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[Display omitted] • The dynamic evolution of multistep inter- and intramolecular cross-linking and formation of different cured species were realized and studied detailedly. • The mechanical properties depended largely on the crosslinking degrees and cured species. • Oxygen permeation behavior was investigated. • The hydrogen bonds mainly consolidated the curing networks of PN resin, especially in phenol-containing PN75. • Thermal pyrolysis behavior was studied by theoretical analysis and experimental techniques. The curing process and mechanism of high-performance phthalonitrile (PN) thermosetting resin are crucial for its excellent mechanical and thermal-oxidation properties. The complicated polymerization reactions of PN resins were effectively controlled to obtain three types of major cured products (triazine, polyindoline and phthalocyanine). The corresponding cross-linking evolution processes and atomistic microstructural models were dynamically constructed and visualized through a creatively self-compiled procedure. Molecular dynamics simulations combined with experiment were properly conducted on two typical PN resins to illustrate the relationship between cross-linked microstructures and performance. The cross-linking degree and types of final products are positively correlated with the mechanical strength of PN resin. The hydrogen bonds present in the cross-linked structure mainly consolidate the cured networks of PN resin and the unique phenolic hydroxyl groups in PN75 enhance the hydrogen bonding contribution. The dominant pyrolysis gases were experimentally detected and distinguished as H 2 O, NH 3 , HCN, CH 4 , CO 2 and CO during the thermal pyrolysis process of PN resin as the temperature rose. This work provides a new approach to probe the curing process and generated species, and further tailor the mechanical behavior and thermal-oxidation properties of resins or composites. [ABSTRACT FROM AUTHOR]

Abstract: Previous investigations have revealed that it is difficult to acquire the infrared spectra of M+(H2O)2 (M=Cu, Ag, Au) through one infrared photon absorption. To explore whether the infrared spectra can be obtained using Ar atom tagging method, ab initio electronic structure calculations are carried out to investigate the geometrical structures, binding energies and infrared spectra of M+(H2O)2Ar (M=Cu, Ag, Au). Theoretical studies predict that for M+(H2O)2Ar (M=Cu, Ag) there are two isomeric structures which result from different binding sites of Ar atom, while only one isomer structure with the Ar atom bound to H atom is found for Au+(H2O)2Ar. With regard to Ag+(H2O)2Ar complex, the Ar atom tends to attach to M+, while Cu+(H2O)2Ar prefers the isomer which the Ar atom binds to H atom of the water molecular. Moreover, the calculated binding energies of the Ar atom are smaller than the infrared photon energy, and so it is possible to obtain the infrared spectra for M+(H2O)2Ar (M=Cu, Ag, Au) complexes. The changes in the spectra caused by the attachment of Ar atom to M+(H2O)2 are discussed. [ABSTRACT FROM AUTHOR]

(Zr, Hf, Nb, Ta, W)C high entropy carbides nano-powders (average particle size less than 65 nm) with low oxygen content (0.2–1.4 wt%) were synthesized through pyrolysis of polymer precursors. Bulk ceramics were fabricated through the process of hot-pressing, and their characteristics were studied by both computational and experimental methods. Density functional theory was carried out to analyze the effects of defects and the W element on the melting point, hardness, modulus, and toughness of ceramics. The results showed that W content had limited influence on the melting points and the aforementioned mechanical properties of ceramics, while carbon vacancies lead to a significant decrease in melting point, hardness and modulus, with slight increases in toughness. Experimental results indicated that the grain size, grain boundary strength, and lattice distortion of ceramic bulks increased with the increase of W content, which affected the mechanical properties consequently. Ceramics with W content of 5 mol% exhibited the best experimental comprehensive performances, with the Vickers hardness, fracture toughness and flexural strength of 23.82 ± 0.49 GPa, 5.66 ± 0.22 MPa·m1/2 and 593.95 ± 15.71 MPa, respectively. [ABSTRACT FROM AUTHOR]

