Conformation and aggregation of poly-L-proline in solution

Although a large number of physical techniques have been successfully used to investigate many of the properties of poly-L-proline, the work reported here has used a combination of osmometry, light scattering, viscometry, and sedimentation studies to reveal a new aspect of this model biopolymer. Experiments were made both on solutions (propionic acid) of poly-L-proline Form I and Form II and on solutions (propionic acid with a threefold dilution of n-propanol) in which Form II was in the process of converting to Form I. The results indicate that an increase in the measured molecular weight accompanies known optical activity and intrinsic viscosity changes which occur as Form II becomes Form I. It appears that the molecular weight determined at infinite dilution for poly-L-proline I is approximately twice that found for poly-L-proline II, and evidence for concentration-promoted aggregation beyond the level of a dimer has been noted in these Form I solutions. Based on these facts and on the information obtained about the particle shapes, it is proposed that this association occurs by a side-to-side binding of two macro-molecules. Discussion is directed toward how these experimental findings can be incorporated into the established concept of the Form I conformation. Light, scattering experiments were also performed on solutions (propionic acid 3 M in LiBr) in which the high-salt, or collapsed, form of poly-L-proline had been generated from either Form II or Form I material. These results showed that the dissolved particles are composed of single chains and are significantly smaller in size than found in solutions of either form and it appears possible that in the collapsed form poly-L-proline might be represented by rodlike macromolecules possessing trans-peptide bonds and a conformation with a relatively small helical pitch.     

Mechanical relaxation in radiation-polymerized vinyl stearate

Mechanical relaxation studies, over the temperature range from about 100[ddot] K to or above the melting point, were performed on unannealed and annealed samples of vinyl stearate, on as-received and purified samples of poly(vinyl stearate), and on a series of radiation-polymerized samples having different monomer-polymer ratios. These latter samples were polymerized in the solid state by exposure of the crystalline monomer to a ⁶⁰Co source and were characterized by density measurements, X-ray diffraction, and differential thermal analysis (DTA). The modulus-temperature and loss-temperature curves of the monomer showed a mechanical relaxation centered at 210[ddot]K (959 Hz) which appeared to decrease toward a limiting value with increase of annealing time. This behavior is similar to that observed in dielectric studies of vinyl stearate. With increase of radiation time and greater conversion to polymer, the magnitude of the 210[ddot] K loss peak increased while the peak position shifted to lower temperatures. The loss-peak is located at 172[ddot] K (864 Hz) in the polymer and is analogous to the γ -relaxation peak in polyethylene in height, breadth, and temperature location. It is attributed to local reorientational motions of (side-chain) —CH₂— segments in disordered or defect regions. X-Ray and DTA data indicate that solid solutions of the monomer and polymer are present in many of the radiation-polymerized samples. The effects of large radiation exposures on the relaxation behavior of the polymer were examined and compared with similar data on polyethylene. The implications of the current data concerning molecular interpretations of the polyethylene relaxations are discussed.     

Surface disorder in c-Si induced by swift heavy ions

The disorders induced in crystalline silicon (c-Si) through the process of electronic energy loss in the swift heavy ion irradiation were investigated. A number of silicon <1 0 0> samples were irradiated with 65 MeV oxygen ions at different fluences, 1×10¹³ to 1.5×10¹⁴ ions/cm², and characterized by the Raman spectroscopy, the optical reflectivity, the X-ray reflectivity, the atomic force microscopy (AFM) and the X-ray diffraction (XRD) techniques. The intensity, redshift, phonon coherence length and asymmetric broadening associated with the Raman peaks reveal that stressed and disordered lattice zones are produced in the surface region of the irradiated silicon. The average crystallite size, obtained by analyzing Raman spectrum with the phonon confinement model, was very large in the virgin silicon but decreased to<100 nm dimension in the ion irradiated silicon. The results of the X-ray reflectivity, AFM and optical reflectivity of 200–700 nm radiation indicate that the roughness of the silicon surface has enhanced substantially after ion irradiation. The diffusion of oxygen in silicon surface during ion irradiation is evident from the oscillation in the X-ray reflectivity spectrum and the sharp decrease in the reflectivity of 200–400 nm radiation. The rise in temperature, estimated from the heat spike model, was high enough to melt the local silicon surface. The results of XRD indicate that lattice defects have been induced and a new plane <2 1 1> has been formed in the silicon <1 0 0>after ion irradiation. The results of the present study show that the energy deposited in crystalline silicon through the process of electronic energy loss ～0.944 keV/nm per ion is sufficient to induce disorders of appreciable magnitude in the silicon surface even at a fluence of ～10¹³ ions/cm².     

Non-isothermal crystallization behaviors of polyamide 6/clay nanocomposites

The non-isothermal crystallization behaviors of polyamide 6/clay nanocomposite (PA6CN) were investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). DSC results showed that the nanometric silicate layers in PA6CN acted as effective nucleation agents. The addition of silicate layers influenced the mechanism of nucleation and the growth of PA6 crystallites. The DSC results also implied an unusual phenomenon, in contrast to PA6, the crystallinity degree of PA6CN increased with increasing cooling rate. XRD results verified this phenomenon and indicated that the addition of silicate layers favored the formation of the γ crystalline form.     

