Self-assembly of inorganic nanoparticle vesicles and tubules driven by tethered linear block copolymers

Controllable self-assembly of nanoscale building blocks into larger specific structures provides an effective route for the fabrication of new materials with unique optical, electronic, and magnetic properties. The ability of nanoparticles (NPs) to self-assemble like molecules is opening new research frontiers in nanoscience and nanotechnology. We present a new class of amphiphilic "colloidal molecules" (ACMs) composed of inorganic NPs tethered with amphiphilic linear block copolymers (BCPs). Driven by the conformational changes of tethered BCP chains, such ACMs can self-assemble into well-defined vesicular and tubular nanostructures comprising a monolayer shell of hexagonally packed NPs in selective solvents. The morphologies and geometries of these assemblies can be controlled by the size of NPs and molecular weight of BCPs. Our approach also allows us to control the interparticle distance, thus fine-tuning the plasmonic properties of the assemblies of metal NPs. This strategy provides a general means to design new building blocks for assembling novel functional materials and devices.     

Temperature‐dependent location of a weakly segregated block copolymer in binary blends of block copolymers

The phase behaviors of binary blends of poly(styrene‐b‐butadiene) block copolymers were investigated by a small‐angle X‐ray scattering technique. The blends were composed of weakly segregated one in a random micellar phase and the other in a cylindrical phase with similar molecular weights and complementary volume fractions. Morphologies, domain spacings, and order–disorder transition temperatures of the blends indicated that the junctions of the constituent block copolymers share the interface at low temperatures. The domain spacing decreased as temperature increased in a blend with a small amount of the weakly segregated block copolymer. In the cases of the blends with a large amount of the weakly segregated constituent, domain spacing increased with increasing temperature. These results implied that some of the weakly segregated block copolymer moved from the interface to one microdomain at higher temperatures. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 470–476     

Self‐assembly in solutions of block and random copolymers during metal nanoparticle formation

The self‐assembly behavior of poly(isoprene‐b‐acrylic acid) and poly(styrene‐b‐2‐vinylpyridine) amphiphilic block copolymers, as well as a poly(styrene‐r‐2‐vinylpyridine) amphiphilic random copolymer was investigated in slightly selective organic solvents (tetrahydrofuran and toluene) in the presence of Ag and Au ions and subsequently Ag, Au metal nanoparticles, by means of dynamic light scattering. In the range of concentrations studied the copolymers exist in the form of micelles with cores composed of acrylic acid and 2‐vinylpyridine segments in equilibrium with unimers. The addition of metal ions and their subsequent transformation to metal nanoparticles shifts the equilibrium in favor of the micelles. The concentration of the inorganic components has also a considerable effect on the size of the polymeric aggregates. A similar behavior is observed for the random copolymer. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR‐FTIR), UV‐visible spectroscopy, and transmission electron microscopy (TEM) give valuable additional information on the nature of the interactions between the polymeric and inorganic components, as well as on the characteristics of the metal nanoparticles and the hybrid micelles formed in each case. The presented results have a direct relation to the synthesis of metal nanoparticles under confinement by utilization of copolymer nanoreactors and appropriate solution conditions. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1515–1524, 2008     

Electroactive block copolymers

Synthesis, characterization and properties of microphase separated mixed (ionic and electronic) conducting or MIEC block copolymers are reported. Poly{[ω-methoxyocta(oxyethylene) methacrylate]-block-(4-vinylpyridine)}, abbreviated as P[MG8–4VP], and poly{(3-methylthiophene)-block-[(ω-methoxyocta(oxyethylene) methacrylate]}, abbreviated as P[3MT-MG8], have been synthesized. Differential scanning calorimetry (DSC) studies indicate that the polymers form a microphase separated structure. P[3MT-MG8] can be doped with I₂ and LiClO₄ to generate electronic and ionic conducting microdomains, respectively. For the P[3MT-MG8] series, bulk electronic conductivity as high as 1×10⁻³ S cm⁻¹ and bulk ionic conductivity as high as 6.6×10⁻⁷ S cm⁻¹ is observed at 30°C. This work represents a new concept in the area of electroactive polymers and should impact the microelectrochemical device industry.     

