Microstructure and Strengthening Mechanism of Die-Cast Mg-Gd Based Alloys

Mg-8Gd-2Y-Nd-0.3Zn (wt%) alloy was prepared by the high pressure die-cast technique. The microstructure, mechanical properties in the temperature range from room temperature to 573 K, and strengthening mechanism were investigated. It was confirmed that the Mg-Gd-based alloy with high Gd content exhibited outstanding die-cast character. The die-cast alloy was mainly composed of small cellular equiaxed dendrites and the matrix. The long lamellar-shaped stacking compound of Mg3X (X: Gd, Y, Nd, and Zn) and polygon-shaped.

Time-resolved synchrotron SAXS observations on sheared syndiotactic poly(propylene) crystallization process

The in situ crystallization kinetics of syndiotactic poly(propylene) (sPP) has been investigated by synchrotron small-angle X-ray scattering (SAXS). The structure evolutions during the isothermal crystallization of sPP with different shear rates have been observed. The results show that shear accelerates the process of crystallization kinetics. Even under low shear rate, the lamellae can be distinctly oriented. In contrast, the lamellar parameters such as the long period, lamellar thickness, and the scattering invariant 0 can change obviously only under high shear rate.

Rigidity effect on phase behavior of symmetric ABA triblock copolymers: A Monte Carlo simulation

The phase behavior of symmetric ABA triblock copolymers containing a semiflexible midblock is studied by lattice Monte Carlo simulation. As the midblock evolves from a fully flexible state to a semiflexible state in terms of increase in its persistence length, different phase behaviors are observed while cooling the system from an infinite high temperature to a temperature below T-ODT (order-disorder transition temperature). Within the midblock flexibility range we studied (l(p)/N-c <= 0.105), a lamellar structure is formed at equilibrium state as the situation for fully flexible chains. The fraction of bridge chain is evaluated for the lamellar structures. We find that the increase in midblock rigidity indeed results in the increase in bridge chain fraction within the range from 44.9% to 51.8%.

Self-assembly of rod-coil-rod ABA-type triblock copolymers

Self-assembled behavior of symmetric ABA rod-coil-rod triblock copolymer melts is studied by applying self-consistent-field lattice techniques in three-dimensional space. The phase diagram is constructed to understand the effects of the chain architecture on the self-assembled behavior. Four stable structures are observed for the ABA rod-coil-rod triblock, i.e., spherelike, lamellar, gyroidlike, and cylindrical structures. Different from AB rod-coil diblock and BAB coil-rod-coil triblock copolymers, the lamellar structure observed in ABA rod-coil-rod triblock copolymer melts is not stable for high volume fraction of the rod component (f(rod)=0.8), which is attributed to the intramolecular interactions between the two rod blocks of the polymer chain.

Novel morphology of polyethylene crystals created upon melt crystallization of spin-coated film

We demonstrate a strikingly novel morphology of high-density polyethylene (HDPE) crystal obtained upon melt crystallization of spin-coated thin film. This crystal gives windmill-like morphology which contains a number of petals. A detailed inspection on this morphology reveals that each petal is actually composed of terrace-stacked PE lamellae, in which the polymer chains within crystallographic a-c planes adopt similar to 45 degrees tilting around b-axis. The surrounding domains associated with a petal of the windmill composed of twisted lamellar overgrowths with an identical orientation of their long axis, which is the crystallographic b-axis shared by the petal and its corresponding twisted lamellar overgrowths.

Monodisperse Co-oligomer Approach toward Nanostructured Films with Alternating Donor-Acceptor Lamellae

A series of donor-acceptor (D-A) co-oligomers with oligo(fluorene-alt-bithiophene) and perylene diimide as donor and acceptor segments, respectively, have been designed and synthesized. They can self-assembly into alternating D-A lamellar nanostructured films with the periods depending on the molecular length. These films have been successfully used in fabrication of high-performance single-molecular solar cells with power conversion efficiency up to 1.50%.

Dendritic Superstructures and Structure Transitions of Asymmetric Poly(L-lactide-b-ethylene oxide) Diblock Copolymer Thin films

The evolution of morphologies of isothermally crystallized thin films with different thicknesses of poly(L-lactide-bethylene oxide) diblock copolymer was observed by optical microscopy (OM) and atomic force microscopy (AFM). Dendritic superstructures stacked with lamellae were investigated in thin films with similar to 200 nm to similar to 400 nm thickness. The lamellar structure was a lozenge- or truncated-lozenge-shaped single crystal of PLLA confirmed by AFM observations. The contour of the dendritic superstructures is hexagonal, and two types of sectors, [110] and [100], can be classified in terms of the chain-folding and crystal growth directions. These phenomena Are due to the interplay of the crystallization of the PLLA block, the microphase separation of the block copolymer, and the effect of the film thickness.

Self-assembly of T-shaped rod-coil block copolymer melts

Self-assembled behavior of T-shaped rod-coil block copolymer melts is studied by applying self-consistent-field lattice techniques in three-dimensional space. Compared with rod-coil diblock copolymers with the anchor point positioned at one end, the copolymers with the anchor point at the middle of the rod exhibit significantly different phase behaviors. When the rod volume fraction is low, the steric hindrance of the lateral coils prevents the rods stacking into strip or micelle as that in rod-coil diblock copolymers. The competition between interfacial energy and entropy results in the formation of lamellar structures and the increasing thickness of the lamellar layer with increasing rod volume fraction.

