Synthesis and spectroscopy of CdS nanoparticles in amphiphilic diblock copolymer micelles

Amphiphilic diblock copolymers with the same hydrophilic but different hydrophobic blocks were used as stabilizing agents to prepare cadmium sulfide nanoparticles in aqueous solutions containing 5% of different nonaqueous solvents: methanol, THF, and acetone. Nearly spherical nanoparticles with a fair degree of monodispersity and quantum yields of 1.5%-2% were obtained. Optical absorption band edge of the CdS nanoparticles shows a >0.5 eV blueshift compared to that of bulk CdS, indicating a high degree of quantum confinement. The absorption spectra, while insensitive to the nature of the hydrophobic blocks, exhibited a clear dependence on the nature of the minor, nonaqueous solvents. The photoluminescence in all cases was broad and redshifted, indicating a predominance of surface trap-state emission. Time-resolved photoluminescence demonstrates that the trap states are populated within the first 500 fs, followed by decay with a broad range of time constants from 0.1 to >10 ns, low energy traps decaying at a slower rate than high-energy ones. Time-resolved photoluminescence anisotropy revealed that the nanoparticles experience a local microviscosity very similar to that of bulk water. The experimental observations suggest that nanoparticle formation takes place predominantly in the hydrophilic corona region of the micelles, around specific points with high local concentration of the Cd+2-coordinating basic amine groups of hydrophilic block and/or the minor, nonaqueous solvent component.     

Dynamic Monte Carlo simulation of aggregation of nanoparticles in the presence of diblock copolymer

The aggregation of hydrophobic nanoparticles in the presence of diblock copolymers is investigated using dynamic Monte Carlo simulation on a simple cubic lattice. One nanoparticle occupies one lattice site, one block copolymer (A(m)B(m)) occupies 2m sequentially linked sites with m segments of A and m segments of B, and solvents are represented by any unoccupied sites. All of them are self-avoiding and nearest-neighbor interactions are considered. A compact big aggregate, dispersed aggregates wrapped by polymer chains, and an ordered lamellar structure are obtained by varying the concentration of copolymer. The structures are seen to be controlled by competing forces between the interaction of copolymer with nanoparticles and the self-assembly of copolymer in solution. The critical concentration of copolymer needed to form the lamellar structure, C(p,L), decreases with the chain length. It is also found that C(p,L) decreases roughly linearly with the concentration of nanoparticles C(n), which can be approximately expressed as C(p,L)=0.764-0.857C(n) when m=2. The simulation demonstrates that addition of diblock copolymer can effectively control the aggregation of nanoparticles and lead to the formation of a variety of nanostructures.     

Surface-initiated, ring-opening metathesis polymerization: formation of diblock copolymer brushes and solvent-dependent morphological changes

In this article, we report the formation of diblock copolymer brushes on a gold surface by surface-initiated, ring-opening metathesis polymerization (SI-ROMP) with the newly developed ruthenium catalyst [(H2IMes)(3-Br-py)2(Cl)2Ru=CHPh]. Taking advantage of the highly improved activity of the ruthenium catalyst and the rapid initiation step of ROMP, we successfully formed thin films of well-defined block copolymers with 5-norbornene-2-endo,3-endo-dimethanol and norbornene carboxylic acid methyl esters (44:56 endo/exo). The catalyst was found to be active enough to polymerize endo isomers of norbonene derivatives from the surface as well as to form diblock copolymer brushes. SI-ROMP of diblock copolymers from the surface was confirmed by ellipsometry, infrared spectroscopy, and X-ray photoelectron spectroscopy. After the formation, the polymer-grafted substrates were immersed in various solvents, and the selective swelling characteristics of polymer brushes were investigated by atomic force microscopy.     

