Preparative size-exclusion chromatography for purification and characterization of colloidal quantum dots bound by chromophore-labeled polymers and low-molecular-weight chromophores

We explore the use of preparative size-exclusion chromatography (SEC) and high-performance liquid chromatography (HPLC) to purify quantum dots (QDs) after surface modification. In one example, in which Bio-Beads (S-X1) were used as the packing material for the preparative SEC column, CdSe QDs treated with a functional coumarin dye could be separated from the excess free dye by using tetrahydrofuran (THF) as the mobile phase. This column was unable to separate polymer-coated QDs from free polymer (M ∼ 8000) because of the relatively low cutoff mass of the column. Here a preparative HPLC column packed with TOYOPEARL gel allowed the effective separation of polymer-bound QDs from the excess free polymer by using N-methyl-2-pyrrolidinone (NMP) as the mobile phase. When other solvents such as absolute ethanol, acetonitrile, THF, and THF–triethylamine mixtures were used as the eluent, QDs stuck to the column. While NMP was an effective medium to remove excess free polymer from the QDs, it was difficult to transfer the purified QDs to more volatile solvents and maintain colloidal stability.     

The influence of PEG macromonomers on the size and properties of thermosensitive aqueous microgels

We describe the preparation and thermal response of aqueous microgels based on poly(N-vinyl caprolactam) containing grafted poly(ethylene glycol) (PEG) chains. These microgels were synthesized by free radical copolymerization of vinyl caprolactam and acetoacetoxyethyl methacrylate in the presence of methoxy-capped poly(ethylene glycol)methacrylate macromonomers. We show that variation of the amount of PEG macromonomer or the length of the PEG chain provides effective control of the microgel diameter in the range 60–220 nm. The presence of the grafted PEG chains improves the colloidal stability of the microgels. The incorporation of the PEG macromonomers into microgel structure decreases the swelling degree and induces a shift of the volume phase transition to higher temperatures. This paper is dedicated to Professor Haruma Kawaguchi in honor of his many contributions to the field of polymer particle synthesis and applications.     

Effect of Topology on the Properties of Poly(N-isopropylacrylamide) in Water and in Bulk

Three macrocyclic poly(N-isopropylacrylamide)s (PNIPAM) with molecular weight (MW) ranging from 6 to 19 kg/mol were synthesized by ‘click’ ring closure of the corresponding α-azido ω-propargyl telechelic linear PNIPAMs, themselves prepared by reversible addition fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide. Differential scanning calorimetry (DSC) studies revealed that both the thermal phase separation in water and the glass transition in bulk of PNIPAM were affected by polymer topology. In aqueous solution, the cyclic polymers exhibit a higher phase separation temperature and broader phase transition range than the corresponding linear counterparts. In bulk, the cyclic polymers display a higher glass transition temperature of lesser molecular weight dependence, as compared to their linear precursors.     

Carbon nanotubes with small and tunable diameters from poly(ferrocenylsilane)-block-polysiloxane diblock copolymers

Iron-containing nanostructures produced from various self-assembled poly(ferrocenylsilane)-block-polysiloxane thin films are catalytically active for the initiation and growth of high density, small diameter carbon nanotubes (CNTs). Moreover, the tube diameter and density can be tuned by adjusting the chain lengths of the block copolymer. Iron-containing nanostructures from poly(ferrocenylmethylethylsilane)-b-poly(methylvinylsiloxane) polymer with 25 repeat units of an iron-containing segment and 265 repeat units of a non-iron-containing segment are able to produce CNTs with diameters around or less than 1 nm. Lithographically selective growth of CNTs across a large surface area has been demonstrated using this polymer system. Under the same growth condition, it has been found that the yield of defect-free CNTs varies with the size of the catalytically active nanostructures, which are dictated by the chain lengths of the two blocks. This result indicates that, for a specific-sized catalyst nanocluster, a unique set of growth conditions is required for synthesizing high yield, defect-free CNTs. This finding further addresses the importance of using uniform-sized catalyst-containing nanostructures for consistently achieving high-yield and high-quality CNTs with a minimum number of defects and amount of amorphous carbon.     