Exploitation of symmetry in periodic Self-Consistent-Field ab initio calculations: application to large three-dimensional compounds

Symmetry can dramatically reduce the computational cost (running time and memory allocation) of Self-Consistent-Field ab initio calculations for crystalline systems. Crucial for running time is use of symmetry in the evaluation of one- and two-electron integrals, diagonalization of the Fock matrix at selected points in reciprocal space, reconstruction of the density matrix. As regards memory allocation, full square matrices (overlap, Fock and density) in the Atomic Orbital (AO) basis are avoided and a direct transformation from the packed AO to the SACO (Symmetry Adapted Crystalline Orbital) basis is performed, so that the largest matrix to be handled has the size of the largest sub-block in the latter basis. We here illustrate the effectiveness of this scheme, following recent advancements in the CRYSTAL code, concerning memory allocation and direct basis set transformation. Quantitative examples are given for large unit cell systems, such as zeolites (all-silica faujasite and silicalite MFI) and garnets (pyrope). It is shown that the full SCF of 3D systems containing up to 576 atoms and 11136 Atomic Orbitals in the cell can be run with a hybrid functional on a single core PC with 500 MB RAM in about 8 h.     

In situ Nucleation-Growth Strategy for Controllable Heterogeneous Supramolecular Polymerization of Liquid Crystalline Block Copolymers and Their Hierarchical Assembly

The self-assembly morphologies of subunits are largely governed by thermodynamics, which plays a less important role in dimensional control. Particularly for one-dimensional assemblies from block copolymers (BCPs), the negligible energy difference between short and long ones imposes great challenges in length control. Herein, we report that by incorporating additional polymers to induce in situ nucleation and trigger the subsequent growth, controllable supramolecular polymerization driven by mesogenic ordering effect could be realized from liquid crystalline BCPs. The length of the resultant fibrillar supramolecular polymers (SP) is controlled by tuning the ratio between nucleating and growing components. Depending on the choice of BCPs, the SPs can be homopolymer-like, heterogeneous triblock, and even pentablock copolymer-like. More interestingly, with insoluble BCP as a nucleating component, amphiphilic SPs are fabricated, which can undergo spontaneous hierarchical assembly.     

Amorphous Mo-doped NiS0.5Se0.5 Nanosheets@Crystalline NiS0.5Se0.5 Nanorods for High Current-density Electrocatalytic Water Splitting in Neutral Media

It is vitally important to develop highly active, robust and low-cost transition metal-based electrocatalysts for overall water splitting in neutral solution especially at large current density. In this work, amorphous Mo-doped NiS_0.5Se_0.5 nanosheets@crystalline NiS_0.5Se_0.5 nanorods (Am−Mo−NiS_0.5Se_0.5) was synthesized using a facil one-step strategy. In phosphate buffer saline solution, the Am−Mo−NiS_0.5Se_0.5 shows tiny overpotentials of 48 and 209 mV for hydrogen evolution reaction (HER), 238 and 514 mV for oxygen evolution reaction (OER) at 10 and 1000 mA cm⁻², respectively. Moreover, Am−Mo−NiS_0.5Se_0.5 delivers excellent stability for at least 300 h without obvious degradation. Theoretical calculations revealed that the Ni sites in the defect-rich amorphous structure of Am−Mo−NiS_0.5Se_0.5 owns higher electron state density and strengthened the binding energy of H₂O, which will optimize H adsorption/desorption energy barriers and reduce the adsorption energy of OER determining step.     

Covalent Organic Frameworks for Extracting Water from Air

Water scarcity is becoming an increasingly pressing issue due to global population growth and industrialization. One effective approach to addressing this issue is sorption-based atmospheric water harvesting (SAWH). Covalent organic frameworks (COFs) are a type of porous crystalline material that have emerged as promising sorbents for water harvesting due to their high surface area, tunable pore size, and customizable pore chemistry. In this mini-review, we provide an overview of the different types of COFs, their structural characteristics, and the diverse linkage chemistries used to construct them. Then, we summarize recent advances in using COF-based sorbents for atmospheric water harvesting, including strategies for controlling sorption properties and optimizing performance in terms of thermodynamics and dynamics. Finally, we discuss prospects and challenges associated with improving the efficiency of COF-based SAWH systems.     

SYNTHESIS OF LIQUID CRYSTALLINE COPOLYESTERS WITH T-SHAPED TWO-DIMENSIONAL MESOGENIC UNIT AND CROWN ETHER CYCLE

A novel series of liquid crystalline copolyesters with T-shaped two-dimensional mesogenic unit and crown ether cycle of cis-4,4′-bis(4-hydroxyphenylazo)dibenzo-18-crown-6 was prepared via solution condensation polymerization from 4,4′-(α,ω-hexanedioyloxy)dibenzoyl dichloride(M_1),2-(4′-ethoxyphenyl)hydroquinone(M_2)and cis-4,4′-bis(4- hydroxyphenylazo)dibenzo-18-crown-6(M_3).The molecular weights of copolyesters are not high,and the intrinsic viscosity [η]of copolyesters ranges from 0.29-0.43.The monomer...