Polymer-water mixtures, dielectric

Its higher dielectric constant raises the overall value of the polymer-water mixture. 

The first mention of the a(x) dependence was in experimental work [265], The dielectric relaxation data of water in mixtures of seven water-soluble polymers was presented there. It was found that in all these solutions, relaxation of water obeys the CC law, while the bulk water exhibits the well-known Debye-like pattern [270,271], Another observation was that a is dependent not only on the concentration of solute but also on the hydrophilic (or hydrophobic) properties of the polymer. The seven polymers were poly(vinylpyrrolidone) (PVP weight average molecular weight (MW) = 10,000), poly (ethylene glycol) (PEG MW = 8000), poly(ethylene imine) (PEI MW = 500,000), poly(acrylic acid) (PAA MW = 5000), poly(vinyl methyl ether) (PVME MW = 90,000), poly(allylamine) (PA1A MW = 10,000), and poly(vinyl alcohol) (PVA MW = 77,000). These polymers were mixed with different ratios (up to 50% of polymer in solution) to water and measured at a constant room temperature (25°C) [265]. 

Using these Va values (va 3 x 10 molcm" ) together with other molecular parameters, i.e. molar mass of monomeric unit M = 71 gmol , density of the dry polymer p = 1.35 gcm , T = 298 K, (p = 0.037, dielectric constant D = 80, molar volumes Vi of the mixtures acetone-water (taken from [6,423), and the degrees of ionizations i = Xmns> the dependences of the interaction parameter x 9i were calculated from experimental (pz values by means of Eq. 

In the previous section, drug stability was shown to depend on the physical state of water in excipients. Detailed information on the physical state of water can be obtained by measuring the dynamics or the mobility of water molecules. The effect of water mobility on drug stability has been studied by determining water mobility in mixtures of water and polymers used as pharmaceutical excipients. Methods used include the measurement of spin-lattice relaxation time and spin-spinrelaxation time by nuclear magnetic resonance (NMR) spectroscopy as well as of dielectric relaxation time by dielectric relaxation spectroscopy. 

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