Seung Joon Park

Dilution exponent in the dynamic dilution theory for polymer melts

Two values, unity and 4/3, of the dilution exponent α in the Milner–McLeish dynamic dilution theory for relaxation of entangled polymers are tested thoroughly against experimental data for linear, star, and star-linear blends of 1,4-polybutadiene using the same set of values for the parameters: the plateau modulus GN0, the entanglement spacing Me, and the equilibrium time τe for each α. We find that α=4/3 gives good agreement with all data sets using the literature value for the plateau modulus and an entanglement spacing that is very close to the value calculated from the plateau modulus. While we can also predict linear and star polymer rheology with adjusted Me and τe for α=1, the required value of Me is 40% higher than the one calculated from the plateau modulus.

Numerical simulation results of the nonlinear coefficient Q from FT-Rheology using a single mode pom-pom model

In previous experimental observations, Hyun and Wilhelm [Macromolecules 42, 411–422 (2009)] proposed a nonlinear coefficient Q (≡ I3/1/γ02) that was determined from Fourier transform rheology experiments under dynamic oscillatory shear flow. Additionally an intrinsic zero-strain nonlinearity, Q0 (≡ limγ0→0Q), was defined. It was found that this nonlinear coefficient Q(ω,γ0), also given as Q0(ω), is a very promising parameter to quantify nonlinear mechanical properties that are highly affected by polymer topology, e.g., branched structures. In this study, we systematically investigated the effect of polymer topology on the nonlinear parameters Q(ω,γ0) and Q0(ω) with a single mode differential pom-pom model. A number of parameters are affecting the topology of a pom-pom polymer, such as the number of dangling arms (q) and the dimensionless molecular weight of both the backbone (sb) and the arms (sa). In the here presented work, the linear viscoelastic properties G′(ω) and G″(ω) were compared with the nonlin...

Modeling the linear viscoelastic properties of metallocene-catalyzed high density polyethylenes with long-chain branching

The hierarchical model modified by Park et al. (2005) is applied to single-site metallocene-catalyzed high density polyethylene (mHDPE) to predict the effect of long-chain branching (LCB) on the linear viscoelastic properties. In this work we generate the distributions of molecular weight and LCB of mHDPE for the hierarchical model predictions using a Monte Carlo simulation of Costeux et al. (2002), where the simulation parameters are obtained from the average molecular weight and the degree of LCB. The parameters needed in the hierarchical model are determined from experimental data for linear polyethylenes. The model predicts the experimental data well, reveals the effect of LCB on the linear viscoelastic properties of mHDPE, and indicates the possibility of inferring branching levels from measurements of linear viscoelastic properties of mHDPEs.

Long-chain dynamics in binary blends of monodisperse linear polymers

The linear viscoelastic properties of binary blends of monodisperse linear polymers of various polymers, including polyisoprene, polybutadiene, and polystyrene, are compared with the predictions of the model of Park and Larson (2004), which is an extension of the Milner-McLeish model [Milner and McLeish (1998)]. For each polymer system the model parameters needed in the predictions of the binary blend are obtained from experimental data for monodisperse polymers. For well entangled blends of nearly monodisperse polymers, the model predictions are in good agreement with experimental data for a wide range of values of the Graessley parameter Gr, which is proportional to the ratio of the reptation time of the long chain in the undilated tube to the constraint-release Rouse time. Our results show that for polymers of multiple chemical types, the terminal behavior of the long chain is controlled by reptation in an undilated tube for small Gr Grc, where Grc is arou...

Flavoprotein autofluorescence detection of early ocular dysfunction.

Metabolic stress occurs at disease onset and causes altered flavoprotein redox activity that increases flavoprotein autofluorescence (FA). Using a new method to measure ocular FA, we studied women with subtle visual dysfunction due to pseudotumor cerebri. Each FA value was greater in the more affected eye of each woman with pseudotumor cerebri, permitting identification of that eye in each case. Flavoprotein autofluorescence values averaged 60% greater in more affected eyes of women with pseudotumor cerebri, but not between eyes of healthy women (control subjects). These results demonstrate the clinical utility of FA and may permit early detection and monitoring of retinal and optic nerve diseases.

Detection of retinal metabolic stress resulting from central serous retinopathy.

