Compositional asymmetry in a crystalline-amorphous block copolymer influences the phase and crystallization behaviors of its blend with an amorphous block copolymer

This study determined the phase and crystallization behaviors of blends composed of asymmetric polystyrene-block-poly(ethylene oxide) (PS-PEO) and symmetric polystyrene-block-poly(methyl methacrylate) (PS-PMMA). The PS blocks in the various binary block copolymers exhibited nearly identical molecular weights, whereas the molecular weight ratios of PEO and PMMA varied. The compatibility of the PEO and PMMA chains aided the binary block copolymers in co-ordering in a lamellar microdomain morphology, with the PEO and PMMA blocks sharing a common microdomain. Adding short tethered PMMA chains to long tethered PEO chains led to a decrease in the common microdomain spacing and an increase in the grafting density. These behaviors increased PEO chain stretching, causing macrophase separation. The mismatch in PEO and PMMA block lengths divided the common PEO/PMMA microdomain into two sections: the coexisting PEO/PMMA section close to the microdomain interface and the neat PEO section far away from it. The high-glass-transition-temperature PMMA reduced PEO chain mobility, inhibiting PEO crystallization in the coexisting PEO/PMMA section but not in the neat PEO section. When the block length ratio of PEO to PMMA decreased, the neat PEO section narrowed. The increase in the extent of PEO confinement led to a reduction in PEO crystallizability.

In Situ Thermal Safety Aspect of the Electrospun Polyimide-Al2O3 Separator Reveals Less Exothermic Heat Energies Than Polypropylene at the Thermal Runaway Event of Lithium-Ion Batteries

Polyimide-Al₂O₃ membranes are developed as a direct alternative to current polyolefin separators by the electrospinning technique and their chemical structures confirm the carbonyl group with the presence of asymmetric and symmetric stretching and bending vibrations at 1778, 1720, and 720 cm^-1 and stretching vibration at 1373 cm^-1 for the imide group. Porous nanofiber architecture morphology is realized with a nanofiber thickness of ∼200 nm and shows an ultrasmooth surface and >1 μm pore size in the architecture, built with the chemical constituents of carbon, nitrogen, aluminum, and oxygen elements. The galvanostatic cycling study of the Li/PI-Al₂O₃/LiFePO₄ lithium cell delivers stable charge-discharge capacities of 144/143 mAh g^-1 at 0.2 C and 110/100 mAh g^-1 at 1 C for 1-100 cycles. The fabricated MCMB/PI-Al₂O₃/LiFePO₄ lithium-ion full-cell reveals less charge transfer resistance of R_ct ∼ 25 Ω and yields stable charge-discharge capacities of 125/119 mAh g^-1. The thermogravimetric curve for the PI-Al₂O₃ separator discloses thermal stability up to 525 °C, and the differential scanning calorimetric curve shows a straight line until 300 °C and depicts high thermal stability than the PP separator. In situ multimode calorimetry analysis of the MCMB/PP/LiFePO₄ full-cell showed a pronounced exothermic peak at 225 °C with a higher released heat energy of 211 J g^-1 at the thermal runaway event, while the MCMB/PI-Al₂O₃/LiFePO₄ full-cell revealed an almost 8-fold less exothermic released heat energy of 25 J g^-1 than the Celgard polypropylene separator, which was because the MCMB anode and LiFePO₄ cathode can be mechanically isolated without any additional separator's melting and burning reactions, as a fire-suppressant separator for lithium-ion batteries.

Hydrogen Bonding-Induced Assembled Structures and Photoresponsive Behavior of Azobenzene Molecule/Polyethylene Glycol Complexes

We investigated the self-assembled structures and photoresponsive and crystallization behaviors of supramolecules composed of 4-methoxy-4'-hydroxyazobenzene (Azo) molecules and polyethylene glycol (PEG) that were formed through hydrogen-bonding interactions. The Azo/PEG complexes exhibited the characteristics of photoresponse and crystallization, which originated from Azo and PEG, respectively. When Azo/PEG complexes were dissolved in solvents, hydrogen-bonding interaction hindered the rotation and inversion of mesogens, causing a reduction in the photoisomerization rate compared with the photoisomerization rate of the neat Azo. The confinement of Azo/PEG complexes in thin films further resulted in a substantial decrease in the photoisomerization rate but an increase in the amounts of H-aggregated and J-aggregated mesogens. Regarding PEG crystallization, ultraviolet irradiation of Azo/PEG complexes increased the quantity of high-polarity cis isomers, which improved the compatibility between mesogens and PEG, subsequently increasing the crystallization temperature of PEG. Moreover, the complexation of Azo and PEG induced microphase separation, forming a lamellar morphology. Within the Azo-rich microphases, mesogens aggregated to form tilted monosmectic layers. By contrast, PEG crystallization within the PEG-rich microphases was hard confined, indicating that the domain size of the lamellar morphology was unchanged during PEG crystallization.

