1,721,065 research outputs found
Inter- and intramolecular complexation of copolymers : On the way to non-equilibrium micelles
No full textMacromolecular non-equilibrium self-assemblies hold great promise for on-demand transformations, which result in substantial changes in their physico-chemical properties. For these non-equilibrium self-assemblies, the pathway for the self-assembly determines the final metastable morphology. In contrast to equilibrium structures, it is therefore possible to create different well-defined morphologies at the same conditions. At the moment, stimuli-sensitive polymers are used extensively to introduce structural variations in dependence of adaptable conditions, like temperature or pressure. Triggered structural transformations at constant conditions have been much less investigated. They require the involvement of metastable states, which are able to occupy another local energetic minimum after small deviations from metastability. Hereby changes in the morphology of self-assembled structures are of high interest. A change from e.g. spherical structures to worm-like structures will also induce a change in macroscopic properties. Here, we are interested in systems which allow a switch between different defined self-assembled structures via a small trigger, or in systems where we are able to generate different defined self-assembled structures depending on the used pathway. To achieve this, we use a ternary star-shaped polymeric system. This system is based on a polymer which is water-soluble under all investigated conditions and which stabilizes the self-assembled structures in water. It further consists of several polycationic arms, which form an interpolyelectrolyte complex (IPEC) in presence of an anionic polymer. The complex phase-separates from water and, therefore, causes aggregation and self-assembly upon complexation. Due to the nature of this complex, the possibility for structural rearrangements can be adjusted. Finally, the star-shaped polymer has a temperature-sensitive arm. We observe that the temperature response of this arm is hindered due to weak complex between the temperature-responsive arm and the polycationic arm. Hence, we further investigate ways to regain the temperature response. While placing the polymers at an oil/water interface even enhances the complexation, we find via experiments and simulation a dependence of the amount of formed weak complex on the topology and composition of the star-shaped polymer. This enables the synthesis of a star with the same chemical constituents, but still showing a temperature response, possibly suitable for the generation of non-equilibrium structures. Further, a less capricious diblock copolymer system is studied. By tuning the synthesis concentration and/or relative block lengths, the copolymer can be designed to form self-assembled spherical micelles, spherical crew-cut micelles, worm-like micelles or vesicles. Within certain parameters, it is anticipated that the equilibrium nanostructures are produced, while other parameters lead to kinetically trapped structures. Despite their non-equilibrated nature, these dispersions are kinetically stable for months, as long as certain triggers are avoided. Then, the kinetically trapped structures undergo a morphological transition resulting in a pronounced change in the macroscopic properties
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
Uniform and highly charged microgels via emulsion polymerization
No full textThis thesis deals with the synthesis and solution properties of uniform and highly charged (cationic, anionic and polyampholyte) microgels via emulsion polymerization with tunable size and degree of crosslinking (swelling). The intermediately synthesized hydrophobic latex particles are later post-modified into hydrophilic charged microgels via post-modification or deprotection techniques. Apart from this, polyamphiphatic microgels form hydrophobic bridges or nanostructured domains in aqueous media visualized via cryogenic (cryo)-TEM.Microgels with cationic charged groups are synthesized by the post-functionalization of uniform latex particles via Cu-mediated Huisgen-type alkyne/azide click chemistry. The particle hardness and swelling can be tuned as a function of the used ratio of monomer/crosslinker. Post modifications in the interior of the particles are conducted in the swollen state in organic solvent. Combined Raman spectroscopy and elemental analysis reveal the kinetics and degree of modification. After modification, the resulting microgels display a pH-dependent ionization and swelling behavior in water. Additionally, it is shown that degree of functionalization depends on the crosslinking degree. In terms of anionically charged, strong polyelectrolyte microgels, the synthesis of highly uniform microgels is performed via emulsion polymerization with monomer (poly(styrene sulfonic ethyl ester) (PSSE)) carrying protecting groups, which are later transformed into highly charged strong polyelectrolyte (poly(styrene sulfonic acid) (PSSA)) microgels. The tuning of particle size, as well as particle softness is demonstrated with solution state characterization via dynamic light scattering (DLS) and cryo-TEM.Apart from that, microgels with internal and reconfigurable complex nanostructure are synthesized by incorporating permanent (poly(methyl methacrylate) (PMMA)) and switchable hydrophobic pockets (poly(N,N’-diethylaminoethyl methacrylate) (PDEAEMA)) in highly charged poly(methacrylic acid) (PMAA) microgels via emulsion polymerization. The tuning of the size, crosslinking density and tailored incorporation of functional comonomers into the polyacid microgels is shown. Analysis