Minmin Tian

Release of Hydrophobic Substances from Polystyrene-Methacrylic Acid Block Copolymer Micelles into Aqueous Media

Abstract Block copolymer micelles are potential carriers for controlled-release drug delivery. In this study, the fundamental aspects of the release of a model compound from the micelles into the surrounding media were explored. A fluorescence-based technique was developed for following the release of the model compound from the cores of block copolymer micelles into aqueous media. The experimental release curves were compatible with theoretical dependences for diffusion controlled release from spherical particles. The diffusion coefficients found were of the order of 1018 cm2/s, which was expected for molecules with sizes of our probes and glassy polystyrene. The partition coefficients are also of the expected order of 105 magnitude. The fraction of the probe present outside of the cores at the start of the experiment may serve to characterize the efficiency of various procedures for the loading of various agents into the core.

Intermolecular contact interactions and their temperature dependence

A model, based on inverse gas chromatography experiments, has been developed for intermolecular interaction and its temperature dependence. The model ascribes to each substance a four-component solubility parameter; the four components reflect the van der Waals, polar, electron donor, and electron acceptor interactive properties. Their values depend on temperature in the same manner as does the cohesive energy. The latter was found to depend only on the critical temperature, the acentric factor, and the reduced temperature. The model was used for evaluation of the solubility parameters from polymer-solvent interaction coefficients and their temperature dependence with high accuracy. For binary solventsolvent mixtures, the free energy of mixing and its temperature dependence, as well as enthalpy of mixing can be derived from this model with a good accuracy.

Neglected turbidity correction in light scattering experiments—a source of erroneous values of the second virial coefficient

Abstract Neglecting turbidity leads to an overestimation of the second virial coefficient in light scattering experiments. Although this effect is totally negligible for polymers in good solvents, it is dominant for systems consisting of large compact particles and for polymers at theta conditions. The turbidity correction is proportional to the characteristic constant of light scatterning K and thus inversely proportional to λ4. The true virial coefficient can be obtained by extrapolation of Kc/R to vanishing value of K. Light scattering of a well characterized sample of polystyrene in cyclohexane at three different wavelengths and several temperatures has been used to demonstrate the correction for turbidity. The true second virial coefficient for polystyrene in cyclohexane at various temperatures was obtained. Theta temperature and the entropic factor ψ were found to be 35.4°C and 0.21, respectively. Alternative procedures for evaluating the correction using light scattering data measured at a single wavelength were also offered.