Gerald Guerin

Self-seeding in one dimension: an approach to control the length of fiberlike polyisoprene-polyferrocenylsilane block copolymer micelles.

Self-seeding is a phenomenon unique for polymer crystallization. Polymers have difficulty in crystallizing and, in most cases, only part of each polymer chain can be accommodated in the crystal lattice. As a result, polymers form crystals with lamellar structures terminated by surfaces containing chain folds. If long chains have to be integrated into the crystal in a short time, they will do so at the expense of lower crystallinity. As a consequence, polymer crystals inevitably consist of regions with different chain order and conformational entropy. Polymer crystals have a broad range of melting temperatures whose values depend upon the details of the crystallization process. In a typical self-seeding experiment, a crystalline polymer in the bulk state or suspended in a solvent is heated slightly above its normal melting point (as determined, for example, by differential scanning calorimetry; DSC) so that no residual crystals can be detected optically or spectroscopically. Cooling this melt or solution leads to the formation of polymer single crystals, normally in the form of thin plates uniform in size and thickness, which can be ideally suited for further applications. These single crystals are thought to be initiated by submicroscopic nuclei that survived the dissolution procedure. Since the discovery of self-seeding in the 1960s the process has attracted attention as a means of controlling the nucleation step of polymer crystallization without the need for external nucleating agents, to form uniform single crystals of homopolymers and block copolymers and also for materials applications. Polyferrocenyldimethylsilane (PFS) is a crystalline metalcontaining polymer with a range of interesting properties. PFS block copolymers and closely related materials selfassemble to form elongated micelles with a semicrystalline core. They are the only currently known synthetic polymers to form fiberlike micelles by a mechanism resembling that for the formation of amyloid fibers from soluble protein. Thus soluble polymeric “monomers” consisting of block copolymer unimers condense onto both ends of seed structures present in, or intentionally added to, the solution. PFS block copolymer fiber formation involves a conformation change driven by epitaxial crystallization of PFS moieties onto the open ends of the PFS core of existing micelles or seeds obtained by subjecting preformed fiberlike micelles to mild sonication. Thus, the number of micelles at the end of the growth process is determined by the number of seeds present at the beginning. Moreover the seeded growth experiments permit exquisite control over the types of structures obtained. For example, one type of PFS block copolymer such as PI-PFS (PI = polyisoprene) can be used to form the seed structure, and a different type of PFS block copolymer such as PFSPDMS (PDMS = polydimethylsiloxane) can be grown off the ends. In this way striking novel architectures referred to as “triblock co-micelles” can be prepared. 10] Our recent work targets a deeper understanding of the self-assembly process for PFS block copolymers in order to develop principles that can be extended to other coil-crystalline block copolymers. This may allow access to processable suspensions of semiflexible nanowires with useful optical or electronic properties. With proper control over their length and dimensions, such structures could be incorporated into optoelectronic devices or used in other applications. In 2009, Reiter and co-workers examined the mechanism of self-seeding in the melt for PFS homopolymer single crystals and for single crystals formed by P2VP-PEO block copolymers. (P2VP = poly(2-vinylpyridine), PEO = poly(ethylene oxide)). They showed for these two systems that the number density of the regenerated crystals decreased exponentially with the increase of the dissolution temperature but did not vary with the dissolution time. They also found a correlation in molecular orientation between a starting single crystal and the regenerated crystal clones formed through the self-seeding process. Their experiments established that single-crystal growth by self-seeding operates under thermodynamic control, consistent with the idea that upon heating, the less perfect crystals will melt and more perfect crystallites will survive. It is not a kinetic effect associated with polymer conformational memory effects. [*] J. S. Qian, Dr. G. Guerin, Y. J. Lu, G. Cambridge, Prof. M. A. Winnik Department of Chemistry, University of Toronto 80 St. George Street Toronto, Ontario, M5S 3H6 (Canada) Fax: (+ 1)416-978-0541 E-mail: mwinnik@chem.utoronto.ca

Highly luminescent lead sulfide nanocrystals in organic solvents and water through ligand exchange with poly(acrylic acid).

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.