Polyethylene-based polyurethane copolymers and block copolymers

Low molecular weight hydroxy terminated polyethylene (HTPE) containing on average an ethyl group every 16–18 carbon atoms, and a hydroxy functionality of 2.6, has been used to prepare polyurethane copolymers and block copolymers which have good solvent resistance. The polymers show somewhat complicated thermal behavior, including T_g's at around −40°C due to the HTPE and diffuse endotherms between 40 and 60°C. The simple copolymers, containing only the polyol and a diisocyanate, show infrared evidence for two phases in the case where CHDI (trans-1,4-diisocyanatocyclohexane) was used, and poorer phase separation where other diisocyanates were used. Dynamic mechanical spectra show very broad tan delta transitions for the copolymers in the range of -9 to −23°C. All the polymers exhibit another transition in the G” curve above room temperature. SAXS reveals a microphase separated structure at 30°C for the simple copolymers which increases in spacing, then disappears in the 60–70°C range. With cooling, the microphase separated structure reappears readily for the CHDI-based copolymer, while its reappearance shows a hysteresis resulting from rate effects for the other copolymers.     

Ferroelectric block copolymers

A block copolymer consisting of polystyrene and a side chain ferroelectric liquid crystalline polymer was synthesized using polymer analogous chemistry on a monodisperse poly(styrene-b-isoprene). Composition was adjusted to give lamellar microstructure after addition of the mesogenic side groups. If placed in an LC cell without orientation of domains, no ferroelectric response was observed. After shearing the thin film, presumably due to alignment of lamellae, a bistable ferroelectric switching could be detected.     

Self-assembly of block copolymers

Block copolymer (BCP) self-assembly has attracted considerable attention for many decades because it can yield ordered structures in a wide range of morphologies, including spheres, cylinders, bicontinuous structures, lamellae, vesicles, and many other complex or hierarchical assemblies. These aggregates provide potential or practical applications in many fields. The present tutorial review introduces the primary principles of BCP self-assembly in bulk and in solution, by describing experiments, theories, accessible morphologies and morphological transitions, factors affecting the morphology, thermodynamics and kinetics, among others. As one specific example at a more advanced level, BCP vesicles (polymersomes) and their potential applications are discussed in some detail.     

Solvent-regulated ordering in block copolymers

Studies of the structural polymorphism exhibited by block copolymers in the presence of selective solvents are relatively recent, but very promising in terms of fundamental understanding and practical applications. Highlighting recent advances, this review progresses from `dry' ordered copolymers to solvated ordered (gel-like) copolymers and lastly to solvated disordered (but locally organized) copolymers. Organic- and aqueous-solvent-based systems are concurrently examined to cross-fertilize polymer and colloid science.     

A general strategy for highly efficient nanoparticle dispersing agents based on hybrid dendritic linear block copolymers

A modular approach to the synthesis of a library of hybrid dendritic‐linear copolymers was developed based on RAFT polymerization from monodisperse dendritic macroRAFT agents. By accurately controlling the molecular weight of the linear block, generation number of the dendrimer and the nature of the dendritic chains ends, the performance of these hybrid block copolymers as dispersing agents was optimized for a range of nanoparticles. For titanium dioxide nanoparticles, dispersion in a poly(methyl methacrylate) matrix was maximized with a second generation dendrimer containing four carboxylic acid end groups, and the quality of dispersion was observed to be superior to commercial dispersing agents for TiO₂. This approach also allowed novel hybrid dendritic‐linear dispersing agents to be prepared for the dispersion of Au and CdSe nanoparticles based on disulphide and phosphine oxide end groups, respectively. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1237–1258, 2009     

Conformationally asymmetric block copolymers

The standard parameters controlling AB diblock copolymer phase behavior are χN and f_A, where χ is an A-B segment interaction parameter, N is the overall degree of polymerization, and f_A is the volume fraction of the A block. Recently, it has been recognized that the ratio of the A and B statistical segment lengths α_A/α_B also represents another important parameter. Here, we theoretically examine the effects of this latter parameter on the phase behavior using the standard Gaussian chain model. Calculations are performed using both self-consistent field theory (SCFT) and strong segregation theory (SST). The ratio α_A/α_B is shown to have strong effects on order-order phase boundaries. Furthermore, it significantly affects the relative stability of the complex phases. In particular, it enhances the metastability of the perforated lamellar phase and may actually cause it to become an equilibrium structure. We also illustrate that varying α_A/α_B produces large changes in the relative domain spacings at order-order phase boundaries, which could strongly affect the kinetics of these transitions. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 945–952, 1997     

