Living Growth Kinetics of Polymeric Micelles on a Substrate.

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.

Anchorage-Dependent Living Supramolecular Self-Assembly of Polymeric Micelles.

Anchorage-dependent contact-inhibited growth usually refers to on-surface cell proliferation inhibited by the proximity of other cells. This phenomenon, prominent in nature, has yet to be achieved with polymeric micelles. Here, we report the control living supra-macromolecular self-assembly of elongated micelles with a liquid crystalline core onto a hydrophobic substrate via the synergetic interactions between the substrate and aggregates dispersed in solution. In this system, seed formation is a transient phenomenon induced by the adsorption and rearrangement of the core-swollen aggregates. The seeds then trigger the growth of elongated micelles onto the substrate in a living controllable manner until the contact with the substrate is disrupted. Brownian dynamic simulations show that this unique behavior is due to the fusion of the aggregates onto both ends of the anchored seeds. More important, the micelle length can be tuned by varying the substrate hydrophobicity, a key step toward the fabrication of intricate structures.

Ultralight Aerogels with Hierarchical Porous Structures Prepared from Cellulose Nanocrystal Stabilized Pickering High Internal Phase Emulsions.

Cellulose nanocrystal (CNC)-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 the net surface charges of CNCs, enhance their aggregation, and therefore achieve Pickering HIPEs stabilized by the particles of ultralow content (∼0.1 wt %). A series of CNC aerogels was then obtained by freeze drying these emulsions. The resulting aerogels were ultralight 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 extremely 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.

Visualizing Nanoscale Coronal Segregation in Rod-Like Micelles Formed by Co-Assembly of Binary Block Copolymer Blends.

Mixed micelles formed by co-assembly of pairs of block copolymers (BCPs) can develop novel morphologies and generate useful properties not accessible from homomicelles. For micelles consisting of two different polymers in the corona, identifying the location of the corona chains is a critical part of morphology characterization. Coronal segregation in mixed micelle is often characterized by transmission electron microscopy in combination with selective staining of individual polymers. In this study, Karstedt's catalyst is used for selective Pt(0)-olefin coordination staining of polyisoprene (PI) and poly(methylvinylsiloxane) (PMVS) corona chains in the presence of poly(dimethylsiloxane) (PDMS) corona chains in cylindrical mixed micelles with a crystalline poly(ferrocenyldimethylsilane) (PFS) core. Previous experiments using OsO4 as a stain did not enable visualization of nanoscale coronal segregation in mixed micelles obtained from co-assembly of PFS-b-PI and PFS-b-PDMS, as well as PFS-b-PMVS and PFS-b-PDMS.

Creating Biomorphic Barbed and Branched Mesostructures in Solution through Block Copolymer Crystallization.

Branched and barbed structures are common in nature but rare in nanoscale or mesoscale objects formed by bottom-up self-assembly. Key characteristics of the morphology of natural objects, such as various types of insects and conifer branches, is that despite their similarities no two individual objects are exactly the same. Here we report the self-assembly conditions for a series of poly(ferrocenyldimethylsilane)-block-polyisoprene (PFS-b-PI) diblock copolymers that generate structures with biomorphic shapes. All of these polymers yield long uniform fiber-like micelles with a crystalline PFS core in decane. Injection of a concentrated THF solution of these polymers into THF/decane mixtures, however, leads to barbed and branched mesostructures, with shapes that depend upon the final THF content of the mixed solvent. Interestingly, evaporation of the THF from suspensions of the colloidal biomorphic structures led to elongated fiber-like structures.

Temperature-Invariant Aqueous Microgels as Hosts for Biomacromolecules.

