Complex and Hierarchical 2D Assemblies via Crystallization-Driven Self-Assembly of Poly(L-lactide) Homopolymers with Charged Termini

Poly­(l-lactide) (PLLA)-based nanoparticles have attracted much attention with respect to applications in drug delivery and nanomedicine as a result of their biocompatibility and biodegradability. Nevertheless, the ability to prepare PLLA assemblies with well-defined shape and dimensions is limited and represents a key challenge. Herein we report access to a series of monodisperse complex and hierarchical colloidally stable 2D structures based on PLLA cores using the seeded growth, “living-crystallization-driven self-assembly” method. Specifically, we describe the formation of diamond-shaped platelet micelles and concentric “patchy” block co-micelles by using seeds of the charge-terminated homopolymer PLLA₂₄[PPh₂Me]I to initiate the sequential growth of either additional PLLA₂₄[PPh₂Me]I or a crystallizable blend of the latter with the block copolymer PLLA₄₂-<i>b</i>-P2VP₂₄₀, respectively. The epitaxial nature of the growth processes used for the creation of the 2D block co-micelles was confirmed by selected area electron diffraction analysis. Cross-linking of the P2VP corona of the peripheral block in the 2D block co-micelles using Pt nanoparticles followed by dissolution of the interior region in good solvent for PLLA led to the formation of novel, hollow diamond-shaped assemblies. We also demonstrate that, in contrast to the aforementioned results, seeded growth of the unsymmetrical PLLA BCPs PLLA₄₂-<i>b</i>-P2VP₂₄₀ or PLLA₂₀-<i>b</i>-PAGE₈₀ alone from 2D platelets leads to the formation of diamond-fiber hybrid structures

Uniform “Patchy” Platelets by Seeded Heteroepitaxial Growth of Crystallizable Polymer Blends in Two Dimensions

Rectangular platelets formed by the self-assembly of block copolymers in selective solvents are of interest for a range of applications. Recently, we showed that the seeded growth of crystallizable blends of a block copolymer and homopolymer yields well-defined, low area dispersity examples of these two-dimensional (2D) structures. The key feature was the use of the same crystallizable polymer segment in the seed and blend components to enable an efficient homoepitaxial growth process. Herein we demonstrate that this 2D crystallization-driven self-assembly approach can be extended to <i>heteroepitaxial</i> growth by the use of different crystallizable polymers with compatible crystal structures. This allows the formation of well-defined “patchy” rectangular platelets and platelet block comicelles with different core chemistries. The use of scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy provided key information on the spatial location of the components in the resulting assemblies and thereby valuable insight into the 2D heteroepitaxial growth process

Dimensional control and morphological transformations of supramolecular polymeric nanofibers based on cofacially-stacked planar amphiphilic platinum(II) complexes

Square-planar platinum­(II) complexes often stack cofacially to yield supramolecular fiber-like structures with interesting photophysical properties. However, control over fiber dimensions and the resulting colloidal stability is limited. We report the self-assembly of amphiphilic Pt­(II) complexes with solubilizing ancillary ligands based on polyethylene glycol [PEG_<i>n</i>, where <i>n</i> = 16, 12, 7]. The complex with the longest solubilizing PEG ligand, <b>Pt-PEG</b>_<b>16</b>, self-assembled to form polydisperse one-dimensional (1D) nanofibers (diameters <5 nm). Sonication led to short seeds which, on addition of further molecularly dissolved <b>Pt-PEG</b>_<b>16</b> complex, underwent elongation in a “living supramolecular polymerization” process to yield relatively uniform fibers of length up to <i>ca</i>. 400 nm. The fiber lengths were dependent on the <b>Pt-PEG</b>_<b>16</b> complex to seed mass ratio in a manner analogous to a living covalent polymerization of molecular monomers. Moreover, the fiber lengths were unchanged in solution after 1 week and were therefore “static” with respect to interfiber exchange processes on this time scale. In contrast, similarly formed near-uniform fibers of <b>Pt-PEG</b>_<b>12</b> exhibited dynamic behavior that led to broadening of the length distribution within 48 h. After aging for 4 weeks in solution, <b>Pt-PEG</b>_<b>12</b> fibers partially evolved into 2D platelets. Furthermore, self-assembly of <b>Pt-PEG</b>_<b>7</b> yielded only transient fibers which rapidly evolved into 2D platelets. On addition of further fiber-forming Pt complex (<b>Pt-PEG</b>_<b>16</b>), the platelets formed assemblies <i>via</i> the growth of fibers selectively from their short edges. Our studies demonstrate that when interfiber dynamic exchange is suppressed, dimensional control and hierarchical structure formation are possible for supramolecular polymers through the use of kinetically controlled seeded growth methods

