Top PDF Self-Assembly of Brush Polymers

Self-Assembly of Brush Polymers

Self-Assembly of Brush Polymers

The fact that no special steps were taken to modify the substrate in order to control interfacial interactions, 34–37 and the orientation of the lamellar microdomains normal to the substrate, even with solvent annealing, was surprising, because PLA has strongly preferential interactions with the oxide layer on the silicon substrate, and the orientation found requires that the PS block be in contact with the substrate. Consequently, the orientation of the lamellar microdomains normal to the substrate may arise from the screening of the interactions of the blocks with the substrate, coupled with more favored parallel alignment of the sterically hindered, rigid blocks at the polymer/substrate interface, which again is an entropic-type of preferred chain orientation. Unlike linear BCPs, the many chain ends of the side chains attached to the backbone would preferentially segregate to the surface and substrate interfaces. This orientation allows the microphase-separated brush BCPs to have more conformational degrees of freedom, in comparison to the case where microdomains of brush BCPs are oriented parallel to the substrate.
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Synthesis and Self Assembly of Bottlebrush Block Polymers: Molecular Architecture and Materials Design

Synthesis and Self Assembly of Bottlebrush Block Polymers: Molecular Architecture and Materials Design

The work presented in this thesis has benefited from the contributions of many other members of the Caltech community. Dr. Hsiang-Yun Chen and Niklas Thompson have made key contributions to our studies of ROMP kinetics: Hsiang-Yun wrote the code to fit our copolymerization data and calculate reactivity ratios (Chapter 2-4), and Nik performed density functional theory (DFT) calculations to provide insight into the ROMP mechanism (Chapter 2-9). I would like to thank Prof. Jonas Peters for providing access to computational resources. I would also like to thank Prof. Julie Kornfield and her student, Joey Kim, for access to their rheometer and for valuable discussions. In addition, I would like to thank Prof. Zhen-Gang Wang and members of his group, including Prof. Liquan Wang, Dr. Pengfei Zhang, and Dr. Jian Jiang, for their patience in discussing self-consistent field theory (SCFT) and polymer phase behavior with me. Liquan, with input from Pengfei and JJ, built SCFT up from scratch in his explanations, and our conversations helped me gain a deeper appreciation for the theory.
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Exploring the Chemistry of η5-Cyclopentadienyl-Cobalt-η4-Cyclobutadiene Containing Polymers; Synthesis, Properties, and Self-Assembly

Exploring the Chemistry of η5-Cyclopentadienyl-Cobalt-η4-Cyclobutadiene Containing Polymers; Synthesis, Properties, and Self-Assembly

Mixed sandwich cobaltocene featuring η 5 -cyclopentadienyl-cobalt-η 4 -cyclobutadiene (CpCoCb) is a neutral, 18 electron species, isoelectronic with ferrocene and cobaltocenium with physical and chemical properties more closely aligned with ferrocene, such as excellent solubility in a wide range of common organic solvents. 70,71 CpCoCb also has the important advantage of the facile preparation of a wide variety of derivatives using well-established cyclodimerization chemistry from substituted alkynes, to install a functionalized cyclobutadiene (Cb) ring, which opens doors for novel, onwards chemistry, or in imposing additional chemical functionality to the sandwich complex. 72,73 The Ragogna Group has previously reported the first metallopolymer derived from polymerization of such a mixed sandwich CpCoCb containing monomer (1.10; Figure 2.1). In this study, the polymerization reaction time was lengthy (days) and despite up to 90% monomer conversion, only low molecular weight polymers were produced. 74 These results have motivated us to optimize the polymerization conditions by utilizing controlled radical polymerization methods for the synthesis of well-defined homo- and block copolymers. The ultimate goal was to take advantage of the neutral, organic soluble materials to access self-assembled architectures and cobalt containing nanomaterials. In this context, we have utilized reversible addition fragmentation transfer (RAFT) polymerization to prepare a new class of homo- and block copolymers, containing CpCoCb repeat unit. Through extensive studies on various RAFT conditions
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Expanding the scope of the crystallization driven self assembly of polylactide containing polymers

