Polyurethane material is widely utilized in industry and daily life due to its versatile chemistry and relatively easy handling. Here, we focused on a novel thermally reversible cross-linked polyurethane with comprehensive remarkable mechanical properties as reported in our recent work (Adv. Mater. 2013, 25, 4912). The microphase-separated structure and heterogeneous segmental dynamics were well revealed by T2 relaxometry experiments, which was also first utilized to in situ monitor the reversible cross-linking associated with Diels-Alder (DA) and retro-Diels-Alder (RDA) reactions. On the basis of T2 relaxometry results, we determined the actual temperature of the (R)DA reaction as well as the corresponding activation energies of the motion of soft segments. Besides, the roles of the temperature and cross-linker contents on the microdomain structure and dynamics are discussed in detail. It is found that the microphase separation is enhanced by the increase of temperature as well as the incorporation of cross-linkers. Also, the polyurethane samples are still thermal-stable even at a high temperature beyond the disassociation of the cross-linkages. Furthermore, Baum-Pines and three-pulse multiple-quantum NMR experiments are utilized to investigate the heterogeneous structures and dynamics of the mobile and rigid segments, respectively. Both the results obtained from the T2 relaxometry and multiple-quantum NMR experiments are in good agreement with the macroscopic mechanical properties of the polyurethane. Finally, it is also well demonstrated that proton T2 relaxometry combined with multiple-quantum NMR is a powerful method to study the heterogeneous structures and dynamics of a multiphase polymer system.

The structure and dynamic behavior of mobile components play a significant role in determining properties of solid materials. Herein, we propose a novel real-time spectrum-editing method to extract signals of mobile components in organic solids on the basis of the polarization inversion spin exchange at magic angle (PISEMA) pulse sequence and the difference in (13)C T(1) values of rigid and mobile components. From the dipolar splitting spectrum sliced along the heteronuclear dipolar coupling dimension of the 2D spectrum, the structural and dynamic information can be obtained, such as the distances between atoms, the dipolar coupling strength, the order parameter of the polymer backbone chain, and so on. Furthermore, our proposed method can be used to achieve the separation of overlapped NMR signals of mobile and rigid phases in the PISEMA experiment. The high efficacy of this 2D NMR method is demonstrated on organic solids, including crystalline L-alanine, semicrystalline polyamide-6, and the natural abundant silk fibroin.

The CODEX (center-band only detection of exchange) NMR experiment is widely used for the detection of slow motions in organic solids, especially polymers. However, the RIDER (relaxation-induced dipolar exchange with recoupling) effect may result in artificial exchange signals in the CODEX pure exchange spectrum, which greatly limits the application of CODEX method. Herein, we investigate the distance range that the RIDER effect can reach by performing CODEX experiments on two typical organic solids, hexadecyltrimethylammonium bromide (CTAB) and semi-crystalline polyamide-6 (PA6) where there are no slow molecular motions at room temperature. Our experimental results demonstrate that generally two-bond distance is far enough to ignore the RIDER effect resulted from the dipolar interactions between 13C and the fast relaxing heteronucleus 14N. From the built-up curve of RIDER signals as a function of recoupling time and mixing time, it is clearly revealed that the RIDER effect can greatly affect the signal from 13C directly bonded with 14N. However, this RIDER effect accounts less than 3% of the reference intensity for signals from 13C not directly bonded with 14N if typical recoupling (∼0.5ms) and mixing times (∼0.5s) are used for the investigation of slow motions. When longer recoupling and mixing time are used, there are small RIDER signals even for the 13C far away from the 14N. These signals, to a large degree, result from the spin diffusion effect and/or the special microscopic molecule arrangement. However, they are so small compared to the reference signal (∼5%) that they can be ignored. Finally, according to the simulation results, it is worth noting that the RIDER signal is still generally negligible compared to the signals due to slow motions if the chemical shift anisotropy reorientation during the mixing time is not too small(larger than 20°) under the condition of 4tr recoupling time at the magic-angle-spinning speed of 6.5kHz.

The effect of finite compressibility on the lamellar phase of diblock copolymers is studied in the Strong Stretching Limit(SST).The results are presented by comparing with those of the incompressible system.For the compressible system,the density profile is no more homogeneous in the both A and B domains.The compressing energy in each domain and the free energy results from the distribution of the free end is much smaller than the interfacial energy and the stretching energy,and the latter two terms are still the dominant ones.