Immobilization of enzymes on solid supports has been widely used to improve enzyme recycling, enzyme stability, and performance. We are interested in using aqueous microgels (colloidal hydrogels) as carriers for enzymes used in high-temperature reactions. These microgels should maintain their volume and colloidal stability in aqueous media up to 100 °C to serve as thermo-stable supports for enzymes. For this purpose, we prepared poly(N-hydroxyethyl acrylamide) (PHEAA) microgels via a two-step synthesis. First, we used precipitation polymerization in water to synthesize colloidal poly(diethylene glycol-ethyl ether acrylate) (PDEGAC) particles as a precursor. PDEGAC forms solvent swollen microgels in organic solvents such as methanol and dioxane and in water at temperatures below 15 °C. In the second step, these PDEGAC particles were transformed to PHEAA microgels through aminolysis in dioxane with ethanolamine and a small amount of ethylenediamine. Dynamic laser scattering studies confirmed that the colloidal stability of microgels was maintained during the aminolysis in dioxane and subsequent transfer to water. Characterization of the PHEAA microgels indicated about 9 mol % of primary amino groups. These provide functionality for bioconjugation. As proof-of-concept experiments, we attached the enzyme horseradish peroxidase (HRP) to these aqueous microgels through (i) N-(3-(dimethylamino)propyl)-N'-ethylcarbodiimide hydrochloride (EDC) coupling to the carboxylated microgels or (ii) bis-aryl hydrazone (BAH) coupling to microgels functionalized with 6-hydrazinonicotinate acetone (PHEAA-HyNic). Our results showed that HRP maintained its catalytic activity after covalent attachment (87% for EDC coupling, 96% for BAH coupling). The microgel enhanced the stability of the enzyme to thermal denaturation. For example, the residual activity of the microgel-supported enzyme was 76% after 330 min of annealing at 50 °C, compared to only 20% for the free enzyme under these conditions. PHEAA microgels in water show great promise as hosts for enzymatic reaction, especially at elevated temperatures.

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.

Synthesis, self-assembly and photophysical properties of oligo(2,5-dihexyloxy-1,4-phenylene vinylene)-block-poly(ethylene glycol).

We describe the synthesis and characterization of a family of diblock copolymers with 5 units of a dihexyloxy-phenylenevinylene block (OHPV) connected to a series of poly(ethylene glycol) (PEG) chains of different average lengths (12, 45 and 115 PEG units: OHPV5-b-PEG12, OHPV5-b-PEG45, OHPV5-b-PEG115). All three polymers underwent self-assembly in ethanol, a good solvent for the PEG units, but poor for the OHPV segment. The nature of the structures formed depends sensitively on the length of the PEG block. OHPV5-b-PEG115 formed long fiber-like micelles of uniform width, whereas OHPV5-b-PEG45 formed fragile broad ribbons. We also obtained thin ribbons with OHPV5-b-PEG12 but they tend to fold and twist upon themselves. The structures obtained were characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM), as well as by wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). In addition, their photophysical properties were examined by UV-vis, steady state fluorescence and fluorescence decay measurements. The results of these experiments indicate that the OHPV groups pack differently in the fiber-like micelles of OHPV5-b-PEG115 than in the lamellar structures formed by OHPV5-b-PEG45.

Polymer-like conformation and growth kinetics of Bi2S3 nanowires.

One-dimensional inorganic crystals (i.e., crystalline nanowires) are one of the most intensely investigated classes of materials of the past two decades. Despite this intense effort, an important question has yet to be answered: do nanowires display some of the unique characteristics of polymers as their diameter is progressively decreased? This work addresses this question with three remarkable findings on the growth and form of ultrathin Bi(2)S(3) nanowires. (i) Their crystallization in solution is quantitatively describable as a form of living step-growth polymerization: an apparently exclusive combination of addition of "monomer" to the ends of the nanowires and coupling of fully formed nanowires "end-to-end", with negligible termination and initiation. (ii) The rate constants of these two main processes are comparable to those of analogous processes found in polymerization. (iii) The conformation of these nanowires is quantitatively described as a worm-like conformation analytically analogous to that of semiflexible polymers and characterized by a persistence length of 17.5 nm (shorter than that of double-stranded DNA) and contour lengths of hundreds of micrometers (longer than those of most synthetic polymers). These findings do not prove a chemical analogy between crystals and polymers (it is unclear if the monomer is a molecular entity tout court) but demonstrate a physical analogy between crystallization and polymerization. Specifically, they (i) show that the crystallization of ensembles of nanoscale inorganic crystals can be conceptually analogous to polymerization and can be described quantitatively with the same experimental and mathematical tools, (ii) demonstrate that one-dimensional nanocrystals can display topological characteristics of polymers (e.g., worm-like conformation in solution), (iii) establish a unique experimental model system for the investigation of polymer-like topological properties in inorganic crystals, and (iv) provide new heuristic guidelines for the synthesis of polymer-like nanowires.