Expanding the scope of the crystallization driven self assembly of polylactide containing polymers

We report the crystallization-driven self-assembly of diblock copolymers bearing a poly( L -lactide) block into cylindrical micelles. Three di ff erent hydrophilic corona-forming blocks have been employed: poly(4- acryloyl morpholine) (P4AM), poly(ethylene oxide) (PEO) and poly( N , N -dimethylacrylamide) (PDMA). Optimization of the experimental conditions to improve the dispersities of the resultant cylinders through variation of the solvent ratio, the polymer concentration, and the addition speed of the selective solvent is reported. The last parameter has been shown to play a crucial role in the homogeneity of the initial solution, which leads to a pure cylindrical phase with a narrow distribution of length. The hydrophilic characters of the polymers have been shown to direct the length of the resultant cylinders, with the most hydrophilic corona block leading to the shortest cylinders.
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Modulation of the cooperativity in the assembly of multistranded supramolecular polymers

Modulation of the cooperativity in the assembly of multistranded supramolecular polymers

Many molecules assemble into long 1D structures by means of weak intermolecular interactions, leading to supramolecular polymers. 1-3 On account of the weak intermolecular interactions that hold the building blocks together, these polymers are dynamic structures , i.e., capable of self-organise into large scale assemblies in response to specific stimuli. 4,5 This process plays a key role in the function of biological systems. Examples are cell division and motility which are regulated by dynamic assembly of microtubules and actin filaments, respectively. 6-8 Some small synthetic molecules also
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Polymerization induced thermal self assembly (PITSA)

Polymerization induced thermal self assembly (PITSA)

Polymerization-induced self-assembly (PISA) is a versatile technique to achieve a wide range of polymeric nanoparticle morphologies. Most previous examples of self-assembled soft nanoparticle synthesis by PISA rely on a growing solvophobic polymer block that leads to changes in nanoparticle architecture during polymerization in a selective solvent. However, synthesis of block copolymers with a growing stimuli- responsive block to form various nanoparticle shapes has yet to be reported. This new concept using thermoresponsive polymers is termed polymerization-induced thermal self-assembly (PITSA). A reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide from a hydrophilic chain transfer agent composed of N,N-dimethylacrylamide and acrylic acid was carried out in water above the known lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAm). After reaching a certain chain length, the growing PNIPAm self-assembled, as induced by the LCST, into block copolymer aggregates within which dispersion polymerization continued. To characterize the nanoparticles at ambient temperatures without their dissolution, the particles were crosslinked immediately following polymerization at elevated temperatures via the reaction of the acid groups with a diamine in the presence of a carbodiimide. Size exclusion chromatography was used to evaluate the unimer molecular weight distributions and reaction kinetics. Dynamic light scattering and transmission electron microscopy provided insight into the size and morphologies of the nanoparticles. The resulting block copolymers formed polymeric nanoparticles with a range of morphologies (e.g., micelles, worms, and vesicles), which were a function of the PNIPAm block length.
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Self-assembly of metal–organic polyhedra into supramolecular polymers with intrinsic microporosity

Self-assembly of metal–organic polyhedra into supramolecular polymers with intrinsic microporosity

Porosity of the supramolecular polymers. While many crystal- line MOFs are sufficiently rigid to avoid structural collapse after guest removal or activation and have been extensively studied in terms of porous properties 6 , the porosity of amorphous coordi- nation polymers is far less studied, perhaps due to their inability to withstand desolvation or a lack of structural data associated with the resulting amorphous phases. Although there are reports describing the synthesis of soft materials based on MOPs, con- firmation of permanent porosity by gas adsorption experiments has never been demonstrated 42 – 45 . To prepare the supramole- cular polymers for gas adsorption measurements, the materials were treated with supercritical CO 2 , and finally activated by
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The synthesis, self assembly and analysis of amphiphilic polymers : developing microscopy techniques using graphene oxide and building catalytic palladium nanostructures

The synthesis, self assembly and analysis of amphiphilic polymers : developing microscopy techniques using graphene oxide and building catalytic palladium nanostructures

