Core-crosslinked compartmentalized cylinders

We present a detailed study on the preparation of compartmentalized cylindrical nanoparticles via a templated approach: the polybutadiene part of a linear polybutadiene-block-poly(2-vinyl pyridine)-block-poly(tert-butyl methacrylate) block terpolymer, B420V280T790, having a bulk microstructure with PB cylinders covered by a P2VP double helix and embedded in a PtBMA matrix was selectively crosslinked. Subsequent sonication-assisted dissolution and chemical modifications such as quaternization (P2VP to P2VPq) and ester hydrolysis (PtBMA to poly(sodium methacrylate), PMANa) resulted in core-crosslinked cylinders soluble in organic and aqueous media. Different amounts of crosslinker and the influence of the sonication treatment on size and shape of the cylindrical aggregates were investigated. The cylinders always exhibit a compartmentalized corona. Under certain conditions, in particular quaternization of P2VP in mixtures of THF and MeOH, the helical arrangement of the P2VPq shell could be preserved even in solution, whereas in most other cases randomly distributed P2VP/P2VPq patches were observed. In aqueous solution at high pH, intramicellar interpolyelectrolyte complex (im-IPEC) formation occurred between the positively charged P2VPq shell and the negatively charged PMANa corona. We further show that different noble metal nanoparticles can be generated either selectively within the im-IPEC compartments (Pd) or randomly distributed among shell and corona of the cylinders (Au and Pt).

Calcium phosphate mineralization beneath a polycationic monolayer at the air-water interface

The self-assembly of the amphiphilic block copolymer poly(n-butyl methacrylate)-block-poly[2-(dimethylamino)ethyl methacrylate] at the air-water interface has been investigated at different pH values. Similar to Rehfeldt et al. (J. Phys. Chem. B 2006, 110, 9171), the subphase pH strongly affects the monolayer properties. The formation of calcium phosphate beneath the monolayer can be tuned by the subphase pH and hence the monolayer charge. After 12 h of mineralization at pH 5, the polymer monolayers are still transparent, but transmission electron microscopy (TEM) shows that very thin calcium phosphate fibers form, which aggregate into cotton ball-like features with diameters of 20 to 50 nm. In contrast, after 12 h of mineralization at pH 8, the polymer film is very slightly turbid and TEM shows dense aggregates with sizes between 200 and 700 nm. The formation of calcium phosphate is further confirmed by Raman and energy dispersive X-ray spectroscopy. The calcium phosphate architectures can be assigned to the monolayer charge, which is high at low pH and low at high pH. The study demonstrates that the effects of polycations should not be ignored if attempting to understand the colloid chemistry of biomimetic mineralization. It also shows that basic block copolymers are useful complementary systems to the much more commonly studied acidic block copolymer templates.

Calcium phosphate growth beneath a polycationic monolayer at the air-water interface: effects of oscillating surface pressure on mineralization

The self-assembly of the amphiphilic block copolymer poly(butadiene)-block-poly[2-(dimethylamino)ethyl methacrylate] at the air-water interface and the mineralization of the monolayers with calcium phosphate was investigated at different pH values. As expected for polyelectrolytes, the subphase pH strongly affects the monolayer properties. The focus of the current study, however, is on the effect of an oscillating (instead of a static) polymer monolayer on calcium phosphate mineralization. Monitoring of the surface pressure vs. mineralization time shows that the monolayer is quite stable if the mineralization is performed at pH 8. In contrast, the monolayer at pH 5 shows a measurable decrease of the surface pressure already after ca. 2 h of mineralization. Transmission electron microscopy reveals that mineralization at low pH under constant oscillation leads to small particles, which are arranged in circular features and larger entities with holes of ca. 200 nm. The larger features with the holes disappear as the mineralization is continued in favor of the smaller particles. These grow with time and form necklace-like architectures of spherical particles with a uniform diameter. In contrast, mineralization at pH 8 leads to very uniform particle morphologies already after 2 h. The mineralization products consist of a circular feature with a dark dot in the center. The increasing contrast of the precipitates in the electron micrographs with mineralization time indicates an increasing degree of mineralization vs. reaction time. The study therefore shows that mechanical effects on mineralization at interfaces are quite complex.

Water-soluble organo-silica hybrid nanotubes templated by cylindrical polymer brushes

We report the preparation of water-soluble organo-silica hybrid nanotubes templated by core-shell-corona structured triblock terpolymer cylindrical polymer brushes (CPBs). The CPBs consist of a polymethacrylate backbone, a poly(tert-butyl acrylate) (PtBA) core, a poly(3-(trimethoxysilyl)propyl acrylate) (PAPTS) shell, and a poly(oligo(ethylene glycol) methacrylate) (POEGMA) corona. They were prepared via the "grafting from" strategy by the combination of two living/controlled polymerization techniques: anionic polymerization for the backbone and atom transfer radical polymerization (ATRP) for the triblock terpolymer side chains. The monomers tBA, APTS, and OEGMA were consecutively grown from the pendant ATRP initiating groups along the backbone to spatially organize the silica precursor, the trimethoxysilyl groups, into a tubular manner. The synthesized core-shell-corona structured CPBs then served as a unimolecular cylindrical template for the in situ fabrication of water-soluble organo-silica hybrid nanotubes via base-catalyzed condensation of the PAPTS shell block. The formed tubular nanostructures were characterized by transmission electron microscopy (TEM), cryogenic TEM, and atomic force microscopy.