The phase behavior of single homopolymer chain systems is studied by Wang-Landau algorithm,based on a simple cubic lattice model.First of all,the density of states is calculated for systems with different chain length(N),and then free energy,internal energy,entropy and specific heat are calculated as a function of temperature(T).Meantime,the micro-canonical average and canonical average of gyration radius and end-end distance are calculated.From the dependence of specific heat on T,as T decreases,the large N system undergoes transitions from a coil to a globule phase and then to a solid phase.By analyzing the coil-globule and liquid-solid transition temperatures for systems with different chain lengths,two transition temperatures do not coincide in the thermodynamic limit(infinite chain length).This result is not consistent with that of Binder K et al based on bond-fluctuation model,but agrees with the result from sophisticated chain-growth methods by Vogel T et al.[KH*3/4]

Self-assembly of symmetric diblock copolymer AB/homopolymer A or C blends confined in spherical nanopores is studied by using a simulated annealing method. Three kinds of blends with different homopolymer chain lengths are investigated. Simulation results show that the self-assembly behavior of the blends depends on the content of the homopolymer, the chain length of the homopolymer, the degree of confinement (characterized by D/L-0, where D is the diameter of the spherical nanopore and L-0 is the period of the bulk lamellar phase of symmetric diblock copolymer),and the interaction strength between A-C monomers. With the variation of the content of homopolymer and the degree of confinement, the blends exhibit a variety of morphologies and morphological transitions. The observed morphologies include onion-like structures (with both concentric lamellae and nonconcentric lamellae), cage-like structures, deformed dome-like structures, helices or stacked toroids, a disk-like layer and spherical structures. Furthermore, the obtained simulation results also show that both the chain length of the homopolymer and the A-C interaction strength have a large effect on the compatibility of the blends. When the A-C interaction strength is relatively weak, the shorter the chain length of homopolymer, the better the compatibility of the blend, and the corresponding structures occur at a slightly smaller value for the homopolymer content. When the interaction strength between the A-C monomers is strong enough, the effects caused by homopolymer chain length become less pronounced and eventually disappear. In this case, the blend has a good compatibility. The simulation results are consistent with recent experimental results of similar systems.

The effects of confining geometries on the self-assembly of cylinder-forming asymmetric diblock copolymers are studied using a simulated annealing technique. Morphological transitions of block copolymers confined inside two parallel flat walls, cylindrical channels, as well as spherical and ellipsoidal cavities are systematically investigated. Depending on the copolymer composition, confining geometry and degree of structural frustration, a very rich array of confinement-induced morphologies is predicted by the simulations. The results reveal that the dimensionality of the confinement can affect the structure, symmetry and degeneracy of the self-assembled structures. In particular, the effect of spherical confinement is much stronger than that of thin film or cylindrical confinement.

Phase behavior of blends of two AB diblock copolymers, with the long one at relatively strong segregation, is studied using the self-consistent field theory, focusing on the effect of compositions of the two block copolymers and their length ratio. In order to carry out extensive calculations on the large parameter space, a unit-cell approximation is employed, in which the mean-field equations are solved using a Bessel function expansion. Phase diagrams are constructed for four typical series of blends by comparing the free energies of the different ordered phases including lamellae, cylinders, and spheres. The results reveal that the competition between macro- and microphase separation leads to complex phase behavior. When the length ratio of the two block copolymers is small, the short copolymers tend to segregate to the A/B interfaces, inducing multiple order-order phase transitions including reentrant phase transitions in some blends. When the length ratio of the two diblock copolymers is sufficiently large, macrophase separation may take place. The predicted phase diagrams are compared with available experiments. Density profiles of typical ordered structures are presented to understand the self-organization of the polymer chains. The energetics of the blends is introduced to account for the appearance of the macro- and microphase separations.

The adsorption and charge inversion by flexible polyelectrolytes (PEs) onto an oppositely charged spherical surface from a bulk solution of finite PE concentration are studied via numerically solving the coupled ordinary differential equations derived from the continuum self-consistent field (SCF) theory under the ground-state dominance approximation. The effects of various parameters, including the particle radius (r(0)) and its surface charge density (sigma(sf)), PE charge fraction (p), short-range surface-PE inter-action, solvent quality, and bulk PE concentration and salt concentration, on the amount of adsorbed PEs (Gamma) and charge inversion ratio are investigated in detail. It is found that in salt-free solutions where the electrostatic interaction is dominant a relationship of approximate to sigma(sf) (0) (2)/p is generally satisfied. The critical particle radius and its surface charge density for PE adsorption are computed as a function of the bulk salt concentration. It is also found that PE adsorption occurs in most cases, whereas strong charge inversion cannot occur either in salt-free solutions or for nonadsorbing surfaces. For attractive surfaces, increasing the bulk salt concentration, or decreasing the surface charge density and the particle radius, generally enhances the charge inversion. Our results on the charge inversion are consistent with previous SCF calculations for planar and cylindrical surfaces.