Main-chain supramolecular block copolymers

Block copolymers are key building blocks for a variety of applications ranging from electronic devices to drug delivery. The material properties of block copolymers can be tuned and potentially improved by introducing noncovalent interactions in place of covalent linkages between polymeric blocks resulting in the formation of supramolecular block copolymers. Such materials combine the microphase separation behavior inherent to block copolymers with the responsiveness of supramolecular materials thereby affording dynamic and reversible materials. This tutorial review covers recent advances in main-chain supramolecular block copolymers and describes the design principles, synthetic approaches, advantages, and potential applications.     

Organoboronium amphiphilic block copolymers

A new class of amphiphilic organometallic block copolymers with cationic organoboron pendant groups was developed. Selective replacement of one of the bromine substitutents on each boryl group of the block copolymer PSBBr₂‐b‐PS with an organometallic reagent ArM (ArM = 2,4,6‐trimethylphenyl copper, 4‐t‐butylphenyltrimethyl tin) followed by treatment with 2,2′‐bipyridine gave the novel block copolymers [ 3Ar ](Br)_n as light yellow solid materials that show good stability in air and moisture and high solubility in most organic solvents. Their structure and composition were confirmed by multinuclear NMR, GPC, and elemental analysis. Highly regular micellar aggregates form in block‐selective solvents (e.g., MeOH, toluene) as demonstrated by ¹H NMR, dynamic light scattering, and transmission electron microscopy. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6612–6618, 2009     

Chain conformations of block copolymers

An analytical theory of block copolymer conformations is developed for systematically studying the effects of the chain length, chain architecture, and segment interactions. The main results obtained for AB diblock and ABA triblock chains are as follows: (i) In the absence of AA and BB interactions, both diblock and triblock chains collapse to a dense form if the AB interaction is attractive. In the collapsed coil form, the mean-square end-to-end distance 〈R²〉 is proportional to the square root of the number of segments n^1/2. (ii) The diblock chain has a dumbbell form if AA and BB interactions are attractive and AB interaction repulsive, but the triblock chain collapses. In the dumbbell form, 〈R²〉 is proportional to n.     

Synthesis of azidourethane block copolymers

Methods of structuring thermoreversible urethane block copolymers on the basis 3,3-bis-(azidomethyl)oxetane (BAMO) and 3-(azidomethyl)-3-methyloxetane (AMMO) were developed for the first time. Type (AB)_n copolymers of BAMO and AMMO have an amorphous crystalline structure but differ from the type B(AB)_n copolymers in that they contain less crystal phase and have a higher glass transition temperature and better physicomechanical properties.     

Theory of block copolymers. I. Domain formation in A–B block copolymers

Microphase separation occurs in many block copolymers to give domain structures. In this first paper in a series dealing with domain formation and the consequences there of, a theory is presented for the formation of spherical domains in A-B block copolymers. The theory establishes criteria for the formation of domains and their size in terms of molecular and thermodynamic variables. It is shown that the considerable loss in configurational entropy due to the constraints on the spacial placement of chains in a domain structure requires that the critical block molecular weights required for domain formation are many-fold greater than required for phase separation of a simple mixture of the component blocks. The relation between domain radius R and molecular dimensions is obtained from the requirement that space in the domain must be filled with a constant density of segments. Segment densities are evaluated from solutions of the diffusion equation, treating the constraints on chain placement as boundary value problems. This gives the relationship R = 4/3 <L²>^1/2, where <L²>^1/2 is the root-mean-square end-to-end chain length. Because of chain perturbations in a domain system, <L²>^1/2 is larger than the unperturbed value $ < {\rm L}^2 > _0^{1/2}$ normally expected for bulk polymers. A means to evaluate the perturbations is shown. The agreement between the predictions of the present theory and the limited published experimental information appears quite satisfactory.