1,721,105 research outputs found
Preparation of Doxorubicin-Loaded Amphiphilic Poly(D,L-Lactide-Co-Glycolide)-b-Poly(N-Acryloylmorpholine) AB(2)Miktoarm Star Block Copolymers for Anticancer Drug Delivery
No full textOwing to their unique topology and physical properties, micelles based on miktoarm amphiphilic star block copolymers play an important role in the biomedical field for drug delivery. Herein, we developed a series of AB(2)-type poly(D,L-lactide-co-glycolide)-b-poly(N-acryloyl morpholine) (PLGA-b-PNAM(2)) miktoarm star block copolymers by reversible addition-fragmentation chain-transfer polymerization and ring-opening copolymerization. The resulting miktoarm star polymers were investigated by(1)H NMR spectroscopy and gel permeation chromatography. The critical micellar concentration value of the micelles increases with an increase in PNAM block length. As revealed by transmission electron microscopy and dynamic light scattering, the amphiphilic miktoarm star block copolymers can self-assemble to form spherical micellar aggregates in water. The anticancer drug doxorubicin (DOX) was encapsulated by polymeric micelles; the drug-loading efficiency and drug-loading content of the DOX-loaded micelles were 81.7% and 9.1%, respectively. Acidic environments triggered the dissociation of the polymeric micelles, which led to the more release of DOX in pH 6.4 than pH 7.4. The amphiphilic PLGA-b-PNAM(2)miktoarm star block copolymers may have broad application as nanocarriers for controlled drug delivery.11Nsciescopu
Synthesis and self-assembly of amphiphilic and biocompatible poly(vinyl alcohol)-block-poly(l-lactide) copolymer
No full textWell-defined amphiphilic and biocompatible poly(vinyl alcohol)block-poly(L-lactide) copolymers (PVA-b-PLLA) were synthesized by the combination of reversible addition-fragmentation chain transfer (RAFT), ring opening polymerization (ROP), and click chemistry. First, an innovative azide-functionalized xanthate mediated chain transfer agent (CTA) [S-(3-azidopropyl propanamide)-(O-butyl xanthate)] was synthesized and used for vinyl acetate (VAc) polymerization through RAFT. Then, azide-terminated poly(vinyl acetate) (azide-PVAc) was converted to the corresponding azide-terminated poly(vinyl alcohol) (azide-PVA) through hydrolysis. Alkyne-terminated poly (L-lactide) (alkyne-PLLA) was synthesized via ROP using a bifunctional initiator, having an alkyne and a hydroxyl group. Finally, azide-PVA and alkyne-PLLA were coupled by 1,3 cycloaddition click reaction to get PVA-b-PLLA. Synthesized azide-PVAc, azide-PVA, alkyne-PLLA and PVA-b-PLLA were characterized by size exclusion chromatography and H-1 NMR spectroscopy. The self-assembly of PVA-b-PLLA in water was investigated by transmission electron microscope, atomic force microscopy, dynamic light scattering and H-1 NMR spectroscopy. Spherical micelles in water were clearly observed. The critical micelle concentration of PVA-b-PLLA in water was determined by fluorescence spectroscopy, and it was decreased with increasing the molecular weight of PLLA block. PVA-b-PLLA exhibited low cytotoxicity which could be used in biomedical application. (C) 2016 Elsevier Ltd. All rights reserved.1142sciescopu
A platform for accelerated continuous-flow radical polymerization of acrylates and styrene with copper-wire threads
No full textEach vinyl monomer of acrylates and styrene requires a ligand, initiator, and solvent set for its controlled radical polymerization because of the difference in the reactivity of the vinyl monomers. In this work, we present a platform for accelerated Cu(0)-mediated polymerization of the vinyl monomers in which a common ligand, initiator, and solvent set is used for all the vinyl monomers in a continuous-flow process with a copper metal-wire catalyst threaded through a transparent perfluoroalkoxy alkane (PFA) tube. The monomer conversion, molecular weight and its distributions (MWDs) were tuned by controlling the contact time between the catalyst and reaction solution, i.e., controlling the rate of comproportionation/disproportionation between Cu(0), Cu(i) and Cu(ii) as a function of the retention time (flow rate) as well as the length/thickness of the Cu(0)-wire. The versatile platform possessing intrinsic microfluidic merits delivered a monomer to polymer conversion up to 60-70% with a well-controlled MWD within a few minutes to an hour for polymerizations that traditionally take tens of hours, even with green and inexpensive reagents. The reliable platform preserves the end group fidelity and livingness of the polymer, which readily allows for serial synthesis of homo- and hetero-chain extensions as pseudoblock copolymers.11Nsciescopu
Sequential synthesis of well-defined poly(vinyl acetate)-block-polystyrene and poly(vinyl alcohol)-block-polystyrene copolymers using difunctional chloroamide-xanthate iniferter
No full textThe sequential synthesis of well-defined poly(vinyl acetate)-block-polystyrene copolymer (PVAc-b-PS) and poly(vinyl alcohol)-block-polystyrene copolymer (PVA-b-PS) is challenging due to inactive vinyl acetate monomer and active styrene monomer. In this study, sequential polymerizations of vinyl acetate and styrene monomer were carried out by reversible addition-fragmentation chain transfer (RAFT) and atom transfer radical polymerization (ATRP) using an innovative amide based difunctional chloroamide-xanthate iniferter [S-(1-((3-(2-chloropropanamido)propyl) amino)-1-oxopropan-2-yl) O-ethyl xanthate]. Synthesized difunctional iniferter was very stable under basic condition and allowed the sequential reactions without chain-end modification and purification steps. Synthesized iniferter was first used for xanthate-mediated RAFT polymerization of VAc, followed by ATRP of styrene to achieve controlled molecular weight of PVAc-b-PS with lower polydispersity (≤1.3). Then, PVAc-b-PS was successfully converted to the corresponding PVA-b-PS by hydrolysis. Synthesized block copolymers were characterized by size exclusion chromatography, nuclear magnetic resonance spectroscopy, transmission electron microscopy, and dynamic light scattering.111sciescopu
Fabrication of Sub-3 nm Feature Size Based on Block Copolymer Self-Assembly for Next-Generation Nanolithography
No full textFor Ultrahigh-density storage media and D-RAM, the feature size of lithography should be much reduced (say less than 10 nm): Though some-research groups reported feature site of 5-6 rim, further reduced feature size is needed. for next generation lithography. We synthesized, via a reversible addition-fragmentation chain-transfer polymerization, polydihydroxystyrene-block-polystyrene (PDHS-b-PS) copolymers showing lamellar and cylindrical microdomains by adjusting the volume fraction of PS block-Ups). We found that the Flory-Huggins interaction parameter (chi) between PDHS and PS was very large, 0.7 at 170 degrees C. Because of-the hugex chi, the lamellar domain spacing (L) of PDHS-b-PS with a total molecular weight of 2.1 kg mol(-1) and f(PS) = 0.5 was only 5.9, nm thus, a sub-3 nm feature size (half-pitch) was successfully obtained, Furthermore, PDHS-b-PS with a molecular weight of 4.2 kg mol(-1) and f(PS) = 0.79 showed hexagonally packed cylinders with 4 nm diameter. We also obtained thin films of PDHS-b-PS with cylindrical microdomains, showing 8.8 nm center-to-center spacing, Furthermore, we fabricated ultrahigh-density ZrO2 nanowire arrays from the cylindrical monolayer thin films via atomic layer deposition, indicating an applicability of PDHS-b-PS for next-generation lithography.1120Nsciescopu
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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