Chaobin He

Polyimide/POSS nanocomposites: interfacial interaction, thermal properties and mechanical properties

Abstract A series of functional polyhedral oligomer silsesquioxnae (POSS)/polyimide (PI) nanocomposites were prepared using a two-step approach, first, the octa(aminophenyl)silsesquioxane (OAPS)/NMP solution was mixed with polyamic acid (PAA) solution prepared by reacting 4,4′-diaminodiphenylmethane and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride in NMP, and second, the polycondensation solution was treated by thermal imidization. The well-defined ‘hard particles’ (POSS) and the strong covalent bonds between the PI and the ‘hard particles’ lead to a significant improvement in the thermal mechanical properties of the resulting nanocomposites. The glass transition temperature dramatically increases while the coefficient of thermal expansion (CTE) decreases, owing to the significant increase of the cross-linking density in the PI–POSS nanocomposites. The thermal stability and mechanical property of the nanocomposites were also improved.

Graphene-Wrapped Polyaniline Hollow Spheres As Novel Hybrid Electrode Materials for Supercapacitor Applications

Polyaniline hollow spheres (PANI-HS)@electrochemical reduced graphene oxide (ERGO) hybrids with core-shell structures have been fabricated via a solution-based coassembly process. The hollow nanostructured designing for the PANI-HS greatly enlarges the specific surface area, providing high electroactive regions and short diffusion lengths for both charge and ion transport. The wrapping of ERGO sheets on the PANI-HS can offer highly conductive pathways by bridging individual PANI-HS together, thus facilitating the rate and cycling performance of supercapacitors. The specific capacitance of PANI-HS36@ERGO hybrids can reach 614 F g(-1) at a current density of 1 A g(-1). Furthermore, the capacitance of the PANI-HS36@ERGO hybrids maintains 90% after 500 charging/discharging cycles at a current density of 1 A g(-1), indicating a good cycling stability. The greatly enhanced electrochemical performance can be ascribed to the synergic effects of the two components of PANI-HS and ERGO, suggesting that the PANI-HS@ERGO hybrids as novel electrode materials may have potential applications in high-performance energy storage devices.

Morphology, thermal and mechanical behavior of polyamide 6/layered-silicate nanocomposites

The microstructure, morphology, thermal and mechanical properties of polyamide 6/layered-silicate nanocomposites (PLSN) have been studied using X-ray diffraction (XRD), tapping-mode atomic force microscopy (AFM), differential scanning calorimetry (DSC) and tensile tests. The XRD and AFM results indicate that the organically-modified layered silicates are intercalated in the polyamide 6 (PA6) matrix. Also, the crystallization and thermal behavior of PA6 are influenced by the addition of layered silicates into polymer matrix. A silicate-induced crystal transformation from the a-form to the g-form of PA6 is confirmed by XRD and DSC, i.e., the formation of g crystal is enhanced by the presence of silicates. In comparison to PA6, the thermal property of PLSN has been investigated using DSC. The possible origins of a new silicate-induced endothermic peak are discussed. Tensile tests show that the tensile modulus and yield strength increase while the strain-at-yield decreases with increasing clay loading. # 2002 Elsevier Science Ltd. All rights reserved.

