Hongzan Song

Liquid Crystalline Phase Behavior and Sol–Gel Transition in Aqueous Halloysite Nanotube Dispersions

The liquid crystalline phase behavior and sol-gel transition in halloysite nanotubes (HNTs) aqueous dispersions have been investigated by applying polarized optical microscopy (POM), macroscopic observation, rheometer, small-angle X-ray scattering, scanning electron microscopy, and transmission electron microscopy. The liquid crystalline phase starts to form at the HNT concentration of 1 wt %, and a full liquid crystalline phase forms at the HNT concentration of 25 wt % as observed by POM and macroscopic observation. Rheological measurements indicate a typical shear flow behavior for the HNT aqueous dispersions with concentrations above 20 wt % and further confirm that the sol-gel transition occurs at the HNT concentration of 37 wt %. Furthermore, the HNT aqueous dispersions exhibit pH-induced gelation with more intense birefringence when hydrochloric acid (HCl) is added. The above findings shed light on the phase behaviors of diversely topological HNTs and lay the foundation for fabrication of the long-range ordered nano-objects.

Studies on the Morphological, Rheological, Electrical, Mechanical and Thermal Properties of the PTT/SCF Composites:

Short carbon fiber reinforced poly(trimethylene terephthalate) composites (PTT/SCF) were investigated in their rheology behaviors, phase morphology, electrical, mechanical, and thermal properties by capillary rheometer, scanning electron microscope, universal tester, high insulation resistance meter, Q meter, and thermal gravimetric analyzer. PTT/SCF melt is the dilating fluid at a shear rate below 130 s-1, while it becomes pseudo-plastic fluid at a shear rate higher than 130 s- 1. Melt apparent viscosity and viscous flow activation energy increase initially, and then decrease as SCF content increases, which have maximum values with 2 wt% SCF. The interaction between SCF and PTT matrix is strong based on the observation from the fracture surface of composites. The tensile strength and rupture strength are all increased obviously with increasing SCF; while the impact strength reaches a maximum value as SCF contents is 5%. With increasing SCF content in matrix, the electrical resistivity is decreased, while the dielectric constant, and the dielectric loss tangent were increased.

Spherulite Morphology and Thermal Behaviors of Short Carbon Fiber/Poly(Trimethylene Terephthalate) Composites

The spherulite morphology and thermal behavior of short carbon fiber/poly (trimethylene terephthalate) (SCF/PTT) composites were investigated by polarized optical microscopy (POM), wide-angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC). The micrographs of the growing spherulites suggest the composites with more SCF content have faster nucleation rates; thus, their spherulite sizes are much smaller. Under the same cooling process, banded spherulites are detected in pure PTT as well as in SCF/PTT composites shown with decreasing band spacing with increasing radius. Melt-crystallization and melting behaviors in DSC indicate the SCF served as nucleating agents, increasing the crystallization rate of the PTT. When the samples were quenched from melt some microcrystals formed in SCF/PTT composites but not in pure PTT. When the SCF/PTT composites were heated to melt, they showed double melting peaks due to some less perfect crystals or thinner lamella formed during DSC scanning.

Dynamic rheological and dynamic thermomechanical properties of poly(trimethylene terephthalate)/short carbon fibre composites

The dynamic rheology and thermomechanical properties of poly(trimethylene terephthalate) (PTT)/short carbon fibre (CF) composites at different mechanical states were investigated by a rotational rheometer and a dynamic mechanical analyzer (DMA). At molten state, the composite melts were pseudo-plastic fluids, and the complex viscosity of the composite melts decreased much with increasing CF content because of the poor adhesion at the fiber/matrix interface. The viscous behavior was predominant rather than elastic behavior in the composites melt and viscous behavior was increased with increasing CF at low shearing frequency. An apparent slope change in storage modulus and loss modulus plot suggested that a structure change occurred in the melt that was dependent on shearing frequency. At glassy state, the storage modulus increased with increasing CF content, suggesting that CFs had good reinforcing effect on PTT. At glass transition region, the increasing loss modulus indicated a better toughness of the composites, and the elastic behavior was predominant rather than viscous behavior. Moreover, the glass-transition temperatures of the composites increased with 10% CF content. The composites have larger cold-crystallization rate than pure PTT.