Sehyun Kim

Enhanced interfacial adhesion between an amorphous polymer (polystyrene) and a semicrystalline polymer [a polyamide (nylon 6)].

We studied enhanced interfacial adhesion between an amorphous polymer (polystyrene, PS) and a semicrystalline polymer (a polyamide, Ny6). The fracture mechanism for this system was investigated to elicit a universal description on the fracture mechanism. The surface modification of PS to provide functional groups that can react with the functional groups of Ny6 was carried out with ion-beam and/or plasma treatment. These surface modifications were found to alter the interfacial adhesion strength between PS and Ny6. A remarkable enhancement was found with the surface functionalization of PS. Though the fracture toughness was varied depending on the process, its overall behavior was quite similar to that of others; the fracture toughness increased with increasing bonding temperature and bonding time, passed through a peak, and then decreased with a further increase of the bonding time or temperature. The variation of the fracture toughness with the bonding time and temperature can be plausibly explained in terms of two different failure mechanisms of adhesive failure and cohesive failure. This change appears more evidently for the interface between an amorphous polymer and a semicrystalline polymer than the interface between semicrystalline polymer pairs. Surface functionalization could exclude the effect of diffusion, thus clarifying the failure mechanisms occurring at the interface.

Nonisothermal crystallization behaviors of nanocomposites prepared by in situ polymerization of high-density polyethylene on tungsten oxide particles

Morphology development and thermal properties of polyolefin composites (tungsten oxide (WO3)/high density polyethylene (HDPE)) were characterized by differential scanning calorimetry, polarized optical microscopy, and nonisothermal crystallization kinetics. Tungsten oxide (WO3) and HDPE composites were prepared by the in situ metallocene polymerization method which consisted of attaching a metallocene catalyst complex onto the surface of the nuclei (WO3) and followed by the surface-initiated polymerization. A kinetic equation proposed by Seo was employed to analyze the nonisothermal crystallization characteristics of the composites. The polarized optical microscopy and the Avrami exponent verified the importance of the interaction between the HDPE molecules and the nuclei surface for the HDPE molecular ordering around the metal powder as well as the morphological development in the early stage. The Avrami exponent, n, determined from the nonisothermal crystallization kinetics analysis indicates random 3-dimentsional morphology development for the WO3/HDPE composites. Dispersed WO3 accelerated the crystallization rate due to heterogeneous nucleation effect as indicated by the shift in the crystallization peaks to higher temperatures. The obtained crystalline structures were compared to those for the neat polyethylene resins and carbon nanotube (CNT)/HDPE composites.

Enhancement of β-phase in PVDF by electrospinning

Poly(vinylidene fluoride) (PVDF), a polymorphic material, has useful electro active properties such as piezo-, pyro-, and ferroelectric properties. The piezoelectric crystallization polymorph is the β-phase which has an all-trans (TTTT) conformation, i.e., all of its dipoles aligned in the same direction normal to the chain axis. Elongational deformation was applied to poly(vinylidene fluoride)(PVDF) / multiwalled carbon nanotube (MWCNT) nanocomposites. Its effect on the conversion of the α-phase crystals into the β-phase crystals and on the piezoelectric propertes was investigated. Elongational deformation of PVDF molecules made it easier for the CF2 groups to arrange themselves in the same conformation by reducing the steric hindrance. However, macro-deformation was dominant in the drawn films, though the interfacial interaction between the functional groups on the MWCNTs and the CF2 dipole of PVDF chains helped the conversion of α-phase. Uniform distribution of MWCNT in the electrospun films helps effi...