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

Comprehensive Dynamics in a Polyelectrolyte Complex Coacervate

The linear viscoelastic response, LVR, of a hydrated polyelectrolyte complex coacervate, PEC, was evaluated over a range of frequencies, temperatures, and salt concentrations. The PEC was a nearly stoichiometric blend of a quaternary ammonium poly([3-(methacrylamido)propyl]trimethylammonium chloride), PMAPTAC, and poly(2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt), PAMPS, an aliphatic sulfonate, selected because they remain fully charged over the conditions of use. Narrow molecular weight distribution polyelectrolytes were prepared using fractionation techniques. A partially deuterated version of PMAPTAC was incorporated to determine its coil radius of gyration, Rg, within PECs using small-angle neutron scattering. Chain dimensions were determined to be Gaussian with a Kuhn length of 2.37 nm, which remained constant from 25 to 65 °C. The LVR for a series of matched molecular weight PECs, mostly above the entanglement threshold, exhibited crossovers of modulus versus frequency classically attributed to the reptation time, relaxation between entanglements, and the relaxation of a Kuhn length of units (the “monomer” time). The scaling for zero shear viscosity, η0, versus chain length, N, was η0 ∼ N3.1, in agreement with “sticky reptation” theory. The lifetime and activation energy, Ep, of a pair between polyanion and polycation repeat units, Pol+Pol–, were determined from diffusion coefficients of salt ions within the PEC. The activation energy for LVR of salt-free PECs was 2Ep, showing that the key mechanism limiting the dynamics of undoped PECs is pair exchange. An FTIR technique was used to distinguish whether SCN– acts as a counterion or a co-ion within PECs. Doping of PECs with NaSCN breaks Pol+Pol– pairing efficiently, which decreases effective cross-linking and decreases viscosity. An equation was derived that quantitatively predicts this effect

Single-Walled Carbon Nanotube-Induced Lyotropic Phase Behavior of a Polymeric System

We report for the first time a new single-walled carbon nanotube (SWNT)-induced lyotropic phase behavior of a F108 block copolymer/water system. As the concentration is increased by evaporation, the F108-SWNT/water system exhibits isotropic–hexagonal–FCC–BCC–lamellar transitions. This is in clear contrast with the F108/water system (isotropic–BCC–lamellar transitions), indicating that the hexagonal and the FCC phases are newly induced by the presence of one-dimensional SWNTs. The SWNTs maintain their individuality or very small bundle state in all the phases except the lamellar phase. In the hexagonal phase, the SWNTs are located in the hydrophobic core of F108 cylinders oriented parallel to the [001] direction. The epitaxial transitions between the phases allowed us to identify the possible orientation of SWNTs in each phase: [110] in the FCC and either ⟨100⟩ or ⟨111⟩ in the BCC. In the lamellar phase, the SWNTs exist most likely in the hydrophobic layers forming aggregations among them. This new SWNT-induced lyotropic phase behavior in a block copolymer system may provide a new scalable route to fabricate SWNT superstructures with well-defined architecture and new functionalities

Tunable Encapsulation Structure of Block Copolymer Coated Single-Walled Carbon Nanotubes in Aqueous Solution

Nanosized and shape-tunable molecular building blocks can provide great opportunities for the fabrication of precisely controlled nanostructures. In this work, we have fabricated a molecular building block of single-walled carbon nanotubes (SWNTs) coated by PPO–PEO–PPO block copolymers whose encapsulation structure can be controlled via temperature or addition of small molecules. The structure and optical properties of SWNT block copolymers have been investigated by small-angle neutron scattering (SANS), ultraviolet–visible (UV–vis) spectroscopy, atomic force microscopy (AFM), and molecular dynamics (MD) simulation. The structure of the hydrated block copolymer layer surrounding SWNT can be controlled reversibly by varying temperature as well as by irreversibly adding 5-methylsalicylic acid (5MS). Increasing hydrophobicity of the polymers with temperature and strong tendency of 5MS to interact with both block copolymers and π orbitals of the SWNTs are likely to be responsible for the significant structural change of the block copolymer encapsulation layer, from loose corona shell to tightly encapsulating compact shell. Our result shows an efficient and simple way to fabricate and manipulate carbon-based nano building blocks in aqueous systems with tunable structure

