Block copolymers consist of two or more chemically different polymers connected by covalent linkages. In solution, repulsion between the blocks leads to a variety of morphologies, which are ...thermodynamically driven. Polyferrocenyldimethylsilane block copolymers show an unusual propensity to forming cylindrical micelles in solution. We found that the micelle structure grows epitaxially through the addition of more polymer, producing micelles with a narrow size dispersity, in a process analogous to the growth of living polymer. By adding a different block copolymer, we could form co-micelles. We were also able to selectively functionalize different parts of the micelle. Potential applications for these materials include their use in lithographic etch resists, in redox-active templates, and as catalytically active metal nanoparticle precursors.
Enhanced control over crystallization-driven self-assembly (CDSA) of coil-crystalline block copolymers has led to the formation of intricate structures with well-defined morphology and dimensions. ...While approaches to build those sophisticated structures may strongly differ from each other, they all share a key cornerstone: a polymer crystallite. Here we report a trapping technique that enables tracking of the change in length of one-dimensional (1D) polymer crystallites as they are annealed in solution at different temperatures. Using the similarities between 1D polymeric micelles and bottle-brush polymers, we developed a model explaining how the dissolving crystallites reach a critical size independent of the annealing temperature, and then explode in a cooperative process involving the remaining polymer chains of the crystallites. This model also allows us to demonstrate the role of the distribution in seed core crystallinity on the dissolution of the crystallites.
The emergence of one-dimensional (1D) micelles obtained from the crystallization driven self-assembly (CDSA) in solution of crystalline-coil block copolymers has opened the door to the fabrication of ...a variety of sophisticated structures. While the development of these fascinating nanomaterials is blossoming, there is very little fundamental work dedicated to understanding the morphological evolution of these 1D micelles in solution. Here, using a combination of transmission electron microscopy, electron tomography, and static and dynamic light scattering, we studied the effect of annealing on a colloidal suspension of 1D micelle fragments formed by the self-assembly of a crystalline-coil poly(ferrocenyldimethylsilane)-block-poly(isoprene) (PFS-b-PI) block copolymer in decane, a solvent selective for PI. We are particularly interested in studying the evolution of the rectangular cross-section of the crystalline core of these micelle fragments. By electron tomography, we observed that the shorter dimension of the cross-section became even thinner upon annealing at elevated temperatures, while the longer dimension increased. In parallel, we observed an increase in packing density of the crystalline block as the fragments were annealed at temperatures above 60 °C. From these results, we concluded that annealing the micelle fragments induces a thinning of the crystalline core coupled with a lateral growth.
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.
Short fragments of the core-crystalline micelles formed by a sample of poly(ferrocenyldimethylsilane)-block-poly(isoprene) (PFS-b-PI) block copolymer (BCP) underwent self-seeding in decane when ...heated above its dissolution temperature. Variable temperature (VT) 1H NMR and diffusion-ordered pulsed-gradient spin–echo (DOSY) NMR were used to monitor the behavior of micelles that dissolved as a function of increasing temperature. We examined a sample of micelle fragments of PFS65-b-PI637 characterized by L n = 39 nm and L w/L n = 1.13. The PI corona had high mobility and gave a 1H NMR signal in both micellar and unimer forms. In contrast, the PFS component could only be detected for the dissolved unimer. We found from 1H NMR that essentially all the BCP molecules were incorporated into the micelles at temperatures up to and including 50 °C, at the limit of NMR detection. Both PFS and PI resonances could be detected between 70 and 100 °C, and the integration ratio of the PFS-to-PI peaks increased with temperature. DOSY NMR measured the self-diffusion coefficients (D s) of the micelle fragments and unimer at these temperatures. The hydrodynamic radii (R h) for these species were calculated from D s using the Stokes–Einstein equation. The PFS signals gave R h values in the range of 5–6 nm at temperatures between 80 and 100 °C, consistent with unimer diffusion. PI signals were fitted by an exponential decay at 25 °C with R h = 38 nm characteristic of the micelle fragments and at 90, 95, and 100 °C with R h ≈ 6 nm, corresponding to unimer. At intermediate temperatures (70–85 °C), PI signals were fitted to a sum of two exponential terms, consistent with a fast diffusing species and a slow diffusing species. Interestingly, we noticed that the size of the micelle fragments at elevated temperatures (80 and 85 °C) was sensitive to sample history; samples heated directly to the elevated temperatures were found to be shorter than those heated stepwise.