Thermal degradation behavior of polyamide 6/clay nanocomposites

This paper focuses on the study of thermal degradation and evolved gas analysis using thermogravimetric analysis (TGA) coupled to Fourier transform infrared (FTIR) spectroscopy, i.e., TG–IR, to study polyamide 6 (PA6) and PA6-clay nanocomposites prepared by melt compounding. The thermal decomposition of PA6 and its clay nanocomposites takes place with the evolution of the cyclic monomer (caprolactam) first, followed by other volatile gases like CO2 and NH3, which are characterized by the presence of oligomeric products with nitrile and vinyl chain ends in the infrared (IR) spectra. The onset temperature for degradation is 12 � C higher for PA6 with 2.5 wt.% of clay loading than that for neat PA6, whereas, the onset temperature for degradation remained almost unchanged for samples with higher clay loading (i.e. 5, 7.5 and 10 wt.% clay). The above findings are related to the morphological observations that show an optimal exfoliated structure only for the nanocomposite with 2.5 wt.% clay, and distinct clay agglomeration in those with higher clay loadings. Our study suggests that only exfoliated polymer/clay nanocomposites exhibit improved thermal stability. Agglomerated clay particles do not significantly affect thermal stability of the polymer matrix. The activation energies for degradation, Ea, estimated by Kissinger method, for PA6 and PA6–2.5 wt.% clay nanocomposite were found to be 175 and 199 kJ/mol in N2, and 228 and 223 kJ/mol in air, respectively. # 2003 Elsevier Science Ltd. All rights reserved.

Some recent developments of polyhedral oligomeric silsesquioxane (POSS)-based polymeric materials

Polyhedral Oligomeric Silsesquioxane (POSS) has attracted considerable interest in materials science due to its well-defined nano-scale organic–inorganic structure, which makes it an ideal building block for constructing nano-structured hybrid materials and nanocomposites. In this article, we highlight some recent developments in applications of POSS materials: 1) improving thermal and mechanical properties of polymers through incorporation of POSS into polymer matrices to form nanocomposites; 2) using POSS as a building block for design and synthesis of POSS-containing organic semiconductor materials to achieve high photo-luminescence/electron luminescence quantum efficiency in organic light emitting diodes and enhanced performance in electrochromatic devices; 3) exploiting POSS as a hydrophobic unit to develop amphiphilic polymers for drug/gene delivery and formation of hydrogels. A future direction for the development of POSS-containing materials is also proposed for applications in organic photovoltaic and other high-performance materials.

Lignin-Derived Fused Electrospun Carbon Fibrous Mats as High Performance Anode Materials for Lithium Ion Batteries

A novel biomass-based nitrogen-doped free-standing fused carbon fibrous mat was fabricated from lignin-polyethylene oxide (PEO) (90:10) blend via electrospinning followed by carbonization and thermal annealing in the presence of urea. The morphology and structure of the carbon fibers were characterized by field-emission scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and elemental analysis, and their electrochemical properties were investigated for the first time as anode in lithium ion batteries (LIBs). The fused carbon fibers without nitrogen doping exhibited high specific capacity up to 445 mA h g(-1) at a current density of 30 mA g(-1) (comparable to polyacrylonitrile (PAN) derived carbon nanofibers) and good cyclic stability at different current rates. After thermal annealing in the presence of urea, the charge capacity was further improved to as high as 576 mA h g(-1) and still maintained a good capacity of about 200 mAh g(-1) even at a high current rate of 2000 mA g(-1). This research demonstrates the great promise of lignin-derived nanocarbon materials for applications in energy storage systems.

Thermomechanical properties of polyimide-epoxy nanocomposites from cubic silsesquioxane epoxides

Novel polyimide-epoxy organic–inorganic nanocomposites are prepared by in situ curing of polyamic acid macromolecules with two POSS epoxides, octa[(epoxycyclohexylethyl)dimethylsilyloxy]silsesquioxane (OC) and octa[(epoxyhexyl)dimethylsilyloxy]silsesquioxane (OH). Comparison between two series of nanocomposites from OC and OH provides a typical example of controlling the macro-scale properties by adjusting the nano-tether structure of the POSS molecules. DMA studies conclude that the crosslink densities of the polyimide-epoxy nanocomposites increase significantly; the nanocomposite networks are predominantly formed by the linkage between the terminal amine groups of the polyimide molecules and the epoxide groups of the POSS molecules. Furthermore, it is interesting to observe that although the crosslink densities of nanocomposites increase significantly, the compressive moduli, hardness and coefficients of thermal expansion change only slightly.

