Immigration to Brazil is the movement to Brazil of foreign peoples to reside permanently. It should not be confused with the colonisation of the country by the Portuguese, or with the forcible bringing of people from Africa as slaves. Throughout its history, Brazil has always been a recipient of immigrants, but this began to gain importance in the late 19th century and throughout the 20th century when the country received massive immigration from Europe , the Middle East , and Japan , which left lasting marks on demography, culture, language and the economy of Brazil. In general, it is considered that people who entered Brazil up to , the year of independence, were colonizers. Since then, those who entered the independent nation were immigrants.
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These metrics are regularly updated to reflect usage leading up to the last few days. Citations are the number of other articles citing this article, calculated by Crossref and updated daily.
Find more information about Crossref citation counts. The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric. Find more information on the Altmetric Attention Score and how the score is calculated. In this Perspective, we discuss the recent development of polymerization-induced self-assembly mediated by reversible addition—fragmentation chain transfer RAFT aqueous dispersion polymerization.
This approach has quickly become a powerful and versatile technique for the synthesis of a wide range of bespoke organic diblock copolymer nano-objects of controllable size, morphology, and surface functionality. Given its potential scalability, such environmentally-friendly formulations are expected to offer many potential applications, such as novel Pickering emulsifiers, efficient microencapsulation vehicles, and sterilizable thermo-responsive hydrogels for the cost-effective long-term storage of mammalian cells.
Scheme a A water-soluble stabilizer block is chain-extended using a water-miscible monomer via RAFT polymerization. Initially, a soluble diblock copolymer is obtained, but at some critical degree of polymerization the growing second block becomes water-insoluble, which causes in situ self-assembly.
In this case only a spherical morphology is depicted, but other morphologies are also possible see later. Scheme a Such monomers form a relatively small subset of building blocks that fulfill the essential requirements for an aqueous dispersion polymerization formulation.
Figure 1. Adapted with permission from ref Figure 2. RAFT aqueous dispersion polymerization of 2-hydroxypropyl methacrylate using either a poly glycerol monomethacrylate , poly[2- methacryloyloxy ethyl phosphorylcholine], or poly ethylene glycol macromolecular chain transfer agent to produce spheres, worms, or vesicles by judicious variation of the packing parameter, P , which is determined by the relative volume fractions of the stabilizer and core-forming blocks. Figure 3. Adapted with permission from refs 43 , 52 , and Figure 4.
According to TEM studies, the five morphological regimes are as follows: molecularly dispersed copolymer chains M , spherical micelles S , worms W , branched worms BW , jellyfish J , and vesicles V. The inset shows a semilogarithmic plot for a subset of these data, which confirms the five-fold nucleation-induced rate enhancement observed after micellar aggregation.
Figure 5. Figure 6. These SAXS plots overlay almost perfectly, indicating excellent reversibility for this thermal transition. Clearly, substantial bacterial growth has occurred in the unfiltered copolymer gel.
In contrast, no bacterial growth is observed for the ultrafiltered copolymer gel right-hand image , indicating complete removal of S. Figure 7.
Figure 8. Figure 9. Figure The striking similarities between these structures suggest that such jellyfish are generic intermediates, rather than being merely an esoteric feature of the PISA process.
Very recently, Monteiro and co-workers reported the preparation of diblock copolymer worms and rods decorated with a wide range of functional groups via the PNIPAM stabilizer chains using alkyne—azide chemistry:. However, close reading of the experimental protocol used in this work indicates that this formulation is actually best described as a surfactant-stabilized RAFT emulsion polymerization with styrene as the water-immiscible monomer and sodium dodecyl sulfate utilized as the anionic surfactant rather than the RAFT aqueous dispersion polymerization claimed by the authors.
Van Hest and co-workers have also recently explored the use of vesicles as novel Pickering stabilizers of water-in-oil emulsions, although in this case the vesicles were prepared via a traditional post-polymerization processing technique:. View Author Information. Cite this: J. Article Views Altmetric -.
