Chengbi Zhao

Synthesis and Curing of Hyperbranched Poly(triazole)s with Click Polymerization for Improved Adhesion Strength

We successfully synthesized hyperbranched poly(triazole)s by in situ click polymerization of diazides 1 and triyne 2 monomers on different metal surfaces (copper, iron, and aluminum) and characterized their adhesive properties. Optimizations were performed to obtain high adhesive strength at different temperatures by analyzing the effects of curing kinetics, annealing temperature and time, catalyst, monomer ratio, surface conditions, alkyl chain length of diazides 1, etc. The adhesive bonding strength with metal substrate is 2 orders of magnitude higher than similar hyperbranched poly(triazole)s made by click polymerization and clearly higher than some commercial adhesives at elevated temperatures. With the same conditions, adhesives prepared on aluminum and iron substrates have higher adhesive strength than those prepared on copper substrate, and an excess of triyne 2 monomer in synthesis has greater adhesive strength than an excess of diazide 1 monomer. Tof-SIMS experiment was employed to understand these phenomena, and the existence of an interphase between the polymer and metal surface was found to be critical for adhesive bonding with thicker interphase (excess of triyne 2 monomer) and the higher binding energy between polymer atoms and substrate atoms (e.g., aluminum substrate) generating the higher bonding strength. In addition, the light-emitting property of synthesized polymers under UV irradiation can be used to check the failure mode of adhesive bonding.

Dynamic mechanism of phase differences in One degree-of-freedom vortex-induced vibration of a cylindrical structure

The purpose of this study is to provide some insights into the phase mechanism of a cylindrical vortex–induced vibration. A transient coupled fluid–structure interaction numerical model is adopted to simulate a cylindrical vortex–induced vibration. The vortex shedding around the cylinder is investigated numerically by a two-dimensional large eddy simulation approach which can catch more details of the flow field and more accuracy on computing hydrodynamic forces. The vortex shedding modes and response and hydrodynamic forces of a cylindrical vortex–induced vibration are acquired with varied frequency ratios. According to differences in the vortex shedding location, the vortex wake can be characterized by two kinds of mode, the “first mode” and the “second mode.” The mechanisms behind the phases of the first mode and the second mode vortex wakes are investigated, and it is found that the flow speed induced by a cylindrical transverse vibration and the position of a vortex release are the root causes of the phase difference between the lift coefficient and transverse displacement. The speeds caused by a cylinder vibration and a cylinder-shed vortex are the reasons that the lift amplitude of an oscillatory cylinder is different from that of a fixed cylinder.

An immersed boundary method with an approximate projection on nonstaggered grids to solve unsteady fluid flow with a submerged moving rigid object

An immersed boundary method is presented, which uses an approximate projection method on nonstaggered grids for computing flows with submerged and moving boundaries. The incompressible Navier–Stokes equations are discretized using a second-order accurate finite difference technique on a nonstaggered grid system, and the new immersed boundary method is proposed based on an approximate projection method with two pressure correction techniques, the Armfield method and the geometrical grid Reynolds modified method on nonstaggered grids. By using this method, the results obtained from (1) flow past a rigid cylinder in two dimensions with different Reynold numbers and (2) flow around an oscillatory circular cylinder in flow at low Keulegan–Carpenter numbers are in agreement with the reported experimental and numerical data, demonstrating that this convenient method of constraining the interface is a reliable and robust numerical approach for solving unsteady fluid flow with a submerged moving rigid object. This method has the advantage of using significantly less computation time and lower computation sources than the traditional immersed boundary methods.

Hydrodynamic Analysis of Floating Marine Structures Based on an IBM-VOF Two-Phase Flow Model

In this study, we develop an immersed boundary method - volume of fluid (IBM-VOF) two-phase flow solver to simulate two-phase flow problem contains solid boundaries and free surface and use it to solve the typical problems for floating marine structures. In the solver, the IBM method is adapted to solve the problems of the moving marine structures and the VOF method for solving the problems of a free surface flow. The free surface at the fluid is considered as the mixed fluid of sea water and air in the solver. Base on this IBM-VOF two-phase flow model, hydrodynamic analysis of a floating marine structure with forced heave motion is done, and hydrodynamic force coefficients are computed. In this case, we firstly calculate the forces of the floating marine structure under different frequencies, and then we get added mass and added damping through fitting the data of the forces by the least square method. The results obtained from the present model are compared, which verified the reliability and accuracy of this numerical model.

Natural Frequency Ratio Effect on 2 DOF Flow Induced Vibration of Cylindrical Structures

In this study, the vortex-induced vibration (VIV) of a circular cylinder at the low Reynolds number of 200 is simulated by a transient coupled fluid-structure interaction numerical model using the combination of FLUENT and ANSYS platforms. Considering VIV with low reduced damping parameters, the trend of the lift coefficient, the drag coefficient and the displacement of the cylinder are analyzed under different oscillating frequencies of the cylinder. The frequency ratio α is a very important parameter, which has been intensively investigated here. The typical nonlinear phenomena of locked-in, beat and phases switch can be captured successfully. The evolution of vortex shedding from the cylinder and the trajectory of the 2 DOF case with varied frequency ratio is also discussed.

Organoclay/thermotropic liquid crystalline polymer nanocomposites. Part I: Effects of concentration on morphology, liquid crystallinity and thermal properties

Abstract Thermotropic liquid crystalline polymer (TLCP) nanocomposites with different organoclay contents were prepared by a method combining ultrasonication, centrifugation and solution casting methods. The effects of organoclay concentration on morphology, liquid crystallinity and thermal properties of nanocomposites were characterized. Molecular level interactions existed in nanocomposites. With a small amount of organoclay (nominal 3.0 wt %), it was enough to change TLCP properties with enhanced thermal stability though with a small negative effect on liquid crystallinity. When high concentration organoclay was added, thermal stability decreased and liquid crystallinity lost with shearinduced phase separation occurrence.

Organoclay/thermotropic liquid crystalline polymer nanocomposites. Part VI: Effects of intercalated organoclay on nanocomposite morphology, thermal and rheological properties

A study of a typical intercalated structure of a thermotropic liquid crystalline polymer (TLCP) with organoclay was performed to elucidate the influence of intercalated organoclay on the TLCP molecules, especially on their liquid crystallinity, thermal and rheological properties. The intercalated structures were confirmed in TLCP and organoclay formed molecular interactions with TLCP molecules in the system. Such intercalated structures caused the glass transition temperature of the nanocomposite to become invisible in thermal measurement and also caused loss of liquid crystallinity. The TLCP molecules inside the organoclay galleries showed higher thermal stability and transition temperatures, but the orderly structure of the TLCP molecules outside the galleries was destroyed by the organoclay, causing the TLCP to display lower thermal stability and transition temperatures than pristine TLCP. At 185°C, where TLCP is in the nematic phase, the nanocomposite had three orders of magnitude higher viscosity in the linear viscoelastic region than that of TLCP, with chain mobility and relaxation time slowed due to the intercalated effects in the nanocomposite. Steady shear altered the domain sizes and oriented the highly anisotropic organoclay layers or tactoids along the shear direction.