Dujin Wang

Influence of polyvinylpyrrolidone on the precipitation of calcium carbonate and on the transformation of vaterite to calcite

Abstract The precipitation process of calcium carbonate (CaCO 3 ) in the absence and presence of poly (N-vinyl-2-pyrrolidone) (PVP) was investigated using scanning electron microscopy, powder X-ray diffraction, and Fourier transform infrared spectroscopy methods at room temperature. The results indicate that PVP does not affect the polymorphy, but has influence on their morphology and size of CaCO 3 crystals. With the addition of PVP, the amorphous CaCO 3 could aggregate into bigger amorphous spherulites before transforming to crystalline state. Also, the transformation from the thermodynamically unstable vaterite to the stable calcite was investigated in the presence of PVP. As a particle-stabilizing agent, PVP molecules inhibit the formation of vaterite, however, promote the formation of calcite as well as the rate of the solvent-mediated transformation from vaterite to calcite.

Tailoring Crystallization: Towards High-Performance Poly(lactic acid)

Poly(lactic acid) (PLA) is one of the most promising alternatives for petrochemical-based plastics. Crystallization mediation provides the simplest and most practical approach for enhancing the properties of PLA. Here, recent advances in understanding the relationship between crystalline structure and properties of PLA are summarized. Methods for manipulating crystallization towards high-performance PLA materials are introduced.

Toward rapid aqueous RAFT polymerization of primary amine functional monomer under visible light irradiation at 25 °C

We report the rapid and well-controlled aqueous RAFT polymerization of primary amine functional monomer under visible light irradiation at 25 °C. N-(2-aminoethyl)methacrylamide hydrochloride (AEMA) and N-(6-aminohexyl)methacrylamide hydrochloride (AHMA) were synthesized as prototypes of primary amine functional monomers. The results demonstrated that this polymerization proceeded rapidly in a well-controlled manner in acidic aqueous media upon irradiating with mild visible light at 25 °C. Moreover, improving the ion-pairing or association of the ionized monomers via adding less polar alcohols or improving the concentration of ionized monomers could shorten the initialization period and accelerate chain propagation, thus remarkably accelerating this polymerization. This polymerization was immediately switched on or off via turning on or off visible light. Dithioester residues at polymer chain ends could be retained or removed simply by adjusting pH. Accordingly, this paper provides a facile approach toward direct and rapid RAFT synthesis of well-defined primary amine functional polymers at room temperature.

Mechanical properties of PNIPAM based hydrogels: A review.

Materials which adjust their properties in response to environmental factors such as temperature, pH and ionic strength are rapidly evolving and known as smart materials. Hydrogels formed by smart polymers have various applications. Among the smart polymers, thermoresponsive polymer poly(N-isopropylacrylamide)(PNIPAM) is very important because of its well defined structure and property specially its temperature response is closed to human body and can be finetuned as well. Mechanical properties are critical for the performance of stimuli responsive hydrogels in diverse applications. However, native PNIPAM hydrogels are very fragile and hardly useful for any practical purpose. Intense researches have been done in recent decade to enhance the mechanical features of PNIPAM hydrogel. In this review, several strategies including interpenetrating polymer network (IPN), double network (DN), nanocomposite (NC) and slide ring (SR) hydrogels are discussed in the context of PNIPAM hydrogel.

Crystallization behaviors of n-octadecane in confined space: crossover of rotator phase from transient to metastable induced by surface freezing.

In this paper, the confined crystallization and phase transition behaviors of n-octadecane in microcapsules with a diameter of about 3 microm were studied with the combination of differential scanning calorimetry (DSC), temperature dependent Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The main discovery is that the microencapsulated n-octadecane crystallizes into a stable triclinic phase via a mestastable rotator phase (R I), which emerges as a transient state for the bulk n-octadecane and is difficult to be detected by the commonly used characterization methods. As evident from the DSC measurement, a surface freezing monolayer, which is formed at the interface between the microcapsule inner wall and n-octadecane, induces the crossover of the R I from transient to metastable. We argue that the existence of the surface freezing monolayer decreases the nucleating potential barrier of the R I phase, and consequently the lower relative nucleation barrier in the confined geometry turns the transient R I phase into a metastable one.