Non‐equilibrium phase behavior of diblock copolymer melts and binary blends in the intermediate segregation regime

The phase behavior of intermediately segregated (χN = 45) poly(ethylene)‐poly(ethylethylene) (PE–PEE) diblock copolymers and PE–PEE binary blends are characterized using transmission electron microscopy and small‐angle X‐ray scattering. Surprisingly, the preparation‐dependent, nonequilibrium phase behavior can be overwhelming even at this degree of segregation. A pure diblock with a poly(ethylene) volume fraction of f_PE = 0.46 exhibited coexisting lamellae and perforated layers when prepared using a precipitation technique, but contained only the lamellar morphology when solvent cast. This preparation dependence was more dramatic in binary diblock copolymer blends with average compositions of 〈f_PE〉 = 0.44, 0.46, and 0.48. Precipitated blends exhibited a microphase separated structure that was disordered and bicontinuous; however, solvent cast samples exhibited either a cylindrical, coexisting cylindrical and lamellar, or lamellar morphology. This nonequilibrium behavior is attributed to the high degree of segregation and the proximity to the cylinder/lamellae phase boundary. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2229–2238, 1999     

Photoresponsive soft materials: Synthesis and photophysical studies of a stilbene‐based diblock copolymer

An amphiphilic diblock copolymer composed of a photoresponsive dialkoxycyanostilbene polymethacrylate and poly(ethylene oxide) (PDACS‐b‐PEO) was synthesized and its photophysical and aggregation properties were investigated. The amphiphilic nature of the polymer caused it to self‐assemble in water, and dynamic light scattering studies indicated formation of spherical aggregates with an average size of 160 nm. Atomic force microscopy images of dried films cast from solutions containing the polymer aggregates revealed supramolecular aggregates with a spherical morphology. Photoisomerization of the stilbene chromophore in PDACS‐b‐PEO on UV irradiation resulted in the destruction of the self‐assembled superstructures which could be attributed both to change in shape of the chromophore from the linear trans isomer to the bent cis isomer which would hinder self‐aggregation of the molecules and the higher dipole moment of the cis isomer leading to a reduction of the hydrophobic nature of the stilbene containing block of PDACS‐b‐PEO. It was observed that hydrophobic dyes such as curcumin could be encapsulated within the hydrophobic interior of the spherical micellar aggregates from which the encapsulated dye could be released on UV irradiation. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

pH-responsive diblock copolymer micelles at the silica/aqueous solution interface: Adsorption kinetics and equilibrium studies

The adsorption behavior of two examples of a weakly basic diblock copolymer, poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA), at the silica/aqueous solution interface has been investigated using a quartz crystal microbalance with dissipation monitoring and an optical reflectometer. Dynamic and static light scattering measurements have also been carried out to assess aqueous solution properties of such pH-responsive copolymers. In alkaline solution, core-shell micelles are formed above the critical micelle concentration (cmc) by both copolymers, whereas the chains are molecularly dissolved (as unimers) at all concentrations in acidic solution. As a result, the adsorption behavior of PDMA-PDEA diblock copolymers on silica is strongly dependent on both the copolymer concentration and the solution pH. Below the cmc at pH 9, the cationic PDMA-PDEA copolymers adsorb as unimers and the conformation of the adsorbed polymer is essentially flat. At concentrations just above the cmc, the initial adsorption of copolymer onto the silica is dominated by the unimers due to their faster diffusion compared to the much larger micelles. Rearrangement of the adsorbed unimers and/or their subsequent displacement by micelles from solution is then observed during an equilibration period, and the final adsorbed mass is greater than that observed below the cmc. At concentrations well above the cmc, the much higher proportion of micelles in solution facilitates more effective competition for the surface at all stages of the adsorption process and no replacement of initially adsorbed unimers by micelles is evident. However, the adsorbed layer undergoes gradual rearrangement after initial adsorption. This relaxation is believed to result from a combination of further copolymer adsorption and swelling of the adsorbed layer.     

Controlling the size of magnetic nanoparticles using pluronic block copolymer surfactants

We have successfully controlled the size of magnetic nanoparticles by adjusting the surfactant/solvent ratio. Gamma-Fe(2)O(3) nanoparticles of 5.6 and 12.7, and Fe(0) nanoparticles of 22.3 nm in diameter were prepared, all having spherical shape and uniform size as confirmed by TEM. M?ssbauer spectra confirmed Fe(3+) for the 5.6 and 12.7 nm particles and Fe(3+) and Fe(0) for 22.3 nm particles, in good agreement with synchrotron XRD patterns. Both room temperature and 5 K H-M measurements show that 22.3 nm particles have much higher magnetization than their oxide counterparts, in agreement with their being Fe(0). T-M measurements show superparamagnetism for 5.6 and 12.7 nm particles and ferromagnetism for 22.3 nm particles.     

