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I am very much interested in polymer processing and rheology, especially in the field as descried below. While I was at the Korea Institute of Science and Technology, I performed research on molecular structural effects on the miscibility of polymer blend.  I did polyarylate synthesis and blending with other commercial polymers to study crystalinity and mechanical property change of the blending,  During that time, I learned the most common experimental RDS, DMA, DSC, TGA, SEM, GPC, Instron, and Blending Mixer.  During my Master program at the Seoul National University, I studied polymer rheology and developed experimental and computational skills in rheological characterization of polymers.  See attached list of related masters and PhD courses.

a) mathematical  & numerical approach to polymer dynamics

Using mathematical and numerical tools, I opened a new area of polymer relaxation research with a new model I proposed in my master thesis at Seoul National University.  Polymer relaxation has been studied for a long time by many people, but still unknown behavior and contradictory results remained.  Here I propose a new continuous relaxation model which can be manipulated as a discrete relaxation model. 

Simple description of my new model is as follows.  A continuous relaxation time spectrum following a modified generalized Maxwell model (MGMM) was proposed[1] and investigated from simulated data and experimental G' and G" of polystyrenes (PS), which were then compared with a discrete relaxation time spectrum obtained by using the generalized Maxwell model (GMM)[2]. The relaxation spectra of both the MGMM and GMM were calculated using linear and nonlinear regression methods. The nonlinear regression method produced better results than the linear method. In the case of simulated data, the MGMM and GMM have shown similar results in reproducing the original spectrum when the number of relaxation times used was sufficient, but the relaxation spectrum of MGMM was closer to original spectrum than that of GMM when the number of relaxation times used was small. In the case of nearly monodisperse PS, the relaxation spectra of MGMM and GMM were independent of molecular weight in the region of short relaxation time, but shifted to the direction of increasing relaxation time in the region of long relaxation time as the molecular weight increased. Relaxation spectrum of polydisperse PS did not show distinct terminal relaxation time observed in nearly monodisperse PS. Parameter m of MGMM was highly dependent on the molecular weight distribution. Discrete relaxation spectrum calculated from the continuous spectrum of MGMM was in good agreement with the spectrum of GMM.

b) molecular dynamics approach to polymer relaxation

With the tool of molecular dynamics for polymer chains, I fundamentally studied the relaxation phenomena of long polymer chains.  For instance, for linear viscoelastic properties of dilute polymer solutions, I studied the following variables: bead number (N), hydrodynamic interaction (HI), excluded volume (EV), and non-linearspring (NS) effects by using the bead-spring chain model (BSM)[3]

Linear viscoelastic properties were derived by modifying Fixman's model and introducing NS concept.  Among them, I investigated the magnitude and the phase angle of complex viscosity.  I determined bead number by the breadth of plateau region of the phase angle, and HI parameter from the exponent of Mark-Houwink equation. By contrast I fit data in order to acquire the dynamic expansion parameter which is a function of EV and NS parameters.  Once the EV parameter is known through an experiment such as light scattering, the NS parameter can be predicted.  Moreover we studied the discrete relaxation time spectra of BSM, and found out that the effects of BSM parameters on the spectra are clearer than those on the linear viscoelastic properties.  In this paper the method of accommodating bead number, HI, EV, and NS effects on BSM was suggested and resulted in the excellent agreement of linear viscoelastic properties with experimental data.

c) scaling approach to a polymer chain analysis on surfaces

Using scaling theory, I analyze the polymer chain behavior on surfaces in a relatively simple way.  As well as the experimental work of understanding the adhesion at polymer-solid interface: sticker and receptor group effects, I use a simple scaling analysis of free energy of a chain on surfaces to predict more complicated polymer and substrate systems[4]. 

For example, the model polymer is renormalized into a blob model, where each blob contains one sticker. The model substrates are modeled as pure energetic surfaces contaminated with inert impurities. The enthalpic driving force and the entropic constraints are related to the adhesive potential increase and the cohesive potential decrease between an adsorbed chain and a solid surface and between an adsorbed chain and neighboring chains, respectively. Based on the conformational dimension by the scaling analysis of free energy, the Entanglement Sink Probability (ESP) is defined to quantify the entanglement density between adsorbed chains and neighboring chains.  Failure of interfaces occurs at the weaker of the two adhesion potentials, which are functions of ESP.

[1].  I. Lee, R. P. Wool, K. H. Ahn, S. J. Park, S. J. Lee, ”°Continuous Relaxation Time Spectrum Derived from Generalized Maxwell Model: Modified Maxwell Spectrum,”± In preparation.

[2].  S. J. Park, I. Lee, S.  J. Lee, and S. J. Lee, ”°The Determination of Continuous Relaxation Time Spectrum of Linear Polymer,”± The Korean J. of Rheology, 8(2), pp103-118 (1996).

[3].  I. Lee, K. H. Ahn, and S. J. Lee, ”°Linear Viscoelastic Properties and Relaxation Time Spectrum of Dilute Polymer Solutions,”± The Korean J. of Rheology, 7(3), pp211-224 (1995).

[4]. I. Lee and R. P. Wool, ”°Thermodynamic Analysis of Polymer Adhesion: Sticker Group and Receptor Group Effects,”± The Journal of Polymer Science: Polymer Physics,  40, pp2343-2353 (2002).


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