40 nm in diameter. As the polymers 3.04-3.06 have the same hydrophobic end group as the polymers 2.09 and 2.10, the core sizes should be the same. The cryo-TEM and SANS analysis in Chapter 2 indicated the core of these SCS pincer structures should be on the order of a few nm, which means there must be significant scattering from the PNIPAM chains and that the corona is visible in addition to the core. While polymeric coronas often are not visible in cryo-TEM due to their hydrated nature, the direct visualisation of micelle coronas (including PNIPAM) has been reported previously. 23-27 Being able to visualise the NIPAM corona makes determining an accurate size difficult as the particles do not have a well-defined edge. The polymer chain density should decrease from the core of the micelle to the ends of the polymer chains. As this density decreases the contrast against the vitrified water layer will be reduced making it difficult to define the particle edge.
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Solution-Phase Assembly of Nanoparticles and Amphiphilic Polymers: Controlling the Morphology From Vesicles to Micelles

Solution-Phase Assembly of Nanoparticles and Amphiphilic Polymers: Controlling the Morphology From Vesicles to Micelles

a high dielectric medium. Thus, the degree of PAA chain stretching is the largest in dioxane. 48 Consequently, the relative volume taken up by PAA becomes the largest in DMF and the smallest in dioxane (DMF>THF>dioxane), which explains the formation of micelles in DMF and THF and vesicles in dioxane without nanoparticles. 48 Thus, when nanoparticles are passively incorporated, it is expected to form magneto-micelles in THF and magneto-polymersomes in dioxane. The TEM and DLS data presented in Figure 2.2 show that the expected structures were indeed formed in THF and dioxane/THF (96.8% dioxane). However, it is important to note that magneto-polymersomes were not the major product of the dioxane sample. The polymersome peak of the DLS data presented in Figure 2.2 appears to be substantial only because bigger polymersomes scatter light more strongly than smaller micelles. When the DLS data shown in Figure 2.2 was converted into the number distribution, the vesicle population was rather small. Also note that unique radial assemblies were formed instead of typical micelles when DMF/THF (96.8% DMF) was used as the initial solvent. These results underscore that it is important to consider the effect of nanoparticles on the self-assembly formation in order to obtain the hybrid particle with the desired structure and properties.
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Development and applications of polyglyoxylate self-immolative polymers

Development and applications of polyglyoxylate self-immolative polymers

Amphiphilic block copolymers can self-assemble in aqueous solution to form a wide range of morphologies including spherical micelles, cylindrical micelles, and vesicles. 1-3 Such assemblies have been of significant interest in recent years for the encapsulation and controlled release of drugs. 4-6 These delivery vehicles can enhance the water-dispersibility of hydrophobic drugs and selectively target them to sites of action such as tumors via the enhanced permeation and retention effect or using active targeting groups. 7-9 Ideally, the assembly would be stable in the blood stream, but selectively release its payload at the target site. To achieve this, polymer assemblies responsive to the conditions associated with various disease states have been developed. For example, systems responsive to the acidic pH encountered in tumor tissue or within the endosomal compartments of cells have been introduced. 10-13 Reducing conditions, associated with the intracellular environment and hypoxic tumor tissue, have also been used to disrupt polymer assemblies. 14-16
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Algorithmic self-assembly of DNA

Algorithmic self-assembly of DNA

Since Adleman’s original paper, every proposal for DNA-based computation has made use of the sequence-specific hybridization of Watson-Crick complementary oligonucleotides. Most applica- tions have been very straightforward, and the the most sophisticated use of this self-assembly is still Adleman’s original technique for creating duplex DNA representing paths through a graph. However, much more elaborate DNA constructs are possible, as epitomized by Seeman’s exten- sive experimental research in DNA nanoconstructions: in addition to duplex DNA, hairpins, n -arm junctions, and double-crossover molecules are all possible. Using this expanded vocabulary, what computations can be done with self-assembly alone? To answer this question, I use the frame- work of formal language theory to develop a model of DNA self-assembly in which such ques- tions can be rigorously answered. The surprising result is that in the two-dimensional case the self-assembly model is Turing-universal, and that natural restrictions of the model reproduce the Chomsky Hierarchy of language families. These restrictions relate to the types of DNA building- blocks used, and the form of their arrangement into larger structures: the self-assembly of linear duplex DNA into linear polymers produces regular languages; the self-assembly of duplexes, hair- pins and 3-arm junctions into dendrimers produces context-free languages; and the self-assembly of double-crossover molecules into two-dimensional lattices achieves Turing-universality, produc- ing recursively enumerable languages.
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Multivalence cooperativity leading to “all or nothing” assembly: the case of nucleation growth in supramolecular polymers