The catanionic surfactant-assisted syntheses of 26-faceted and hexapod-shaped Cu2O and their electrochemical performances

Crystalline materials with a well-defined morphology and/or a narrow size distribution might exhibit a specific shape- and/or size-dependent performances. In the first instance, the catanionic surfactants of anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethylammonium hydroxide (CTAOH) were added as crystal modifiers into the 60 °C reaction systems of copper chloride and sodium hydroxide for the syntheses of cuprous oxide (Cu2O). Then, the reversible reaction activity of crystalline Cu2O with metal lithium was conducted to investigate its electrochemical performance as rechargeable lithium ion battery anodes. The presence of SDS-rich catanionic surfactants could induce the formation of polyhedral Cu2O structures with 8 triangular {111}, 6 square {100}, and 12 rectangular {110} faces outside, while the presence of CTAOH-rich catanionic surfactants, especially the doping methanol in CTAOH, led to the generation of hexapod-shaped Cu2O mesostructures with tiny nanoparticles on these symmetrical branches. At a discharge–charge cycling current of 80 mA g−1, the 26-faceted Cu2O crystals with rough {110} faces displayed an initial capacity of 756 mA h g−1 and a reversible capacity of 280 mA h g−1 at the first cycling. In comparison with the electrochemical performance of Cu2O hexapod-shaped mesocrystals at the same cycling current, the 26-faceted crystals of Cu2O could be capable of remaining a relatively high capacity (∼145 mA h g−1) and keeping an excellent Coulombic efficiency (∼100%) over 50 discharge–charge cycles. As a whole, the catanionic surfactants at different anionic/cationic molar ratios were used as additives to highlight the secondary nucleation and growth mechanism for the formation of Cu2O, then, the resulting 26-faceted crystallites and hexapod-shaped mesoparticles were separately used as active materials in the assembled Cu2O/Li half-cells to study their shape-dependent electrochemical performances.

A tough hydrogel–hydroxyapatite bone-like composite fabricated in situ by the electrophoresis approach

Mechanically strong hydrogel-HAp composites have been successfully fabricated through in situ formation of hydroxyapatite (HAp) in a tough polyacrylamide (PAAm) hydrogel with a modified electrophoretic mineralization method. The pre-swelling of the PAAm hydrogels in CaCl2 buffer solutions makes the electrophoresis method able to produce large area (10 × 8 cm2) hydrogel-HAp composites. At the same time the CaCl2 solution with different concentrations could control the HAp contents. The obtained hydrogel-HAp composites exhibit enhanced mechanical properties, namely higher extensibility (>2000%), tensile strength (0.1-1.0 MPa) and compressive strength (up to 35 MPa), in comparison to the as-synthesized PAAm hydrogels. FTIR and Raman characterizations indicate the formation of strong interactions between PAAm chains and HAp particles, which are thought to be the main reason for the enhanced mechanical properties. The hydrogel-HAp composite also shows excellent osteoblast cell adhesion properties. These composite materials may find more applications in biomedical areas, e.g. as a matrix for tissue repair especially for orthopedic applications and bone tissue engineering.

