He Jiasong

An investigation on the microcellular structure of polystyrene/LCP blends prepared by using supercritical carbon dioxide

Abstract Foamed PS/LCP blends with cell diameter less than 7xa0μm are prepared by using supercritical CO2, 25xa0MPa, and 80°C for 6xa0h. Characterization of the microcellular structure of these blends is conducted to reveal the influence of LCP addition, LCP ratio and compatibilizer used on the microcellular blends. Due to poor adsorption of supercritical CO2 by LCP under the experimental conditions, the microvoids only exist in the polystyrene phase of the blends. Where in the LCP phase, the microfibrils and spheres retain their original morphology and a skin-core structure exist as in the unfoamed PS/LCP blends. The LCP ratio and the compatibilizer, zinc neutralized lightly sulfonated polystyrene ionomer (ZnSPS), influence the cell size of the microcellular blends. A significant decrease of cell diameter in low LCP composition is observed, and then the change is much less and levels off in higher LCP composition. An increase of cell size is found from skin to core, which is resulted from the effect of the skin-core structure of the PS/LCP blends and the effect of competition between gas diffusing in the cells and diffusing out of the skin. The microcellular blends with ZnSPS has larger cell size than those without ZnSPS, which is the consequence of the improvement of interfacial adhesion, where CO2 could easily diffuse out through the gap between poor adhesion interface of blends without ZnSPS. It is also found that the cell density in the microcellular blends is slightly larger than that in the microcellular polystyrene. This implies an additional heterogeneous nucleation of LCP to the homogeneous nucleation of polystyrene.

Transparent and flame retardant cellulose/aluminum hydroxide nanocomposite aerogels

Cellulose-based nanocomposite aerogels were prepared by incorporation of aluminum hydroxide (AH) nanoparticles into cellulose gels via in-situ sol-gel synthesis and following supercritical CO2 drying. The structure and properties of cellulose/AH nanocomposite aerogels were investigated by Fourier transform infrared spectroscopy, scanning electron microscopy, ultraviolet-visible spectrometry, N2 adsorption, thermogravimetric analysis, and micro-scale combustion calorimetry. The results indicated that the AH nanoparticles were homogeneously distributed within matrix, and the presence of AH nanoparticles did not affect the homogeneous nanoporous structure and morphology of regenerated cellulose aerogels prepared from 1-allyl-3-methylimidazolium chloride solution. The resultant nanocomposite aerogels exhibited good transparency and excellent mechanical properties. Moreover, the incorporation of AH was found to significantly decrease the flammability of cellulose aerogels. Therefore, this work provides a facile method to prepare transparent and flame retardant cellulose-based nanocomposite aerogels, which may have great potential in the application of building materials.

Preparation of conductive polypyrrole (PPy) composites under supercritical carbon dioxide conditions

Electrically conductive composites were prepared via the chemical oxidative polymerization of the pyrrole monomer in polystyrene (PS) and zinc neutralized sulfonated polystyrene (Zn-SPS) films under supercritical carbon dioxide (SC-CO2) conditions. The strong swelling effect of SC-CO2 made polypyrrole (PPy) particles not only form on the surface, but also become incorporated into the film, resulting in a homogeneous structure with a relatively higher conductivity. By comparison, the composite prepared in aqueous solutions shows a skin-core structure and a conductivity of 3 to 4 orders of magnitude lower than that of the former due to the diffusion-controlled process of the pyrrole monomer. The percolation thresholds of PS/PPy and Zn-SPS/PPy composites were 6.2% and 2.7% of the volume fraction of PPy, respectively, much lower than the theoretically predicted value of 16%. Moreover, the conductive composites prepared under SC-CO2 conditions showed higher thermal stability, especially in the high-temperature region.

Understanding cellulose dissolution: effect of the cation and anion structure of ionic liquids on the solubility of cellulose

The effect of ionic liquids (ILs) on the solubility of cellulose was investigated by changing their anions and cations. The structural variation included 11 kinds of cations in combination with 4 kinds of anions. The interaction between the IL and cellobiose, the repeating unit of cellulose, was clarified through nuclear magnetic resonance (NMR) spectroscopy. The reason for different dissolving capabilities of various ILs was revealed. The hydrogen bonding interaction between the IL and hydroxyl was the major force for cellulose dissolution. Both the anion and cation in the IL formed hydrogen bonds with cellulose. Anions associated with hydrogen atoms of hydroxyls, and cations favored the formation of hydrogen bonds with oxygen atoms of hydroxyls by utilizing activated protons in imidazolium ring. Weakening of either the hydrogen bonding interaction between the anion and cellulose, or that between the cation and cellulose, or both, decreases the capability of ILs to dissolve cellulose.

Homogeneous synthesis of cellulose derivatives in ionic liquids

Cellulose derivatives are important fine chemicals with a wide range of applications in modern industries, so that cellulose derivatization has been one of hot topics for a long time. However, the existence of numerous hydroxyls in cellulose also generates a well-developed intra- and inter-molecular hydrogen bonding network, thus natural cellulose is neither meltable nor soluble in conventional solvents. Until today, the preparation method for cellulose esters begins with a heterogeneous reaction between cellulose and a large excess of the acylation reagent in the presence of pyridine or sulfuric acid as the catalyst. These heterogeneous reactions cause some problems, such as uneven distribution of substituents, side reactions, time consumption and considerable degradation of cellulose. Homogeneous esterification of cellulose in appropriate media, the alternative procedure for heterogeneous process, has been one focus in cellulose chemistry. It not only provides opportunities to obtain the effective control of degree of substitution (DS), distribution and uniformity of the functional groups along cellulose chains, but also creates more options to induce novel and functional groups. Recently, the advent of ionic liquids that can dissolve cellulose provides a new and versatile platform for the efficient and homogeneous derivatization of cellulose. A variety of well-known, novel and functional cellulose esters have been successfully synthesized in ionic liquids. Meanwhile, development of esterification techniques, utilization of agricultural residues, and pilot scheme of up-scaling esterification in ionic liquids have been in progress continuously. This review summarizes the development in homogeneous synthesis of cellulose esters in ionic liquids published in the recent decade.