ReaxFF molecular dynamics (ReaxFF-MD) simulations and X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman, Fourier transform infrared spectroscopy (FT-IR), 29Si NMR were used to investigate the temperature-dependent pyrolysis gas release behavior, dynamic evolution of microstructure, and fundamental carbonization mechanism of silicon-modified phenolic resin (SiPR). An improved and effective algorithm for model building was used to construct the cross-linked SiPR simulation model by considering three reactions in the actual experimental cross-linking process with adjustable occurrence rates. Tracking and analysis of the structures showed that with the continuous release of pyrolysis gas, the residual Si and C-containing components decomposed, rearranged, and aggregated into different phase structures. The Si-units agglomerated and underwent phase separation stages. The carbonization process was divided into three periods: initial aromatization, merging and flattening, and residual healing. These findings may shed light on the polysilicon phase distribution and carbonization mechanism of SiPR. [Display omitted] • The morphology changes of SiPR during pyrolysis are studied by ReaxFF-MD and experimental characterization. • The polycondensation reactions in cured SiPR are simulated by a pseudoreaction algorithm considering the different reaction types and reaction rates. • The high-temperature driven Si-units phase separation is divided into self-aggregation and phase separation stages. • The graphitization of SiPR undergoes initial aromatization, merging and flattening, and residual healing periods. [ABSTRACT FROM AUTHOR]

The role of hydrogen bonding (H-bonding) in polymer nucleation and crystallization is not yet so clear. To understand this physical issue, a series of poly (L-lactide) (PLLA) with different amount of terminal oxamide segments (OXA, -NHCOCONH-) were tailor-made as model H-bonding polymers (PLLA nOXA , n is the number of –NHCO–), and then the influence of H-bonding on the polymer nucleation and crystallization were investigated by a combination of experiments and molecular dynamics (MD) simulations. Compared with PLLA, much more types of H-bonds are created between the OXA segments and either the PLLA chains or the other OXA segments in the PLLA nOXA systems. A segment can be immobilized in the melt by H-bonding induced topological constraint and/or steric hindrance leading to a certain extent of reduction in entropy. Thus, the pair of H-bonded segments acting as a precursor of the "first stems" is expected to inducing local ordered structures (LOS) at elevated temperatures, which further facilitates the formation of both primary and secondary nuclei. As a consequence, the nOXA segments induced H-bonds not only significantly enhanced the crystallization temperature (T c , from 100 °C to 112 °C) and the crystallinity (X c , from 10% to 52%) of PLLA, but also increases the crystal growth rate (G) by nearly 10 times (from 0.25 to 2.66 μm min−1 at 150 °C). Moreover, a HI-MS (H-bond induced multi-stage) nucleation phenomenon in the melt of the PLLA nOXA was revealed by in-situ small-angle X-ray scattering (SAXS) under both non-isothermal (1 °C/min) and isothermal (135 °C) conditions. The HI-MS nucleation mechanism is well correlated to the nOXA induced multi-types of H-bonds which possess different thermodynamical stability at a certain high temperature. Therefore, this work not only provides a new route to make rapid crystallizing PLLA via hybridizing H-bonding segments, but also provides a molecular-level insight in the H-bonding assisted nucleation of semi-crystalline polymers in general. PLLA with H-bonding segments (OXA) were tailor-made. The H-bonds create ample immobilized segmental pairs at high (er) temperatures and improves their topological conformation which promote the nucleation of PLLA chains via a formation of stable local ordered structures. Consequently, the minor modification of PLLA not only results in nucleation at much higher temperature but increases the spherulitic growth rate as well, ascribing to an H-bonding assistanted nucleation mechanism. [Display omitted] • The terminal H-bonding were designed and introduced into the backbone of the PLLA. • The H-bonds strongly enhanced the crystallization temperature and crystallinity of the PLLA. • A H-bonds assisted nucleation mechanism is revealed in the PLLA nOXA system. [ABSTRACT FROM AUTHOR]