Dissolution of polymers

As we have seen previously not all polymers are capable of being dissolved. In principle the capacity to dissolve is restricted to linear polymers only crosslinked polymers, while they may swell in appropriate solvents, are not soluble in the fullest sense of the word. While individual segments of such polymers may become solvated the crosslinks prevent solvent molecules from establishing adequate interactions with the whole polymer, thus preventing the molecules being carried off into solution. 

Dissolution of polymers is a very slow process it can take days or even weeks for particularly high relative molar mass substances. Two stages are discernible during the process of dissolution. Firstly, a swollen gel is produced by solvent molecules gradually diffusing into the polymer. Secondly, this gel gradually disintegrates as yet more solvent enters the 

Crosslinking is not the only feature that may influence solubility. Such features as crystallinity, hydrogen bonding, or the absence of chain branching may all increase the resistance of a given specimen of polymer to dissolve. Some of these features are discussed later in the chapter. 

AB cements are not only formulated from relatively small ions with well defined hydration numbers. They may also be prepared from macromolecules which dissolve in water to give multiply charged species known as polyelectrolytes. Cements which fall into this category are the zinc polycarboxylates and the glass-ionomers, the polyelectrolytes being poly(acrylic acid) or acrylic add copolymers. The interaction of such polymers is a complicated topic, and one which is of wide importance to a number of scientific disciplines. Molyneux (1975) has highlighted the fact that these substances form the focal point of three complex and contentious territories of sdence , namely aqueous systems, ionic systems and polymeric systems. 

The conformations adopted by polyelectrolytes under different conditions in aqueous solution have been the subject of much study. It is known, for example, that at low charge densities or at high ionic strengths polyelectrolytes have more or less randomly coiled conformations. As neutralization proceeds, with concomitant increase in charge density, so the polyelectrolyte chain uncoils due to electrostatic repulsion. Eventually at full neutralization such molecules have conformations that are essentially rod-like (Kitano et al., 1980). This rod-like conformation for poly(acrylic acid) neutralized with sodium hydroxide in aqueous solution is not due to an increase in stiffness of the polymer, but to an increase in the so-called excluded volume, i.e. that region around an individual polymer molecule that cannot be entered by another molecule. The excluded volume itself increases due to an increase in electrostatic charge density (Kitano et al., 1980). 

In a study of the transition in conformation from random coil to stiff rod by poly(acrylic acid), it was found that the point of transition depended on a number of factors, including the nature of the solvent, the temperature, the particular counterion used and the degree of dissociation (Klooster, van der Trouw Mandel, 1984). 

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In order to perform extraction of additives or dissolution of polymers, solvents that absorb microwave energy are necessary. This is more important than direct absorption of microwave energy by the polymer or additives. When microwave extraction of additives... 

A British Standard method [635] describes dissolution of polymers in refluxing toluene, with reprecipitation of the polymer by addition of ethanol. 

RPLC-PDA is frequently used for quality control, such as the determination of free Irganox 1098 in PA4.6 (at 278 nm after dissolution/precipitation), of free Irganox 1010/1076 in PP (at 278 nm after extraction with MTBE, thus avoiding dissolution of polymer waxes), of Luperco 802 in PP (at 218 nm, after extraction with HCC13), and of Tinuvin 122 in HDPE (at 225 nm as diol). The advantages of the use of HSLC over conventional LC in QC of plastics and additives have been demonstrated, e.g. for AOs in PE, mixed phthalate esters and residual terephthalic acid in PET and partially cured epoxy resins [557],... 

It was found from the absorption spectrum that 1.1 % of the incident photons were absorbed at 2537 A by a PMMA film of 0.5 ym thickness (Fig. 5). The molecular weight distribution and the average molecular weight of the coated polymer which was irradiated for the least irradiation time required for the dissolution of polymer coating in the developer were measured by gel-permeation chromatography (Fig. 7). 

The least irradiation required for the dissolution of polymer coating in the developer. 

The dissolution of polymers, regardless of whether they are cellulose based, methacrylates, or other, depends on a variety of factors that may also influence the release of the drug. These are discussed in detail following. Some of these factors are important in vivo, whereas others play a role in vitro. 

Dissolution of polymers intended for enteric targeting depends on the pH of the dissolution medium [33, 34], This is mainly influenced by the composition of the polymer, the monomers, or the type and degree of substitution. pH dissolution profiles can also be modified by the addition of other polymers, as demonstrated for EUDRAGIT L 100 and EUDRAGIT S 100 [35] (Figure 6). Such mixtures provide a variety of different pH dissolution profiles, which allows for specific targeting anywhere between the pylorus and the colon. This is also illustrated in Figure 2. 

It could be shown [5] that the dissolution of polymers depends on the type of ions present in the dissolution medium. Dissolution is base catalyzed and can be described by the Bronstedt dissolution law [38], At a given pH, a linear relationship exists between the logarithm of the dissolution rate and the pKa of the acidic component of the salt present in the dissolution medium. Cellulose acetate phosphate, especially, showed a strong dependency of the dissolution rate on the type of ions added. Sodium chloride prevented the dissolution of some polymers, because the base catalysis was at a minimum level. 

Dissolution of polymers is controlled by processes of diffusion and convection. The rate of diffusion may be estimated from the intrinsic properties of the polymer and the Reynolds number of the dissolving liquid. 

The last category of mass transfer problems is the dissolution of polymers by liquids. The study of this category is still in its infancy. 

Dissociation energy of the weakest bond, 764 Dissolution of polymers, 696 Distortional waves, 506 Distribution of molecular mass, 17 Ditonic system, 66 Doolittle equation, 621 n-doping, 338 p-doping, 338... 

Potential interactions of a supercritical fluid with a polymeric material may include the following (i) sorption of carbon dioxide by polymers (ii) swelling of polymers by carbon dioxide (Hi) dissolution of polymers in carbon dioxide (iv) dissolution of carbon dioxide in polymers (v) plasticization and decrease of the glass transition... 

The dissolution of polymers into a solvent produces little heat effect for polymers and solvent of similar chemical structure. However, the solvents in these solutions are much less volatile than in an ideal solution with activity coefficients much lower than 1. 

The solutions are also used to measure of the thermodynamic interactions between the polymer segments and solvent molecules. The latter is best discussed in terms of the virial coefficients. A.. The change of solvent chemical potential upon dissolution of polymer is given by ... 

Another drawback of the H-F theory was the initial assumption that all lattice cells are occupied by either solvent molecules or polymeric segments that are of equal size. As a consequence the free volume contribution was neglected. Maron [1959] pointed out that dissolution of polymer is associated with volume changes — his modification of... 

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