via cryo-TEM and pyrene probe measurements reveal switchable hydrophobic pockets inside the microgels as a function of pH. The particles show a rich diversity of internal phase-segregation that adapts to the surrounding conditions. Large amounts of hydrophobic pockets even lead to hydrophobic bridging between particles. The study shows ways towards tailored polyelectrolyte microgels with narrow dispersity, high charge density, as well as tailored and reconfigurable hydrophobic compartments and interactions.Furthermore, microgels having well-defined core-shell morphology with opposite charged groups in core and shell are synthesized via surfactant-free seeded emulsion polymerization. The swelling behavior of microgels are measured at different pH via DLS and cryo-TEM. Cryo-TEM reveals the core-shell morphology for PMAAcore-P2VPshell microgels due to ionization of PMAA shell and P2VP core at low and high pH. The homogenous shell around the core allows to understand the pH-dependent swelling behavior of core-shell microgels.In general, emulsion polymerization technique allows to synthesize highly uniform latex particles with different monomers carrying hidden functionalities, which are readily converted into charged units via post-functionalization by click-chemistry, deprotection of protected groups or modifications
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Supramolecular engineering of adaptive bioinspired nanocomposites
No full textBiological materials such as nacre, bone and crustaceans fascinate us with their synergistic combination of strength, stiffness, high toughness and light weight. Their high mechanical performance originates from the combination of soft and hard building blocks, high fraction of inorganic reinforcements, and perfectly ordered structures. Replication of those structural features and transferring the high mechanical properties, especially the combination of high stiffness and high toughness would undoubtedly benefit a wide field of areas. Nacre is among the most extensively studied biological materials, due to its high mechanical performance and unique structure. Different approaches have been employed to mimic the inorganic/organic brick and mortar structure, and in this work we utilize the most recently developed ‘self-assembled nacre mimetics’, in that it is easy and simple and allows for large area production of thick films. Well defined polymers, despite its low fraction (usually below 5 vol% in nacre), play very important roles in the mechanical properties, such as integrating the inorganic reinforcements, providing appropriate frictional sliding between the platelets, and giving sacrificial bonds and hidden length mechanisms to enhance the toughness. However, all of the previous work only concentrated on commercially available, high Tg polymers, and no efforts have so far been devoted to careful macromolecular engineering of the polymer phase. I am going to address this challenge in the first part of my PhD work. Dynamic polymers were designed with low glass-transition temperature and bonded by quadruple hydrogen-bonding motifs, and subsequently assembled them with high-aspect-ratio synthetic nanoclays to generate nacre-mimetic films. The high dynamics and self-healing of the polymers render transparent films with a near-perfectly aligned structure. Varying the polymer composition allows molecular control over the mechanical properties up to very stiff and very strong films (E ≈ 45 GPa, σUTS ≈ 270 MPa). The amount of supramolecular bonds in the nacre mimetic material governs the mechanical properties in a large extent. Stable crack propagation and multiple toughening mechanisms occur in situations of balanced dynamics, enabling synergistic combinations of stiffness and toughness. In the second part, I transfer the supramolecularly engineered nacre mimetic composites into a light adaptive material via doping a small fraction of reduced graphene oxide. Supramolecular interactions of the nanoconfined polymer phase govern the mechanical tensile properties of all nacre-mimetic films. The materials containing higher molar amount of supramolecular motifs are very stiff and strong, whereas those with lower amount realize interesting combination of stiffness and toughness/ductility. Co-assembly of 1 wt% of RGO imparts a strong photo-thermal effect, the material quickly reach a steady state temperature where heat generation and dissipation are balanced. The amount of supraomolecular bonds and more importantly the laser intensity governs the stress relaxation mechanism in the RGO doped nacre mimetic materials. In situ digital image correlation (DIC) analysis shows that we can modulate the strain field at will by using localized laser irradiation. Most importantly, the material is light adaptive. The bulk material turns from strong/stiff to soft/tough when we globally irradiate it and readily opens up the supramolecular bonds. In the third part, I explore other possibilities of our supramolecular copolymers as the soft phase of a different type of bioinspired nanocomposite materials. The synthesized low Tg, hydrophilic copolymers with varying functionalization of supramolecular bonding were self-assembled with cellulose nanocrystals, to give ordered cholesteric phases with characteristic photonic stop bands. The dimensions of the helical pitch are controlled by the ratio of polymer/CNC. We demonstrate that the supramolecular motifs regulate the swelling when exposing the biomimetic hybrids to water, and they allow engineering the photonic response. Moreover, the amount of hydrogen bonds and the polymer fraction are decisive in defining the mechanical properties. The molecular engineering allows us to span an unprecedented mechanical property range from highest inelastic deformation (strain-to-failure, εb up to ∼13%) to highest stiffness (E ∼ 15 GPa) and combinations of both