Crystallization-Driven Self-Assembly of Block Copolymers with a Short Crystallizable Core-Forming Segment: Controlling Micelle Morphology through the Influence of Molar Mass and Solvent Selectivity

Three well-defined asymmetric crystalline-coil poly­(ferrocenyldimethylsilane-<i>block</i>-2-vinylpyridine) (PFS-<i>b</i>-P2VP) diblock copolymers (PFS₄₄<b>-</b><i>b</i>-P2VP₂₆₄, PFS₇₅<b>-</b><i>b</i>-P2VP₄₅₄, and PFS₁₀₂<b>-</b><i>b</i>-P2VP₆₂₅) with similar block ratios (<i>r</i> = <i>N</i>_P2VP/<i>N</i>_PFS = ca. 6.0 ± 0.1) but different overall molar masses (<i>M</i>_n = 38 700, 65 800, and 90 400 g mol^–1) were synthesized by sequential anionic polymerization, and their solution self-assembly behavior was explored as a function of (i) molar mass and (ii) the ratio of common to selective solvent. When self-assembly was performed in isopropanol (i-PrOH), a selective solvent for P2VP, a decrease in the rate of the crystallization-driven transition from the initially formed spheres (with amorphous PFS cores) into cylinders (with crystalline cores) was detected with an increase in molecular weight. This trend can be explained by a decrease in the rate of crystallization for the PFS core-forming block as the chain length increased. In contrast, when a mixture of i-PrOH with increasing amounts of THF, a common solvent for both blocks, was used, spheres, cylinders, and also narrow lenticular platelets consisting of crystallized PFS lamellae sandwiched by two glassy coronal P2VP layers were formed from the same PFS_<i>x</i>-<i>b</i>-P2VP_6<i>x</i> sample. The most likely explanation involves the plasticization of the PFS core-forming block which facilitates crystallization, possibly complemented by contraction of the coils of the P2VP coronal block which otherwise limit of the lateral growth of the crystalline PFS core as THF is a poorer solvent for P2VP than i-PrOH. Selected area electron diffraction studies indicated that the PFS cores of the spherical micelles were amorphous but were consistent with those of the cylindrical micelles existing in a state approximating to that of a monoclinic PFS single crystal. In contrast, in the platelets formed in THF/i-PrOH, the PFS cores were found to be polycrystalline. The formation of narrow lenticular polycrystalline platelets rather than a regular, rectangular single crystalline morphology was attributed to a poisoning effect whereby the interference of the long P2VP coronal blocks in the growth of a rectangular PFS single crystalline core introduces defects at the crystal growth fronts

Optimizing Seed Aging for Single Crystal Gold Nanorod Growth: The Critical Role of Gold Nanocluster Crystal Structure

Controlled synthesis of single crystal gold nanorods (Au NRs) with desired dimensions, narrow polydispersity, and minimal byproducts depends critically on the state of the seed solution. Herein, we clarify the growth of cetyltrimethylammonium bromide stabilized Au seeds and their impact on conventional seed-mediated growth of Au NRs. UV–vis spectroscopy, TEM microscopy, and small-angle X-ray scattering reveal that during aging of the seed solution, constituents evolve from solubilized Au-salt precursors (Au­[III]) to Au nanoclusters (Au_<i>n</i>) to multitwinned Au nanocrystals. The most consistent single crystal Au NR growth, with minimal byproducts, occurs when the seed solution contains the maximum concentration of small (<1 nm) nanoclusters (<i>n</i> < 55, 1 min < <i>t</i>_age < 10 min at <i>T</i> = 25 °C). This point can be identified spectroscopically between 300 and 480 nm. Additionally, the optimal clusters coincide approximately with a relative shift in structural equilibrium of closed-shell Au nanoclusters from predominately cuboctahedral to icosahedral. Geometrically, a cuboctahedral cluster can evolve into a single crystal fcc lattice, whereas an icosahedral cluster must adopt a twinned structure upon growth. This shift is consistent with the increased number fraction of polycrystalline byproducts (e.g., twinned spherical particles and rods) if the seeds are aged beyond this point. Therefore, these observations suggest that the unique structural properties of Au_<i>n</i> nanoclusters are paramount to the initial formation of single crystal seeds and their subsequent anisotropic growth