An efficient method for identifying different types of carbon groups (CH(3), CH(2), CH, and quaternary carbons) in organic solids is proposed by utilizing the combination of a two-dimensional (2D) (13)C-(1)H polarization inversion spin exchange at magic angle (PISEMA) NMR experiment and numerical simulation results of simple isolated (13)C-(1)H dipolar coupling models. Our results reveal that there is a unique line shape of the (13)C-(1)H dipolar splitting pattern and a corresponding characteristic splitting value for each carbon group, based on which different carbon types can be distinguished unambiguously. In particular, by using this method, the discrimination and assignment of overlapped signals from different types of carbons can be achieved easily. The efficacy of this method is demonstrated on typical solid small molecules, polymers, and biomacromolecules.

Mesh-like bismuth oxide single crystalline nanoflakes were synthesized via a bismuth oxalate precursor way. During the synthesis procedure bismuth oxalate precipitate was hydrothermally treated, and variation of the pH value of synthesis solution as well as the addition of l-lysine could effectively tune the crystal phase and morphology of the bismuth oxalate precursor. Depending on the crystalline phase of the precursors and the calcination temperature, monoclinic α-Bi2O3 and tetragonal β-Bi2O3 were selectively obtained. Optical properties of the Bi2O3 materials were investigated by UV–vis absorption and the results proved that bismuth oxides were visible light responsive. Photodegradation of rhodamine B was used as a model reaction to test the photocatalytic activity of the Bi2O3 samples. The photocatalytic activity of bismuth oxides was related to its crystalline phase and morphology.

Hierarchically nanoporous single-crystal mesoporous silica was fabricated by using mesomorphous polyelectrolyte−surfactant complexes as dynamic template. During the kinetic self-assembly, the order of the mesomorphous complexes was realigned and evolved into Pm3̅n mesostructured silica, and the phase-separated polymer served as the template for secondary nanopores within the single crystal SBA-1.

1H spin-diffusion solid-state NMR, in combination with other techniques, was utilized to investigate the effect of molecular architecture and temperature on the interphase thickness and domain size in poly(styrene)-block-poly(butadiene) and poly(styrene)-block-poly(butadiene)-block-poly(styrene) copolymers (SB and SBS) over the temperature range from 25 to 80°C. These two block copolymers contain equal PS weight fraction of 32wt%, and especially, polystyrene (PS) and polybutadiene (PB) blocks are in glass and melt state, respectively, within the experimental temperature range. It was found that the domain sizes of the dispersed phase and interphase thicknesses in these two block copolymers increased with increasing temperature. Surprisingly we found that the interphase thicknesses in these two block copolymers were obviously different, which was inconsistent with the theoretical predictions about the evolution of interphase in block copolymer melts by self-consistent mean-field theory (SCFT). This implies that the interphase thickness not only depends strongly on the binary thermodynamic interaction (χ) between the PS and PB blocks, but also is influenced by their molecular architectures in the experimental temperature range.

First principles calculations of the total energy of CaTe as a function of unit cell volume have been carried out for the NaCl, MnP and CsCl structures on the basis of density functional theory (DFT). All these calculations are performed with the CRYSTAL06 program package. The sequence of high-pressure phases for CaTe transforms from NaCl phase to an intermediate state with a mixture of NaCl and MnP phases and then to the CsCl phase is obtained, which is in good agreement with the previous experimental results. Several structural properties (equilibrium lattice constant, bulk modulus, etc.) of NaCl structure have been calculated, which are also in agreement with the previous experimental results.

