Hyungsu Kim

Effect of ultrasonic wave on the degradation of polypropylene melt and morphology of its blend with polystyrene

Abstract A new method using high intensity ultrasonic wave has been developed for the degradation of polymer melt in an intensive mixer. For the effective transfer of ultrasonic energy, experimental apparatus was specially designed so that polymer melt can be in direct contact with the ultrasonic horn. In this research, we observed that the significant degradation of polypropylene (PP) melt in a mixer occurs due to the action of ultrasonic wave without any aid of additives or solvents. To compare the degradation efficiency of ultrasonic irradiation with that of peroxide, additional experiments were performed to practice the peroxide-aided degradation by mixing 1 part of dicumyl peroxide with PP in the mixer. It was also found that the direct sonication on polymer mixture in melt state reduces the domain sizes and stabilizes the phase morphology of the blend. It is suggested that ultrasound assisted melt mixing can lead to in situ copolymer formation between the components and consequently provide an effective route to compatibilize immiscible polymer blends.

Microdroplet formation of polyvinylpyrrolidone/carbon nanotube by ultrasonic atomization

Ultrasonic atomization is conducted to produce composite particles of polyvinylpyrrolidone (PVP) and multi-walled carbon nanotube (MWCNT) from an aqueous suspension. The rheological properties of the suspensions are varied in a systematic manner by changing the molecular weight of the polymers, the polarity, and the concentration of MWCNT in the given composite system. A particular interest of this study is to investigate how the ultrasonic atomization of PVP/MWCNT suspensions is affected by their shear and extensional properties. As a characteristic of the fluid under extensional flow, the fluid relaxation time is measured from capillary breakup extensional rheometry. It is found that the average size of the composite droplets is increased as the relaxation time in the capillaries of PVP/MWCNT increases, while the number of available particles is mainly limited by the shear viscosity of the suspension. With the available materials and analyses adopted in this study, the observed trends are explained in terms of two dimensionless groups: the Deborah and Ohnesorge numbers, which encompass the material parameters (viz, relaxation time, shear viscosity, density, and surface tension) associated with the atomization process of viscoelastic fluids.

Development of Branched Polycarbonate by an Ultrasound‐Assisted Melt Mixing Process with Multifunctional Agents

The chain structure of polymer molecules is an important characteristic of polymers. In the polymer industry, in‐situ processing during polymerization or a post‐processing is applied to alter the chain structure as an attempt to produce polymers with tailored properties. Among various methods to control the chain structure, ultrasound‐induced polymer chain scission is a useful route which can either be used as a post‐processing step or can be used during ultrasound‐induced polymerization. In our previous studies, we intended to induce degradation of polymer melts in a sonicated intensive mixer and extruder. By combining high intensity ultrasound which causes chain scission of polymer molecules and a multifunctional agent (MFA) having double bonds at its ends, we were able to modify the molecular structure of polycarbonate (PC) from linear to branched structure during melt processing. The three double bonds in chain ends of MFA were expected to act as sites for trapping macroradicals of PC during the cours...

Thermally induced crystallization kinetics of uncrosslinked and unfilled synthetic cis-1,4-polyisoprene rubber monitored by shear rheological tests

This study demonstrates the unique capability of a shear rotational rheometer for studying the thermally induced crystallization (TIC) of uncrosslinked and unfilled cis-1,4-polyisoprene rubber (IR). At temperatures below −15°C, a crystallization phenomenon (TIC) occurred in a quasi-unstrained IR specimen. Such a distinguished phenomenon was determined from the steady and sharp changes of both tanδ and the modulus. The changing ratio of those parameters with time characterizes the crystallization rate, on which the effects of the compressive force magnitude, testing repeat, and temperature are studied. The crystallization rate was shown to depend less on the magnitude of normal force, but depended largely on the specimen’s previous testing history. A specimen not fully recovered from the previous crystallized memory showed a faster rate than before. More cooling to −25°C increased the crystallization rate, but the slow crystallization helped increase the final crystallinity. The crystallization rate was further interpreted by the Avrami equation to propose the crystal structure, whose morphological feature was shown in agreement with the reported TEM and X-ray results. However, our study found a thermo-mechanically aged specimen showed a very different rheological behavior at the late stage of crystallization suggesting the crystalline metamorphosis. But this unexpected behavior turned out to be unrecoverable indicating a property failure due to material aging more plausibly. All these findings were successfully monitored by the rheometer. It is expected the well-organized rheometric measurements can sufficiently supplement some instrumental limitations of the traditional crystallization monitoring analyzers on soft materials.

A simple derivation of the critical condition for the ultrasonic atomization of polymer solutions

A simple model is proposed for the ultrasonic atomization of polymer solutions. In this model, the atomization process is approximated as an equilibrium process. It is shown that the minimum attainable droplet size is determined by two parameters, the (Rayleigh) acoustic pressure acting on the surface of the liquid, and the surface tension of the liquid. Increasing the viscosity of the liquid suppresses the formation of small-sized droplets because of increased attenuation of the sound wave and thus decreased acoustic pressure. Lowering the surface tension of the liquid (e.g., by spreading a surfactant film on the liquid surface) has the opposite effect of enhancing the formation of smaller droplets. Also, there exists a maximum limit for the droplet size, because when the produced droplet is too large, the aspiration flow is unable to carry the droplet against sedimentation. These predictions are supported by experimental observations.

New Approach of Ultrasonic Blend Process on PP/PC and Its Nanocomposites

In previous research, it was possible to induce chain scission of the PP in a molten state by high intensity ultrasonic wave. Also, with the aid of a multifunctional agent (MFA) and ultrasound, a long‐chain branched PP and its nanocomposites can be created without any decomposing agent. And recent paper is introduced a ultrasonic method by imposing ultrasound and MFA to modify linear PC into branched grade. From these results, It was expected that we may be able to provide a blend information associated with the development of ultrasonic process on structural modification. The MFA used in this study includes three arms and a double bond exists at the end of each arm. So, the three double bonds in chain ends of MFA were expected to act as sites for trapping macroradiclas of polymer during ultrasound‐assisted blend process. When macroradicals of ultrasonic blend react with the end group of MFA, formation of branched structure can be prepared and enhanced chain combination of polymer blend during sonication....