Undulating the Lamellar Interface of Polymer–Surfactant Complex by Dendrimer

Self-assembly of the supramolecules formed by the complexation between poly­(amidoamine) (PAMAM) generation four (G4) dendrimer and the surfactant, dodecylbenzene­sulfonic acid (DBSA), generated a hexagonal columnar phase and a typical flat lamellar phase at low and high surfactant binding ratio, respectively, due to the dominance of the dendrimer to attain its natural curvature and the surfactant alkyl tails to reduce their hydrophobic interaction energy and packing frustration. Most strikingly, the delicate balance between these two free energy components at the intermediate binding ratios resulted in an undulated lamellar structure characterized by the centered rectangular unit cell. The finding demonstrates the power of dendrimer as a building block for expanding the morphological window of polymeric assemblies by modulating the interfacial curvature of microphase-separated structure

Variable Crystal Orientation of Poly(ethylene oxide) Confined within the Tubular Space Templated by Anodic Aluminum Oxide Nanochannels

The use of nanoscale templates to confine the crystallization process is an effective approach for creating large-scale crystal orientation. Here we incorporated poly­(ethylene oxide) (PEO) into anodic aluminum oxide (AAO) nanochannels by a solution infiltration process to generate PEO nanotubes confined in the channels. The preferred crystal orientation of the PEO crystallites developed in the tubular space was then studied as a function of crystallization temperature (<i>T</i>_c), PEO molecular weight (<i>M</i>_PEO), and AAO channel diameter (<i>D</i>_AAO = 23 and 89 nm). Two distinct types of crystal orientation, i.e., perpendicular and tilt orientation with the (120) plane aligning along and tilting 45° away from the channel axis, respectively, were identified. Crystallization in the nanotubes templated by the AAO with <i>D</i>_AAO of 23 nm led predominantly to perpendicularly oriented crystallites except for the high molecular weight PEO (<i>M</i>_PEO = 95 000 g/mol), where a significant fraction of the crystallites showed tilt orientation. In the nanochannels with <i>D</i>_AAO = 89 nm, however, perpendicular orientation only dominated at the higher <i>T</i>_c (≥20 °C), as most crystallites developed at the lower <i>T</i>_c adopted tilt orientation with the invariant tilt angle of the (120) plane of 45°. On basis of the observed effects of the three parameters, the preferred crystal orientation attained was proposed to be governed by the strength of confinement to the crystal growth prescribed by the confinement geometry and the nucleation density of crystallization

Conformation and Fluorescence Property of Poly(3-hexylthiophene) Isolated Chains Studied by Single Molecule Spectroscopy: Effects of Solvent Quality and Regioregularity

Poly­(3-alkylthiophene)­s (P3AT) are among the most widely used conjugated polymers for applications in polymer solar cells and thin-film transistors. In this study, the influence of the casting solvent on the conformational structure and fluorescence properties of the isolated chains of a regioregular and a regiorandom poly­(3-hexylthiophene) (denoted by rr-P3HT and ra-P3HT, respectively) has been systematically investigated by single molecule spectroscopy. The isolated chains were dispersed in polystyrene matrix by casting from their ultradilute solutions with tetrahydrofuran (THF) or toluene. ra-P3HT chains were found to contain mostly high-energy emitting species with short conjugating length, while the low-energy emitting sites existed predominantly in rr-P3HT chains irrespective of the casting solvent. The measurement of the modulation depth revealed that rr-P3HT chains cast from THF formed defect cylinder structure. The regioregular chains cast from toluene exhibited relatively regular folding to form rod structure as a result of the amplification of the segmental attraction in the poorer solvent. The casting solvent had insignificant effect on the conformational structure of ra-P3HT, where the isolated chains predominantly showed defect coil conformation irrespective of the casting solvent

Accessing the Frank–Kasper σ Phase of Block Copolymer with Small Conformational Asymmetry via Selective Solvent Solubilization in the Micellar Corona