Synthesis and network-like self-assembly of porphyrin-polyselenophene complexes.

A hexagonal network structure fabricated by self-assembly of a branched conjugated polymer with a porphyrin core and P3HT or P3HS arms is presented (see picture). Polymer symmetry is very important to the network structure formation probably due to the different viscosities in linear and branched polymers.

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.

Smart polymer nanoparticles designed for environmentally compliant coatings.

We describe the synthesis, characterization, and film-forming properties of two-component nanoparticles that undergo a reversible morphology transformation in water as a function of pH. The particles consist of a high molecular weight acrylate copolymer and an acid-rich oligomer designed to be miscible with the polymer when its -COOH groups are protonated. Attaching a fluorescence resonance energy transfer (FRET) pair to components inside the nanoparticles enabled us to assess morphology at the molecular level. By inspecting changes in the donor fluorescence decay profile at different pH values, we established miscibility of the components in acidic solution but with charge-induced phase separation when the oligomers were neutralized to their carboxylate form. Complementary titration experiments revealed that the nanoparticles adopt a core-shell structure when the acid groups are deprotonated. We studied the effect of the acid-rich oligomer on the diffusion rate of the high molecular weight polymers following film formation. Our results show that the carboxylated oligomer enhanced the rate of diffusive mixing between high molecular weight molecules by more than 2 orders of magnitude. FRET measurements carried out on partially dried films using a low-resolution microscope showed that the carboxylate oligomer shell can delay coalescence for ca. 30 min after passage of the drying front. This delay is expected to help with increasing the 'open time' of latex paints, a desirable property of solvent-based paints that remains difficult to achieve with (environmentally compliant) waterborne paints. Use of ammonia as a volatile base resulted in synergistic effects: initial retardation of coalescence followed by acceleration of diffusive mixing as the ammonium salts dissociated and ammonia evaporated from the film.

In-Depth Analysis of the Effect of Fragmentation on the Crystallization-Driven Self-Assembly Growth Kinetics of 1D Micelles Studied by Seed Trapping.

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.

Triggered instability of liposomes bound to hydrophobically modified core-shell PNIPAM hydrogel beads.

The ability to trigger a destabilization of the membrane integrity of liposomes bound to environmentally sensitive hydrophobically modified core-shell hydrogel beads is demonstrated. Hydrogel beads with a core composed of poly(N-isopropylacrylamide) lightly cross-linked with bisacrylamide (BA) (pNIPAM) and a shell composed of NIPAM highly cross-linked with BA and containing varying amounts of acrylic acid (AA) [p(NIPAM-co-AA)] undergo a volume phase transition (VPT) at approximately 32 degrees C, as determined from (1)H magic angle spinning (MAS) NMR, regardless of the AA content of the shell. When the shell was hydrophobically modified with either decylamine or tetradecylamine, binding of extruded large unilamellar vesicles (eLUVs) composed of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) was quantitative, as determined via fluorescence spectroscopy. Fluorescence microscopy showed that such bound eLUVs did not fuse. Hydrogel-bound eLUV membrane permeability was assessed using (31)P MAS NMR in the presence of the chemical shift agent praseodymium and demonstrated that only at lower degrees of hydrophobic modification of the core-shell hydrogels was eLUV membrane barrier integrity maintained when T < VPT. At a low degree of hydrophobic modification, cycling the temperature above the VPT even for short periods caused the eLUV membranes to become leaky. Hence, eLUV membrane permeability was coupled to the hydrogel VPT, a situation that would be useful in applications requiring triggered release of liposomal contents.

