Apparent solubilities polymers

Sorption curves obtained at activity and temperature conditions which have been experienced to be not able to alter the polymer morphology during the test, i.e. a = 0.60 and T = 75 °C, for as cast (A) and for samples previously equilibrated in more severe conditions, a = 0.99 and T = 75 °C (B), are shown in Fig. 13. According to the previous discussion, the diffusion coefficient, calculated by using the time at the intersection points between the initial linear behaviour and the equilibrium asymptote (a and b), for the damaged sample is lower than that of the undamaged one, since b > a. The morphological modification which increases the apparent solubility lowers, in fact, the effective diffusion coefficient. 

The extent of hydration or solvation of a molecule also has a profound effect on the transport of the substance. The apparent solubility of the drug in both aqueous and nonaqueous media may be influenced by the absence or presence of moisture. Diffusion of drugs in polymeric systems may also be influenced by the hydration of the polymers and hydration of the membrane through which transport is occurring for example, skin hydration may enhance the diffusion of drug molecules significantly. 

Another method by which carbazole has been incorporated into polymers is by acid-catalyzed condensation with formaldehyde. It was shown (79MI11103) that linear, soluble polymers (37) could be prepared when the substituent on the 9-position was propyl or larger. With methyl or ethyl substitution there is apparently little steric inhibition to condensation at the 1-position as well, and crosslinked polymers are the result. 

Vasilenko and coworkers reported the hydrosilylation polymerization of AB2 monomers containing extremely long spacers between the SiH and vinyl groups (19)189. For the monomer with n = 10, polymers of 15,000-30,000 molecular weight were obtained in neat polymerizations. In solution, the monomer apparently cyclized intramolecularly. In the monomers with = 50 or 100, crosslinked polymers were obtained in neat polymerizations, but soluble polymers were obtained in solution polymerizations. 

For example, Soga et al. > found, by examining propylene polymerization in toluene in the temperature range 0-65 °C with an apparently soluble catalyst such as tetrabenzylzirconium, that the isotactic index and the polydispersity of the polymer increased as the polymerization temperature increased. Furthermore, the value of Q > 30 was much greater than that predicted for homogeneous active centres. From these results the authors concluded that the catalyst should be made up of small invisible colloid-type particles. The same interpretation is valid for the surprising results (Q 40) obtainedfor polyethylene, in Isopar solution at 200 °C, with the originally soluble tetrabenzylzirconium-water catalyst. 

The sorption isotherms for most gases in glassy polymers tend to have concave shapes l ke those shown in Figure 4. As a result, the apparent solubility (S = C2/p2) is a decreasing function of the upstream penetrant pressure. On the other hand, the diffusion coefficients, D, of gases in glassy polymers typically increase with sorbed concentration even in the absence of plasticization (17). 

The apparent solubilities and average diffusivities, S and D, for CO2 and CH in a number of glassy polymers at 30"C and 20 atm are shown in Table I. The values reported for cellulose acetate were estimated from various sources in the literature (., 6,37). In the case of cellulose acetate where C02 plasticization is apparently significant, it was assumed that the CH4 permeability in CO2/CH4 mixtures will increase by at least the same percentage as the CO2 permeability. This assumption seems reasonable since the plasticized matrix becomes more rubber-like and less discriminating for penetrants of different sizes and shapes (see Figure 1). 

Figure 5. Correlation of the apparent solubility at 20 atm and 35 °C with the critical temperatures of various penetrants in a number of glassy polymers polycarbonate,...

Typical results of an ultrafiltration experiment also reflect the presence of concentration polarization. This phenomenon, l.e. accumulation of solute in front of the membrane, was described in great detail by others (Refs. 3, 4). A consequence of concentration polarization is a strong dependence of measured rejection coefficients on transmembrane fluxes. An illustration of the effect is presented in Figure 9, which shows the measured "apparent" rejection coefficients (Rg) as a function of transmembrane flux for two water-soluble polymers (Tetronic 707 and Carbowax 4000). It is clear from Figure 9 that if we want to minimize the effects of concentration polarization, we have to conduct experiments at very low values of transmembrane flux. 

Finally, the same Italian group studied monocarboxy-functionalized poly-N-acryloylmorpholine (PAcM) (MW 6000) as a new soluble polymer support for liquid-phase oligonucleotide synthesis [322] and a possible alternative to PEG, although its advantages over the latter were not immediately apparent. Preparation and antisense properties of oligonucleotide-polyacryloylmorpholine conjugates were also described [323]. 

As seen in Table 1, the decrease in permeability can be directly attributed to a dramatic reduction in the effective diffusion coefficient, while there is a much smaller effect on the apparent solubility. A similar dependence of the solubility and diffusion coefficients on the draw ratio has been observed in other uniaxially oriented polymers (35-37). Because the glass transition and density of the polystyrene samples were found independent of the draw ratio, they concluded that the reduction in diffusivity was due to anisotropic redistribution of the free volume during drawing. Using an expansion coefficient related to draw ratio, the polystyrene data were successfully correlated using the Cohen-Turnbull free volume theory. However, the situation was found to be more complex for PVC (i ) ... 

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