Supracolloidal co-assemblies of microgels based on orthogonal host-guest interactions
No full textColloids are valuable model systems to understand structure and dynamics of matter, explore new self-assembly concepts, and realize advanced materials. A key to simulate complex colloidal systems is to furnish colloids with defined, orthogonal, and switchable interactions beyond classical electrostatic stabilization. Microgels as a special class of colloids are ideal models to investigate the relationship of the interparticle interactions because of their stable swollen states in good solvents. The major body in microgel self-assembly focuses on harnessing ionic interactions and self-assemblies based on single components. In fact, there are very few studies about hetero-aggregation or co-assembling microgel systems, and they mostly focus on simple cationic/anionic microgel partners. Such ionic interactions yet offer only limited selectivity in multicomponent systems, and also limited pathways of switchability. On the other hand, supramolecular chemistry is highly developed and supplies a range of hetero or self-complementary interactions up to the level of DNA programming. Few attempts have been made to incorporate supramolecular interactions into microgel systems, but the prospects are significant as it for instance allows selective structure formation and new and orthogonal triggers. To address this challenge I focus on building supracolloidal co-assemblies of microgels based on orthogonal host-guest interactions and investigating the relationships of interparticle interactions. The entire thesis contains three parts of work. In the first part, I explore a simple and versatile synthesis route for the preparation of highly uniform and densely functionalized aqueous microgels by modification of latex particles composed of an active ester monomer (pentafluorophenyl acrylate; PFPA). The hydrophobic nature of the PFPA allows synthesizing very uniform latex particles via emulsion polymerization, whose size can be controlled by the surfactant concentration, while the degree of swelling is a function of the added crosslinker. The high reactivity of the PFPA groups toward nucleophilic substitution delivers a platform method to synthesize functional microgels by reaction with functional amines. This study demonstrates this process for the dense functionalization of the entire particle with an amine carrying a pH-responsive unit. This study further describes the influence of the crosslinking degree on the ability for swelling of the resulting microgels in aqueous dispersion.In the second part I demonstrate social self-sorting of co-assembled families of colloids by orthogonal host/guest recognition using cyclodextrins. It builds on the expertise gained on functionalization in the first study, but uses advanced core-shell microgel architectures enabling adavanced imaging using confocal microscopy. Mixtures of up to four partners can self-sort into their respective families without mutual interference. Additionally, the self-assemblies and their interactions are switchable using orthogonal triggers. This study goes beyond previous possibilities of molecular self-sorting, and opens the design space for future self-sorting colloidal systems via rationally designed molecular recognition.In the third part, I investigate one of the co-assembling microgel families in greater detail, and show how electrostatic repulsion needs to be balanced for the supramolecular recognition to take place. A gradual change from repellent microgels to stable clusters and ordered flocculates upon decreasing electrostatic repulsion are observed. The adaptive nature of the multivalent interactions embedded in the soft microgel shell leads to kinetically trapped scenarios and fibril formation from spherical building blocks
Responsive chitosan-based microgels
No full textRegarding the development of stimuli-responsive microgels as drug delivery systems for cancer therapies, improvements in biocompatibility, stability, and controlled release are the major challenges due to the generally limited dosages of anticancer drugs capable of being loaded, poor drug bioavailability, and non-specialized drug administration. To overcome those challenges, this Thesis presents various pH-sensitive biopolymer-based microgel systems which exhibit good biocompatibility and biodegradability (whilst producing non-toxic degradation by-products), thus demonstrating the great potential for the incorporation of various active agents including drugs and biologics. Based on these properties, microgels can be utilized as drug delivery vehicles for stimulus-triggered degradation and controlled drug delivery, thus suggesting that the presented microgel systems are good candidates for site-specific cancer therapies. This Thesis focuses on conductive polymer-based, as well as biopolymer-based microgels, for use in drug delivery systems. Chapter 1 provides an overview of different functional microgels. These microgels exhibit good biocompatibility, biodegradability, non-toxicity, pH-sensitivity, redox-activity, and adjustable chemical and mechanical properties. These properties endow them with a wide variety of applications, such as drug encapsulation, which facilitates their use as delivery systems, electrical sensors, and functional coatings, as well as their application in areas such as tissue regeneration and wastewater filtration. Chapter 2 introduces a controlled drug release system: drug-loaded