We report an extensive simulation study of the self-assembly of amphiphilic ABA triblock copolymers dissolved in solvents selective for the middle B-block. The effects of copolymer composition, copolymer concentration, and A-solvent interactions on the morphologies and morphological transitions of the aggregates are examined systematically. The simulations reveal that a rich variety of aggregates, ranging from spherical and rodlike micelles and vesicles to toroidal and net-cage micelles, can be formed spontaneously from a randomly generated initial state. Phase diagrams are constructed and rich morphological transitions are predicted. Chain packing in different micelles is investigated. The simulation results are compared with previous observations or predictions for related copolymer systems.

Total energy of SiO2 as a function of unit cell volume has been calculated for α-quartz,rutile and CaCl2 structures using the density function theory(DFT) and Hartree-Fork(HF) method.According to the Murnaghan's equation of state,the bulk modulus B0 and B′=dB/dp for these three structures have been obtained.The calculated results are all in good agreement with experimental and other theoretical data available in the literature,which indicate that SiO2 will transform from α-quartz structure to CaCl2 structure as pressure increasing,and there is no phase transition between rutile structure and CaCl2 structure,namely they can coexist.By analyzing the changes of the lattice parameters,the electronic density of states,and the band gap under pressure,it is found that in α-quartz SiO2 the energy band width and charge transfer increase with increasing pressure.Moreover,the Si─O bond length is shortened and the electric charges are redistributed.

Quaternary ammonium surfactants were generally used as templates in the synthesis of mesoporous materials, and the surfactants were removed by extraction or calcination to give the space of mesopores. Here with the cetyltrimethylammonium bromide templated mesostructured materials, we directly utilized the surfactants as capturing and stabilizing agents to prepare supported noble metal nanoparticle catalysts. The Pd-supported catalysts exhibited high and stable activity in hydrogenation of allyl alcohol (with a remarkably high turnover frequency of 5676 h(-1)) and aerobic oxidation of benzyl alcohol. During the recycle use of the catalyst, though surfactants were gradually leached into the reaction medium, the amount of Pd in the catalyst remained constant within experimental error. It is facile and energy-saving to use directly the as-synthesized mesostructured materials as support for noble metal nanoparticles without further calcination or further modification treatment. This method is versatile to prepare other noble metal (such as Au and Pt) supported catalysts by altering the metal precursors. (C) 2010 Elsevier Inc. All rights reserved.

Binary blends of a diblock copolymer (AB) and an incompatible homopolymer (C) confined in spherical cavities are studied using a simulated annealing technique. The phase behavior of the blends is examined for four typical cases, representing the different selectivity of the pore surface to the A, B, and C species. The internal morphology of the spherical polymeric particles is controlled by the homopolymer volume fraction, the degree of confinement, and the composition of the copolymer. Inside a particle, the homopolymers segregate to form one or, under some conditions, two domains; thus, the homopolymers may act as an additional controlling parameter of the shape and symmetry of the copolymer domain. A rich array of confinement-induced novel diblock copolymer morphologies is predicted. In particular, core-shell particles with the copolymers as the shell wrapping around a homopolymer core or a copolymer-homopolymer combined core and Janus-like particles with the copolymers and the homopolymers on different sides are obtained.

Equation of state and phase transformation of ZnO in wurtzite structure and NaCl structure under high pressures are studied using density function theory(DFT) formalism.We analyze structural properties,including lattice constant,electronic density of states and band gap.The results are in good agreement with experimental and other theoretical results.It is found that the calculated phase trasformation pressure of metallic oxides using local density approximation(LDA) is generally higher than experimental results.Calculated results using generalized gradient approximation(GGA) are in good agreement with experiments.

Porous lanthanide oxides were fabricated by a precursor-thermolysis method. The precursors were synthesized by a hydrothermal reaction with lanthanide (La, Ce, Pr and Nd) salts, sodium oxalate and asparagine (or glutamine). Under hydrothermal conditions asparagine and glutamine exhibited greatly different complexation abilities with lanthanide cations. The competitive interactions of lanthanide cations with oxalate anions and asparagine (or glutamine) gave rise to the formation of precursors with different structures and morphologies. ESI-MS detection further confirmed the different complexation abilities of asparagine or glutamine with lanthanide cations at the molecular level. Variation of oxalate anion concentration or the pH value of the reaction solution could tune the morphology of the products. After calcination, porous lanthanide oxides were obtained with the morphologies of their corresponding precursors. Our work suggests that the complexation ability of organic molecules with metal cations could be a crucial factor for morphological control of the precursors. Moreover, considering the diversity of organic additives and metal salts, other metal oxides with complex composition and morphology could be fabricated via this organic molecule-modified precursor method.