Large conformational asymmetry has been considered as an essential ingredient for accessing the Frank–Kasper (FK) phase in a neat block copolymer (bcp) melt. This criterion can be relaxed by blending the bcp with the corresponding homopolymer, where the homopolymer is incorporated into the micellar core in the dry-brush regime. While the solubilization of the homopolymer in the micellar corona in the wet-brush regime has also been demonstrated to yield the FK σ phase in a bcp system bearing a large conformational asymmetry parameter (ε > 1.5), we demonstrate here that this is also true for the system with small conformational asymmetry. A cylinder-forming poly(ethylene oxide)-block-poly(1,2-butadiene) (PEO-b-PB) with a ε of 1.2 was blended with a small amount of selective solvent, which was either a PB homopolymer (h-PB) or dodecylbenzene (DB) to yield the mixtures forming the spherical micelles composed of the PEO core and the corona containing the wet-brush mixture of PB blocks and the selective solvents. Both types of mixtures were found to form the FK σ phase in a narrow region near the cylinder-sphere phase boundary. We argue that, under a given volume fraction (fPEO) and size of the PEO core, the shorter PB coronal blocks in the bcp/solvent mixtures are more strained than the longer PB blocks in the corresponding neat diblock; therefore, the spherical phase is accessed at a higher PEO core fraction to alleviate the excess entropic penalty. The formation of a larger core (at a higher fPEO) and the stronger propensity to alleviate the packing frustration of the more strained PB coronal blocks in the bcp/selective solvent mixture creates an effect equivalent to increasing the conformational asymmetry and hence promotes the core to adopt the polyhedral geometry templated by the Voronoi cells of the lattice. The micelles approaching the polyhedral interface limit then favor the packing in the FK σ phase to minimize the total free energy

Spatial Distributions of Guest Molecule and Hydration Level in Dendrimer-Based Guest–Host Complex

Using the electrostatic complex of G4 poly­(amidoamine) (PAMAM) dendrimer with an amphiphilic surfactant as a model system, contrast variation small angle neutron scattering (SANS) is implemented to resolve the key structural characteristics of dendrimer-based guest–host system. Quantifications of the radial distributions of the scattering length density and the hydration level within the complex molecule reveal that the surfactant is embedded in the peripheral region of dendrimer and the steric crowding in this region increases the backfolding of the dendritic segments, thereby reducing the hydration level throughout the complex molecule. The insights into the spatial location of the guest molecules as well as the perturbations of dendrimer conformation and hydration level deduced here are crucial for the delicate design of dendrimer-based guest–host system for biomedical applications

Mechanism of Hierarchical Structure Formation of Polymer/Nanoparticle Hybrids

Exploiting the assembly of metallic nanoparticles (NPs) in their hybrids with polymers is an important task for creating new properties or functionalities via collective interactions or dynamics of the constituents. In this work, we present a detailed study of the process and mechanism of the reaction-induced hierarchical assembly of NPs in the hybrid wherein the metallic Pd NPs were synthesized and their numbers in the system increased with time by reducing the metal precursor, Pd­(acac)₂, originally dissolved uniformly in the solution of poly­(2-vinyl­pyridine) (P2VP) with benzyl alcohol (BA) as a solvent and reduction agent. The time-resolved small-angle X-ray scattering (SAXS) experiment using synchrotron radiation revealed that the structural evolution process from the beginning of NP formation to the establishment of a fractal structure built up by the clusters of Pd NPs was constituted of four distinct stages, Stages 1–4, governed by the increasing overall NP volume fraction (ϕ_overall) and hence by the increasing reduction time. At Stage 1, the NPs were uniformly distributed in the matrix of P2VP and BA with negligible interparticle interaction due to the low particle volume fraction (ϕ_overall ≤ ca. 3 × 10^–4). The structural evolution advanced to Stage 2 when ϕ_overall was increased above 3 × 10^–4. In this stage, the NPs experienced the sticky hard sphere (SHS) type attractive interaction, and as a consequence the NPs underwent a phase separation into a particle-poor phase (Phase I) and a particle-rich phase (Phase II) strikingly even in such a small ϕ_overall. The net interparticle attraction created the dynamic aggregates distributed uniformly within Phase II. As the ϕ_overall continued to increase to ca. 4.7 × 10^–4, the structural development entered Stage 3, in which the dynamic aggregates started to form a higher-order organization, generating a larger-scale heterogeneity in Phase II. The structure developed at Stage 3 served as a precursor directing the subsequent formation of the large-scale mass-fractal network built up by the static clusters of NPs within Phase II at Stage 4. The particles within the clusters were proposed to be bridged by the P2VP chains due to pyridine–Pd coordination interaction

Evolution of Crystal Orientation in One-Dimensionally Confined Space Templated by Lamellae-Forming Block Copolymers

Polymer crystallites may exhibit preferred orientation when the crystallization is allowed to occur under the influence of spatial confinement. Using time-resolved wide-angle X-ray scattering (WAXS), we explore the time evolution of the preferred crystal orientation within one-dimensionally confined space constructed by the lamellar microdomains of two crystalline block copolymers, polyethylene-<i>block</i>-poly­(dl-lactide) (PE-<i>b</i>-PDLLA) and poly­(l-lactide)-<i>block</i>-polyethylene (PLLA-<i>b</i>-PE), where the developments of the parallel and the perpendicular orientation of PE and PLLA crystallites, respectively, were monitored from the early stage of crystallization. Both types of crystallites were randomly oriented at the early stage of formation. As crystallization proceeded further, the ensemble-average orientation progressively improved toward the preferred orientation type, and the rate of establishing the orientation exhibited the same dependence on crystallization temperature (<i>T</i>_c) as the crystallization kinetics. Further examination of the effectiveness of enhancing the average orientation with respect to the increase of crystallinity supported the postulate that the perpendicular orientation of PLLA crystallites arises from the tendency to attain long-range crystal growth, while the parallel crystal orientation of PE is driven by the excluded volume interaction between the crystallites as a result of the intrinsically high nucleating power of PE

