Phase transition of asymmetric diblock copolymer induced by nanorods of different properties

We investigate the microphase transition of asymmetric diblock copolymer induced by nanorods of different properties using cell dynamics simulation and Brown dynamics.The results show the phase diagram and representative nanostructures of the diblock copolymer nanocomposite.Various structures such as sea-island structure(SI),sea-island and lamellar structure(SI-L),and lamellar structure(L) are observed in the phase diagram.The system undergoes phase transition from SI-L to SI or from L to SI with increasing length of A-like sites for all numbers of nanorods except 10 and 300,and from SI to L with increasing number of nanorods for all lengths of A-like sites.Notably,the polymer system transforms from a tilted layered structure to a parallel lamellar,perpendicular lamellar,and subsequently sea-island structure with increasing length of A-like sites for a rod number of 240.To gain more detailed insight into these structural formation mechanisms,we analyze the evolution kinetics of the system with various lengths of A-like sites of the rods.The pattern evolution and domain growth of the ordered parallel/perpendicular lamellar structure are also investigated.Furthermore,the effects of the wetting strength,rod-rod interaction,polymerization degree,and length of nanorods on the self-assembled structure of asymmetric diblock copolymer/nanorods are studied.Our simulations provide theoretical guidance on the construction of complex-assembled structures and the design of novel functional materials.     

Influence of Grafting Density of Diblock Copolymers on Interfacial Assembly Behavior and Interfacial Shear Strength of Glass Fiber/Polystyrene Composites

To investigate the influence of the grafting density and the molecular structure of block copolymers on the interfacial assembly behavior and interfacial shear strength, macromolecular coupling agents, hydroxyl-terminated poly(n-butyl acrylate-b-styrene) (HO-P(BA-b-S)) were synthesized by atom transfer radical polymerization, and then chemically anchored on the glass fiber surfaces to form a well-defined monolayer. The phase separation and 'hemispherical' domain morphologies of diblock copolymer brushes at the polystyrene/glass fiber interface were observed. The interfacial assembly morphology differs with changes in the grafting density of diblock copolymers. When the grafting density is greatest, the highest height difference of the hemispherical domain and the largest surface roughness are achieved, as well as the best interface shear strength. It was also found that the copolymer brush with a PBA block of the polymerization degree (Xn) about 77 is the optimal option for the interfacial adhesion of PS/GF composites. Thus, the grafting density and molecular structure of diblock copolymers determines the interfacial assembly behavior of copolymer brushes, and therefore the interfacial shear strength.     

Multiple shear-banding transitions in a supramolecular polymer solution

We report on the nonlinear rheology of a reversible supramolecular polymer based on hydrogen bonding. The coupling between the flow-induced chain alignment and breakage and recombination of bonds between monomers leads to a very unusual flow behavior. Measured velocity profiles indicate three different shear-banding regimes upon increasing shear rate, each with different characteristics. While the first of these regimes has features of a mechanical instability, the second shear-banding regime is related to a shear-induced phase separation and the appearance of birefringent textures. The shear-induced phase itself becomes unstable at very high shear rates, giving rise to a third banding regime.     

Surface-field-induced effects on morphologies of lamella-forming diblock copolymers in nanorod arrays

The surface-induced effect on the morphologies of lamella-forming diblock copolymers in nanorod arrays is studied by using the self-consistent field theory.In the simulation study,a rich variety of novel morphologies are observed by variations in the strength of the surface field for the diblock copolymers.Different surface-field-induced effects are examined for the diblock copolymers in the arrays with distinct preferential surfaces.It is observed that the majority-block preferential surfaces have more obvious induced effects than those of minority-block preferential surfaces.The strong surface fields exhibit different behaviours from those observed in the weak surface fields,by which the morphologies possess cylindrical symmetries.Results from this research deepen the knowledge of surface-induced effects in a confinement system,which may aid the fabrication of polymer-based nanomaterials.     