biopolymer-based microgels. Due to the present problems faced with its use, such as insufficient cellular uptake as well as the numerous drug resistance mechanisms in cells, an anticancer drug, doxorubicin (DOX), has been developed to be capable of being encapsulated into nanocarriers. Moreover, this drug can also be released under the control of the microenvironment, most notably in tumor tissues. The Thesis details the preparation of cross-linked chitosan-poly(hydroquinone) (CHHQ) microgels with pH and redox sensitivity. Due to their pH-sensitivity, redox-activity, and biodegradability, CHHQ microgels have previously been exploited to load and release DOX. The loading of the active ingredient is achieved by means of physical entrapment of both π-π stacking and hydrogen bonding between chitosan, poly(hydroquinone), and DOX. The drug loading profiles were investigated and an encapsulation efficiency of 80.9% was observed. The drug release profiles show that approximately 43% of DOX is released over one hour at pH 6; contrastingly, very little DOX release is observed over the same time period at pH 7.4. These results suggest that CHHQ microgels are a promising anti-tumor drug carrier for anticancer drug delivery systems. Chapter 3 describes the development of chitosan-poly(aniline) (CH-PANI) microgels. These microgels exhibit both pH-sensitivity and redox-activity. The CH-PANI microgels are composed of chitosan and poly(aniline), using glutaraldehyde as the cross-linker. The degradation results show that CH-PANI microgels can be degraded in an acidic environment, in the presence of lysozyme. The results suggest that the prepared CH-PANI microgels hold great potential as drug delivery carriers for the selective delivery of therapeutics to acidic tissues, such as tumors. Chapter 4 details and explores how novel pH-sensitive dual-degradable dextran-chitosan (DE-CH) microgels are suitable as drug carriers for the efficient, targeted delivery of drugs to the colon. A series of DE-CH microgels were synthesized by cross-linking two modified biopolymers, alkyne-modified chitosan, and azide-modified dextran with varying azide:alkyne molar ratios from 1:0.5, 1:1, 1:1.5 to 1:2. The microgels were cross-linked via copper(II)-catalyzed azide-alkyne cycloaddition (CuAAC) without a cross-linker. By conducting dynamic light scattering (DLS) and electrophoretic mobility studies, it was demonstrated that the microgels were pH-sensitive. Under slightly acidic conditions, the microgels can be degraded in the presence of dextranase, an enzyme present in the colon. In addition, the prepared DE-CH microgels are capable of loading vancomycin hydrochloride (VM), an antibiotic effective against many gram-positive bacteria. The results showed an encapsulation efficacy of up to 93.7%, indicating a possible application for the microgels as an effective platform for site-specific targeted drug delivery (e.g., to the colon)
Electrochemical switching and synthesis of microgels
No full textStimuli-responsive polymer systems can adapt their properties in response to environmental changes. Here, the electrochemical stimulus is, despite its simple and precise controllability, less often investigated compared to e.g. pH or temperature stimuli. This is also true for microgels being mostly spherical, cross-linked polymer networks in the range of nanometres to micrometres. To tailor the properties of these microgels, the distribution of functional comonomers in the polymer network is highly important. As soft anisotropic colloids are exciting building blocks for self-assembly processes, also the control over the shape of the resulting microgels attracts attention.Starting from a poly(N-isopropylacrylamide-co-vinylferrocene) (P(NIPAM-co-VFc)) microgel with a VFc-rich core and a NIPAM-rich shell, the synthesis protocol is adapted within this thesis to control the distribution of the VFc monomer. By combining model-based synthesis predictions with experimental work, microgels with a homogeneous VFc distribution and a VFc-rich shell were easily synthesised. Placing the electro-active VFc units in the shell enhances their electrochemical addressability in organic media. In aqueous solution, the low dynamics in the VFc-rich spots seem to hinder or even prevent electrochemical addressability. Additionally, the formation of anisotropic microgels via spatially resolved electrodeposition is studied. To realise a polymer film that replicates a 2D electrode structure, different polymer systems and electrode properties were investigated. The deposition performance of the quaternized homopolymers poly[2-(methacryloyloxy)ethyl]trimethylammonium chloride (PMOTAC), poly[2-(acryloyloxy)ethyl]trimethylammonium chloride (POTAC) and poly[3-(methacrylamido)propyl]trimethylammonium chloride (PMAPTAC) and their copolymers with N-(3-aminopropyl)methacrylamide hydrochloride (APMA) was studied. In a further step, P(MAPTAC-co-APMA) was modified with thermally cross-linkable units to be able to ensure the structural integrity of the electrodeposited films. After investigating different materials, platinum ring electrodes embedded in a, with poly(ethylene glycol) brushes, silanized silicon dioxide matrix seem to be most suitable for the electrodeposition process. When dipping this electrode from above into the deposition solution, the electrode shape seems to dictate the shape of the electrodeposited polymer film. With increasing charge transfer the preservation of the ring cavity fails probably as the adhesion of polymer film onto the insulating silicon dioxide layer cannot be fully prevented
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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