Phosphorus-containing block copolymer templates can control the size and shape of gold nanostructures

Amphiphilic block copolymers containing phosphine moieties in the main chain are employed as macromolecular ligands for gold(I). The sequential living anionic copolymerization of isoprene (I) and the phosphaalkene, MesP CPh2 (Mes = 2,4,6-trimethylphenyl) affords the block copolymer [PI]404-b-[MesP-CPh2]32 (1a). The incorporation of gold(I) moieties into this functional copolymer is accomplished by treating 1 with THT.AuCl (THT = tetrahydrothiophene) which affords [PI]404-b-[MesP(AuCl)-CPh2]32 (2a). Remarkably, dissolution of gold-functionalized 2 in n-heptane, a block-selective solvent for isoprene, affords spherical micelles with gold(I)-rich cores. Micelles were examined by transmission electron microscopy (TEM) and dynamic light scattering (DLS). We also prepared two additional copolymers with longer phosphine blocks and shorter PI segments: [PI]222-b-[MesP(AuCl)-CPh2]77 (2b) and [PI]164-b-[MesP(AuCl)-CPh2]85 (2c). When assembled in isoprene-selective solvents, 2b forms wormlike structures and 2c, with the longest phosphine block, forms fascinating micron sized intertwined wormlike structures. This represents a new method to control the shape and size of gold(I) nanostructures.     

Evolution of size and shape in the colloidal crystallization of gold nanoparticles

The addition of dodecanethiol to a solution of oleylamine-stabilized gold nanoparticles in chloroform leads to aggregation of nanoparticles and formation of colloidal crystals. Based on results from dynamic light scattering and scanning electron microscopy we identify three different growth mechanisms: direct nanoparticle aggregation, cluster aggregation, and heterogeneous aggregation. These mechanisms produce amorphous, single-crystalline, polycrystalline, and core-shell type clusters. In the latter, gold nanoparticles encapsulate an impurity nucleus. All crystalline structures exhibit fcc or icosahedral packing and are terminated by (100) and (111) planes, which leads to truncated tetrahedral, octahedral, and icosahedral shapes. Importantly, most clusters in this system grow by aggregation of 60-80 nm structurally nonrigid clusters that form in the first 60 s of the experiment. The aggregation mechanism is discussed in terms of classical and other nucleation theories.     

Water‐soluble complex formation of fullerene and thermo‐responsive diblock copolymer

A diblock copolymer (P₉₈N₁₀₀) composed of a biocompatible water‐soluble block (PMPC) and a lower critical solution temperature (LCST) type thermo‐responsive block (PNIPAM) was prepared via controlled radical polymerization. To dissolve fullerene (C₆₀) in water, the C₆₀/P₉₈N₁₀₀ complex was prepared by mixing C₆₀ and P₉₈N₁₀₀ powders. The maximum solubilized C₆₀ concentration in water was 1.39 g/L, as estimated from UV–vis adsorption, when the polymer concentration was 5.0 g/L. The percent transmittance of the aqueous solution of the C₆₀/P₉₈N₁₀₀ complex decreased above 36 °C due to inter‐complex association above the LCST for the PNIPAM block. While the hydrodynamic radius of C₆₀/P₉₈N₁₀₀ complex was 135 nm at 20 °C, it increased to 161 nm at 50 °C. Despite the observation of ¹H NMR signals from PMPC and PNIPAM blocks for the C₆₀/P₉₈N₁₀₀ complex in D₂O at room temperature, the signals from PNIPAM disappeared above 35 °C due to restricted motion of PNIPAM. Generation of singlet oxygen (¹O₂) from the C₆₀/P₉₈N₁₀₀ complex by photo‐irradiation was confirmed using 9,10‐anthracene dipropionic acid (ADPA). The absorbance of ADPA decreased with increasing irradiation time due to oxidation of ADPA by ¹O₂. It is expected that the C₆₀/P₉₈N₁₀₀ complex can be applied as a thermo‐responsive carrier for photodynamic therapy. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2432–2439     

Spontaneous formation of vesicles of diblock copolymer EO6BO11 in water: a SANS study

