Polymers solvation solvents

The unconventional applications of SEC usually produce estimated values of various characteristics, which are valuable for further analyses. These embrace assessment of theta conditions for given polymer (mixed solvent-eluent composition and temperature Section 16.2.2), second virial coefficients A2 [109], coefficients of preferential solvation of macromolecules in mixed solvents (eluents) [40], as well as estimation of pore size distribution within porous bodies (inverse SEC) [136-140] and rates of diffusion of macromolecules within porous bodies. Some semiquantitative information on polymer samples can be obtained from the SEC results indirectly, for example, the assessment of the polymer stereoregularity from the stability of macromolecular aggregates (PVC [140]), of the segment lengths in polymer crystallites after their controlled partial degradation [141], and of the enthalpic interactions between unlike polymers in solution (in eluent) [142], as well as between polymer and column packing [123,143]. 

Many of the considerations discussed above in connection with organic chemistry are, of course, also valid for polymer technology. Solvents for polymers, used in such materials as paints and lacquers, and as media for the polymerization reaction, provide examples of industrial applications of solvating media. 

Flexible polymer chains expand with increasing solvent power of the medium, leading to an increase in [77] with increasing polymer solvation. For chains of a similar kind, varying in length (homologous series), the relationship between [77] and molecular weight, M, may be represented by the Mark-Houwink relationship [19-22],... 

The influence of solvating solvents, in our case THF, has been discussed previously (1). There is some ambiguity about the state of solvation of the initiator in polymerization in nonpolar solvents, in our case toluene. These systems, in which highly isotactic polymers are formed, are sometimes described as "ether-free" or "de-etherated". The basic procedure is to pump... 

In general, to explain the observed cosolvent effects, the preferential adsorption phenomena have been invoked. Flowever few topics in the physical chemistry of polymers have evoked so many theories but so little consensus as preferential adsorption. When a polymer is dissolved in a binary solvent mixture, usually one of the solvents preferentially solvates the polymer. This solvent will then be found in a greater proportion in the proximities of the macromolecule with respect to the bulk solution composition. This variation of the solvent composition can cause interesting phenomena such as cosolvency as was discussed before, [11, 91, 92] non - cosolvency [93, 94], and some times variation of the unperturbed polymer dimensions [95,96]... 

In this chapter we will mostly focus on the application of molecular dynamics simulation technique to understand solvation process in polymers. The organization of this chapter is as follow. In the first few sections the thermodynamics and statistical mechanics of solvation are introduced. In this regards, Flory s theory of polymer solutions has been compared with the classical solution methods for interpretation of experimental data. Very dilute solution of gases in polymers and the methods of calculation of chemical potentials, and hence calculation of Henry s law constants and sorption isotherms of gases in polymers are discussed in Section 11.6.1. The solution of polymers in solvents, solvent effect on equilibrium and dynamics of polymer-size change in solutions, and the solvation structures are described, with the main emphasis on molecular dynamics simulation method to obtain understanding of solvation of nonpolar polymers in nonpolar solvents and that of polar polymers in polar solvents, in Section 11.6.2. Finally, the dynamics of solvation with a short review of the experimental, theoretical, and simulation methods are explained in Section 11.7. 

We now consider the second alternative, the concentrated solutions of polymers in solvents, where the concentration of solvent can be changed over a wide range. Here the polymer molecules will evenly distribute among the solvent molecules and a new set of interactions between solvent and solute molecules sets up, which results in a solvation structure. There are many interaction configurations, called solvation structures. Specification of solvation structures is very important in such disciplines as bioscience [75], pharmacy [76], and lavation [77], The polymer solvation structure has been the subject of studies in recent years. In the concept of polymer solvation, since the overall size of polymer also changes in solution, therefore, the solvation... 

One of the most important phenomena in the polymer solvation is the change in the overall size of the polymer chain upon solvation. In fact at equilibrium the average size of isolated polymer molecules in solution is a function of solvent quality and varies from expanded conformations in good solvents to random walk conformations in poor solvents. This is referred to as collapse transition and was first predicted by Stockmayer [82] more than 45 years ago. The phenomenon was observed by Nishio et al. [83] and Swislow et al. [84] more than 25 years ago and is still a subject of much experimental, computational, and theoretical research today. So far many investigators have tried to study the chain size with solvation using a variety of methods. 

Assuming an incompressible part of the solution, Passynsky derived an equation relating the adiabatic compressibility P, observed for the solution, to the number n of grams of solvating solvent per g of polymer 73) ... 

Fig. 34. Temperature dependence of the unperturbed chain dimension A, estimated by method 2B (Eq. (32)), for a cellulose diacetate (CDA) fraction in acetone (open circles), and of the number of the solvating solvent molecules at infinite dilution si of a CDA whole polymer in acetone (black circles) 7)...

Highly polar polymers like polyfvinyl chloride) and some cellulose derivatives require polar liquids as solvents, in which dipole interactions or hydrogen bonding between polymer and solvent molecules occur. However, solvation does not necessarily lead to solution because the liquid, if it is to act as a solvent, must dissolve the solvated polymer. This process may be very slow because of the high viscosity of the partially solvated system. 

The solvent effect on kp might be caused by the specific interaction between polymer and solvent (selective solvation of polymer), because the concentration of mono-... 

Initiation by Electron Transfer. This mechanism of initiation operates in polymerizations by alkali metals or their complexes and was best elucidated in the case of the sodium naphthalene complex (13). which was used to form the well-known "living" polymers. In these complexes, the naphthalene is a radical anion (35) formed by transfer of an electron from sodium in the presence of a highly solvating solvent such as H -furan ... 

A series of quantitative data for solvent effect on the aminolysis of nitrophenyl esters attached to polyacrylamides have also been reported [41b]. These data are in broad agreement with the above-mentioned observations. However, the apparent solvent effects in chemical transformation of polymers must be interpreted in terms of a dual function, i.e. polymer solvation and solvent catalysis . For example, DMSO is a poor solvent for copol3 AOTq)-styrene), but a good solvent for polymers carrying amide residues. It should also be noted that alcohols and water are not usually suitable as solvent for chemical transformation of activated esters, because they may them lves enter the reaction as nucleophiles. 

A specific feature of the pol3Tuer-solvent system is positive values of the entropic member (TAS) of (1.9) because of the large number of possible conformations of macromolecules in diluted polymer solutions. The enthalpy member value AH) can be below zero if dissolving is reached via strong interactions between the polymer and solvent, e.g. solvation, generation of hydrogen and coordination bonds. Moreover, there is a variant when AH > 0, i.e. the energy of interactions between molecules in pure substances and the polymer exceeds that in the polymer-solvent system. In this case, the polymer solution in the solvent or its combination with PI, liquid Cl or other liquids and polymers will take place only if AH > 0 and AH < TAS. 

Unlike sodium naphthalene, which requires the presence of highly solvating solvents, such as tetrahydrofuran (THF), the organolithium systems can operate in various polar and nonpolar solvents such as ethers or hydrocarbons. However, the rates are much slower in the latter than in the former solvents. Hence, if the initiation reaction (Eq. (2.67)) is very much slower than the propagation reaction, the molecular weight distribution may be considerably broadened (Hsieh and McKinney, 1966). This does not, of course, vitiate the living polymer aspect of the polymerization, which has been shown (Morton et al., 1963) to operate in these systems, regardless of type of solvent, if side reactions do not intervene. 

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