Hydrothermal and plasma nitrided electrospun carbon nanofibers for amperometric sensing of hydrogen peroxide

Nitrogen-doped carbon nanofibers (CNFs) were prepared by an electrospinning method, this followed by a hydrothermal reaction or nitrogen plasma treatment to obtain electrode for non-enzymatic amperometric sensing of H₂O₂. The hydrothermally treated electrode performs better. Its electrochemical surface is 3.7 × 10^-3 mA cm^-2, which is larger than that of a nitrogen plasma treated electrode (8.9 × 10^-4) or a non-doped CNF (2.45 × 10^-4 mA cm^-2). The hydrothermally treated CNF with rough surface and a complex profile with doped N has a higher sensitivity (357 μA∙mM^-1∙cm^-2), a lower detection limit (0.62 μM), and a wider linear range (0.01-0.71 mM) than N-CNF_P at a working potential of -0.4 V (vs. Ag/AgCl). The electrode gave high recoveries when applied to the analysis of milk samples spiked with H₂O₂. Graphical abstract Nitrogen-doped carbon nanofibers prepared by an electrospinning method followed by a hydrothermal reaction (N-CNF_ht) or nitrogen plasma treatment (N-CNF_P) are directly used as non-enzymatic amperometric H₂O₂ sensors.

Morphology-Mediated Photoresponsive and Fluorescence Behaviors of Azobenzene-Containing Block Copolymers

We investigated the relationship between the self-assembled morphology of poly( tert-butyl acrylate)- block-poly(6-[4-(4'-methoxyphenylazo)phenoxy]hexyl methacrylate) (P tBA- b-PAzoMA) block copolymers and their photoresponsive and fluorescence behaviors. The morphology of P tBA- b-PAzoMA copolymers was manipulated by dissolving them in mixed dimethylformamide (DMF)/hexanol solvents. When P tBA- b-PAzoMA was dissolved in DMF-rich (neutral) solvents, a favorable interaction between the DMF molecules and both blocks resulted in a random-coiled conformation. The unconfined morphology facilitated the formation of both nonassociated and head-to-head organized azobenzene mesogens, which promoted fluorescence emission. When hexanol, a P tBA-selective solvent, was added to DMF, the solvency of P tBA- b-PAzoMA worsened, leading to its assembly into micelles, with PAzoMA in the micelle core. The confinement of azobenzene moieties in the micelle core hindered their trans-to- cis photoisomerization, thereby considerably decreasing the kinetics of photoisomerization and the population of cis isomers. Additionally, a nanoconfined geometry resulted in compactly packed chromophores, causing fluorescence loss. When P tBA- b-PAzoMA was exposed to UV light, the increased number of cis isomers hampered the closely packed mesogens, resulting in a substantial enhancement of fluorescence emission. When the mole fraction of the PAzoMA block was increased, P tBA- b-PAzoMA formed clusters, causing the slow kinetics of photoisomerization and fluorescence quenching.

Binder-Free N- and O-Rich Carbon Nanofiber Anodes for Long Cycle Life K-Ion Batteries