Synchrotron investigation on the sheared structure evolution of syndiotactic polypropylene crystallization process

The final structure of molten syndiotactic polypropylene (sPP) sheared under different conditions was investigated by synchrotron small-angle x-ray scattering (SAXS) and wide-angle x-ray diffraction (WAXD) techniques to elucidate the shear effects on sPP crystalline structure. The results obtained from the WAXD show that there is no variation on crystalline form but a little difference on the orientation of the 200 reflection. The SAXS data indicate that the lamellar thickness and long period have not been affected by shear but the lamellar orientation is dependent on shear. The experimental data of sPP crystallization from sheared melt may indicate a mesophase structure that is crucial to the shear effects on the final polymer multiscale crystalline structures.

Effect of Shear Flow on the Formation of Ring-Shaped ABA Amphiphilic Triblock Copolymer Micelles

We have investigated the effect of Shear flow on the formation of rill.-shaped ABA triblock copolymer (P4VP(43)-b-PS260-b-P4VP(43)) micelles. The results reveal that Shear flow Plays an important role in the formation of the rings Both ring size and its, distribution are found to be dependent sensitively on the stirring rate. Sizable rings are more likely to be formed at moderate stirring rate, Interestingly, the ring formation mechanism is also dependent oil the Shear flow. Copolymers are likely to form rings via end-to-end cylinder connection at low stirring rates, whereas they tend to form rings via the pathway of the rod-sphere-vesicle-ring it high stirring rates.

Structural evolution of tensile deformed high-density polyethylene at elevated temperatures: Scanning synchrotron small- and wide-angle X-ray scattering studies

The structural evolution of an ice-quenched high-density polyethylene (HDPE) subjected to uniaxial tensile deformation at elevated temperatures was examined as a function of the imposed strains by means of combined synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques. The data show that when stretching an isotropic sample with the spherulitic structure, intralamellar slipping of crystalline blocks was activated at small deformations, followed by a stress-induced fragmentation and recrystallization process yielding lamellar crystallites with their normal parallel to the stretching direction. Stretching of an isothermally crystallized HDPE sample at 120 degrees C exhibited changes of the SAXS diagram with strain similar to that observed for quenched HDPE elongated at room temperature, implying that the thermal stability of the crystal blocks composing the lamellae is only dependent on the crystallization temperature.

Structure stability and strengthening mechanism of die-cast Mg-Gd-Dy based alloy

Mg-8Gd-1Dy-0.3Zn (wt.%) alloy was prepared by high-pressure die-casting technique. The thermal stability, mechanical properties at temperature range from room temperature to 573 K and strengthening mechanism was investigated. The results showed that the die-cast state alloy was mainly composed of fine cellular equiaxed grain. The fine porosity-free skin region was related to the aggregation of rare earth elements. The long lamellar-shaped stacking compound containing Zn and polygon-shaped precipitate were observed along the grain boundaries. The die-cast sample exhibited high mechanical properties and good thermal stability until 523 K.

The Study of Electroactive Block Copolymer Containing Aniline Pentamer Isolated from Different Solvents

For the films and powder of polymers containing conductive oligomer are usually obtained from solution, the choice of better solvents for the regular arrangement of oligomers is very important for the higher conductivity. Because of the poor solubility of the oligomers, it is difficult to study the arrangement directly in most common solvents, so, we synthesized a triblock copolymer, mPEG2k-aniline pentamer-mPEG2k, as the model to investigate the arrangement-solvent relationship. For the poor solubility of the AP block in common solvents, the copolymer self-assembled into spheric micelles in toluene and into lamellar crystals in water and THF. The crystallinity (X-c) and crystallization temperature (T-c) values of mPEG blocks in powders prepared from different solvents differed obviously, which may be the effect of different self-assembled structures. From the two-phase model of one-dimensional electron density correlation function of SAXS, the long period of copolymer prepared from THF was presumably equal to the long period of pure mPEG plus the chain length of AP, which demonstrates that the AP blocks arrange regularly in the noncrystalline regions.

Insight into growth details and characteristics of windmill-like polyethylene crystals

The crystallization detail of polyethylene (PE) has been scarcely studied via in-situ approach since it is an extremely fast process. In this work, optical microscopy is used to investigate crystallization details and characteristics of windmill-like polyethylene crystals. It has been shown that the straight edges of the petals appear firstly and grow in pairs from their central junctions, which subsequently induce the surrounding domains in between each pairs of petals to nucleate and crystallize into twisted lamellar overgrowths. The remaining terrace-stacked lamellae which form curved edges of the petals start to develop only after the straight edges of the petals together with the twisted lamellar overgrowths have completed their growth. It is confirmed that the preferential growth direction of these petals are along crystallographic [113] axis, which has an angle of 65, with the typical direction along b-axis adopted also by the twisted lamellar overgrowths.

Crystal Growth Transition from Flat-On to Edge-On Induced by Solvent Evaporation in Ultrathin Films of Polystyrene-b-Poly(ethylene oxide)

The transition of lamellar crystal orientation from flat-on to edge-on in ultrathin films of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) via solvent vapor (toluene) treatment Was investigated. When the as-prepared film was treated in saturated solvent vapor, breakout crystals could form quickly, and then they transformed from square single crystals (flat-on lamellae) to dendrites and finally to nanowire crystals (edge-on lamellae). Initially, heterogeneous nucleation tit the polymer/substrate interface dominated the structure evolution, leading to flat-on lamellar crystals orientation. And the transition from faceted habits to dendrites indicated a transition of underlying mechanism from nucleation-controlled to diffusion-limited growth. As the solvent molecules gradually diffused into the polymer/substrate interface, it will subsequently weaken the polymer-substrate interaction.