Thermoreversible Gels Composed of Colloidal Silica Rods with Short-Range Attractions

Dynamic arrest transitions of colloidal suspensions containing nonspherical particles are of interest for the design and processing of various particle technologies. To better understand the effects of particle shape anisotropy and attraction strength on gel and glass formation, we present a colloidal model system of octadecyl-coated silica rods, termed as adhesive hard rods (AHR), which enables control of rod aspect ratio and temperature-dependent interactions. The aspect ratios of silica rods were controlled by varying the initial TEOS concentration following the work of Kuijk et al. (<i>J. Am. Chem. Soc.</i>, <b>2011</b>, <i>133</i>, 2346–2349) and temperature-dependent attractions were introduced by coating the calcined silica rods with an octadecyl-brush and suspending in tetradecane. The rod length and aspect ratio were found to increase with TEOS concentration as expected, while other properties such as the rod diameter, coating coverage, density, and surface roughness were nearly independent of the aspect ratio. Ultrasmall angle X-ray scattering measurements revealed temperature-dependent attractions between octadecyl-coated silica rods in tetradecane, as characterized by a low-q upturn in the scattered intensity upon thermal quenching. Lastly, the rheology of a concentrated AHR suspension in tetradecane demonstrated thermoreversible gelation behavior, displaying a nearly 5 orders of magnitude change in the dynamic moduli as the temperature was cycled between 15 and 40 °C. The adhesive hard rod model system serves as a tunable platform to explore the combined influence of particle shape anisotropy and attraction strength on the dynamic arrest transitions in colloidal suspensions with thermoreversible, short-range attractions

Controlled Pd(0)/<i>t</i>‑Bu₃P‑Catalyzed Suzuki Cross-Coupling Polymerization of AB-Type Monomers with ArPd(<i>t</i>‑Bu₃P)X or Pd₂(dba)₃/<i>t</i>‑Bu₃P/ArX as the Initiator

Controlled Pd­(0)/<i>t</i>-Bu₃P-catalyzed Suzuki cross-coupling polymerizations of AB-type monomers via the chain-growth mechanism with a series of ArPd­(<i>t</i>-Bu₃P)­X (X = I, Br, Cl) complexes as initiators were described. Both isolated PhPd­(<i>t</i>-Bu₃P)­X (X = I, Br) complexes and ArPd­(<i>t</i>-Bu₃P)­X (X = I, Br, Cl) complexes <i>in situ</i> generated from Pd₂(dba)₃/<i>t</i>-Bu₃P/ArX (X = I, Br, Cl) were employed as initiators for the controlled Pd(0)/<i>t</i>-Bu₃P-catalyzed Suzuki cross-coupling polymerization. The <i>in situ</i> generated ArPd­(<i>t</i>-Bu₃P)­X complexes were found to be better initiators in general. Among them, the combinations of <i>p</i>-BrC₆H₄I, <i>p</i>-HOCH₂C₆H₄Br and <i>p</i>-PhCOC₆H₄Br with Pd₂(dba)₃/<i>t</i>-Bu₃P were identified as highly robust initiator systems, resulted in polymers with narrow PDIs (1.13–1.20). In addition, the Pd₂(dba)₃/<i>t</i>-Bu₃P/<i>p</i>-HOCH₂C₆H₄Br and Pd₂(dba)₃/<i>t</i>-Bu₃P/<i>p</i>-PhCOC₆H₄Br initiator systems also offered heterobifunctional chain ends with high fidelity. Our study showed that an additional amount of <i>t</i>-Bu₃P in the initiator system helped to achieve the narrow PDIs, likely by stabilizing Pd(0) species in the initiator system via coordination to form more stable Pd­(<i>t</i>-Bu₃P)_<i>n</i> (<i>n</i> ≥ 2) complexes. Our study opens a new avenue toward well-defined conjugated polymers

Dielectric and Mechanical Investigations on the Hydrophilicity and Hydrophobicity of Polyethylene Oxide Modified on a Silicon Surface

Polyethylene oxide (PEO) has been widely used in biomedical fields. The antibiofouling property of the PEO-modified surface has been extensively investigated but is far from being fully understood. A series of PEOs with narrowly distributed molecular weight (<i>M</i>_w), synthesized with the technique of high vacuum anionic polymerization, have been successfully grafted onto the surface of silicon wafers. The power-law relationship between the thickness of the monolayer versus the <i>M</i>_w of the grafted PEO shows a scaling of 0.3, indicating compact condensing of the chains. The static contact angles show higher hydrophobicity for the layer of PEO with higher <i>M</i>_w, which can be attributed to the closely packed conformation of the chains with high density. The frequency shift of the contact resonance indicates that the Young’s modulus decreases and the loss factor increases with the increase in the <i>M</i>_w of PEO and the thickness of the PEO layers. Dielectric spectroscopy of bare or PEO-grafted wafers in the aqueous solutions reveals an interfacial polarization, which results from compositional and structural changes in the interface layer and depends on temperatures and salt concentrations. At a given grafting density, the PEO chains are swollen in pure water, demonstrating hydrophilic behavior, whereas they collapse in salt solutions, showing hydrophobic characteristics

