Hanmin Zeng

Microstructure and tribological behavior of polymeric nanocomposites

Nanocomposites represent a new prospective branch in the huge field of polymer materials science and technology. It has been shown that an overall enhancement of properties of polymers can be achieved under certain conditions by the addition of nanoparticles. To examine the influence of microstructure on the tribological performance of nanocomposites, different ways of compounding were used in this study. It was found that the friction and wear behavior of polymeric nanocomposites under sliding environment was rather sensitive to the dispersion states of the nanoparticles. When the microstructural homogeneity of the nanocomposites was improved, their wear resistance could be increased significantly. The present work demonstrates the importance of TiO2‐nanoparticles dispersion in an epoxy resin matrix, on the materials’ tribological properties, when sliding against a smooth steel counterpart.

Synthesis of silver nanoparticles and their self-organization behavior in epoxy resin

Silver nanoparticles were chemically synthesized by means of microemulsions techniques. Their self-organization behavior, including detailed description of the mechanism, the process and the resultant structure changes, was investigated by examining the electrically conductive characteristics of silver nanoparticles/solvent suspension, silver nanoparticles/epoxy resin mixture and its cured version, respectively. For silver nanoparticles suspension, higher particle concentration was found to be unfavorable to the formation of conducting paths owing to particles aggregation. For silver/epoxy mixture, transition-like changes in conductivity as a function of temperature were observed, manifesting two-stage self-organization of the nanoparticles: one related to the formation of some texture and the other related to melting and growth of the particles. As a result, conductive networks can be established by freezing the nanoparticles aggregation at the first stage under a particle loading even lower than the percolation threshold estimated conventionally. Accordingly, curing reaction of epoxy resin should be so designed as to adequately match the self-organization process.

Effect of transcrystallinity on tensile behaviour of discontinuous carbon fibre reinforced semicrystalline thermoplastic composites

Short carbon fibre reinforced polyetheretherketone (PEEK) composites with and without transcrystalline interphase are prepared under different conditions, and are used to investigate the effect of transcrystalline interphase on the tensile behaviour of semicrystalline thermoplastic polymer based composites. Volume dilatometry and scanning electron microscopy (SEM) are also employed as supplementary means. It is found that the formation of transcrystalline interphase improves such important characteristics of the composites as tensile stiffness, strength and toughness. The transcrystalline interphase enhances fibre-matrix adhesion and reduces stress concentration and cavitation at the fibre ends. The predominant deformation mechanism of the composite is changed from cavitation to shearing process in the presence of transcrystalline interphase, and moreover, significant local plastic deformation that absorbs more energy occurs, resulting in cohesive failure of the composite instead of adhesive failure happening to the composite without transcrystalline interphase.

Effect of silane-grafted polypropylene on the mechanical properties and crystallization behavior of talc/polypropylene composites

Talc-filled polypropylene (PP) composites coupled with silane-grafted polypropylene (PP-g-Si) were prepared. Effect of PP-g-Si on the mechanical properties, crystallization, and melting behavior of PP composites was investigated. Compared with the uncoupled composites, the mechanical properties of Talc/PP composites coupled with a small amount of PP-g-Si were increased to some extent. Meanwhile, PP-g-Si can promote crystallization rate and increase crystallization temperature of PP in the composites.

Effect of macromolecular coupling agent on the property of PP/GF composites

Silane-grafted polypropylene manufactured by a reactive grafting process was used as the coupling agent in polypropylene/glass-fiber composites to improve the interaction of the interfacial regions. Polypropylene reinforced with 30% by weight of short glass fibers was injection-molded and the mechanical behaviors were investigated. The results indicate that the mechanical properties (tensile strength, tensile modulus, flexural strength, flexural modulus, and Izod impact strength) of the composite increased remarkably as compared with the noncoupled glass fiber/polypropylene. SEM of the fracture surfaces of the coupled composites shows a good adhesion at the fiber/matrix interface: The fibers are coated with matrix polymer, and a matrix transition region exists near the fibers.

Radial growth rate of spherulites in polypropylene/barium sulfate composites

Abstract The effect of barium sulfate (BaSO4) on the spherulite radial growth rates of isotactic polypropylene (PP) was studied by means of optical microscopy. The PP–BaSO4 interface was modified by treating the fillers with different couple agents. The addition of BaSO4 depresses the spherulite growth rate, and the effect is more dramatic when the interfacial interaction between the PP and BaSO4 was enhanced. The data were analyzed with the Lauritzen–Hoffmann secondary nucleation theory, and the results demonstrated that the enhanced PP–BaSO4 interaction promotes the particles to serve as physical crosslinking points, which hinder the movement of the polymer chains and results in a decrease of the pre-exponential factor, G0.