Anomalistic Self-Assembled Phase Behavior of Block Copolymer Blended with Organic Derivative Depending on Temperature

Amphiphilic Pluronic block copolymers have attracted great attention in a broad spectrum of potential applications due to the excellent phase behaviors in an aqueous solution, and many efforts have been made to investigate their phase behaviors under various external conditions. With a variety of external conditions, however, the closed looplike phase behaviors of a Pluronic block copolymer in an aqueous solution have not been reported yet. Herein, we report the closed looplike (CLL) phase behavior of a Pluronic P65 triblock copolymer blended with an organic derivative, 5-methylsalicylic acid (5mS), in aqueous solution, which is very unique for block copolymers. As the 5mS concentration increases, the isotropic to ordered phase or back to isotropic phase transition temperature is decreased while the number of closed loops is increased to two. To the best of our knowledge, this is the first demonstration of a CLL phase transition of a Pluronic block copolymer in an aqueous solution, which is readily applicable to optical devices such as optical sensors or optoelectronics, and nanotemplates for a highly ordered superlattice. Furthermore, this provides new insight into the understanding on the phase behavior of a Pluronic block copolymer blended with additives

Polymer Chain Shape of Poly(3-alkylthiophenes) in Solution Using Small-Angle Neutron Scattering

The chain shape of polymers affects many aspects of their behavior and is governed by their intramolecular interactions. Delocalization of electrons along the backbone of conjugated polymers has been shown to lead to increased chain rigidity by encouraging a planar conformation. Poly­(3-hexylthiophene) and other poly­(3-alkylthiophenes) (P3ATs) are interesting for organic electronics applications, and it is clear that a hierarchy of structural features in these polymers controls charge transport. While other conjugated polymers are very rigid, the molecular structure of P3AT allows for two different planar conformations and a significant degree of torsion at room temperature. It is unclear, however, how their chain shape depends on variables such as side chain chemistry or regioregularity, both of which are key aspects in the molecular design of organic electronics. Small-angle neutron scattering from dilute polymer solutions indicates that the chains adopt a random coil geometry with a semiflexible backbone. The measured persistence length is shorter than the estimated conjugation length due to the two planar conformations that preserve conjugation but not backbone correlations. The persistence length of regioregular P3HT has been measured to be 3 nm at room temperature and decreases at higher temperatures. Changes in the regioregularity, side chain chemistry, or synthetic defects decrease the persistence length by 60–70%

Atomistic Structure of Bottlebrush Polymers: Simulations and Neutron Scattering Studies

We have used small angle neutron scattering (SANS) measurement and atomistic molecular dynamics (MD) simulations to investigate the conformation of bottlebrush polymers with poly­(norbornene) (PNB) backbone and different sizes of poly­(lactide) (PLA) side chains (PNB₂₅-<i>g</i>-PLA₅, PNB₂₅-<i>g</i>-PLA₁₀, and PNB₂₅-<i>g</i>-PLA₁₉). At early stage of simulations, stretched side chains with visible spatial-correlations of about 30 Å were observed. The experimentally measured SANS data, on the other hand, does not exhibit any correlation peaks in the corresponding length scale indicating a compact form rather than a stretched-hairy polymer conformation. As the simulation continued, the spatial correlations between side chains disappeared after about 40 ns of chain relaxation, and the scattering intensity calculated for the simulated structure becomes reasonably close to the measured one. Statistical approach is used to overcome the time scale limitation and search for optimal conformation space, which also provides a good agreement with the experimental data. Further coarse-grained simulation results suggest that the side chain conformation strongly depends on the solubility competition among side chain, backbone, and solvent. Significant changes of backbone dynamics due to the side chain encapsulation have been revealed and discussed

Pronounced Dielectric and Hydration/Dehydration Behaviors of Monopolar Poly(<i>N</i>‑alkylglycine)s in Aqueous Solution