Monodisperse Copper Nanocubes: Synthesis, Self-Assembly, and Large-Area Dense-Packed Films

In comparison to the well-characterized bottom-up synthesis of Au and Ag nanomaterials, the synthesis of Cu nanocrystals with well-defined and controllable shapes is still in need of improvement. Among the many shapes, a cube covered by six {100} facets can be regarded as a standard model to study the surface properties of {100} facets. Herein, we have prepared monodisperse Cu nanoparticles having a slightly truncated cubic shape with an average edge length of 75.7 nm and a standard deviation of 3.87% by using CuCl as the precursor, oleylamine as the reaction solvent, and trioctylphosphine and octadecylamine as shape control agents. The as-prepared Cu nanocubes tend to self-assemble on transmission electron microscopy grids or silicon substrates. Electron microscopy and small-angle X-ray scattering reveal that the Cu nanocubes prefer to self-assemble into 2D or 3D rhombohedral structures (RS). Large-area dense-packed films (1.5 cm × 2.5 cm) composed of monodisperse Cu nanocubes were fabricated by immersing a Si substrate in a dispersion of dodecanethiol-capped Cu nanocubes in toluene and evaporating the toluene at a controlled rate while holding the substrate at an angle. The electrical properties of the Cu films with various thickness and annealing temperatures were studied

Interplay between the Phase Transitions at Different Length Scales in the Supramolecular Comb–Coil Block Copolymers Bearing (AB)_<i>n</i> Multiblock Architecture

We introduced the concept of comb–coil supramolecule into linear (AB)_<i>n</i>-type multiblock copolymer and investigated the self-assembly behavior of the copolymers as a function of the unit number <i>n</i>. Linear (polystyrene-<i>block</i>-poly­(2-vinylpyridine))_<i>n</i> (denoted as (PS-<i>b</i>-P2VP)_<i>n</i>, where <i>n</i> = 1, 2, 3) was complexed with a surfactant, dodecylbenzenesulfonic acid (DBSA), to yield the comb–coil multiblock copolymers, in which DBSA bound stoichiometrically with P2VP block via physical bonds. All three comb–coil block copolymers, including diblock (<i>n</i> = 1), tetrablock (<i>n</i> = 2), and hexablock (<i>n</i> = 3), self-organized to form cylinder-<i>within</i>-lamellae morphology at the lower temperature, where the cylindrical microdomains formed by the PS block embedded in the matrix composed of the lamellar mesophase organized by the P2VP­(DBSA) comb block. The disordering of the smaller-scale lamellar mesophase formed by the comb block occurred upon heating; at the same time, the larger-scale cylindrical domains transformed to body-centered cubic-packed spheres in the diblock complex and to another hexagonally packed cylinder structure with smaller domain spacing in tetrablock and hexablock complexes, indicating that the order–disorder transition (ODT) of the smaller-scale structure drove an order–order transition (OOT) of the larger-scale structure irrespective of <i>n</i>. The transition temperatures were found to increase with increasing <i>n</i> due to the introduction of more interfacial area in the microphase-separated state of the multiblock with larger unit number

Gelation of a Solution of Poly(3-hexylthiophene) Greatly Retards Its Crystallization Rate in the Subsequently Cast Film

We compared the crystallization rate of poly­(3-hexylthiophene) (P3HT) in the film cast from the gel (called “gel-cast film”) with that in the film cast from the liquid solution (called “solution-cast film”) to understand the effect of solution structure on the structural development in the subsequently cast film of conjugated polymer. P3HT was found to form a homogeneous liquid solution with xylene at elevated temperature. When the freshly prepared semidilute solution was allowed to age at room temperature, the solution transformed into a gel in which a significant amount of nanowhiskers formed. The nanowhiskers in the gel were effectively transferred to the corresponding cast film, while the film cast from the freshly prepared solution only contained a small amount of such a morphological entity. The in situ small-angle X-ray scattering (SAXS) measurement and thermal analysis revealed that both the cold and melt crystallization of P3HT in the gel-cast film were much slower than those in the solution-cast counterpart. The retardation of crystallization rate in the gel-cast film was attributed to the abundance of the nanowhiskers. In this case, the crystallization of P3HT occurred predominantly within the individual nanowhiskers and the mesh regions in the networks of the whiskers, where their limited sizes in at least one dimension imposed a strong spatial constraint to the crystal growth and chain motion for crystallization