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

Small-Angle Neutron Scattering Analysis of Bottlebrush Polymers Prepared via Grafting-Through Polymerization

Bottlebrush polymers are highly branched macromolecules with potential applications in antifouling coatings, rheological modifiers, and drug delivery systems. However, the solution conformation of bottlebrush polymers has been studied in only a limited set of materials made primarily by grafting-from polymerization. Here we present small-angle neutron scattering (SANS) measurements on a series of polystyrene bottlebrush polymers with varying side-chain and backbone lengths in <i>d</i>₈-toluene to analyze their size, shape, and conformation. Bottlebrush polymers with 2–7 kg mol^–1 polystyrene side chains (degree of polymerization DP = 14–54) and poly­(oxanorbornene) backbones (DP = 10–264) were synthesized using reversible addition–fragmentation chain transfer (RAFT) followed by a ring-opening metathesis polymerization (ROMP) grafting-through synthesis scheme. Analysis by Guinier–Porod, rigid cylinder, and flexible cylinder models provided estimates of the bottlebrush polymer length, radius, and stiffness. The bottlebrush polymer cross-sectional area depends primarily on side-chain DP, and the radius of gyration <i>R</i>_g exhibits a power-law dependence with side-chain DP. We also observe a sphere-to-cylinder transition with increasing backbone DP, with the transition occurring at a backbone DP of approximately 120 for the polystyrene bottlebrush polymers studied. The maximum molecular dimension for the series studied varies from 25 to 350 nm

2‑Isopropenyl-2-oxazoline: Well-Defined Homopolymers and Block Copolymers via Living Anionic Polymerization

Poly­(2-isopropenyl-2-oxazoline) (PIPOx) has drawn significant attention for numerous applications. However, the successful living anionic polymerization of 2-isopropenyl-2-oxazoline has not been reported previously. Herein, we describe how well-defined PIPOx with quantitative yields, controlled molecular weights from 6800 to over 100 000 g/mol and low polydispersity indices (PDI ≤ 1.17) were synthesized successfully via living anionic polymerization using diphenyl­methyl­potassium/diethyl­zinc (DPM-K/Et₂Zn) in tetrahydrofuran (THF) at 0 °C. In particular, we report the precise synthesis of well-defined PIPOx with the highest molecular weight ever reported (over 100 000 g/mol) and low PDI of 1.17. The resulting polymers were characterized by ¹H and ¹³C nuclear magnetic resonance spectroscopy (NMR) along with size exclusion chromatography (SEC). Additionally, the reactivity of living PIPOx was investigated by crossover block copolymerization with styrene (St), 2-vinylpyridine (2VP), and methyl methacrylate (MMA). It was found that the nucleophilicity of living PIPOx is of this order: living PS > living P2VP > living PMMA > living PIPOx. The self-assembly behavior in bulk of PIPOx-<i>b</i>-PS-<i>b</i>-PIPOx triblock copolymers having different block ratios of 10:80:10 and 25:50:25 was studied using transmission electron microscopy (TEM). The formation of spherical and lamellar nanostructures, respectively, was observed

Poly(1-adamantyl acrylate): Living Anionic Polymerization, Block Copolymerization, and Thermal Properties

Living anionic polymerization of acrylates is challenging due to intrinsic side reactions including backbiting reactions of propagating enolate anions and aggregation of active chain ends. In this study, the controlled synthesis of poly­(1-adamatyl acrylate) (PAdA) was performed successfully for the first time via living anionic polymerization through investigation of the initiation systems of <i>sec</i>-butyl­lithium/diphenyl­ethylene/lithium chloride (<i>sec</i>-BuLi/DPE/LiCl), diphenyl­methyl­potassium/diethyl­zinc (DPMK/Et₂Zn), and sodium naphthalenide/dipenyl­ethylene/diethylzinc (Na-Naph/DPE/Et₂Zn) in tetrahydrofuran at −78 °C using custom glass-blowing and high-vacuum techniques. PAdA synthesized via anionic polymerization using DPMK with a large excess (more than 40-fold to DPMK) of Et₂Zn as the ligand exhibited predicted molecular weights from 4.3 to 71.8 kg/mol and polydispersity indices of around 1.10. In addition, the produced PAdAs exhibit a low level of isotactic content (mm triads of 2.1%). The block copolymers of AdA and methyl methacrylate (MMA) were obtained by sequential anionic polymerization, and the distinct living property of PAdA over other acrylates was demonstrated based on the observation that the resulting PAdA-<i>b</i>-PMMA block copolymers were formed with no residual PAdA homopolymer. The PAdA homopolymers exhibit a very high glass transition temperature (133 °C) and outstanding thermal stability (<i>T</i>_d: 376 °C) as compared to other acrylic polymers such as poly­(<i>tert</i>-butyl acrylate) and poly­(methyl acrylate). These merits make PAdA a promising candidate for acrylic-based thermoplastic elastomers with high upper service temperature and enhanced mechanical strength

