Solvent diffusion polymer effect

The solubility parameter concept has been used to correlate many physical phenomena. Miscibility of solvents with polymers, diffusion of solvents within polymers, effects of intermolecular forces on the glass transition temperature and interfacial interactions within copolymer materials would be included, just to mention a few examples. In many cases, meaningful interpretation of results was facilitated with the use of the solubility parameter. 

Figure 48. Evolution of the apparent diffusion coefficient (V as a function of solution ionic conductivity (x ) (Reprinted from H.-J. Grande, T. F. Otero, and 1. Cantero, Conformational relaxation in conducting polymers Effect of the polymer-solvent interactions. J. Non-Cryst. Sol. 235-237, 619, 1998. Figs. 1-3, Copyright 1998. Reproduced with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)...

Intraparticle diffusion limits rates in triphase catalysis whenever the reaction is fast enough to prevent attaiment of an equilibrium distribution of reactant throughout the gel catalyst. Numerous experimental parameters affect intraparticle diffusion. If mass transfer is not rate-limiting, particle size effects on observed rates can be attributed entirely to intraparticle diffusion. Polymer % cross-linking (% CL), % ring substitution (% RS), swelling solvent, and the size of reactant molecule all can affect both intrinsic reactivity and intraparticle diffusion. Typical particle size effects on the... 

The activity of polymer-supported crown ethers depends on solvent. As shown in Fig. 11, rates for Br-I exchange reactions with catalysts 34 and 41 increased with a change in solvent from toluene to chlorobenzene. Since the reaction with catalyst 34 is limited substantially by intrinsic reactivity (Fig. 10), the rate increase must be due to an increase in intrinsic reactivity. The reaction with catalyst 41 is limited by both intrinsic reactivity and intraparticle diffusion (Fig. 10), and the rate increase from toluene to chlorobenzene corresponds with increases in both parameters. Solvent effects on rates with polymer-supported phase transfer catalysts differ from those with soluble phase transfer catalysts60. With the soluble catalysts rates increase (for a limited number of reactions) with decreased polarity of solvent60), while with the polymeric catalysts rates increase with increased polarity of solvent74). Solvents swell polymer-supported catalysts and influence the microenvironment of active sites as well as intraparticle diffusion. The microenvironment, especially hydration... 

In polyisobutylene in the melt and in solution (CC14, CS2), McCall, Douglass, and Anderson 17) found that the activation energies for polymer diffusion increased with polymer concentration from the value at infinite dilution (approaching the pure solvent value) to the value in the melt. Solvent diffusion, and solvent effect on polymer diffusion, were also measured. The Stokes-Einstein model applied to this data yielded molecular dimensions too small by a factor of two or three. 

In his PGSE study of polyethylene oxide and polydimethylsiloxane in QHg and CHC13, Tanner 39) also measured the diffusion of a fixed fraction of solvent in the polymers. He concluded that their diffusion rate in polymers of molecular weight lower than their own was approximately equal to that of the polymers. As the polymer molecular weight exceeded that of the solvent, the solvent diffusion rate approached a constant value, independent of polymer molecular weight. Tanner offered semiempirical explanations for this effect. 

The characteristics of pore structure in polymers is a key parameter in the study of diffusion in polymers. Pore sizes ranging from 0.1 to 1.0 pm (macroporous) are much larger than the pore sizes of diffusing solute molecules, and thus the diffusant molecules do not face a significant hurdle to diffuse through polymers comprising the solvent-filled pores. Thus, a minor modification of the values determined by the hydrodynamic theory or its empirical equations can be made to take into account the fraction of void volume in polymers (i.e., porosity, e), the crookedness of pores (i.e., tortuosity, x), and the affinity of solutes to polymers (i.e., partition coefficient, K). The effective diffusion coefficient, De, in the solvent-filled polymer pores is expressed by ... 