Thermal degradation of electrical conductivity of polyacrylic acid doped polyaniline: effect of molecular weight of the dopants

This paper describes the effect of molecular weight of dopants on thermal degradation behaviour of electrical conductivity of polyacrylic acid (PAA) doped polyaniline (PANI). Two PAA with weight-average molecular weight (Mw) of 5000 and 250,000, respectively, were used as dopants to synthesise PANI–PAA complexes by in situ oxidative polymerisation. PANI doped with the low Mw PAA has lower electrical conductivity, which is attributed to its smaller size of crystal islands due to the end group effect. When annealed at 180 8C, within a period of 2 h the conductivity of PANI doped with the high Mw PAA decreases continuously with the annealing time following s ¼ s0 exp½�ð ta=tÞ 1=2 � law, while the one doped with the low Mw PAA obeys this law only within 1 h of annealing. This indicates that the decrease of conducting island size is responsible for the thermal degradation of the conductivity of the complexes in a certain period and the length of this period depends on Mw of the dopants. TGA study shows that at 180 8C weight loss rates of the low and high Mw PAA doped PANI are about the same, which implies that the faster reduction of the conductivity in the low Mw PAA doped PANI is mainly due to its smaller initial crystal size. FTIR study shows that annealing leads to de-doping, but it is less pronounced in the high Mw PAA doped PANI. # 2002 Elsevier Science B.V. All rights reserved.

Trimeric supramolecular liquid crystals induced by halogen bonds

This paper described a series of trimeric halogen bonded supramolecular thermotropic liquid crystals. The halogen bonding interaction was studied by infrared and X-ray photoelectron spectroscopy. It was observed that the aromatic skeleton infrared vibration of halogen bonded acceptor molecules had moved to higher frequencies relative to their corresponding unbonded counterparts. The halogen-bonded acceptors showed higher N 1s binding energies of 0.89–1.12 eV than those of unbonded components. In contrast, the halogen-bonded donors displayed lower I 3d binding energies than their corresponding unbonded species. The changes in binding energies and in infrared frequencies demonstrated the presence of effective halogen bonding interaction. The thermal behaviors of the halogen-bonded complexes were examined using differential scanning calorimetry and optical polarizing microscopy. Most of the trimeric complexes exhibited thermotropic liquid crystals with smectic A phases. The effect of length in spacers and terminal groups in halogen bond molecules on mesophase transition temperatures was investigated, revealing that there was little correlation between the length in spacers and terminal groups and phase transition temperatures.

Superhydrophobic fluorinated POSS–PVDF-HFP nanocomposite coating on glass by electrospinning

Fluorinated polyhedral oligomeric silsesquioxanes–poly(vinylidene fluoride-co-hexafluoro propylene) (fluoroPOSS–PVDF-HFP) nanocomposite mixtures are prepared by individually mixing two fluorinated POSS materials (FP8 and FPSi8) with PVDF-HFP solution and transparent superhydrophobic coatings on a glass substrate are produced by electrospinning. The fabricated superhydrophobic surface exhibits continuous, uniform and non-beaded nanofibers with a high water contact angle (157.3°) and a low sliding angle (SA < 5°). The superhydrophobic properties including surface energy, water contact angle, and contact angle hysteresis as well as the optical properties are studied by altering the wt% of fluoroPOSS in PVDF-HFP solution. It is observed that due to the increase in the viscosity of the solution, the diameter of the nanofibers increases with increase in the concentration of fluoroPOSS. It is also observed that as the concentration of fluoroPOSS in PVDF solution increases, the amount of fluorine content increases thereby the surface energy of the coating decreases (to around 10 mN m−1) leading to a superhydrophobic surface with low contact angle hysteresis (<5°). The optical properties of the coatings produced using FPSi8 fluoroPOSS–PVDF-HFP mixture are compared with those of FP8 fluoroPOSS, and the former is found to be more transparent due to its relatively high solubility in PVDF-HFP.