Small angle neutron scattering (SANS) is used to study the structures formed in water by a diblock copolymer EO6BO11 (having 6 ethylene oxide, EO, and 11 butylene oxide, BO, units). The data show that polymer solutions over a broad concentration range (0.05-20 wt %) contain vesicular structures at room temperature. Interestingly, these vesicles could be formed without any external energy input, such as extrusion, which is commonly required for the formation of other block copolymer or lipid vesicles. The EO6BO11 vesicles are predominantly unilamellar at low polymer concentrations, whereas at higher polymer concentrations or temperatures there is a coexisting population of unilamellar and multilamellar vesicles. At a critical concentration and temperature, the vesicular structures fuse into lyotropic arrays of planar lamellar sheets. The findings from this study are in broad agreement with the work of Harris et al. (Langmuir, 2002, 18, 5337), who used electron microscopy to identify the vesicle phase in the same system.     

Morphology transformation of hybrid micelles self-assembled from rod-coil block copolymer and nanoparticles

Hybrid polymeric micelles self-assembled from a mixture containing poly(γ-benzyl-L-glutamate)-block-poly(ethylene glycol) (PBLG-b-PEG) block copolymer and gold nanoparticles (AuNPs) were prepared. The effect of AuNPs on the self-assembly behavior of PBLG-b-PEG was studied both experimentally by transmission electron microscopy, scanning electron microscopy, and laser light scattering and computationally using dissipative particle dynamics (DPD) simulations. It was found that, the pure PBLG-b-PEG block copolymer self-assembles into long cylindrical micelles. By introducing AuNPs to the stock block copolymer solution, the formed aggregate morphology transforms to spherical micelles. The DPD simulation results well reproduced the morphological transformations observed in the experiments. And the simulation revealed that the main reason for the aggregate morphology transformation is the breakage of ordered packing of PBLG rods in micelle core by the added nanoparticles. Moreover, from the DPD simulations, the distribution information on nanoparticles was obtained. The nanoparticles were found to prefer to locate near the core/shell interface as well as in the core center of the micelles. The combination of experimental and simulation methods lead to a comprehensive understanding of such a complex self-assembly system.     

Giant phospholipid/block copolymer hybrid vesicles: mixing behavior and domain formation

Lipids and block copolymers can be individually assembled into unsupported, spherical membranes (liposomes or polymersomes), each having their own particular benefits and limitations. Here we demonstrate the preparation of microscale, hybrid "lipopolymersomes" composed of the common lipid POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine) and the commercially available copolymer PBd-b-PEO (polybutadiene-b-poly(ethylene oxide)) with the goal of incorporating the advantageous qualities of the unitary systems into mixed-membrane capsules. We investigate the lipopolymersomes using confocal fluorescence microscopy and demonstrate that these hybrid membranes are well mixed on nanoscopic length scales within the permittable compositional windows for hybrid vesicle formation. We measure the intramembrane dynamics and mechanical properties of these hybrid membranes by fluorescence recovery after photobleaching (FRAP) and micropipet aspiration, respectively. For the first time, we demonstrate the demixing of lipid-rich and polymer-rich membrane domains within the same vesicle membrane. This is achieved by the biotinylation of one of the constituent species and cross linking with the protein NeutrAvidin. The resultant domain patterning is dependent upon which component carries the biotin functionality: cross linking of the copolymer species results in domains that ripen into a single, large, copolymer-rich island, and cross linking of the lipids yields many small, "spot-like", lipid-rich domains within a copolymer-rich matrix. We discuss these morphological differences in terms of the fluidity and mechanical properties of the membrane phases and the possible resultant interdomain interactions within the membrane. These heterogeneous hybrid lipopolymersomes could find applications in fields such as targeted delivery, controlled release, and environmental detection assays where these capsules possess the characteristics of biocompatible lipid membranes combined with enhanced mechanical strength and stability from the copolymer matrix.     

