Belts, stars, 2D lattices, and elongated objects are formed by solution‐based tip‐to‐tip or side‐by‐side self‐assembly of single‐tip Au–CdSe nanorods (see picture). Assembly was mediated by ...attraction between the Au or CdSe segments in selective solvents.
The block copolymer poly(ferrocenyldimethylsilane-b-dimethylsiloxane) (PFS40-b-PDMS480, where the subscripts refer to the mean degree of polymerization) self-assembles in n-decane at 25 °C to form ...long rodlike micelles (Raez et al. J. Am. Chem. Soc. 2002, 124, 10381). Here, we describe a combination of static (SLS) and dynamic (DLS) light scattering measurements on solutions of the micelles in n-decane. By SLS, we find that the micelles are thin, rigid, rodlike structures with a mean length of ∼960 nm in the limit of zero concentration and a length that increases with concentration. Through a combination of SLS and DLS measurements, we find that that the cross-sectional hydrodynamic diameter of the micelle is 18 nm. These self-assembled structures also appear to have a narrow distribution of lengths (with M w/M n ≈ 1.1). These structures are much smaller than those observed on transmission electron microscopy (TEM) grids obtained from solutions of these self-assembled structures.
We describe simultaneous static (SLS) and dynamic light scattering (DLS) measurements on dilute solutions of a series of poly(ferrocenyldimethylsilane-b-isoprene) (PFS50–PI1000) block copolymer ...micelles of uniform length in tert-butyl acetate (tBA) and in decane. The subscripts in the term PFS50–PI1000 refer to the mean degree of polymerization of each block. The SLS experiments show that in both solvents the micelles formed are elongated and rigid. We also observed that the large length of the PI block (1000 units) contributes to the SLS signal. From the SLS data, we calculated the mass per unit length (linear aggregation number), as well as the cross section of the micelles in both solvents. Interestingly, the linear aggregation number and the micelle cross sections, as deduced by SLS, were the same in decane and in tBA. However, the fitting of DLS data indicates that the hydrodynamic cross section of the micelles in tBA is much larger than that in decane, and both values are larger than the values determined by SLS. We hypothesize that the difference between cross sections deduced from SLS and DLS data fitting is related to the shape of the segment density profile of the corona. In tBA, the PI chains are more stretched than in decane, increasing the hydrodynamic radius of the micelle cross section.
Hydrophobic lead sulfide quantum dots (PbS/OA) synthesized in the presence of oleic acid were transferred from nonpolar organic solvents to polar solvents such as alcohols and water by a simple ...ligand exchange with poly(acrylic acid) (PAA). Ligand exchange took place rapidly at room temperature When a colloidal solution of PbS/OA in tetrahydrofuran (THF) was treated with excess PAA, the PbS/PAA nanocrystals that formed were insoluble in hexane and toluene but could be dissolved in methanol or water, where they formed colloidal solutions that were stable for months. Ligand exchange was accompanied by a small blue shift in the band-edge absorption, consistent with a small reduction in particle size. While there was a decrease in quantum yield associated with ligand exchange and transfer to polar solvents, as is commonly found for colloidal quantum dots, the quantum yields determined were impressively high: PbS/OA in toluene (82%) and in THF (58%); PbS/PAA in THF (42%) and in water (24%). The quantum yields for the PbS/PAA solutions decreased over time as the solutions were allowed to age in the presence of air.
Two samples of rod-like micelles in decane were prepared by seeded growth from a sample of a poly(isoprene-b-ferrocenyldimethylsilane) diblock copolymer (PI1000–PFS50, where the subscripts indicate ...the degree of polymerization). These micelles were uniform in length with a mass/length of 1.9 molecules/nm. The longer micelles (L-1250) had a number-average length L n = 1243 nm, whereas the shorter micelles (L-250) had L n = 256 nm. We used transmission electron microscopy (TEM) to examine the behavior of these micelles when dilute solutions of L-1250 or L-250 or their mixtures were heated at temperatures ranging from 40 to 75 °C and then cooled to room temperature. At 55 °C, the L-1250 sample underwent kinetically controlled fragmentation to give a broad distribution of micelle lengths. At this temperature, fragmentation was much less prominent in the L-250 sample. At higher temperatures, micelles with narrow distributions of lengths were obtained in each case (L w/L n ≈ 1.01). This process operates under thermodynamic control, and L n values increased strongly with an increase in temperature. These results indicate that the micelles fragment, and polymer molecules dissolve, as the samples were heated. The fraction of surviving fragments decreased significantly at elevated temperatures, presumably reflecting a distribution of crystallinity in the cores of the micelle precursor. When the solutions were cooled, the surviving fragments served as seeds for the epitaxial growth of the micelles as the polymer solubility decreased. The most striking result of these experiments was the finding that fragments formed from the L-1250 micelles had a distribution of dissolution temperatures shifted by about 5 °C to higher temperature than the shorter L-250 micelles.
