Mahn Won Kim

A novel method for measurements of second-order non-linearities of organic molecules

Abstract The second-order non-linear polarizabilities of a number of organic molecules have been determined by measuring the optical second-harmonic generation from monolayers of such molecules spread on a water surface.

Additional Reduction of Surface Tension of Aqueous Polyethylene Oxide (PEO) Solution at High Polymer Concentration

The surface activity of polyethylene oxide in water is studied by measuring the surface tension as a function of polymer concentration. Conventional adsorption behavior is observed at low polymer concentration, but a novel anomalous surface tension reduction behavior is found at high polymer concentration. The self-assembled adsorbed film created at low polymer concentrations behaves similarly to PEO monolayers at the air/water interface. In the high-concentration regime, the additional surface tension reduction is due to the formation of dense PEO layers. The high-concentration adsorption behavior was mimicked by spreading C16H33-PEO-C16H33(CPC) molecules onto a water surface. The CPC molecule is stable even at high surface polymer concentration, because of the anchoring of the C16H33 at the water surface. The correlation of the surface pressure vs. surface area per molecule of a CPC polymer at the air/water interface shows a behavior similar to a PEO monolayer at low surface concentrations, but also shows additional surface tension reduction at high surface polymer concentrations.

Adsorption kinetics of surfactant molecules at a liquid–air interface

The adsorption of the surfactant sodium–dodecyl naphthalene–sulfonate from the bulk to the water–air interface was studied by optical second harmonic generation. An initial rapid Langmuir type of adsorption to about 80% of a monolayer is observed, followed by a much slower adsorption until a full monolayer is formed.

Influence of an elongational flow field on random coil and rod-like polymers in solution

Abstract The conformational response and spatial arrangement of random coil and rod-like macromolecules in solution was investigated utilizing an opposing jet device. This device is capable of generating a well-defined elongational flow field, which is quite useful for probing intra- and intermolecular interactions of sulphonated polystyrene (random coil) and xanthan (rod-like chain). With regard to the random coil in an aqueous environment, it was determined that its behaviour below the critical overlap concentration ( C∗ ) follows the anticipated trends as predicted by theory. Above C∗ , the elongational behaviour is determined by the intermolecular entanglement structure. However, changes in the ionic strength in these latter solutions reverts behaviour to that observed below C∗ . One implication of this finding is briefly discussed. Interestingly, intermolecular interactions dominate the behaviour of rod-like polymers, such as xanthan, below and above C∗ . Superimposed on these interactions is the subtle influence of the flow field both outside and along the axis connecting the centre of the jets. These results confirm that the xanthan macromolecule behaves as a liquid crystal at all concentrations investigated, even with relatively large changes in ionic strength.

Rod to coil transition and aggregation in soluble polydiacetylenes: Semidilute regime

Visible absorption, rheological measurements, polarizing light microscopy, and temperature–time dependent quasielastic light scattering was used to examine the dilute–semidilute properties of polydiacetylene solutions, specifically poly(4BCMU). A good (tetrahydrofuran) and poor (toluene) solvent were selected for examining solution properties over a broad range of solvent quality. In this particular instance, poly(4BCMU) dissolved in both the pure solvents and their respective mixtures. Based on the observation that dramatic color changes (yellow–orange–red) can occur with the hydrodynamic radius remaining invariant, it is quite likely that the color changes are due to a local modification in the conjugation length. That is, on a molecular level, a local stiffening of the segments occurs as the magnitude of the red absorption component increases. Interestingly, the stability of the solutions towards aggregation/phase separation also correlates well with the absorbance value of the red peak. Furthermore, c...

Light scattering measurements in a dilute microemulsion

We have measured the mutual diffusion coefficient and static light scattering intensity at small angle of a water‐in‐oil microemulsion at low (0.005–0.04) minor component volume fraction. The system studied was AOT/water/decane at 25 °C. We found a linear dependence on volume fraction for both quantities, with virial coefficients of −17 and −11, respectively, for the static structure factor and mutual diffusion coefficient. Using available expressions for these coefficients as a function of the parameters of a model potential consisting of an attractive square well and a hard core, these results can be shown to be in agreement with those previously obtained by neutron scattering.

Surface waves on polymer solutions: Complete capillary wave–elastic wave crossover

We study the surface waves of polyethyleneoxide (PEO) in water by surface heterodyne light scattering. The complete capillary wave–elastic wave crossover is observed as the polymer concentration is increased. At high polymer concentrations, the peak frequencies of the experimental power spectra increase as the polymer concentration is increased, while the corresponding frequency widths decrease as the polymer concentration is increased. This unusual behavior of peak frequency and frequency width results from the strong viscoelasticity of the bulk which gives the additional restoring force of the surface waves at high polymer concentrations, as predicted by Harden et al.’s [J. Chem. Phys. 94, 5208 (1991)] theoretical calculation. Furthermore, the adsorbed monolayer of 1000 K PEO appears to have no effect on the surface waves.

Surface modes on polymer solutions by surface light‐scattering techniques

By using the surface heterodyne spectroscopy, we study the surface modes on polymer solutions of polyisobutylene/decane at different concentrations. For each concentration, we measure the power spectra at various wave numbers (110.0–600.0 cm−1). We find that the relation between the frequency shift ωk of the power spectrum and wave number k changes continuously from ωk∝k3/2 to ωk∝k when concentration of the polymer solution is changed from 0.5 g/dl to 8.0 g/dl. For the low‐concentration polymer solutions, the surface modes are capillary modes (ωk∝k3/2). For the high‐concentration polymer solutions, the surface modes are elastic modes (ωk∝k). This result is compared with the theory [J. L. Harden, H. Pleiner, and P. A. Pincus, J. Chem. Phys. 94, 5208 (1991)].

Two-dimensional properties of surfactant-like polymer monolayrs

A functional polymer monolayer at an air/water interface has been studied systematically by substituting a hydrophobic backbone with a hydrophilic moiety (OH) by measuring the surface pressure as a function of surface concentration for different molecular weight polymers. The characteristic scaling exponent (ν) estimated from the isotherms and the overlap surface concentration suggests that the air/water interface becomes a better solvent as the amount of hydrophilic functional groups is increased. Also, the isotherms show that the area per polymer molecule increases with increasing amounts of OH groups. However, the water surface tension is most effectively reduced at the 50% substitution level, not at 100%. This optimum amount of OH group on the hydrophobic polymer is equivalent to the hydrophic/lypophic balance of a surfactant in the formation of microemulsions. Furthermore, the surface occupied area per OH group (σo) is estimated with the ideal gas law at a dilute surface concentrations. The results show that the σo is a minimum at the same substitution level at which the reduction of water surface tension is a maximum.

Light scattering in a dilute microemulsion. II. Radius dependence of interactions

We have measured the static light‐scattering intensity and mutual diffusion coefficient in dilute water‐in‐oil microemulsions made of sodium diethylhexylsulfosuccinate (AOT), water, and n‐decane. From these measurements we determine the second virial coefficient and linear correction to the mutual diffusion coefficient. We discuss these results in the context of a model potential of a hard core plus attractive square well and determine the parameters of the square well from the data. We find the results consistent with prior small‐angle neutron scattering results: assuming a constant range for the square well of about 3 A, we see a linear dependence of the depth of the well upon the radius of the droplets. Then assuming a ‘‘chain interpenetration model’’ for the interaction, we calculate the interaction per overlapping surfactant to be ≈kBT/5.