The super hydrophobicity of ZnO nanorods fabricated by electrochemical deposition method

Electrochemical deposition method was employed to fabricate ZnO nanorods on zinc foil substrate in this paper. The structural observations of ZnO nanorods with different aspect ratios were carried out by field-emission scanning electron microscopy. The microstructures of ZnO nanorods were also characterized by X-ray diffraction and the changes in surface hydroxyls with electrochemical deposition time were analyzed by X-ray photoelectron spectroscopy. The study results show the aspect ratios of ZnO nanorods and the density of their surface hydroxyls are responsible for their superhydrophobicity. The fluorinated polymer coated ZnO nanorods showed an excellent superhydrophobic behavior with 167° contact angle of water droplet, which is larger than that of fluorinated polymer flat surface. The more the surface hydroxyls are, the more hydrophilic the surfaces are. Meanwhile, the larger the aspect ratio of ZnO nanorod arrays is, the larger its drophobicity is. The results of this study might pave a simple and feasibility pathway to the fabrication of superhydrophobic cleaning materials used in engineering fields.     

Shear effect on the phase behavior and morphology in pmma/san-29.5 blend

The effect of simple shear flow on the miscibility and morphology of blends of poly(methyl methacrylate) (PMMA) and a styrene-acrylonitrile random copoly-mer with 29.5 wt% acrylonitrile (SAN-29.5) has been investigated using shear apparatus and transmission electron microscopy (TEM). The obtained data showed that only shear-induced mixing was observed for all of the composition ratios. The increase of the cloud point (or homogenization temperature) ΔT(γdot; = T(γdot;) - T(0) was investigated as a function of shear rate γdot;; in addition, the normalized shift in the cloud point ΔT(γdot;)/T(0) versus γdot; was also studied and compared with that of simple liquid mixtures and polymer solutions. The results showed that the polymer blends were more sensitive to the shear rate than both simple-liquid mixtures and polymer solutions. The morphology of the PMMN SAN(= 75/25) blend (the critical composition) indicated that shear-induced phase mixing occurred at a critical shear rate value, below which the two phases were highly oriented and elongated in the flow direction. Three regimes, depending on the applied shear rate values, were detected that were in good agreement with the literature data for polymer solutions. The effect of relaxation times after shear cessation showed a decrease in the orientation of the elongated particles, but it did not completely vanish even for 10 min after the shear cessation.     

Diblock copolymers adsorbed at a water-oil interface

We probe the conformation of a diblock copolymer layer adsorbed at the surface of water-in-oil emulsion droplets at various concentration of a molecular surfactant. The diblock copolymer is made of a hydrophobic polybutadiene part linked to a hydrophilic polyethylene oxide one. The measure provides the equilibrium thickness of the polymer layer that is obtained with two different techniques, i.e. dynamic light scattering and force measurements. The structure of the layer is shown to change from a “mushroom” conformation in which the adsorbed chains form independent Gaussian coils to a conformation where they interact and extend in the continuous phase. The transition from one regime to the other is progressive as the ratio surfactant/polymer bulk concentration varies. Received 5 May 2000 and Received in final form 6 July 2000     

Shear-induced nano-macro structural transition in a polymeric bicontinuous microemulsion

Bicontinuous microemulsions arise in a narrow concentration range for ternary blends containing two immiscible homopolymers and the corresponding diblock copolymer. Steady shear reveals four distinct regimes of response as a function of shear rate, corresponding to flow-induced transitions in fluid structure. In situ neutron scattering shows flow-induced anisotropy in the nanometer-scale microemulsion structure at moderate shear rates, while higher rates induce bulk phase separation, with micron-size morphology, which is characterized with in situ light scattering and optical microscopy.     

采用固体NMR研究MPEG-PLA双嵌段共聚物的固态相区结构

采用固体核磁共振方法，研究了AB型聚（L-丙氨酸）聚乙二醇嵌段共聚物(MPEG-b-PLA）的固态微相结构. ¹³C核的交叉极化与直接极化实验表明，MPEG中存在晶态和非晶态两种相区结构，PLA则含有大量的α螺旋与少量的β折叠二级结构. 由交叉极化过程下的¹³C自旋-晶格弛豫时间(T₁)测定结果进一步表明，MPEG链段由于嵌段结构使结晶过程受抑制，结晶度明显下降. PLA链段以结晶态形式存在，并由于大量α螺旋和β折叠有序结构的存在, 链段非常刚性，运动严重受限，而β位甲基因为可以自由旋转，所以运动能力较α位次甲基和羰基强.     