Going beyond the surface: revealing complex block copolymer morphologies with 3D scanning force microscopy

We report on the quasi in situ scanning force microscopy nanotomography which proved to be a key method to effectively obtain a three-dimensional (3D) microdomain structure of a complex ABC triblock morphology. As an example, we studied polybutadiene-block-poly(2-vinyl pyridine)-block-poly(tert-butyl methacrylate) (BVT) thin triblock terpolymer films. We realized a controlled erosion of the material by using low-pressure plasma etching coupled to the scanning force microscope. The 3D reconstruction provides insights into the structural behavior in very thin volume elements revealing morphological details not accessible with other methods.

Direct Synthesis of Poly(potassium 3-sulfopropyl methacrylate) Cylindrical Polymer Brushes via ATRP Using a Supramolecular Complex With Crown Ether

A supramolecular complex between an ionic monomer 3-sulfopropyl methacrylate (SPMAK) and crown ether 18-crown-6 (18C6) has been employed to prepare a strong anionic cylindrical polyelectrolyte brush poly(potassium 3-sulfopropyl methacrylate) (PSPMAK) by atom transfer radical polymerization (ATRP) in polar solvent dimethyl sulfoxide (DMSO). This strategy solved the problem of the solubilities of the incompatible hydrophobic poly-initiator and hydrophilic ionic monomer. The formation of the PSPMAK brush is well proven by (1) H NMR, aqueous gel permeation chromatography (GPC), dynamic light scattering (DLS), static light scattering (SLS), atomic force microscopy (AFM), and cryogenic transmission electron microscopy (cryo-TEM) measurements. Cleavage of the side chains and further analysis reveal that the initiating efficiency of the polymerization is as low as 0.35.

Mixed, multicompartment, or Janus micelles? A systematic study of thermoresponsive bis-hydrophilic block terpolymers

We present a systematic investigation of the extent of compartmentalization in micelles formed by a series of bis-hydrophilic block terpolymers with two outer water-soluble segments. The corona blocks are constructed from poly(ethylene oxide) (PEO) and the thermoresponsive poly(N-isopropyl-acrylamide) (PNiPAAm). The fraction of PNiPAAm is varied to establish its influence on the supramicellar aggregation and corona phase behavior. We demonstrate that--when the collapse of PNiPAAm is triggered--a clustering of micelles into superstructures only occurs when the contour length of the thermoresponsive block is longer than that of the PEO chains. The volume fractions play a minor role. The extent of superstructure formation increases with the amount of heating cycles, pointing to a rearrangement of micelles with a mixed corona into a phase-segregated corona. The collapse of PNiPAAm is exploited to artificially raise the incompatibility and drive phase segregation. A uniform population of biphasic Janus micelles cannot be obtained. After repeated heating cycles, the mixture consists of a range of multicompartment architectures, whose patch distribution can be derived from aggregate structures found in cryo-TEM obtained at high temperature. In the last section, we relate our results to previously studied systems and attempt to derive some generalities. First, we try to answer the question of how likely it is in terms of thermodynamics to obtain truly self-assembled Janus micelles. Furthermore, our results can provide an estimation for the volume ratio or/and block lengths required in micelles composed out of two corona blocks to induce supramicellar aggregation when a hydrophilic-to-hydrophobic phase transition is triggered in one of the blocks.

Interpolyelectrolyte complexation in chloroform

Interpolyelectrolyte complexes (IPECs) were formed in chloroform from complementary polyelectrolyte-surfactant complexes (PESCs), i.e., linear polyelectrolytes whose counterions were substituted by surfactants to dissolve them in the low-polarity organic solvent. The interaction between such complementary PESCs was followed by turbidimetry, (1)H NMR, and dynamic light scattering. The experimental results, together with those from transmission electron microscopy and scanning force microscopy, provide evidence on the formation IPECs in the system. This process is apparently driven by the entropically favorable release of the pairs of the oppositely charged surfactant ions. If the mixing base-molar ratio between the complementary PESCs, Z, is below a certain threshold value, their chloroform mixtures are colloidally stable, containing relatively large aggregates. These aggregates are attributed to particles of the formed IPECs stabilized by the fragments of the excess polymeric component. Otherwise, the mixtures of the PESCs undergo phase separation (most pronounced at Z = 1) with the formation of an insoluble top phase (attributed to insoluble IPEC) and a clear bottom phase enriched with the surfactant counterions. Electron and scanning force micrographs indicate a rather broad size distribution of the soluble macromolecular coassemblies with a close to spherical shape.

Manipulating the morphologies of cylindrical polyelectrolyte brushes by forming interpolyelectrolyte complexes with oppositely charged linear polyelectrolytes: an AFM study

We present a study on water-soluble interpolyelectrolyte complexes (IPECs) formed by cationic cylindrical polyelectrolyte brushes (CPBs) and linear anionic poly(sodium styrenesulfonate) (PSSNa) using atomic force microscopy (AFM). The IPECs were prepared by dialysis of salt-containing solutions of the two polymeric components. The morphologies of the IPECs could be tuned by changing the charge ratio between the two polyelectrolytes, Z(-/+). Addition of increasing numbers of short PSSNa chains induced morphology changes of host CPBs from worms through intermediate pearl-necklace structures to fully collapsed spheres. Extremely long guest PSSNa caused the full collapse of the brushes to spheres even at very low charge ratios without intermediate states. In both cases we observe "disproportionation", that is, inhomogeneous distribution of the PSS chains between the CPB for Z(-/+) < 1. Unexpected micrometer-scale core-shell cylindrical objects were found by directly mixing CPBs with long PSSNa, which might be nonequilibrium structures caused by the kinetically controlled IPEC formation.