Anion Detection by a Fluorescent Poly(squaramide): Self‐Assembly of Anion‐Binding Sites by Polymer Aggregation

The selective recognition and sensing of anions has been the subject of intensive research effort, motivated by applications in medical diagnostics, environmental and industrial monitoring, and nuclear waste cleanup. Selectivity and affinity may be achieved by preorganization of binding groups in an orientation complementary to the geometry and charge distribution of the anionic guest: sulfateand phosphatebinding proteins represent impressive illustrations of this principle. While there has been some progress toward synthetic hosts capable of high-affinity, selective anion recognition in competitive (aqueous) environment, the synthesis of preorganized targets having the ideal number and orientation of binding groups for a given analyte is often difficult, and rates of guest binding and release may become problematic. An attractive solution to this problem employs reversible processes (noncovalent or dynamic covalent interactions) as the basis for receptor self-assembly. Self-assembled capsules, dynamic covalent libraries, and coordination complexes capable of anion binding are successful implementations of this concept. Here, we describe experiments demonstrating that aggregation of an organic polymer composed of repeating hydrogen-bond donor groups may be exploited to achieve remarkable enhancements in anion affinity and selectivity. In particular, a polymer based on the 3,4-diaminocyclobutene1,2-dione (squaramide) functional group shows a selective “turn-on” fluorescence response to dihydrogenphosphate (H2PO4 ) ions in the competitive medium 10% water in Nmethylpyrrolidinone (NMP). The dual role of the squaramide groups in controlling both the aggregation of the polymer and its anion-responsive properties results in complex behavior, including cooperativity in analyte binding. Comparison with a non-polymeric reference compound indicates that incorporation of the squaramide group into a polymer results in an unprecedented alteration of anion selectivity as well as an enhancement in anion sensitivity. Our investigations began with the synthesis of poly(squaramide) 1a (Scheme 1). This material is unlikely to be capable of long-range exciton transport, a property that has been exploited to generate signal “gain” in conjugated polymer-

Self-Seeding in One Dimension: A Route to Uniform Fiber-like Nanostructures from Block Copolymers with a Crystallizable Core-Forming Block

One-dimensional micelles formed by the self-assembly of crystalline-coil poly(ferrocenyldimethylsilane) (PFS) block copolymers exhibit self-seeding behavior when solutions of short micelle fragments are heated above a certain temperature and then cooled back to room temperature. In this process, a fraction of the fragments (the least crystalline fragments) dissolves at elevated temperature, but the dissolved polymer crystallizes onto the ends of the remaining seed fragments upon cooling. This process yields longer nanostructures (up to 1 μm) with uniform width (ca. 15 nm) and a narrow length distribution. In this paper, we describe a systematic investigation of factors that affect the self-seeding behavior of PFS block copolymer micelle fragments. For PI(1000)-PFS(50) (the subscripts refer to the number average degree of polymerization) in decane, these factors include the presence of a good solvent (THF) for PFS and the effect of annealing the fragments prior to the self-seeding experiments. THF promoted the dissolution of the micelle fragments, while preannealing improved their stability. We also extended our experiments to other PFS block copolymers with different corona-forming blocks. These included PI(637)-PFS(53) in decane, PFS(60)-PDMS(660) in decane (PDMS = polydimethylsiloxane), and PFS(30)-P2VP(300) in 2-propanol (P2VP = poly(2-vinylpyridine)). The most remarkable result of these experiments is our finding that the corona-forming chain plays an important role in affecting how the PFS chains crystallize in the core of the micelles and, subsequently, the range of temperatures over which the micelle fragments dissolve. Our results also show that self-seeding is a versatile approach to generate uniform PFS fiber-like nanostructures, and in principle, the method should be extendable to a wide variety of crystalline-coil block copolymers.

Healing of interfaces of amorphous and semi-crystalline poly(ethylene terephthalate) in the vicinity of the glass transition temperature

Abstract Healing of interfaces of amorphous and semi-crystalline poly(ethylene terephthalate) (PET) was carried out above and below the (bulk) glass transition temperature ( T g ) of the samples (two amorphous and two semi-crystalline polymers of different molecular weights). The lap-shear strength of the amorphous/amorphous interface was found to develop without discontinuity in the vicinity of the T g and, in addition, it develops at amorphous/amorphous and amorphous/crystalline interfaces to the one-fourth power of healing time indicating that it is, in both cases, a diffusion controlled process. Similar values of strength were found with these two interfaces whereas the strength at the crystalline/crystalline interface of PET was at least one order of magnitude lower after healing under the same conditions. However, the largest values of strength were obtained by a procedure where diffusion is followed by crystallisation. These results were compared with those measured for an amorphous and incompatible PS/PET interface, below and above the T g s of PS and PET, and it was found that the strength of the incompatible PS/PET interface is close to the strength of the compatible PET/PET interface. A comparison of the strength developed at symmetric amorphous interfaces of crystallisable PET and non-crystallisable atactic PS showed a more rapid growth in strength in the second case, even at temperatures below T g .