Poly­(<i>N</i>-methylglycine) (NMG_<i>n</i>) and poly­(<i>N</i>-ethylglycine) (NEG_<i>n</i>) obtained by polymerization reactions initiated by benzylamine have no carboxy termini, such as those in normal polyamides, but have only amino termini, which exist primarily as cations in aqueous media at a pH value of ca. 9.5, observed in aqueous solutions without any buffer reagents. Therefore, polypeptoids, such as NMG_<i>n</i> and NEG_<i>n</i>, possessing a degree of polymerization (DP) higher than a certain value behave as cationic monopolar polymeric chain molecules in aqueous solution. It has not been clarified so far whether such a monopolar chain molecule exhibits dielectric relaxation (DR) behavior resulting from its molecular motions in aqueous media as dipolar chain molecules. DR measurements revealed that NMG₁₉ and NEG₁₇, possessing DPs of 19 and 17, respectively, dissolved in pure water clearly demonstrated pronounced DR behavior caused by fluctuating molecular motions of cationic termini at relaxation times of ca. 4 and 9 ns at 10 °C (283 K). The hydration numbers of NMG₁₉ and NEG₁₇ per monomeric residue (<i>n</i>_m) in aqueous solution were also evaluated via DR data as functions of temperature, and the <i>n</i>_m value of ca. 4.5 at 10 °C showed a remarkable reduction to ca. 2.0 around 40 °C (313 K) and 30 °C (303 K), depending on differences in the substituted group: methyl and ethyl groups. This temperature-dependent hydration/dehydration behavior found in NMG₁₉ and NEG₁₇ slightly influenced the sizes and molecular dynamics of the monopolar chain molecules in aqueous solution

Solvation Structure of Methanol-in-Salt Electrolyte Revealed by Small-Angle X‑ray Scattering and Simulations

The solvation structure of water-in-salt electrolytes was thoroughly studied, and two competing structuresanion solvated structure and anion networkwere well-defined in recent publications. To further reveal the solvation structure in those highly concentrated electrolytes, particularly the influence of solvent, methanol was chosen as the solvent for this proposed study. In this work, small-angle X-ray scattering, small-angle neutron scattering, Fourier-transform infrared spectroscopy, and Raman spectroscopy were utilized to obtain the global and local structural information. With the concentration increment, the anion network formed by TFSI– became the dominant structure. Meanwhile, the hydrogen bonds among methanol were interrupted by the TFSI– anion and formed a new connection with them. Molecular dynamic simulations with two different force fields (GAFF and OPLS-AA) are tested, and GAFF agreed with synchrotron small-angle X-ray scattering/wide-angle X-ray scattering (SAXS/WAXS) results well and provided insightful information about molecular/ion scale solvation structure. This article not only deepens the understanding of the solvation structure in highly concentrated solutions, but more importantly, it provides additional strong evidence for utilizing SAXS/WAXS to validate molecular dynamics simulations

Reduction-Triggered Self-Assembly of Nanoscale Molybdenum Oxide Molecular Clusters

Understanding the formation mechanism of giant molecular clusters is essential for rational design and synthesis of cluster-based nanomaterials with required morphologies and functionalities. Here, typical synthetic reactions of a 2.9 nm spherical molybdenum oxide cluster, {Mo₁₃₂} (formula: [Mo^VI₇₂Mo^V₆₀O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]^42–), with systematically varied reaction parameters have been fully explored to determine the morphologies and concentration of products, reduction of metal centers, and chemical environments of the organic ligands. The growth of these clusters shows a typical sigmoid curve, suggesting a general multistep self-assembly mechanism for the formation of giant molecular clusters. The reaction starts with a lag phase period when partial Mo^VI centers of molybdate precursors are reduced to form {Mo^V₂(acetate)} structures under the coordination effect of the acetate groups. Once the concentration of {Mo^V₂(acetate)} reaches a critical value, it triggers the co-assembly of Mo^V and Mo^VI species into the giant clusters