We describe the synthesis, characterization, and film-forming properties of two-component nanoparticles that undergo a reversible morphology transformation in water as a function of pH. The particles ...consist of a high molecular weight acrylate copolymer and an acid-rich oligomer designed to be miscible with the polymer when its −COOH groups are protonated. Attaching a fluorescence resonance energy transfer (FRET) pair to components inside the nanoparticles enabled us to assess morphology at the molecular level. By inspecting changes in the donor fluorescence decay profile at different pH values, we established miscibility of the components in acidic solution but with charge-induced phase separation when the oligomers were neutralized to their carboxylate form. Complementary titration experiments revealed that the nanoparticles adopt a core–shell structure when the acid groups are deprotonated. We studied the effect of the acid-rich oligomer on the diffusion rate of the high molecular weight polymers following film formation. Our results show that the carboxylated oligomer enhanced the rate of diffusive mixing between high molecular weight molecules by more than 2 orders of magnitude. FRET measurements carried out on partially dried films using a low-resolution microscope showed that the carboxylate oligomer shell can delay coalescence for ca. 30 min after passage of the drying front. This delay is expected to help with increasing the ‘open time’ of latex paints, a desirable property of solvent-based paints that remains difficult to achieve with (environmentally compliant) waterborne paints. Use of ammonia as a volatile base resulted in synergistic effects: initial retardation of coalescence followed by acceleration of diffusive mixing as the ammonium salts dissociated and ammonia evaporated from the film.
This paper reports a new synthetic strategy for the preparation of polyferrocenylsilane (PFS) block copolymers (BCPs), by conjugation of two independently prepared homopolymers using Diels–Alder ...cycloaddition. The PFS blocks were synthesized by photocontrolled ring-opening polymerization, yielding polymers with a cyclopentadienyl end group that serves as a diene in the conjugation reaction. In this initial study we focused on the synthesis of organometallic–polypeptide block copolymers PFS-b-PBLG (PBLG = poly(γ-benzyl-l-glutamate) using PBLG polymers with a terminal maleimide attached by one-step postpolymerization modification. Five PFS-b-PBLG copolymers with different degrees of polymerization were synthesized and purified by a series of selective precipitations. The self-assembly of these samples was studied in N,N -dimethylformamide, a solvent selective for PBLG. The BCPs with a PFS block longer than the PBLG block after annealing at 90 °C formed highly uniform platelet micelles with a truncated elliptical shape. Experiments at 75 °C showed that disordered elongated structures formed initially, with fiber-like protrusions from the ends. Over time, the structures became shorter and wider, evolving into uniform truncated elliptical micelles. The process was monitored by X-ray diffraction (XRD) measurements and by 1H NMR spectroscopy. AFM analysis showed that the micelles were flat and that their thickness increased with the overall chain length of the BCP. Self-assembly of these BCPs in the presence of PFS homopolymer led to formation of flower-like mesostructures consisting of stacks of lamellar petals.
Scattering techniques (i.e., static light scattering, small angle neutron scattering, or small angle X-ray scattering) are excellent tools to study nanoscopic objects in solution. However, to ...interpret the experimental data, one needs to use the appropriate form factor. While recent progress has been made in the writing of form factors for complex structures, there is still a need to develop a method to evaluate the form factor of inhomogeneous elongated scatterers. Here, we propose an approach based on the principle of “Russian dolls”. Multiblock rods are represented as multi generations of rods (mother, daughter, granddaughter, etc.), where each rod is nested within the rod of the previous generation, like Russian dolls. A shift parameter is used to introduce asymmetry in the rod along its long axis. This approach not only allowed us to write the form factor of multiblock rods, but it also gave us the possibility to account for the polydispersity in length of each block and of the shift parameter. Finally, we applied these equations to the case of a series of solutions of triblock comicelles slightly polydisperse in length.
We describe the room-temperature synthesis and characterization of needle-like lanthanide phosphate (LnPO4) nanocrystals in water, based on in situ precipitation of LnPO4 using functional aqueous ...microgels as a soft nanoreactor. The poly(NIPAm/VCL/MAA) microgels were prepared by the copolymerization of N-isopropylacrylamide, N-vinylcaprolactam, and methacrylic acid. Our goal was to prepare Ln-encoded microgels suitable for bead-based biological assays employing mass cytometry. The low solubility of nanocrystalline LnPO4 avoids the problem of Ln ion leakage from the microgels. The main challenge was to find appropriate conditions to confine the LnPO4 precipitate to the interior of the microgels. Various sources of phosphate ions led to precipitation of LnPO4 on the exterior of the microgels. One approach worked. It involved three steps: neutralization of the MAA groups in the microgels with NaOH, ion exchange with a lanthanide salt, followed by treatment with a large excess of PBS buffer at pH 7.4. In this way, we obtained microgels containing ca. 107 Ln atoms per microgel (Ln = La, Nd, Eu, Tb, Ho, Tm). The microgels were shown to be uniform in size by dynamic light scattering and transmission electron microscopy. The LnPO4-containing microgels showed much higher stability against leakage of metals to acidic buffers compared to the LnF3-containing microgels reported previously Lin W. ; Langmuir 2011, 27, 7265 . Small-angle X-ray scattering measurements and selected area electron diffraction data showed striking differences in the internal structure of hybrid microgels containing TbPO4 from those containing TbF3. The TbF3-containing microgels had a core–shell structure with an amorphous TbF3 core, whereas the TbPO4 microgels contained needle-like nanocrystals (width ca. 2 nm, lengths on the order of 80 nm) distributed throughout the structure. These microgels are very promising materials for biological assays based on mass cytometry.
Poly(ferrocenyldimethylsilane) (PFS) block copolymers form fiberlike micelles by a seeded growth process. This paper describes the effect of adding similar amounts of PFS block copolymers, PFS–PDMS ...and PFS–PI, to a common micelle seed. The lengths of the micelles obtained were strongly influenced by the degree of polymerization of the corona‐forming blocks. The change in length was due to a change in the number of polymer molecules per unit length of the micelle.
Polyferrocenylsilane (PFS) block copolymers form rodlike micelles in selective solvents by a seeded growth mechanism. Micelles of four different PFS block copolymers were grown from suspensions of a common seed micelle. The length of the micelles varied with the nature of the corona‐forming block.