Fragmentation of Fiberlike Structures: Sonication Studies of Cylindrical Block Copolymer Micelles and Behavioral Comparisons to Biological Fibrils

In alkane solvents, poly(isoprene-b-ferrocenyldimethylsilane) (PI-b-PFS) block copolymer forms fiberlike micelles, which show intriguing similarities with biological fibers such as amyloid fibers. Both systems exhibit fiber growth by a nucleated self-assembly mechanism and rapidly fragment upon exposure to the shear forces of ultrasonic irradiation. Sonication of PI-b-PFS cylindrical micelles was studied quantitatively by static light scattering and by electron microscopy. Both techniques are in excellent agreement and show that the weight-average length of sonicated micelles decreases as a function of sonication time. Simulation of the cleavage of micelles using different scission models shows that micelle fragmentation follows a Gaussian model and that the scission is highly dependent on micelle length, in contrast to DNA and polymer chain scission. We speculate that biological fibers, which are similar in length and rigidity to PFS block copolymer micelles, fragment by a similar mechanism when subjected to sonication.

A design strategy for the hierarchical fabrication of colloidal hybrid mesostructures

Advances in nanotechnology depend upon expanding the ability to create new and complex materials with well-defined multidimensional mesoscale structures. The creation of hybrid hierarchical structures by combining colloidal organic and inorganic building blocks remains a challenge due to the difficulty in preparing organic structural units of precise size and shape. Here we describe a design strategy to generate controlled hierarchical organic-inorganic hybrid architectures by multistep bottom-up self-assembly. Starting with a suspension of large inorganic nanoparticles, we anchor uniform block copolymer crystallites onto the nanoparticle surface. These colloidally stable multi-component particles can initiate the living growth of uniform cylindrical micelles from their surface, leading to three-dimensional architectures. Structures of greater complexity can be obtained by extending the micelles via addition of a second core-crystalline block copolymer. This controlled growth of polymer micelles from the surface of inorganic particles opens the door to the construction of previously inaccessible colloidal organic-inorganic hybrid structures.

Templated Fabrication of Fiber-Basket Polymersomes via Crystallization-Driven Block Copolymer Self-Assembly

Immobilizing uniform nanostructures on a mesoscale substrate is a promising approach to prepare nanometer to micrometer sized materials with new functionalities. The hierarchical structures formed depend on both the nature of the substrate and the components deposited. In this paper, we describe the use of colloidal polystyrene microbeads as a sacrificial template to create a nanofibrous network coating consisting of elongated block copolymer micelles. This network has a secondary structure very different from that of conformal coatings obtained by other methods. In addition, the fibers of the network could be elongated by crystallization-driven self-assembly. The network was locked in place by cross-linking the micelles through in situ generation of small Pt nanoparticles. Subsequent removal of the sacrificial template gave an open vesicular structure. To demonstrate further transformation of the membrane, we showed that the cross-linked micelles could also be used to embed silver nanoparticles. The sacrificial template contained known amounts of Tb and Tm ions, allowing us to estimate via atomic mass spectrometry that 85% of the template surface was covered with micelle seeds. This approach to fabricating hierarchical coating structures expands the generality and scope of template-assisted synthesis to build advanced hierarchical materials with precise morphological control.

Fiberlike Micelles Formed by Living Epitaxial Growth from Blends of Polyferrocenylsilane Block Copolymers

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.

Improving Lanthanide Nanocrystal Colloidal Stability in Competitive Aqueous Buffer Solutions using Multivalent PEG-Phosphonate Ligands

The range of properties available in the lanthanide series has inspired research into the use of lanthanide nanoparticles for numerous applications. We aim to use NaLnF(4) nanoparticles for isotopic tags in mass cytometry. This application requires nanoparticles of narrow size distribution, diameters preferably less than 15 nm, and robust surface chemistry to avoid nonspecific interactions and to facilitate bioconjugation. Nanoparticles (NaHoF(4), NaEuF(4), NaGdF(4), and NaTbF(4)) were synthesized with diameters from 9 to 11 nm with oleic acid surface stabilization. The surface ligands were replaced by a series of mono-, di-, and tetraphosphonate PEG ligands, whose synthesis is reported here. The colloidal stability of the resulting particles was monitored over a range of pH values and in phosphate containing solutions. All of the PEG-phosphonate ligands were found to produce non-aggregated colloidally stable suspensions of the nanoparticles in water as judged by DLS and TEM measurements. However, in more aggressive solutions, at high pH and in phosphate buffers, the mono- and diphosphonate PEG ligands did not stabilize the particles and aggregation as well as flocculation was observed. However, the tetraphosphonate ligand was able to stabilize the particles at high pH and in phosphate buffers for extended periods of time.