Kinetics of pH-Induced formation and dissociation of polymeric vesicles assembled from a water-soluble zwitterionic diblock copolymer

The kinetics of pH-induced formation and dissociation of vesicles self-assembled from a biocompatible zwitterionic diblock copolymer, poly(2-(methacryloyloxy)ethyl phosphorylcholine)-b-poly(2-(diisopropylamino)ethyl methacrylate) (PMPC- b-PDPA), was investigated in detail via a combination of stopped-flow light scattering and laser light scattering (LLS). Upon jumping from pH 2 to 10, stopped-flow light scattering reveals three distinct relaxation processes for the early stages of vesicle self-assembly (0-40 s). Kinetic sequences associated with the obtained three characteristic relaxation times have been tentatively proposed. Moreover, the kinetics of vesicle formation in the later stage (from 3 min onward) was investigated by dynamic LLS. It was found that both the intensity-averaged hydrodynamic radius, R h, and the polydispersity, mu2/Gamma (2), decrease exponentially, yielding a characteristic relaxation time of approximately 350 s. To our knowledge, this is the first report on the kinetics of the unimer-to-vesicle transition of a stimulus-responsive diblock copolymer. The kinetics of vesicle dissociation for a pH jump from 12 to 2 was also investigated. The breakdown of polymeric vesicles is extremely fast and is independent of polymer concentration; it is complete within approximately 5 ms and is in marked contrast to the much slower rate of vesicle formation.     

Magnetic nanoparticles for power absorption: Optimizing size, shape and magnetic properties

We present a study on the magnetic properties of naked and silica-coated Fe₃O₄ nanoparticles with sizes between 5 and 110 nm. Their efficiency as heating agents was assessed through specific power absorption (SPA) measurements as a function of particle size and shape. The results show a strong dependence of the SPA with the particle size, with a maximum around 30 nm, as expected for a Néel relaxation mechanism in single-domain particles. The SiO₂ shell thickness was found to play an important role in the SPA mechanism by hindering the heat outflow, thus decreasing the heating efficiency. It is concluded that a compromise between good heating efficiency and surface functionality for biomedical purposes can be attained by making the SiO₂ functional coating as thin as possible.     

Tunable magnetic arrangement of iron oxide nanoparticles in situ synthesized on the solid substrate from diblock copolymer micelles

Hexagonal arrangement of iron oxide nanoparticles was fabricated by utilizing a single-layered film of diblock copolymer micelles. The synthesis was directly performed on the solid substrate by oxygen plasma with preserving the dimensional order of micelles so that separate procedures for synthesis and deposition of nanoparticles were not necessary. Since the oxygen plasma treatment also eliminated polymers, pure patterns of iron oxide nanoparticles were obtained. Moreover, easy control over the size of nanoparticles enabled us to selectively create a ferrimagnetic or a superparamagnetic pattern of iron oxide nanoparticles without altering the fabrication process.     

Phase behavior in mixtures of a homopolymer and a block copolymer. 4. SALS and SAXS studies

Phase behavior of blends of poly(vinyl methyl ether) (PVME) with four styrene-butadiene-styrene (SBS) triblock copolymers, being of various molecular weights, architecture, and compositions, was investigated by small-angle light scattering. Small-angle X-ray scattering investigation was accomplished for one blend. Low critical solution temperature (LCST) and a unique phase behavior, resembling upper critical solution temperature (UCST), were observed. It was found that the architecture of the copolymer greatly influenced the phase behavior of the blends. Random phase approximation theory was used to calculate the spinodal phase transition curves of the ABA/C and BAB/C systems; LCST and resembling UCST phase behavior were observed as the parameters of the system changed. Qualitatively, the experimental and the theoretical results are consistent with each other. © 1996 John Wiley & Sons, Inc.     

A-B diblock copolymer micelles: effects of soluble-block length and component compatibility

Micellization of a diblock copolymer in dilute solution is studied by dissipative particle dynamics. The influence of the compatibility between blocks A and B and the interaction between the insoluble block and solvent on aggregation number P and micellar core radius Rc are examined. The micelle size distribution is obtained, and it is quite polydisperse. Different from the scaling theory for starlike micelles, the mean aggregation number based on weight average w decreases with increasing soluble-block length NA and the power law relation can be obtained, w approximately NA(-alpha). Similarly, the micellar core radius declines with NA, following Rc approximately NA(-beta) with beta=alpha/3. However, the exponent depends on the mutual compatibility between soluble and insoluble blocks. For the same composition, the incompatible diblocks form a smaller micelle and its aggregation number declines with a smaller exponent alpha. When NA approximately NB, the micelles deviate significantly from the spherical shape and solvophilic blocks are observed to be entrapped in the solvophobic core for compatible diblocks.