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Mitchell A Winnik

Publications and source records attributed to Mitchell A Winnik.

At least 19 recordsLinked to original sources
PubMed2006

Energy migration study of random immobile anthracene derivatives by time-resolved fluorescence anisotropy decays.

We describe time-resolved fluorescence anisotropy measurements for a simple 2-substituted anthracene derivative (2-An-M) as a function of concentration in poly(methyl methacrylate) (PMMA) films. The anisotropy decays via energy migration among 2-An-M molecules in the matrix. The data were interpreted in terms of the survival probability of the initially excited chromophores and are in good agreement with the plot predicted using R0 = 1.8 nm, the value of the Förster radius determined independently by the spectral overlap method. For all concentrations, we found r0 = 0.20 +/- 0.01 for the initial anisotropy at time zero, whereas the residual anisotropy, r(infinity), was concentration-dependent and higher in magnitude than the theoretically predicted value, 4% of r0. The higher residual anisotropy values may originate from the possibility that not all of the anthracene molecules are involved in the energy migration process. Similar anthracene anisotropy experiments were performed on anthracene-labeled poly(isoprene-b-methyl methacrylate) (PI-PMMA). The results show an increased depolarization rate for samples containing a higher fraction of polymers labeled at the junction with anthracene chromophores.

Journal Article↗
PubMed2006

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.

Journal Article↗
PubMed2005

Phosphorescence quenching of dyes adsorbed to silica thin-layer chromatography plates.

We describe photoluminescence (PL) quenching experiments by oxygen for a series of transition metal dyes adsorbed to commercial silica thin-layer chromatography plates. These TLC plates have been used by others as rapidly responding PL pressure sensors. The quenching kinetics show interesting differences from the behavior of the same dyes adsorbed to the more well-defined surfaces of SBA-15 mesoporous silica particles adsorbed to a thin layer-by-layer polymer film as reported in Chem. Mater. 2005, 17, 3160. The pore size and the pore size distribution are much larger for the TLC silica than for the mesoporous silica. On the TLC silica, the dye PL decay profiles show smaller deviations from an exponential form. One sees larger differences between the intensity and lifetime Stern-Volmer plots, and most surprisingly, the dyes on the surface of the TLC silica are about three times more sensitive to quenching by oxygen.

Journal Article↗
PubMed2005

The orientation parameter for energy transfer in restricted geometries including block copolymer interfaces: a Monte Carlo study.

We describe Monte Carlo simulations of resonance energy transfer (RET) experiments for immobile donor (D) and acceptor (A) dyes confined to planar, cylindrical, and spherical restricted geometries. We compare values of the quantum efficiency (PhiET) evaluated through consideration of individual donor-acceptor pairs, with values calculated assuming a pre-averaged value of the orientation parameter /kappa/2 = 0.476 appropriate for infinite three dimensional (3D) space. For dyes confined to restricted geometries where the length scale of the confining dimension is less than or equal to the Förster radius R0, the coupling of the orientation parameter and the donor-acceptor distance becomes noticeable. Values of Phi(ET) obtained by proper consideration of the orientation parameter are smaller than those calculated using /kappa/2 = 0.476. We use this Monte Carlo method to reanalyze the fluoresce decay measured from dye-labeled poly(isoprene-b-methyl methacrylate) diblock copolymer with lamellar structure,(1) from which the interface thickness for PI-PMMA lamella can be retrieved. We found the retrieved interface thickness is sensitive to the choice of dipole orientation. If all dipoles in the confined polymer interface have a random orientation, the value of interface thickness was found to be 0.9 +/- 0.2 nm through consideration of individual dipole orientations. Assumption of /kappa/2 = 0.476 in the FRET calculations leads to a larger value of interface thickness (1.3 +/- 0.2 nm) due to the neglect of the coupling between dipole orientation and D-A distance for the dyes confined to lamellar interfaces.

Energy Transfer↗
PubMed2005

The determination of the Förster distance (R0) for phenanthrene and anthracene derivatives in poly(methyl methacrylate) films.

Experiments that employ direct resonance energy transfer (DET) to obtain information about distances or domain sizes in polymer systems require independent information about the magnitude of the characteristic (Förster) energy transfer distance R(0). Values of R(0) are relatively straightforward to obtain by the traditional spectral overlap method (R(0)(SO)) for dyes in fluid solution, but are much more difficult to obtain for dyes in rigid polymer films. Here one can obtain a value for R(0) as a fitting parameter (R(0)(FF)) for donor fluorescence decay experiments for samples containing a random distribution of donor and acceptor dyes in the polymer film. In previous experiments from our group, we needed values of R(0) for various phenanthrene (Phe, donor) and anthracene (An, acceptor) derivatives. In this paper, we describe experiments which determine R(0) values by both methods for a series of Phe-An donor-acceptor pairs in poly(methyl methacrylate) and polystyrene films. Both the location of substituents on the donor and acceptor as well as the choice of the medium had an effect on the measured R(0), which varied between 2.0 and 2.6 nm. We also ascertained that there is some unknown factor, also prevalent in the work of others, which results in the Förster radius being larger when determined by the Förster fit method than by the method of spectral overlap.

Journal Article↗
PubMed2005

Redox-induced synthesis and encapsulation of metal nanoparticles in shell-cross-linked organometallic nanotubes.

A new approach to encapsulate silver nanoparticles inside block copolymer nanotubes is reported and involves an in situ redox reaction between a polyferrocenylsilane (PFS) inner wall and silver ions. Partial preoxidation of the PFS domains was found to be a key step for the efficient formation of one-dimensional arrays of silver nanoparticles confined within the nanotubes.

