Characterization of Polymers in Dilute Solutions
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Basic moleculer characterisitics of polymers are: molecular architecture, molecular weight and size, and their distributions. All these characterisitics are derived from measurements of very dilute solutions. Methods of characterization can be roughly divided into two categories: (i) Direct (also called absolute) methods, in which only measured values and physical constants are needed for the evaluation of a given molecular characterisitics (e.g. static and dynamic or osmometry), and (ii) indirect methods, in which either a calibration with polymer samples of known value of the given property must be employed or a polymer structure and shape of the polymer molecules under study must be known (e.g. viscometry, sedimentation, and gel permeation chromatography). Block copolymers, when dissolved in selective solvents (both organic and aqueous) that are thermodynamically good for one block and poor for the other, and undergo a self- association in which spherical micelles of a nearly uniform size are formed with insoluble blocks in the core and soluble blocks in the shell. A micelle typically consists of ten to a few hundreds of copolymer molecules. Copolymer micelles have been studied for about three decdes and are interesting not only from the viewpoint of academic research, but also for their potential applications as motor oil additives, emulsifiers, dispersing and foaming agents, thickeners, solubilizers and, mainly, as vehicles for a targetted drug delivery Block, copolymers micelles just like micelles of soaps and surfactants, are in a dynamic equilibrium with unassociated molecules, called unimers. Various methods are being used for the characterization of the micellar solutions. Combined experimental data of static light scattering, dynamic light scattering and small- angle X-ray (or neutron) scattering provide basic characteristics of micelles: micellar molar mass, micelle hydrodynamic radius, geometric radius of the core and the thickness of the shell. Sedimentation velocity gives us the unimer/micelle mass ratio and, like steady - state fluorescence, also an information about the dynamics of the unimer - micelle equilibrium. Kinetics of micelle formation and micelle dissociation induced by abrupt changes in temperature or in a solvent selectivity, can be determined by stopped - flow instrumentation with static light scattering detection. Very unique information on segmental dynamics in the core and in the shell can be drawn from either NMR or time - resolved fluorescence.
- Qatar University Science Journal - [From 1981 TO 2007] [768 items ]