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.

Seeded growth and solvent-induced fragmentation of fiberlike polyferrocenylsilane-polyisoprene block copolymer micelles.

Addition of a concentrated solution of PI(1000) -PFS(50) dissolved in THF to a solution of PI(1000) -PFS(50) seed micelles in decane led to the formation of uniform elongated fiberlike micelles with a narrow length distribution. When additional THF (>10 vol.-%) was added to the micelles, the micelle length decreased and the contour-length distribution broadened. This effect was shown to be inconsistent with a transition to an equilibrium, in which individual polymer molecules dissociated from and added to existing micelles. Rather, it appears that the polar solvent induced fragmentation of the fiberlike micelles.

Fabrication of bicontinuous double networks as thermal and pH stimuli responsive drug carriers for on-demand release.

The fabrication, via a two steps approach, of a novel bicontinuous Double Network (BCDN) material is reported. We first use a bicontinuous emulsion as template to obtain a poly(butyl acrylate) (PBA) and poly(acrylic acid) (PAA) bicontinuous amphiphilic material. The material is then swollen with the precursor of a second hydrophilic polymer (PNIPAM, poly(N-isopropylacrylamide)). After polymerization of these precursors, the two responsive polymers, PNIPAM and PAA, form a double-network within a bicontinuous templated material, i.e. a bicontinuous double network (BCDN) material. The advantages of using such unique and complex double network architecture are manifold. PBA increases the mechanical properties of the hydrogel all together with the hydrophilic double network that also decouples the pH and temperature responsiveness. Among different possible applications, we tested this responsive hydrogels for its biomedical application. It can be used as pH and temperature sensitive devices for on-demand drug delivery. In addition, the release of a drug confined in the amphiphilic bicontinuous structure follows different kinetics profiles, depending on pH and temperature. This last result indicates that it is possible to control and regulate the release of an encapsulated drug according to the fluctuations of physiological conditions.

Formation of 2D and 3D multi-tori mesostructures via crystallization-driven self-assembly.

The fabrication of three-dimensional (3D) objects by polymer self-assembly in solution is extremely challenging. Here, multi-tori mesostructures were obtained from the crystallization-driven self-assembly of a coil-crystalline block copolymer (BCP) in mixed solvents. The formation of these structures follows a multistep process. First, the BCP self-assembles into amorphous micrometer-large vesicles. Then, the BCP confined in these mesosized vesicles crystallizes. This second step leads to the formation of objects with shapes ranging from closed 3D multi-tori spherical shells to 2D toroid mesh monolayers, depending on the solvent mixture composition. This approach demonstrates how topological constraints induced by the specific interactions between coil-crystalline BCP and solvents can be used to prepare mesostructures of complex morphologies.

Synergistic self-seeding in one-dimension: a route to patchy and block comicelles with uniform and controllable length.

Self-seeding is a process unique to polymer crystals, which consist of regions of different chain packing order and different crystallinity. Here we report the synergistic self-seeding behaviour of pairs of core-crystalline block copolymer (BCP) micelle fragments and show how this strategy can be employed to control the morphology of these BCP comicelles. Each micelle fragment has a critical dissolution temperature (T c), and unimers of each BCP have a characteristic epitaxial growth rate. The T c value affects the dissolution sequence of the fragments upon heating, while the unimer growth rate affects the growth sequence upon cooling. By carefully choosing micelle fragments having different T c values as well as growth rates, we could prepare patchy comicelles and block comicelles with uniform and controllable length. This synergistic self-seeding strategy is a simple yet effective route to control both length and morphology of core-crystalline comicelles.

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.