Selectivity and temperature dependence of phase and phase transition in diblock copolymer solution

In order to study the effects of solvent selectivity and temperature on phase behavior and transition of diblock copolymer solution, self-consistent field theory is modified to incorporate the short-range interaction and non-local effects. Inhomogeneous free-energy density is shown to be dependent on solvent selectivity, temperature and copolymer concentration. Enthalpic quantity and entropic contributions are crucial to phase diagrams of diblock copolymer solution. Three selective strengths of solvent --weak, moderate and strong-- are chosen for comparison. For a weakly selective solvent, theoretical and experimental results illustrate the same variation tendency in the phase boundary of the order-disorder transition for a symmetric diblock of polystyrene and polyisoprene. Self-consistent field equations can be used to calculate the exact FCC-BCC structural phase transition temperatures in moderately and strongly selective solvents. Detailed comparison with the experimental phase diagrams including lamellar, cylindrical and spherical structures is presented.     

Cylindrical-confinement-induced phase behaviours of diblock copolymer melts

The phase behaviours of diblock copolymers under cylindrical confinement are studied in two-dimensional space by using the self-consistent field theory. Several phase parameters are adjusted to investigate the cylindrical-confinement-induced phase behaviours of diblock copolymers. A series of lamella-cylinder mixture phases, such as the mixture of broken-lamellae and cylinders and the mixture of square-lamellae and cylinders, are observed by varying the phase parameters, in which the behaviours of these mixture phases are discussed in the corresponding phase diagrams. Fur-thermore, the free energies of these mixture phases are investigated to illustrate their evolution processes. Our results are compared with the available observations from the experiments and simulations respectively, and they are in good agreement and provide an insight into the phase behaviours under cylindrical confinement.     

Density-controlled growth of ZnO nanorods using ZnO nanocrystals-embedded polymer composite

This work focuses on novel synthesis of ZnO nanorods for their potential applications to optoelectronic and electronic nanodevices. The growth density of ZnO nanorods was modulated through controlling of the density of ZnO nanocrystals dispersed on Si substrate. For this, ZnO nanocrystals synthesized via a polyol process were blended with a polymer matrix. ZnO nanocrystals-embedded polymer composite film was generated by spin-coating the mixed solution. Subsequent heat treatment of composite film removed a polymer matrix and left ZnO nanocrystals on the substrate, serving as seeds for the following ZnO nanorod growth. The density of grown ZnO nanorods was well controllable, depending on the density of dispersed ZnO nanocrystals on the substrate, which was varied by the concentration of ZnO nanocrystal-polymer solution.     

Nanorod near-field radiative heat exchange analysis

A theoretical method for cylinder-to-cylinder radiative heat exchange is formulated. The method utilized was a modified version of a previously published numerical method for near-field sphere-to-sphere radiative exchange. Modifications were made to the numerical procedure to make it applicable to cylindrical geometry of nanorods. Nanorods investigated had length to diameter ratios of 3:1 and 7:1. The heat exchange of nanorods is plotted vs. gap to assess the impact of near-field radiative transfer as gap decreases. Graphical results of energy vs. nanorod radii are also presented. A nanorod-to-plane configuration is estimated utilizing a nanorod asymptotic method. The nanorod-to-nanorod method approximates a nanorod-to-plane geometric configuration when one nanorod radii is held constant, and the second nanorod radii is iteratively increased until the corresponding radiative exchange converges.     

Enhancement of ethanol sensing of TeO2 nanorods by Ag functionalization

Gas sensors based on Ag–TeO₂ composite nanorods were fabricated using thermal evaporation and sputtering techniques. The morphology, structure and phase composition of the as-prepared nanofibers were characterized by scanning electron microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD), respectively. TEM and XRD showed that the nanorods and nanoparticles on them were tetragonal-structured single crystal TeO₂ and a mainly amorphous phase, respectively. The multiple-networked bare TeO₂ nanorod sensors exhibited a response of ～219% at 25 ppm C₂H₅OH at 300 °C, whereas the Ag-functionalized TeO₂ nanorod sensors showed a response of ～808% under the same conditions. The mechanism by which the sensing properties of the TeO₂ nanorods were enhanced by functionalization with Ag is also discussed.     