The separation of polymers due to thermal diffusion may be quite large. For example, the thermal diffusion ratio for dilute solutions of polystyrene in tetrahydrofuran is around 0.6 K1. This indicates that the change of polystyrene concentration per degree is 60%. The type of solvent and polymer pair may have a considerable effect on both the thermal diffusion ratio and the thermal diffusion coefficient. 

Vrentas JS and Duda JL. Solvent and temperature effects on diffusion in pol3mier solvent systems. J. Appl. Polym. Sci. 1977 21 1715-1728. 

Of course, membrane thickness can be calculated supposing that its volume is given by the arithmetic addition of solvent and polymer volume. Assuming that the most important effect of matrix swelling/shrinking is to modify the solute diffusion coefficient Dg (and the Peppas-Reinhart equation [Equation 15.14] can be considered to account for this), permeation can be described by the continuity equation (Equation 15.1) where the generative term Rj is set to zero and the following initial and boundary conditions hold ... 

SC CO2 used as a carrier of drug molecules into a polymer matrix has a number of advantages such as the plasticizing ability of CO2 (based on specific interactions between CO2 and polymer moieties), which both enhances the diffusion rates of drug molecules into the polymer and facilitates solvent removal. Polymer plasticization is accompanied by the swelling of the polymer matrix, with a concomitant increase in the free volume of the polymer. Moreover, SC CO2 can reduce the melting temperature of semicrystalline polymers. These effects are crucial to the impregnation and modification of polymeric materials. 

The early research of Myers et al. [1,2] shows that polymer thermal field-flow fractionation (ThFFF) retention and thermal diffusion are solvent dependent. Recently, Sisson and Giddings [3] indicated that polymer ThFFF retention could be increased by mixing solvents. Rue and Schimpf [4] extended the molecular-weight range that can be retained by ThFFF to much lower molecular weights (<10 kDa) by using solvent mixtures without using extreme experimental conditions. There are several other reports on the effect of solvents on polymer retention, selectivity, and the universal calibration in FFF in last few years [5]. 

Under these conditions we have a reverse coattail effect. It can be understood as follows A small Kq implies a poor solvent-diffusant interaction (large positive xf) the diffusant prefers to remain in the polymer phase in the absence of solvent absorption. When solvent is absorbed, the diffusant now "sees" a thermodynamic environment in the polymer phase that is similar to the one in the solvent phase, neither of which it likes. 

The polarity of macromolecules and the solvent determine the effect of organic solvents on polymer materials. Polymers containing polar groups are resistant to non-polar substances but can swell and be dissolved in polar substances. The polymer permolecular structure exerts a considerable effect on its stability under aging as well. Crystalline polymers dissolve slower than non-crystalline ones, which is due to different diffusion velocities of low-molecular-weight components in the crystalline and amorphous polymers. 

Response to Catechols in the Presence of Ascorbic Acid. In addition to the enhanced response for most catechol compounds, the voltammetric signals due to species in solution that are not complexed by the polymer are often diminished. Because the solvent-swollen polymer occupies space near the electrode surface, it effectively decreases the concentration of uncomplexed solution species. Furthermore, the polymer hinders diffusion of all species to the electrode surface. In the case of catechols, the increase in concentration in the film offsets this effect, but for species that do not bind with the polymer (e.g. ascorbic acid), the rate of mass transport (and subsequently the oxidation current monitored) is attenuated. This effect can be very useful when determining catechol in biological samples. 

Molecular motion in polymer solutions can have significant effects on the physical properties of the systems formed from these solutions. For example, the rates of drying polymer films can determine the film properties. We have shown that the drying of a polystyrene film from toluene solutions could be predicted with the knowledge of thermodynamic parameters, plus solvent diffusion data.(i) The ability of polymers to respond to changes in conditions is determined by die ability of the polymer and/or its segments to reorient. Solvent diffusion is also correlated to the segmental motions of the polymer chains.(2) The reason for this correlation appears to be that both molecules are coupled to the same fractional free volume. 

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