Multivalence cooperativity leading to “all or nothing” assembly: the case of nucleation growth in supramolecular polymers

All-or-nothing molecular assembly events, essential for the e ffi cient regulation of living systems at the molecular level, are emerging properties of complex chemical systems that are largely attributed to the cooperativity of weak interactions. The link between the self-assembly and the interactions responsible for the assembly is however often poorly de fi ned. In this work we demonstrate how the chelate e ff ect (multivalence cooperativity) can play a central role in the regulation of the all-or-nothing assembly of structures (supramolecular polymers here), even if the building blocks are not multivalent. We have studied the formation of double-stranded supramolecular polymers formed from Co-metalloporphyrin and bi-pyridine building blocks. Their cooperative nucleation – elongation assembly can be summarized as a thermodynamic cycle, where the monomer weakly oligomerizes linearly or weakly dimerizes laterally. But thanks to the chelate e ff ect, the lateral dimer readily oligomerizes linearly and the oligomer readily dimerizes laterally, leading to long double stranded polymers. A model based on this simple thermodynamic cycle can be applied to the assembly of polymers with any number of strands, and allows for the determination of the length of the polymer and the all-or-nothing switching concentration from the pairwise binding constants. The model, which is consistent with the behaviour of supramolecular polymers such as microtubules and gelators, clearly shows that all-or-nothing assembly is triggered by a change in the mode of assembly, from non-multivalent to multivalent, when a critical concentration is reached. We believe this model is applicable to many molecular assembly processes, ranging from the formation of cell – cell focal adhesion points to crystallization.
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Degradation Kinetics and Functional Design of Linear Self-Immolative Polymers

Degradation Kinetics and Functional Design of Linear Self-Immolative Polymers

The functional linear self-immolative block copolymer designs presented in this thesis demonstrate the inherent flexibility of such materials. With much of the synthetic groundwork laid in the design of linear self-immolative polymer backbones, a shift in focus needs to be geared towards the application of these materials through controlled design. The two systems explored in this work are promising examples of the ease at which linear self-immolative polymers can be incorporated into amphiphilic block copolymers capable of self-assembly in aqueous solution. Future work in the development of these materials should also focus on the incorporation of different stimuli-responsive linkages to initiate block separation and self-immolative backbone degradation under a variety of conditions for programmable release. The ideal design for these systems would involve an entirely self-immolative block copolymer system in which triggering groups are contained at the terminus of the hydrophilic block, directly exposed to the external environment. To this end, additional work should be conducted into developing hydrophilic self-immolative homopolymers by modifying the currently available backbones with pendent solubilizing groups such as triethylene glycol. Upon further optimization of the self-immolative block copolymer design, it is presumable these materials will receive widespread application in the field of drug delivery due their ability to form functional nanoparticles with a highly controlled mechanism of
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Polyglyoxylates: a new class of triggerable self-immolative polymers