Ginzburg number of a homopolymer-diblock copolymer mixture covering the 3D-Ising, isotropic Lifshitz, and Brasovskii classes of critical universality

The Ginzburg number Gi of deuterated poly(butadiene) (dPB) and poly(styrene) (PS) homopolymer blend of critical composition mixed with a dPB-PS symmetric diblock copolymer was determined from small angle neutron scattering. A 3 orders of magnitude change of Gi was determined between binary polymer blend and diblock copolymer melt. The strongest change of Gi is observed within the isotropic Lifshitz regime of critical universality occurring over a 3% range of diblock concentration and interpolates the corresponding Gi of the 3D-Ising and Brasovskii regimes. A Lifshitz critical point was not observed consistent with the proposed lower critical dimension d(LCP)=4.     

Fabrication and Characterization of Well-Aligned Zn1-xMnxO Nanorods

Well-Migned Zn1-xMnxO nanorods have been synthesized successfully on bare silicon substrates by a simple evaporation method without using any catalyst. X-ray diffraction and electron microscopy studies demonstrate that the as-grown nanorods are of single wurtzite phase with a preferential growth direction along their c- axes, Quantitative energy-dispersive spectrum analysis reveals that the concentration of manganese is around 4 at,%, Magnetic measurements show the single-phase Zn1-xMnxO nanorod arrays exhibiting the paramagnetic behaviour. Photolumlnescence spectra demonstrate that the Zn1-xMnxO nanorods preserve ultraviolet emission at room temperature.     

Hyperviscous diblock copolymer vesicles

Giant vesicles prepared from the diblock copolymer polybutadien-b-polyethyleneoxide (PB-PEO) exhibit a shear surface viscosity, which is about 500 times higher than those found in common phospholipid bilayers. Our result constitutes the first direct measurement of the shear surface viscosity of such polymersomes. At the same time, we measure bending and stretching elastic constants, which fall in the range of values typical for lipid membranes. Pulling out a tether from an immobilized polymersome and following its relaxation back to the vesicle body provides an estimate of the viscous coupling between the two monolayers composing the polymer membrane. The detected intermonolayer friction is about an order of magnitude higher than the characteristic one for phospholipid membranes. Polymersomes are tough vesicles with a high lysis tension. This, together with their robust rheological properties, makes them interesting candidates for a number of technological applications. Received 2 March 2001 and Received in final form 15 February 2002     

Shear-induced smectic ordering in the melt of isotactic polypropylene

We report an unusual shear-induced smectic phase of high-molecular-mass isotactic polypropylene at temperatures well above its melting point. The necessary stiffness in the polymer chain is provided by shear-induced coil-helix transitions. Coupling between the conformational ordering and the density generates the smectic layering. The periodicity of about 4 nm is built up from a rigid helical sequence of at least 11 monomers and a random-coil part of about 1.4 nm. These results also provide new clues to the question of preordering before polymer crystallization.     

水热法合成ZnO纳米棒图案及其场增强效应

利用水热合成方法在图案化的Au岛上合成了ZnO纳米棒图案,采用的溶液体系为六次甲基四胺和硝酸锌溶液,ZnO纳米棒的基底是ITO导电玻璃上的有序Au岛. 由于ZnO的异相成核速度在Au和ITO基底上具有不同的成核速度,因此ZnO优先生长在成核速度快的Au岛上,同时由于受到了溶液中前驱物种扩散的限制,纳米棒继续生长也被受到了约束. 通过调控六次甲基四胺和硝酸锌的浓度,可以调整不同的图案. 此外,利用X射线衍射、光致发光谱和场发射特性性能对水热合成的ZnO纳米棒图案进行了研究. ZnO纳米棒表现出良好的场增强性