Polycarbonate-epoxy semi-interpenetrating polymer network : 2. Phase separation and morphology

Abstract Bisphenol A-based epoxy resin (DGEBA) was modified with bisphenol A polycarbonate (PC), by either a physical or chemical process, and thermally cured with 4,4′-diaminodiphenyl sulfone, tetraethylenepentamine, and anhydride. The phase separation in the PC/epoxy blends was controlled by varying the composition, and the PC-epoxy reaction, and utilizing curing agents of different reactivity. A single phase morphology was achieved by accelerating the curing reaction, or introducing PC-epoxy reaction, or eliminating crystallization of PC. Under certain conditions, PC spherulites are able to form, in company with the formation of epoxy networks. Curing of the epoxy resin modified the crystallization mechanism, the melting behaviour and the morphology. The nuclei are likely to be pre-existing high molecular weight PC chains and are formed by the special interaction between PC molecules and the primary network. Low molecular weight epoxy resins increase the crystallization rate. The internal fractionation changes by PC chain scission during the melt blending process has an important influence on the crystallization. The spherulite morphology fixed by semi-IPN structure subjected a loss of order by annealing. It is suggested that hydrogen bonding and graft reaction in this kind of PC/epoxy semi-IPN is the key to controlling phase separation and morphology.

Isothermal crystallization behavior and melting characteristics of injection sample of nucleated polypropylene

The isothermal crystallization behavior and melting characteristics of pure polypropylene (PP) and PPs nucleated with a phosphate nucleating agent (A) and a sorbitol derivative (D) have been studied by differential scanning calorimetry (DSC). Compared with pure PP, nucleated PPs show a shorter half-times of crystallization. Dependence of crystallization rate of nucleated PP on the crystallization temperature is stronger than that of pure PP at the higher crystallization temperature, whereas the opposite results are obtained at the lower crystallization temperature. Addition of nucleating agent shifts the temperature at the deviation from the baseline of DSC melting curve, T, and the temperature at the completion of melting, T, to higher temperatures, indicating that nucleated PPs exhibit a higher perfection of PP crystals. A shoulder peak in the high temperature range of melting peak of nucleated PP and a wider low temperature region in the melting peak of pure PP are observed. Obviously, PP and nucleated PPs form different distribution of crystal perfection in the isothermal crystallization process. According to the half-time of crystallization, nucleating agent A is more effective than D.

Microstructures and fracture behavior of glass-fiber reinforced PBT/PC/E-GMA elastomer blends: 1 : Microstructures

Abstract A group of glass-fiber-reinforced polymer composites with controlled morphology were designed and prepared by sequential compounding of poly(butylene terephthalate)/glass-fiber (PBT/GF) composite with a reactive elastomer, ethylene-co-glycidyl methacrylate (E-GMA), and/or polycarbonate (PC). The microstructures of the composites were characterized by means of AFM, SEM and thermal analysis. The results indicate that the glass fiber was surrounded by a dead layer of PBT. In the matrix, the E-GMA particles, of sizes varying between 0.5 and 1 μm, were encapsulated by the PBT phase. The PBT and PC formed an interconnected phase structure with a PBT domain thickness of about 1 μm and the PC domain thickness of less than 0.5 μm. It was found that when the PBT/GF was mixed with the E-GMA in the first step of the sequential blending, the epoxide groups in the E-GMA tended to homo-polymerize through ring-opening rather than to react with the carboxyl and/or hydroxyl groups of the PBT. Consequently, a slightly cross-linked structure formed in the E-GMA phase, which kept the E-GMA domains to stay in the PBT phase during the second step of the sequential blending with the PC. On the other hand, the transesterification between the PBT and the PC resulted in a decrease in the PBT chain regularity, leading to a lower crystallization rate and formation of crystallites with low perfection.

Nonisothermal crystallization of poly(phenylene sulfide) in presence of molten state of crystalline polyamide 6

The nonisothermal crystallization and melting behavior of a poly(phenylene sulfide) (PPS) blend with polyamide 6 (PA6) were investigated by differential scanning calorimetry. The results indicate that the crystallization parameters for PPS become modified to a greater extent than those for PA6 in the blends. The PPS and PA6 crystallize at high temperature as a result of blending. The crystallization temperatures of PPS in its blends are always higher than that of pure PPS and are independent of the melting temperature and the residence time at that temperature. The PPS crystallization peak becomes narrower and the crystallization temperature shifts to a higher temperature, suggesting a faster rate of crystallization as a result of blending with PA6. This enhancement in the nucleation of PPS could be attributed to the possible presence of interfacial interactions between the component polymers to induce heterogeneous nucleation. On the other hand, the increase in the crystallization temperature of PA6 can be attributed to the heterogeneous nucleation provided by the already crystallized PPS. The heterogeneous nucleation induced by interfacial interactions depends on the temperature at which the polymers remain in the molten state and on the storage time at this temperature.