Purpose: To test whether eyes with central serous retinopathy have elevated retinal flavoprotein fluorescence (FPF) using a novel clinical imaging method. Methods: Three male patients with unilateral central serous retinopathy were examined for FPF at 535 nm induced by 1-msec flashes of 467 nm light. FPF was captured with an electron multiplying charged-coupled device camera with a 512 × 512 pixel chip. Average intensity of retinal FPF for each affected eye was compared with the contralateral, unaffected eye and with six age-matched control eyes by analyzing histograms of pixel intensities plotted for each eye. Results: For each patient, the central serous retinopathy-affected eye had a significantly greater retinal FPF when compared with the retinal FPF of the unaffected eye. Eyes affected with central serous retinopathy had retinal FPF values that averaged 98% greater than the retinal FPF of age-matched control eyes. Conclusion: Flavoprotein fluorescence analysis may be useful for rapidly and noninvasively identifying metabolic tissue stress of central serous retinopathy.

Implications of microscopic simulations of polymer melts for mean-field tube theories

Despite the success of the mean-field tube theory in predicting the stress relaxation of linear and branched polymers, several important issues remain unresolved. Recent simulation methods that address some of these issues are shedding light on the dynamics of entangled polymer molecules. In this survey, we consider a class of coarse-grained models for polymer melts called slip-link models, which have been used to study the phenomena of constraint release, branch-point diffusion, and the relationship between the plateau modulus and entanglement spacing. We also consider the bond-fluctuation lattice model, and the pearl-necklace molecular dynamics model in conjunction with recently developed algorithms to identify primitive paths. The ability of these latter models to identify the primitive path network enables molecular simulations to be used to test and improve mean-field tube models. The implications of these simulations for the mean-field tube potential in the absence of constraint release, and for the dilution exponent which controls the strength of constraint release, are examined.

Superensembles of linear viscoelastic models of polymer melts

The idea of incorporating multiple models of linear rheology into a superensemble, to forge a consensus forecast from the individual model predictions, is investigated. The relative importance of the individual models in the so-called multimodel superensemble (MMSE) was inferred by evaluating their performance on a set of experimental training data, via nonlinear regression. The predictive ability of the MMSE model was tested by comparing its predictions on test data that were similar (in-sample) and dissimilar (out-of-sample) to the training data used in the calibration. For the in-sample forecasts, we found that the MMSE model easily outperformed the best constituent model. The presence of good individual models greatly enhanced the MMSE forecast, while the presence of some bad models in the superensemble also improved the MMSE forecast modestly. While the performance of the MMSE model on the out-of-sample training data was not as spectacular, it demonstrated the robustness of this approach.

Modeling of the Stress Distribution of the Pressure Sensitive Adhesive in the Multi‐Layered Structures

The control of the stress distribution in pressure sensitive adhesive (PSA) layer is very important in designing the multi‐layered structures. In this work, a model is developed for calculating the stress distribution of the PSA, which is induced by the dimensional change of substrates. The domains of the multi‐layered structure are divided into sub‐elements. The system of equations of the force balance for each sub‐element is solved simultaneously for the calculation of the stress distribution. We compare the model predictions for the light leakage of the polarizer used in the liquid crystal display (LCD) with experimental data. The results of this work indicate that the developed model can be used for optimizing the stress distribution in the multi‐layered structures and predict well the pattern of the light leakage of the polarizer.

Modeling the Rheology of Commercial Long Chain Branched Polymer Melts

The predictions of a general “hierarchical model” for the rheology of general mixtures of linear and branched polymers are compared to experimental data for well-defined long-chain branched polymers. For a wide range of branched polymer melts, which include star-linear blends, H-polymers, and comb polymers, the predictions of the model agree well with experimental data. We apply the hierarchical model to metallocene-catalyzed polyethylenes (mPEs), in which the branched structures are generated by a Monte Carlo method based on the known reaction kinetics. The hierarchical model captures the shape of the curves of viscoelastic moduli vs. frequency of mPEs well and predicts accurately the effect of long chain branching on the linear viscoelastic properties. The quantitative agreement of the hierarchical model prediction with experimental data of well-defined long-chain branched polymers and mPEs shows that information on branching structure could be inferred from rheological measurements on combinatorial sets of mixtures of an unknown branched with different combinations of linear polymers.