Polyglyoxylates: a new class of triggerable self-immolative polymers

previously reported method 18 and incorporated end-caps including perylen-3-yl methanol, 2-nitrobenzyl alcohol, and diethanol disulfide which are visible light-, UV-light-, and reduction-responsive end-caps respectively (Fig. 1.14) or a noncleavable benzyl alcohol end-cap. 28 The remaining alcohol termini were then functionalized with a reversible addition-fragmentation chain transfer (RAFT) agent. RAFT polymerization was then used to grow hydrophilic poly(N,N-dimethylacrylamide) (PDMA) block, resulting in block copolymers with hydrophilic fractions on the order of 60-70 wt%. Self-assembly of the polymers in aqueous solution was then investigated. The assemblies were studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM), confocal laser scanning microscopy, and dynamic light scattering. It was found that these block copolymers formed vesicles with diameters ranging from 200 to 580 nm. This behavior was despite their relatively high hydrophilic fraction in comparison to the expected volume fraction of ~ 25-45 wt% for vesicle-forming block copolymers. 29 This was attributed to the strong hydrogen-bonding interactions between the carbamate groups. Upon end-cap cleavage with light or reducing conditions, the vesicles were shown by microscopy, NMR spectroscopy, and size exclusion chromatography (SEC) to
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Three-Dimensional Self-Assembly of Brush Block Copolymers to Photonic Crystals

Three-Dimensional Self-Assembly of Brush Block Copolymers to Photonic Crystals

It appears that there is some order in this sample with some portions showing circles, but it is much less conclusive than some of the samples shown earlier. Further testing must be done with other annealing conditions to produce better ordered samples. Solvent evaporation was briefly tested, producing mediocre results to the naked eye, but it may be worth retesting. Another possibility is longer heating times to allow the polymers more time to reach their self- assembled positions. After heating, it is also possible to expose the sample to vapor for an extended period of time. This would increase chain mobility, giving them more opportunity to
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Self assembly of cyclic polymers

Self assembly of cyclic polymers

As a consequence of improved synthetic methods, enabling the preparation of well-defined, high purity cyclic polymers, the self-assembly of amphiphilic cyclic polymers has received increasing attention in recent years. By comparing the aggrega- tion of cyclic polymers with equivalent linear polymers, we can determine the effect of cyclization on self-assembly and increase our understanding of structure – property relation- ships. Indeed, the examples discussed in this review have high- lighted the profound effect cyclization can have on particle dimensions, stability, and morphology, as well as the packing of polymer chains within the assembly. In general, cyclic diblock copolymers form smaller, entropically disfavoured aggregates in comparison to linear diblock copolymers, as a consequence of the confined and looped nature of cyclic poly- mers, resulting in a greater number of unfavourable core- solvent junctions. The self-assembly of cyclic diblock copoly- mers is however similar to that of linear triblock copolymers, which are also required to loop upon aggregation. For the self- assembly of more complex cyclic topologies, assemblies of cyclic polymers are often larger than the equivalent linear polymer assembly as a consequence of poor polymer packing. Furthermore, the cyclization of amphiphilic polymers can lead to unique self-assembly behaviour that cannot be achieved through the self-assembly of linear polymers or can impart improved properties to the resulting nanostructures, for example, greater thermal stability and robustness towards salt additives.
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Effects of Branching on Conformation, Crystallization, and Self Assembly of Polymers

Effects of Branching on Conformation, Crystallization, and Self Assembly of Polymers

Bottlebrush polymers are unique class of polymers consisting of many polymeric units hierarchically arranged into side-chains densely grafted to a main-chain. Neighboring side-chains (SC) repel each other, stretching the backbone along its contour. The degree of SC-SC repulsion can be controlled by architectural parameters, mainly graft spacing and SC length, in addition to the inherent chemical properties of the constituent polymers, e.g., chain flexibility and polymer functional groups. The array of tunable parameters allows bottlebrushes to expand the envelope of accessible properties. Recently, bottlebrushes have been used in applications for super-soft elastomers 3,4 , drug delivery 5 , self-assembling materials 6–8 , protective layers 9 , photonics 10 , lubricants 11 , emulsifiers 12 , and energy storage 13 . In addition, the regular spacing of side-chains in bottlebrushes provides a model architecture for studying the effects of branching relevant to biological functions rooted in branched architecture, e.g., mucin 14 , in which dense branching leads to formation of a regulatory layer, and aggrecans 15 ; in which dense branching is essential to joint lubrication.
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Synthesis and self assembly of nucleobase containing polymers by RAFT techniques

