| Summary: | 博士 === 國立中央大學 === 化學工程與材料工程研究所 === 92 === Mixtures of water-soluble polymers and surfactants in aqueous solutions are common in industrial applications and biological systems. Moreover, many end-products such as shampoos, detergents, and paints contain polymer/surfactant mixtures. An important issue in most applications is the fine-tuning of the solution viscosity by a suitable polymer/surfactant combination. In the present study, we focus on the linear homopolymer poly (ethylene glycol), which is the most commonly used substances in pharmaceutical and other industrial formulations, due to its high water solubility.
The polymer-surfactant interaction leads to the formation of polymer-surfactant complex. The well-accepted morphology of the complex is the necklace model. In this scenario, a "necklace" is formed by the micelles (beads) and the uncharged, water-soluble polymer (string). It is evident that in this model the micelle size must be small compared to the characteristic size of the polymer, which corresponds to high molecular weight. Despite many studies on interactions between neutral polymer and anionic surfactant have supported the necklace scenario, the understanding of the nature of the neutral polymer-surfactant interaction is still incomplete. For example, the radius of gyration of a polymer with molecular weight of O (10³) is less than about 5 nm. One may ask how the necklace model be modified when the "string" is comparable to or smaller than the "bead."
To explore the interaction of low molecular weight polymer with surfactant, we have to know the polymer size. Since the conductometry is used to study the polymer solution, we also have to understand the hindrance to ion mobility due to polymer. Hence, this thesis divided into three topics. In the first topic, we determine the second virial coefficients Bij (nm³) of poly(ethylene glycol) with molecular weight M=600-104 in water by freezing point depression. B12 represents the virial cross coefficient for two PEG solutes with different molecular weights M1 and M2. B11 can be well described by the scaling law M3ν with ν≃0.60. That is, the good solvent behavior is observed even for such low molecular weight. In terms of the hard-sphere model, the effective diameter of PEG ranges from 1.3 to 7.9 nm. Since the second virial coefficient is generally increased with decreasing temperature, our results at freezing point provide an upper bound. We also observe the effective hard-sphere picture is reasonable for dilute solutions of different polymer molecules in good solvents.
In the second topic, we investigate the ion migration in polymer solutions of different molecular weights by conductometry for various inorganic salts. The electric conductivity�n�� declines with increasing the number concentration of polymer cp at a given salt concentration cs. All reduced conductivities for salts of the same valency type collapse into a single curve for a given polymer molecular weight and can be well represented by the simple exponential ��=�菥(cs)exp(-[�菏cp). Here�n�菥 is the conductivity of the salt solution in the absence of polymers and [�菏 can be regarded as an intrinsic conductivity. Our result indicates that the reduction of the ion mobility is mainly attributed to hydrodynamic interactions between the probe ion and polymer segments and the specific ion effect plays a minor role, The intrinsic conductivity is found to be independent of the salt concentration but to vary with polymer molecular weight Mw, [k] �f Mw. This consequence reveals that the ion interacts with all segments of a polymer as it migrates through the coil or the network and hence the conductivity reduction depends mainly on the polymer weight concentration.
On the basis of the above results, we study the neutral polymer-micelle interaction for various surfactants by viscometry and electrical conductometry. In order to exclude the well-known necklace scenario, we consider aqueous solutions of low molecular weight poly(ethylene glycol) (3×103 and 2×104), whose radial size is comparable to or smaller than micelles. The single-tail surfactants consist of anionic, cationic, and nonionic head-groups. It is found that the viscosity of the polymer solution may be increased several times by micelles if the attraction between a polymer segment and a surfactant is weak, ε<kBT. Similarly, ion migration in polymer solutions may be significantly hindered by cooperative interactions between polymers and micelles. Even though ε is small, the interaction energy between a macromolecule and a micelle is sufficiently larger than kBT due to many contacts and thus leads to polymer adsorption on micelles' surfaces. The rapid growth of the viscosity with surfactant concentration is therefore attributed to the substantial cross-links among micelles and polymers (transient network). In addition to substantial alteration of the transport properties, this weak interaction also influences the onset of thermodynamic instability associated with polymer-surfactant solutions. The examples include the decrease of critical aggregation concentration for ionic surfactant and clouding point for nonionic surfactant due to PEG addition.
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