Citations PDF 9 MB. Abstract High Resolution Image. The seminal discovery of living polymerizations by Szwarc and co-workers in provided the synthetic means to prepare well-defined block copolymers. For example, self-assembly of ABA triblock copolymers in the solid state is the basis for thermoplastic elastomers synthetic rubber , 5, 6 while block copolymers based on alkylene oxides are widely used in various commercial formulations as surfactants, dispersants, gelators, and stabilizers.
In principle, amphiphilic diblock copolymers can form a wide range of particle morphologies, , but in practice this is usually achieved via post-polymerization processing in dilute aqueous solution, often with the aid of a water-miscible co-solvent or a pH switch.
Notwithstanding the important advances described above, the efficient synthesis of bespoke block copolymer nanoparticles with well-defined morphologies in concentrated aqueous solution is widely recognized to be a formidable technical challenge.
Recently, Charleux and co-workers have made considerable progress toward this important scientific objective utilizing various emulsion polymerization formulations. Aqueous Dispersion Polymerization. In contrast to aqueous emulsion polymerization, this Perspective is focused on a versatile alternative approach to aqueous emulsion polymerization known as aqueous dispersion polymerization.
An important prerequisite for such formulations is the selection of a water-miscible monomer which, when polymerized, forms a water-insoluble polymer. Normally, this would simply lead to macroscopic precipitation, but stable colloidal dispersions can be obtained if an appropriate colloid stability mechanism prevails. RAFT polymerization is based on rapid reversible chain transfer between polymer radicals and organosulfur-based chain-transfer agents CTAs , such as dithiobenzoates, trithiocarbonates, or xanthates.
Scheme 1. High Resolution Image. In this regard, there is some evidence that dithiobenzoates are more susceptible to in situ hydrolysis than trithiocarbonates. In each case the corresponding homopolymer has relatively weak hydrophobic character, which means that the resulting diblock copolymer nanoparticles exhibit varying degrees of thermo-sensitivity.
This stimulus-responsive behavior is not exhibited by more hydrophobic polymers such as polystyrene or poly methyl methacrylate. Scheme 2. With the exception of HPMA, only spherical nanoparticles have been obtained when using the monomers depicted in Scheme 2 to generate the core-forming block in RAFT aqueous dispersion polymerization formulations. In the same year, Charleux and co-workers 39b described the synthesis of similar nanogels with the core-forming block based on DEAA rather than NIPAM using nitroxide-mediated polymerization.
More recently, An and co-workers have reported the synthesis of further examples of thermo-sensitive nanogels using RAFT aqueous dispersion polymerization. Dynamic light scattering DLS studies indicated relatively narrow size distributions see Figure 1 b and mean hydrodynamic diameters ranging from 40 to 60 nm.
In a second study, the stabilizer block comprised either linear poly ethylene glycol or poly oligo ethylene glycol methyl ether methacrylate , while the core-forming block was a statistical copolymer of oligo ethylene glycol methyl ether methacrylate, di ethylene glycol methyl ether methacrylate , and a small amount of poly ethylene glycol dimethacrylate.
Moreover, high cell viabilities were obtained when A lung cells were exposed to these nanogels for 48 h, suggesting good biocompatibilities. According to DLS studies, the dimensions of such nanogels decrease almost linearly with increasing solution temperature, which is in marked contrast to the sharp thermal transitions exhibited by other thermo-responsive polymers, such as poly N -isopropylacrylamide. In a related FT-IR spectroscopy study conducted by the same research group, these differing volume phase transitions have been interpreted in terms of subtle differences in hydrogen bonding between the core-forming blocks and the surrounding water molecules.
This approach is currently the only protocol that provides access to non-spherical morphologies such as worms or vesicles see TEM images in Figure 3. This in situ self-assembly was confirmed by DLS analysis and TEM studies, which indicated the formation of approximately spherical nanoparticles of around 20—30 nm diameter see Figure 4 b.
As the monomer-swollen copolymer micelles grow, they undergo 1D fusion to form worms. In view of a recent theoretical study of the formation of long chains by spherical nanoparticles, such worm evolution appears to be an entropy-driven process.