Ferrous Compounds↗
PubMed2004

Formation of dispersed nanostructures from poly(ferrocenyldimethylsilane-b-dimethylsiloxane) nanotubes upon exposure to supercritical carbon dioxide.

While incompatible block copolymers commonly assemble into several established classical or complex morphologies, highly asymmetric poly(ferrocenyldimethylsilane-b-dimethylsiloxane) (PFS-b-PDMS) diblock copolymers can also self-organize into high-aspect-ratio nanotubes with PDMS corona in the presence of PDMS-selective organic solvents. Exposure of these nanotubes on a carbon substrate to supercritical carbon dioxide (scCO2), also a PDMS-selective solvent, appears to promote partial dissolution of the copolymer molecules. At sufficiently high copolymer concentrations, the dissolved molecules subsequently re-organize within the scCO2 environment to form new copolymer nanostructures that redeposit on the substrate upon scCO2 depressurization. Transmission electron microscopy reveals that micelles form under all the conditions examined here, whereas nanotubes coalesce and vesicles develop only at relatively high temperatures. The extent to which the copolymer nanotubes dissolve and the size distribution of the replacement micelles are sensitive to exposure conditions. These results suggest that the phase behavior of PFS-b-PDMS diblock copolymers in scCO2 may be remarkably rich and easily tunable.

Carbon Dioxide↗
PubMed2004

Cylindrical micelles from the aqueous self-assembly of an amphiphilic poly(ethylene oxide)-b-poly(ferrocenylsilane) (PEO-b-PFS) block copolymer with a metallo-supramolecular linker at the block junction.

A supramolecular AB diblock copolymer has been prepared by the sequential self-assembly of terpyridine end-functionalized polymer blocks by using Ru(III)/Ru(II) chemistry. By this synthetic strategy a hydrophobic poly(ferrocenylsilane) (PFS) was attached to a hydrophilic poly(ethylene oxide) (PEO) block to give an amphiphilic metallo-supramolecular diblock copolymer (PEO/PFS block ratio 6:1). This compound was used to form micelles in water that were characterized by a combination of dynamic and static light scattering, transmission electron microscopy, and atomic force microscopy. These complementary techniques showed that the copolymers investigated form rod-like micelles in water; the micelles have a constant diameter but are rather polydisperse in length, and light scattering measurements indicate that they are flexible. Crystallization of the PFS in these micelles was observed by differential scanning calorimetry, and is thought to be the key behind the formation of rod-like structures. The cylindrical micelles can be cleaved into smaller rods whenever the temperature of the solution is increased or they are exposed to ultrasound.

Journal Article↗
PubMed2004

Surface passivation of luminescent colloidal quantum dots with poly(dimethylaminoethyl methacrylate) through a ligand exchange process.

Polydimethylaminoethyl methacrylate (PDMAEMA) was used as a multidentate ligand to modify the surface of CdSe/ZnS core-shell colloidal quantum dots in toluene with trioctylphosphine oxide (TOPO) as the surface ligand. Adsorption of PDMAEMA was accompanied by release of TOPO. The process is free of agglomeration, and the modified nanocrystals become soluble in methanol. The photoluminescence properties are well-preserved in either toluene or methanol.

Journal Article↗
PubMed2004

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.

Journal Article↗
PubMed2004

Exchange mechanisms for sodium dodecyl sulfate micelles: high salt concentration.

Solute exchange experiments for the pyrene-labeled triglyceride TG-Py solubilized in sodium dodecyl sulfate (SDS) micelles in the presence and absence of salt show that the "observed" rate constant k(obs) for solute exchange varies by over 6 orders of magnitude as the free sodium ion concentration [Na(+)](aq) is varied between 10 and 850 mM. There is a sharp break in the log-log plot of k(obs) versus [Na(+)](aq) in the range of [Na(+)](aq) = 200 mM, with the exchange rate showing a weaker dependence on [Na(+)](aq) above this concentration. Up to 100 mM added NaCl, this exchange takes place essentially exclusively by a micelle fission mechanism in which each submicelle carries off one of the solutes. At higher salt concentrations, a bimolecular process becomes increasingly important. This fusion process, which involves formation of a transient supermicelle followed by fission back to two normal micelles, becomes the dominant process at high salt concentrations. The fission rate appears to level off for salt concentrations above 300-400 mM. These fission and fusion processes are related in an intimate way to the changes in the size and shape of the SDS micelles with increasing salt concentration.

Algorithms↗
PubMed2003

Shell cross-linked cylinders of polyisoprene-b-ferrocenyldimethylsilane: formation of magnetic ceramic replicas and microfluidic channel alignment and patterning.

Stable organometallic cylindrical nanostructures have been prepared by shell cross-linking of PI320-b-PFS53 wormlike micelles in hexane through metal-catalyzed hydrosilylation at 23 degrees C. The cross-linked structures permit the formation of cylindrical ceramic replicas containing size- and separation-tunable arrays of Fe nanoclusters. In addition, microfluidic alignment of the cross-linked cylinders was possible.

Journal Article↗
PubMed2003

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.

Journal Article↗
PubMed2003

Bulk microphase segregation of an asymmetric organometallic-inorganic diblock copolymer: a remarkable example of concentric cylinders.

We report that an asymmetric diblock copolymer, poly(ferrocenyldimethylsilane-b-dimethylsiloxane) (PFS90-b-PDMS900, PDI = 1.01, volume fraction PFS = 0.20), self-assembles in the bulk state to form a hexagonal periodic structure with a remarkable morphology. Part of the major component forms the core of concentric cylinders with a mean diameter of 35.5 nm surrounded by a 7 nm thick shell of PFS. The remaining PDMS fills the interstitial spaces. The morphology was elucidated by small-angle X-ray scattering, as well as by scanning and conventional transmission electron microscopy.

Journal Article↗