Seung Won Seo

Domain and segmental deformation behavior of thermoplastic elastomers using synchrotron SAXS and FTIR methods

Deformation behavior of the segmented block copolymers was studied with synchrotron small-angle X-ray scattering (SAXS) and Fourier transform infrared spectroscopy (FTIR) methods. Polyurethanes used in this work consist of 4,4′-methylene-bis(phenyl isocyanate) and butanediol as a hard segment, and poly(tetramethylene oxide) of various molecular weights as a soft segment. As expected, the deformation of the domain structure that is macroscopically isotropic before the drawing was anisotropic. Depending on the initial orientation of the hard domains, the deformation behavior was observed to be characteristically different. Whereas the hard domains oriented along the deformation direction underwent the extension of the domain separation distance at the low draw ratio, the perpendicular ones showed the shear compression. Further drawing was found to cause the breakup of the hard domains, followed by the formation of fibril structure oriented along the deformation direction. Based on SAXS and FTIR results, a model is proposed to explain the deformation behavior of the various domains and segments of the segmented block copolymers. By quantitatively analyzing the conformation of the soft segment during the deformation process, the model proposed has been consolidated.

Synchrotron SAXS study on the micro-phase separation kinetics of segmented block copolymer

The phase transition behavior and isothermal micro-phase separation kinetics of polyester-based thermoplastic elastomer were studied using the synchrotron X-ray scattering (SAXS) method. The structural changes occurring during heating period were investigated by determining the changes of the one-dimensional correlation function, interfacial thickness and Porod constant. Based on the abrupt increases of the domain spacing and interfacial thickness, a major structural change occurring well below the melting transition temperature is suggested. Those changes are explained in terms of melting of the thermodynamically unstable hard domains or/and the interdiffusion of the hard and soft segments in the interfacial regions. SAXS profile changes during the micro-phase separation process were also clearly observed at various temperatures and the separation rate was found to be sensitively affected by the temperature. The peak position of maximum scattering intensity stayed constant during the entire course of the phase separation process. The scattering data during the isothermal phase separation process was interpreted with the Cahn-Hilliard diffusion equation. The experimental data obtained during the early stage of the phase separation seems to satisfy the Cahn-Hilliard spinodal mechanism. The transition temperature obtained from the extrapolation of the diffusion coefficient to zero value turned out to be about 147±2°C, which is close to the order-disorder transition temperature obtained from the Porod analysis. The transition temperature was also estimated from the invariant growth rate. By extrapolating the invariant growth rate to zero, a transition temperature of about 145±2°C was obtained.

Thermal properties of copolyetherester/silica nanocomposites

Thermal properties of copolyetherester/silica nanocomposites were examined by using DSC and TGA. The segmented block copolyetheresters with various hard segment structures and hard segment contents (HSC) were synthesized and their silica nanocomposite films were prepared by solution casting method. The nano-sized fumed silica particles were found to act as a nucleating agent of the copolyetheresters. The nanocomposites always showed reduced degree of supercooling or faster crystallization than the corresponding copolyetheresters. The nanocomposites also showed increased hard segment crystallinity except HSC 35 sample which had short hard segment length. In case of 2GT [poly(ethylene terephthalate)] copolyetheresters, which were not developed commercially because of their low crystallization rate, the hard segment crystallinity increased considerably. The copolyetherester/silica nanocomposites showed better thermal stability than copolyetheresters.