This change in copolymer morphology from spheres to worms to vesicles can be rationalized in terms of an increase in the packing parameter, P , which is given by the equation For an amphiphilic AB diblock copolymer such as PGMA-PHPMA, v and l are the volume and the length of the hydrophobic block, respectively, and a is the effective interfacial area of the block junction.
Thus P also necessarily increases during the synthesis, which accounts for the progressive evolution in copolymer morphology from spheres to worms to vesicles.
However, this is merely a qualitative argument: calculation of P for diblock copolymers is non-trivial, particularly when both 1 H NMR studies 51 and small-angle X-ray scattering SAXS analysis 52 indicate that the core-forming or membrane-forming block is partially solvated with water molecules in addition to unreacted monomer for intermediate conversions.
Moreover, the mean diameter of these spherical nanoparticles increases monotonically as the DP of the core-forming block is increased.
To access higher order copolymer morphologies, longer core-forming blocks must be targeted at relatively high copolymer concentrations. This approach enables pure vesicular and worm phases to be generated. The former particles occupy a relatively broad phase region, whereas the latter occupy a relatively narrow phase region. Moreover, the worm phase is bounded by mixed phases i. Thus, reliable targeting of the worm phase usually becomes feasible only after construction of a full phase diagram.
At intermediate copolymer concentrations and core-forming block DPs, there is also a very narrow complex phase in which all three kinetically trapped copolymer morphologies coexist. However, the copolymer prepared at lower concentration forms spheres, whereas that formed at higher concentration forms vesicles. Clearly, only one of these two copolymers can be in its thermodynamically preferred equilibrium morphology. Based on the diblock asymmetry, the preferred morphology must be vesicles.
The initial event in the evolution of the copolymer morphology from spheres is the fusion of two spheres to form a spherical dimer. This is the critical first stage in the formation of worms, which eventually transform into vesicles via the sequence of events described above. On lowering the mean DP of the stabilizer block from 78 to 47, the steric barrier to micelle fusion is significantly reduced. This leads to a strikingly different phase diagram for a series of G 47 -H x diblock copolymers see Figure 5 b.
In this case, there is essentially zero concentration dependence for the final copolymer morphology, which is now dictated solely by the target DP of the core-forming block. Thus, although we currently have no quantitative understanding of the packing parameter P for these formulations, it is possible to qualitatively explain many experimental observations. However, it remains to be seen whether the phase diagram shown for G 47 actually represents the thermodynamically preferred equilibrium states of the various copolymer chains.
More Cost-Effective Formulations. Glycerol monomethacrylate GMA is a commercially available specialty monomer that is used in the manufacture of soft contact lenses. It is prepared on an industrial scale from glycerol via protecting group chemistry using acetone to mask two of the three hydroxy groups. Thus it is worth considering alternative synthetic routes to GMA. No background polymerization was detected by 1 H NMR spectroscopy when this reaction was conducted in the presence of dissolved oxygen, which acts as an inhibitor.
Perhaps more surprisingly, no evidence for methacrylic ester hydrolysis was observed under these conditions. A one-pot formulation was also demonstrated for the overall process, although blocking efficiencies were somewhat lower than those observed for PGMA macro-CTAs isolated at intermediate conversions.
Thermo-responsive Diblock Copolymer Worm Gels. To what extent hydrogen-bonding interactions may be important in this context has not yet been explored.
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Immigration to Brazil
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A controversy exists as to whether de novo-generated neuronal tetraploid cells Mosch et al. Tat: a polypeptide for ; Masuzawa M. Archived from the original on 27 January Permanent visas may be granted to individuals intending to establish residence in Brazil. The sources dw the material used were as follows: The participation of the Japanese increased. Although the ER- and mitochondria-associated apoptotic pathways can be activated independently, it has been suggested that induction of ER stress may lead to the triggering of intrinsic apoptotic processes [ ]. Cleavage of this SRL results in ribosome inactivation leading to protein biosynthesis inhibition and cell death by apoptosis Gasset et al.