All-Acrylic Multigraft Copolymers: Effect of Side Chain Molecular Weight and Volume Fraction on Mechanical Behavior

We present the synthesis of poly­(<i>n</i>-butyl acrylate)-<i>g</i>-poly­(methyl methacrylate) (P<i>n</i>BA-<i>g</i>-PMMA) multigraft copolymers via a grafting-through (macromonomer) approach. The synthesis was performed using two controlled polymerization techniques. The PMMA macromonomer was obtained by high-vacuum anionic polymerization followed by the copolymerization of <i>n</i>-butyl acrylate and PMMA macromonomer using reversible addition–fragmentation chain transfer (RAFT) polymerization to yield the desired all-acrylic multigraft structures. The P<i>n</i>BA-<i>g</i>-PMMA multigraft structures exhibit randomly spaced branch points with various PMMA contents, ranging from 15 to 40 vol %, allowing an investigation into how physical properties vary with differences in the number of branch points and molecular weight of grafted side chains. The determination of molecular weight and polydispersity indices of both the PMMA macromonomer and the graft copolymers was carried out using size exclusion chromatography with triple detection, and the structural characteristics of both the macromonomer and P<i>n</i>BA-<i>g</i>-PMMA graft materials were characterized by ¹H and ¹³C NMR. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was employed for monitoring the macromonomer synthesis. Thermal characteristics of the materials were analyzed using differential scanning calorimetry and thermogravimetric analysis. The mechanical performance of the graft materials was characterized by rheology and dynamic mechanical analysis, revealing that samples with PMMA content of 25–40 vol % exhibit superior elastomeric properties as compared to materials containing short PMMA side chains or <25 vol % PMMA. Lastly, atomic force microscopy showed a varying degree of microphase separation between the glassy and rubbery components that is strongly dependent on PMMA side chain molecular weight

Deuteration as a Means to Tune Crystallinity of Conducting Polymers

The effects of deuterium isotope substitution on conjugated polymer chain stacking of poly­(3-hexylthiophene) is studied experimentally by X-ray diffraction (XRD) in combination with gel permeation chromatography and theoretically using density functional theory and quantum molecular dynamics. For four P3HT materials with different levels of deuteration (pristine, main-chain deuterated, side-chain deuterated, and fully deuterated), the XRD measurements show that main-chain thiophene deuteration significantly reduces crystallinity, regardless of the side-chain deuteration. The reduction of crystallinity due to the main-chain deuteration is a quantum nuclear effect resulting from a static zero-point vibrational energy combined with a dynamic correlation of the dipole fluctuations. The quantum molecular dynamics simulations confirm the interchain correlation of the proton–proton and deuteron–deuteron motions but not of the proton–deuteron motion. Thus, isotopic purity is an important factor affecting stability and properties of conjugated polymer crystals, which should be considered in the design of electronic and spintronic devices

Nanoscale Lipid/Polymer Hybrid Vesicles: Effects of Triblock Copolymer Composition and Hydrophilic Weight Fraction

Hybrid vesicles resulting from combined self-assembly of polymers and lipids have gained interest in recent years as drug delivery systems, biosensors, and model systems for structural scaffolds as artificial organelles. So far, in the literature, more attention has been given to study how the discrepancy of chemical compositions and hydrophobic segment sizes between polymers and lipids affect the vesicle’s nature. In this study, we focused on how the hydrophilic blocks impact the vesicle’s morphology. Therefore, we developed hybrid lipid/polymer vesicle systems with lipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and amphiphilic triblock copolymers composed of poly(ethylene glycol)–poly(dimethylsiloxane)–poly(ethylene glycol) (PEG–PDMS–PEG). Three types of hybrid vesicle systems were investigated in a systematic manner by changing the hydrophilic block length and the polymer–lipid composition. From the data obtained from cryo-transmission electron microscopy (cryo-TEM) and small-angle neutron scattering, we observed that the hydrophilic mass fraction of an amphiphilic polymer can affect the membrane thickness, size polydispersity, and lamellarity of polymer/lipid hybrid vesicles