Studying the growth of 1D structures formed by the self-assembly of crystalline-coil block copolymers in solution at elevated temperatures is a challenging task. Like most 1D fibril structures, they ...fragment and dissolve when the solution is heated, creating a mixture of surviving crystallites and free polymer chains. However, unlike protein fibrils, no new nuclei are formed upon cooling and only the surviving crystallites regrow. Here, we report how trapping these crystallites at elevated temperatures allowed us to study their growth kinetics at different annealing times and for different amounts of unimer added. We developed a model describing the growth kinetics of these crystallites that accounts for fragmentation accompanying the 1D growth process. We show that the growth kinetics follow a stretched exponential law that may be due to polymer fractionation. In addition, by evaluating the micelle growth rate as a function of the concentration of unimer present in solution, we could conclude that the micelle growth occurred in the mononucleation regime.
Many poly(ferrocenyldimethylsilane) (PFS) block copolymers form fiber-like micelles with a semicrystalline core in selective solvents. Solvent effects on micelle formation are not well understood. ...This paper compares micelle formation for a sample of PFS50–PI1000 (the subscripts refer to the number-average degrees of polymerization) in decane with that in tert-butyl acetate (tBA), a more polar solvent. When micelle formation is seeded, by adding block copolymer as a concentrated solution in tetrahydrofuran to solutions of micelle fragments, micelle growth was similar in both solvents. Micelles with a narrow length distribution were formed and the length increased in proportion to the amount of polymer added. In contrast, when micelles were prepared by heating a sample of the block copolymer in decane or tBA to 90 °C and allowing the solution to cool, pronounced differences were observed. In decane, micelles with a uniform width (10 nm) and a length on the order of 5 μm formed after 1 h, and grew to about 10 μm after 5 days. In tBA, aliquots taken from solution 1 h after cooling appeared to undergo microphase separation only upon solvent evaporation. Ribbon-like structures were observed after 1 and 5 days aging, but these evolved into fiber-like structures with a uniform 10 nm width and lengths greater than 30 μm after 25 days. These differences observed in the rate of micelle formation likely reflect differences in the nucleation stage of micelle formation. tBA is a better solvent for the PFS block than decane. As a consequence, it appears to take much longer for semicrystalline micelle nuclei to form in tBA. The seeded growth experiments demonstrate that once seed micelles are present, growth occurs similarly in both solvents.
Living growth of micelles on the substrate is an intriguing phenomenon; however, little is known about its growth kinetics, especially from a theoretical viewpoint. Here, we examine the living growth ...kinetics of polymeric micelles on a hydrophobic substrate immersed in an aqueous solution. The block copolymers first assemble into short cylinder seeds anchored on the substrate. Then, the small aggregates of block copolymers in the solutions fuse onto the active ends of the anchored seeds, leading to micelle growth on the substrate. A theoretical model is proposed to interpret such living growth kinetics. It is revealed that the growth rate coefficient on the substrate is independent of the copolymer concentration and the multistep feedings; however, it is significantly affected by the surface hydrophobicity. Brownian dynamics simulations further support the proposed growth mechanism and the kinetic model. This work enriches living assembly systems and provides guidance for fabricating bioinspired surface nanostructures.
Poly(squaramides) are a novel class of anion-responsive macromolecules that incorporate the diaminocyclobutenedione hydrogen bond donor group into the polymer backbone. Herein, the synthesis and ...properties of a series of fluorene-based poly(squaramides) varying in conformational rigidity, squaramide content, and propensity for aggregation are described. Structure–activity relationships for the anion sensory behavior of these polymers (as probed by fluorescence titrations, dynamic light scattering, confocal fluorescence microscopy, and transmission electron microscopy) indicate that anion-induced polymer aggregation leads to a cooperative response with enhanced levels of sensitivity and selectivity. These observations are consistent with a mechanism involving noncovalent cross-linking of polymer chains through squaramide–anion hydrogen-bonding interactions and point toward new applications of polyamides as stimulus-responsive materials.
Cellulose nanocrystals (CNCs) based aerogels with extremely low density and hierarchical porous structure were constructed via a facile Pickering emulsion-templated strategy. In this method, aminated ...CNCs (CNC-NH2) were synthesized to stabilize o/w Pickering high internal phase emulsions (Pickering HIPEs). Amino groups were introduced to CNCs to decrease of net surface charges of CNCs, and enhanced their aggregation, and therefore to achieve Pickering HIPEs stabilized by the particles of ultra-low content (~0.1 wt%). A series of CNC aerogels was then obtained by freeze-drying these emulsions. The resulting aerogels were ultrahigh-light with a density that reached ca. 0.5 mg/cm3 (an order of magnitude lower than that previously reported for CNC aerogels), and an ultrahigh-porosity (up to 99.969%). Contributed to the extreme low density, the thermal conductivity of the aerogels was around 0.021 W/(m∙K) which is lower than that of air (0.024 W/(m∙K)). This novel strategy could be applied to other materials, such as graphene and carbon nanotubes, to prepare ultralight aerogels with controllable porous structures and unique properties.
PDF
