces friction, greatly surpassing the lubricious properties typically attained by applying linear grafts.
Differently from their nanotribological properties, C-PEOXA brushes always show improved resistance towards the adsorption of proteins if compared to their linear counterparts, irrespective of their grafting density. The enhancement in biopassivity achieved by applying cyclic PEOXAs is ascribed to their intrinsic tendency to hinder protein penetration within a brush layer
Topological Polymer Chemistry Enters Surface Science: The Interfacial, Physico-Chemical Properties of Linear, Cyclic and Loops Brushes
No full textThe application of distinctive polymer topologies, beyond linearity, to yield cyclic and “loops”-forming surfacegrafted
polymer assemblies, enables a broad modulation of highly relevant, interfacial, physicochemical properties
that are difficult to be addressed by linear polymer brushes. On flat surfaces, the ultra-dense and highly compact
character of cyclic poly-2-ethyl-2-oxazoline (PEOXA) brushes provide enhanced steric stabilization of the interface,
improved biopassivity and highly lubricious behavior, if compared to linear brushes with similar molar mass. The
application of random PEOXA-based copolymers with a variable concentration of surface-reactive co-monomers
generates mixtures of grafted loops and “tails”. The precise tuning of the relative surface concentration of these two
polymer topologies allows shifting of the interfacial properties either towards the ones showed by linear brushes, or
near the characteristics of cyclic analogues. An increase in the loops fraction generates an increment of steric
repulsion, a concomitant decrease of friction, and an improvement of the antifouling character of the polymer films. All
these findings highlight how polymer topological effects, typically observed in bulk or in solution are amplified by
adding an additional boundary such as a grafting surface. Their precise tuning translates into materials with
unprecedented characteristics and extremely high applicability
Modulation of Surface-Initiated ATRP by Confinement: Mechanism and Applications
No full textThe mechanism of surface-initiated atom transfer polymerization (SI-ATRP) of methacrylates in confined volumes is systematically investigated by finely tuning the distance between a grafting surface and an inert plane by means of nanosized patterns and micrometer thick foils. The 0 polymers were synthesized from monolayers of photocleavable initiators, which allow the analysis of detached brushes by size exclusion chromatography (SEC). Compared to brushes synthesized under "open" polymerization mixtures, nearly a 4-fold increase in brush molar mass was recorded When SI-ATRP was performed within highly confined reaction volumes. Correlating the SI-ATRP of methyl methacrylate (MMA), with and without "sacrificial" initiator to that of lauryl methacrylate (LMA) and analyzing the brush growth rates within differently confined Volumes, we demonstrate faster grafting kinetics with increasing confinement due to the progressive hindering of Cu-II-based deactivators from the brush propagating, front. This effect is especially noticeable when viscous polymerization mixtures are generated and enables the synthesis: of several hundred nanometer thick brushes within relatively short polymerization times. The faster rates of confined SI-ATRP can be additionally used to fabricate, in. one pot, precisely structured brush gradients, when volume confinement is continuously varied across a single substrate by spatially tuning the vertical distance between the grafting and the confining surfaces
Biocatalytic ATRP in solution and on surfaces
No full textThe promiscuity of enzymes allows for their implementation as catalysts for non-native chemical transformations. Utilizing the redox activity of metalloenzymes under activator regenerated by electron transfer (ARGET) ATRP conditions, well-controlled and defined polymers can be generated. In this chapter, we review bioATRP in solution and on surfaces and provide experimental protocols for hemoglobin-catalyzed ATRP and for surface-initiated biocatalytic ATRP. This chapter highlights the polymerization of acrylate and acrylamide monomers and provides detailed experimental protocols for the characterization of the polymers and of the polymer brushes
Loops and Cycles at Surfaces: The Unique Properties of Topological Polymer Brushes
No full textGrafting synthetic polymers to inorganic and organic surfaces to yield polymer brushes has represented a revolution in many fields of materials science. Polymer brushes provide colloidal stabilization to nanoparticles (NPs), prevent and/or regulate the adsorption of proteins on biomaterials, and significantly reduce friction when applied to two surfaces sheared against each other. Can the performance of polymer brushes as steric stabilizers and boundary lubricants be improved? The answer to this question encompasses the application of polymer grafts presenting different chain topologies, beyond linearity. In particular, grafted polymers forming loops and cycles at the surface have been recently demonstrated to enable the modulation of interfacial physicochemical properties, including nanomechanical and nanotribological, to an extent that is difficultly addressed by using their linear counterparts. Loop and cyclic polymer brushes provide enhanced steric stabilization to surfaces, increase their biopassivity and show superlubricious behavior. Their distinctive structure, the methods applied to fabricate them and their application in several technologically relevant fields of materials science are reviewed in this contribution
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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