Surfactant exudation in the presence of a coalescing aid in latex films studied by atomic force microscopy1

We report atomic force microscopy images of surfactant (SDS) exudation in PBMA latex films, in the presence and the absence of a coalescing aid (Texanol^?, TPM). The exudates appear as hilly islets, and at times as mountains, at the film surface. Their size and number increase upon annealing above the glass-transition temperature of the latex polymer. TPM was found to be a strong promoter of surfactant exudation at the air-polymer interface. In the absence of TPM, annealing the films for several hours at 70°C led to very little migration of surfactant to the surface at most sites in the film. When the films with structures of SDS on their surface were immersed in water, these structures disappeared. Pores, ranging in size from tens to hundreds of nm in diameter, were clearly visible in the surface of the films. These films dry from the edges of the film inward, with a propagation front concentrating the water-soluble species into a turbid, moist region in the center. At this site, the rate at which the surfactant comes to the surface is enormously enhanced over that at other sites in the film. This is likely due to the high concentration of surfactant in this region, transported there by the drying process. © 1995 John Wiley & Sons, Inc.     

The structure of the charge transfer complex in olefin quenching of benzophenone phosphorescence

The phosphorescence lifetimes of ω-alkenyl esters of p-benzophenone-carboxylic acid have been measured in dilute solution in degassed acetic acid solution. The lifetimes were extrapolated to infinite dilution. Intramolecular quenching was not very significant in benzophenone-4-CO₂(CH₂)_nCHCH₂ for n = 1, 2 or 4. For n = 9 the phosphorescence lifetime was found to be 2 μs compared to 70 μs for n = 1, 2. These results are interpreted to mean that the quenching complex has a specific geometric requirement with the CHCH₂ group approaching within a van der Waals distance of the ketone carbonyl group.     

Two-stage dispersion polymerization toward monodisperse, controlled micrometer-sized copolymer particles

We describe a two-stage dispersion polymerization process to prepare monodisperse micrometer-sized polymer particles in which problematic reagents, such as functional monomers and cross-linkers that commonly disrupt the particle-forming process, were added to the reaction after the nucleation stage was complete. Comonomers added after this point became incorporated into the particles without disturbing the final particle size and size distribution. In this way dye-labeled or functional group-containing micrometer-sized particles with a very narrow size distribution were prepared. By varying the amount of monomer added in the second stage, the final particle diameter could be controlled precisely. Most important of all, this synthetic strategy allowed us to prepare cross-linked particles containing up to 3 mol % cross-linking agent.     

A reversible tube-to-rod transition in a block copolymer micelle

A remarkable morphology transition occurs with a change in temperature for a diblock copolymer [poly(ferrocenyldimethylsilane-b-dimethylsiloxane) (PFS40-b-PDMS480, PDI = 1.01)] in n-decane solution. This polymer, which forms nanotubes at 25 degrees C, rearranges to form short dense rods when the solution is heated to 50 degrees C. When the solution is cooled to 25 degrees C, the system evolves back to nanotubes. These experiments demonstrate that both structures are dynamic and represent equilibrium states of the material. Contrast matching static light-scattering measurements on the short dense rods show that the insoluble PFS core is rigid and has a length distribution similar to that seen in electron microscopy images.     

Magnon excitation by spin injection in thin Fe/Cr/Fe films

The technique of Brillouin light scattering is used to observe strong excitation of magnons in antiferromagnetically coupled trilayers of Fe/Cr/Fe at room temperature. The magnons are driven out of equilibrium by a microwave current applied in the trilayer through point contacts. The magnitude of the scattering intensity is investigated as a function of the magnon wave number and applied magnetic field. Confirming recent theoretical predictions, the observations provide strong evidence of electronic spin injection in the rf driving field.