Carbon nanofibers produced by electrospinning of polyacrylonitrile polymer and subsequent carbonization were tested as freestanding potassium-ion anodes. The effect of oxygen functionalization on K-ion carbon anode performance was tested for the first time via plasma oxidation of prepared carbon nanofibers. The produced materials exhibited exceptional cycling stability through the amorphous carbon structuring and one-dimensional architecture accommodating significant material expansion upon K⁺ intercalation, resulting in a stable capacity of 170 mAh g^-1 after 1900 cycles at 1C rate for N-rich carbon nanofibers. Excellent rate performance of 110 mAh g^-1 at 10C rate, as compared to 230 mAh g^-1 at C/10 rate, resulted from the K-ion surface storage mechanism and the increased K⁺ solid diffusion coefficient in carbon nanofibers as compared to graphite. Plasma oxidation treatment augmented surface storage of K⁺ by oxygen functionalities but increased material charge transfer resistance as compared to N-rich carbon fibers. Ex situ characterization revealed that the one-dimensional structure was maintained throughout cycling, despite the increase in graphitic interlattice spacing from 0.37 to 0.46 nm. The carbon nanofibers demonstrate great potential as an anode material for potassium-ion batteries with superior cycling stability and rate capability over previously reported carbon materials.

Plasma oxidation of electrospun carbon nanofibers as supercapacitor electrodes

Electrospun carbon nanofibers modified by oxygen plasma have increased hydrophilicity, resulting in a substantial increase in the specific capacitance.

Effect of molecular properties of random copolymers on the stability and domain dimension of block copolymer/random copolymer blends

The morphological behavior of binary mixtures containing poly(styrene-b-2-vinylpyridine) (PS-b-PVP) diblock copolymer and poly(styrene-r-2-vinylpyridine) (PS-r-PVP) random copolymer was investigated as a function of the molecular weight ratio of PS-b-PVP and PS-r-PVP (R), the PS fraction in PS-r-PVP, and the concentration of PS-r-PVP in the blends (ϕr). When R was high, the addition of symmetric PS-r-PVP caused lateral expansion of microdomains and reduced the interdomain distance of the blend, indicating localization of PS-r-PVP at the PS-b-PVP interface. At high ϕr, packing constraints prevented all PS-r-PVP from assembling at the PS-b-PVP interface, which induced macrophase separation and formed a coexisting morphology composed of ordered polymer phase and random copolymer-rich regimes. Reducing the R value reduced the amount of PS-r-PVP that could be assembled at the PS-b-PVP interface, and macrophase separation occurred at a low PS-r-PVP content. When asymmetric PS-r-PVP was introduced into PS-b-PVP, PS-r-PVP was located in the preferred domain of PS-b-PVP because of the favorable interaction of PS-r-PVP with the particular domain. The enthalpically driven self-assembly rendered to swell the preferred domain and increased the interfacial curvature that, in turn, induced an order-order transition.

Synthesis and characterization of magnetic nanoparticle/block copolymer composites

A simple strategy to improve the particle dispersion and structural ordering of magnetic nanoparticle/block copolymer composites is demonstrated. This method consists of manipulating both the block copolymer structure in a solvent and the relative interactions of particle/solvent and polymer/solvent to facilitate the self-assembly of particles in the preferred domain of the block copolymer. In a neutral solvent, the freely expanded structure of the polymer and the slightly weaker affinity between particle and solvent than between particle and polymer promotes the selection of particles in the preferred domain of the block copolymer. Furthermore, more particles are allowed to be incorporated into the polymer matrix while still obtaining the nicely ordered structure when these particles exhibit smaller magnetization.

pH- and temperature-dependent phase behavior of a PEO-PPO-PEO-based pentablock copolymer in aqueous media

We investigated the structural features of micelles formed by the self-association of the pentablock copolymer poly[ N,N -(diethyl amino)ethyl methacrylate]-block-poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethyleneoxide)-block-poly[ N,N -(diethylamino)ethyl methacrylate] (PDEAEM-PEO-PPO-PEO-PDEAEM) in aqueous solutions by using small-angle neutron scattering SANS. The pentablock copolymer solutions exhibit micellar and gel phases in response to changes in both the temperature and pH by virtue of (1) the lower critical solution temperature of the PPO blocks and (2) the polyelectrolyte character of the pendant PDEAEM blocks. Two modeling schemes were employed to describe the SANS data of semidilute copolymer solutions at higher temperature as they contain interacting charged micelles at pH<7.5 and interacting neutral micelles at higher pH. We have elucidated the structures of the micelles in terms of size, shape, polydispersity, association number, number density, and surface charge. At low pH the charged spherical micelles are less packed with the copolymers presumably due to the electrostatic repulsion between the charged pendant groups. On the other hand, at higher pH the hydrophobic character of the neutral pendant groups enable them to sequester within the micelle core along with the PPO, thus increasing the number density and the core size of the spherical micelles. At higher copolymer concentration reversible thermoresponsive sol-gel transitions were observed at all pH conditions and the rheological behavior of the gels nicely correlates with different organization of micelles with different shapes.