Synthesis and self assembly of nucleobase containing polymers by RAFT techniques

The specific hydrogen bonding interactions between nucleobase pairs play a key role in nature for precise biosynthesis and stereospecific molecular assembly. Inspired by nature, nucleobases have been employed in synthetic polymer chemistry to control polymer tacticity, 1 to mediate polymer composition or sequence 2-4 and to template polymerizations. 5-7 Moreover, nucleobase interactions have also been applied to drive self-assembly 8-12 and to achieve a biomimetic segregation/templating approach to polymerizations. 13 This pioneering work has expanded the window for further investigation into nucleobase materials. 14 However, to our knowledge, although various morphologies, including large vesicles, 8 rods 9 and spheres, 13 have been obtained, there is still very little research into the systematic study of the self- assembly of nucleobase-containing polymers. This might result from the poor solubility of nucleobase-containing copolymers, some of which which are only fully soluble in polar organic solvents. Therefore, nucleobase-containing polymers with a relatively high degree of polymerization (DP) have to be synthesized in polar solvents (e.g., DMSO, DMF) to avoid precipitation or to achieve good control; self- assembly is then achieved by post-polymerization processing. These multiple steps limit the synthesis, self-assembly, and other associated studies of nucleobase- containing polymers. Thus, a facile approach to make well-controlled nucleobase- containing polymers and to prepare their corresponding self-assemblies is worthy of investigation.
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Self-Assembly and Stimuli-Responsive Properties of Amphiphilic Self-Immolative Block Co-Polymers

Self-Assembly and Stimuli-Responsive Properties of Amphiphilic Self-Immolative Block Co-Polymers

Scheme 11: Bovine serum albumin (BSA) selectively cleaves the 4-hydroxybutanone end-cap. An enzyme is labelled with the azaquinone methide. ........................................ 16 Scheme 12: The general synthesis and depolymerization of a polyacetal. ....................... 17 Scheme 13: Preparation of three PPHA SIPs. a) responds to Pd(0) metal catalysts, b) is a control polymer, and c) responds to F - anions. ................................................................. 18 Scheme 14: Random copolymerization with functionalized benzaldehydes gives these PPHAs post-polymerization characteristics aside from depolymerization. ...................... 19 Scheme 15: Depolymerization following end-cap cleavage in polyglyoxylates, resulting in the production of glyoxyl hydrate. ................................................................................ 20 Scheme 16: The NMR-monitored depolymerization of NVOC-end-capped PEtG.......... 21 Scheme 17: Upon removal of the end-cap, this alternating 1,6-elimination cyclization SIP depolymerizes completely into small molecules. ............................................................. 22 Scheme 18: Linear SIP depolymerizations following end-cap removal and their relative degradation kinetics. a) diamine cyclization spacer, b) N-methylaminoethanol cyclization spacer, and c) 2-mercaptoethanol cyclization spacer. ....................................................... 23 Scheme 19: a) representation of liposomes (vesicles) and micelles where the yellow lines represent the hydrophobic tail and the white spheres are the hydrophilic head, b) from left to right: TEM images of vesicles, cylindrical micelles, and spherical micelles. (adapted with permission from reference 74 Copyright 2006 Wiley Online Library.) ................... 25 Scheme 20: The synthesis and self-assembly of PDMA-b-PBC block co-polymers. ...... 27 Scheme 21: The shape regulation of different morphologies after both CO 2 purging
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Magnetic Field-Directed Self-Assembly of Magnetic Nanoparticle Chains in Polymers.

Magnetic Field-Directed Self-Assembly of Magnetic Nanoparticle Chains in Polymers.

possessing typical molecular masses. 4 Some NPs have already shown impressive effects with a small amount of additive. 5 Polymer nanocomposites are incorporated as photovoltaic junctions for semiconductors, light-emitting displays, integrated circuits, and storage devices, and as heterogeneous electrolytes in solid-state lithium batteries. 6-7 Block copolymers provide the added benefit of allowing control over NP spatial distribution, while also fulfilling their own functional attributes. Examples of emergent technologies based on these materials include lithography, 8 membrane technology, 9-10 drug encapsulation, 11-12 catalysis, 13 and optoelectronics. 14 Researchers have likewise been inspired by biological materials to create synthetic materials based on molecular self-assembly for applications in the (bio)nanotechnology fields. 15
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