Crystallization Features of Normal Alkanes in Confined Geometry

How polymers crystallize can greatly affect their thermal and mechanical properties, which influence the practical applications of these materials. Polymeric materials, such as block copolymers, graft polymers, and polymer blends, have complex molecular structures. Due to the multiple hierarchical structures and different size domains in polymer systems, confined hard environments for polymer crystallization exist widely in these materials. The confined geometry is closely related to both the phase metastability and lifetime of polymer. This affects the phase miscibility, microphase separation, and crystallization behaviors and determines both the performance of polymer materials and how easily these materials can be processed. Furthermore, the size effect of metastable states needs to be clarified in polymers. However, scientists find it difficult to propose a quantitative formula to describe the transition dynamics of metastable states in these complex systems. Normal alkanes [CnH2n+2, n-alkanes], especially linear saturated hydrocarbons, can provide a well-defined model system for studying the complex crystallization behaviors of polymer materials, surfactants, and lipids. Therefore, a deeper investigation of normal alkane phase behavior in confinement will help scientists to understand the crystalline phase transition and ultimate properties of many polymeric materials, especially polyolefins. In this Account, we provide an in-depth look at the research concerning the confined crystallization behavior of n-alkanes and binary mixtures in microcapsules by our laboratory and others. Since 2006, our group has developed a technique for synthesizing nearly monodispersed n-alkane containing microcapsules with controllable size and surface porous morphology. We applied an in situ polymerization method, using melamine-formaldehyde resin as shell material and nonionic surfactants as emulsifiers. The solid shell of microcapsules can provide a stable three-dimensional (3-D) confining environment. We have studied multiple parameters of these microencapsulated n-alkanes, including surface freezing, metastability of the rotator phase, and the phase separation behaviors of n-alkane mixtures using differential scanning calorimetry (DSC), temperature-dependent X-ray diffraction (XRD), and variable-temperature solid-state nuclear magnetic resonance (NMR). Our investigations revealed new direct evidence for the existence of surface freezing in microencapsulated n-alkanes. By examining the differences among chain packing and nucleation kinetics between bulk alkane solid solutions and their microencapsulated counterparts, we also discovered a mechanism responsible for the formation of a new metastable bulk phase. In addition, we found that confinement suppresses lamellar ordering and longitudinal diffusion, which play an important role in stabilizing the binary n-alkane solid solution in microcapsules. Our work also provided new insights into the phase separation of other mixed system, such as waxes, lipids, and polymer blends in confined geometry. These works provide a profound understanding of the relationship between molecular structure and material properties in the context of crystallization and therefore advance our ability to improve applications incorporating polymeric and molecular materials.

One-year monthly survey of rotavirus, astrovirus and norovirus in three sewage treatment plants in Beijing, China and associated health risk assessment

To evaluate the presence and distribution of the three main viruses (rotavirus, astrovirus, and norovirus) responsible for human acute gastroenteritis in sewerage system an one-year study was carried out in Beijing, China. A total of 96 samples of influent and effluents from three sewage treatment plants (STPs) were collected from November 2006 to October 2007. Silica was used to concentrate viral particles from water samples and a reverse transcriptase-nested polymerase chain reaction (RT-nested PCR) method was used for detection of viruses. Viruses could be detected in 35.4% (34/96) of the water samples analyzed, where human rotavirus was the most frequently detected (32.3%, 31/96), followed by human astrovirus (6.3%, 6/96) and human norovirus (3.1%, 3/96). According to the quantitation results of rotaviruses, which were gained by the real-time quantitative RT-PCR method with SYBR Green I , it was known that the distributions of rotaviruses in influents and effluents of three STPs were quite similar, i.e., abundant in cold weather (from October to March) and less prevalent in warm weather (from April to September). According to the estimated exposure dose, exposure frequency, as well as the acceptable annual risk level, it was shown that the rotaviruses in the reused wastewater after conventional treatment process presented potential risk to human health through both occupational and accidental exposure.

Double Orthogonal Sample Design Scheme and Corresponding Basic Patterns in Two-Dimensional Correlation Spectra for Probing Subtle Spectral Variations Caused by Intermolecular Interactions

This paper introduces a new approach named double orthogonal sample design scheme (DOSD) to probe intermolecular interactions based on a framework of two-dimensional (2D) correlated spectroscopy. In this approach, specifically designed concentration series are selected according to the mathematical analysis on orthogonal vectors to generate useful 2D correlated spectra. As a result, the interfering portion can be completely removed from both synchronous and asynchronous spectra, and complementary information concerning intermolecular interactions can be obtained from the set of 2D spectra. A model system, where intermolecular interactions occur between two solutes in a solution, is used to investigate the behavior of 2D correlated spectra generated by using the DOSD approach. Simulation results demonstrate that the resultant spectral patterns can reflect subtle spectral variation in bandwidths, peak positions, and absorptivities brought about by intermolecular interaction, which are hardly visualized in conventional 1D spectra because of the severe band-overlapping problem. The ability to reveal a subtle variation in a characteristic peak in detail by using the DOSD approach provides a new opportunity to understand the nature of intermolecular interactions from a molecular structural point of view. Intermolecular interactions between iodine and benzene in CCl(4) solutions were investigated by using the proposed DOSD approach to prove the applicability of the DOSD method in real chemical systems.