Structural characterization using the multiple scattering effects in grazing-incidence small-angle X-ray scattering

The multiple scattering effects present in grazing-incidence small-angle X-ray scattering (GISAXS) data and interference between them are addressed theoretically as well as experimentally with measurement of a series of patterns at different incident angles, referred to as `incident-angle-resolved GISAXS' (IAR-GISAXS). X-ray reflectivity (XR), GISAXS and IAR-GISAXS of virus particles on Si-substrate supported-polystyrene films have been measured and all the data have been analyzed with appropriate formalisms. It was found that under certain conditions it is possible to extract the correct structural features of the materials from the GISAXS/IAR-GISAXS data using the kinematic SAXS formalisms, without the need to use the distorted-wave Born approximation. Furthermore, the Kiessig fringes in GISAXS enable the measurement of the average distance between the particle and the substrate, similar to the measurement of film thickness using the fringes in the XR data. It is believed that the methods developed here will expand the application of GISAXS as they enable the application of model-independent and kinematic SAXS theories to nanostructured two-dimensional ordered films.

Effect of interfacial interaction on the cross-sectional morphology of tobacco mosaic virus using GISAXS

We have investigated the effect of the interfacial interaction on the cross-sectional morphology of the tobacco mosaic virus (TMV) in solution and on two types of solid substrates, SiOx (polar) on Si(100) and polystyrene film (nonpolar) on Si(100), using small-angle X-ray scattering (SAXS) and grazing incidence small-angle X-ray scattering (GISAXS), respectively. Results reveal that the flexible chains at the outer surface of TMV either expand or contract depending on the nature of the substrate. Although the unfavorable interaction between the TMV and the PS causes a minimal effect, the stronger attractive interaction between the outer protein surface of TMV and the SiOx substrate induces pronounced deformation of its cross-sectional morphology.

Electron density map using multiple scattering in grazing-incidence small-angle X-ray scattering

The electron density map of a block copolymer thin film having the hexagonally perforated layer (HPL) structure was directly obtained from the measured grazing-incidence small-angle X-ray scattering (GISAXS) pattern, exploiting the multiple-scattering phenomena present in GISAXS. It is shown that GISAXS is in principle equivalent to three-beam diffraction, which has been used to extract phases of diffraction peaks. In addition, X-ray reflectivity analysis has been performed which, when combined with the GISAXS results, provides full details of the HPL structure.

Silver behenate as a calibration standard of grazing-incidence small-angle X-ray scattering

Grazing-incidence small-angle X-ray scattering (GISAXS) patterns of a silver behenate composite film, which has a typical layered structure, are described. The peak position of the film in the GISAXS pattern was varied depending on the incident angle, which was well described by taking into account the refraction and the reflection effects. Since the refractive index of samples depends on sample preparation, it is recommended that the measurement of silver behenate as a standard be done in conventional transmission mode to avoid any complexity.

Fabrication and characterization of microfluidic probes for convection enhanced drug delivery

Convection enhanced drug delivery (CED) is a promising therapeutic method for treating diseases of the brain by enhancing the penetration of drugs. Most controlled release delivery methods rely on diffusion from a source to transport drugs throughout tissue. CED relies on direct infusion of drugs into tissue at a sufficiently high rate so that convective transport of drug is at least as important as diffusive transport. This work describes the fabrication and characterization of microfluidic probes for CED protocols and the role diffusion plays in determining penetration. Microfluidic channels were formed on silicon substrates by employing a sacrificial photoresist layer encased in a parylene structural layer. Flow in the microchannels was characterized by applying constant upstream pressures from 35 to 310 kPa, which resulted in flow rates of 0.5-4.5 microL/min. The devices were used to infuse Evans Blue and albumin in hydrogel brain phantoms. The results of these infusions were compared to a simple convection-diffusion model for infusions into porous media. In vivo infusions of albumin were performed in the gray matter of rats at upstream pressures of 35, 70, and 140 kPa. The microfabricated probes show reduced evidence of backflow along the device-tissue interface when compared with conventional needles used for CED.