Internal crosslinking

Theoretical Effect of Isocyanate Concentration. If very high isocyanate concentrations were used, we would expect that most of the hydroxyl groups could have reacted before internal crosslinking took place. This argument can be illustrated as follows. 

Other considerations relate to the probable form of the polymer molecule. As the crosslinking proceeds, the volume of the coil will decline so that the internal crosslinking might be expected to accelerate. On the other hand, the occurrence of a cage-like structure might well constrain the molecular motion and slow down the later stages of the reaction substantially. 

This result provides further evidence for an internal crosslinking reaction since intercrosslinking of two polymer molecules and grafting to polymer chains generally increase intrinsic viscosity. 

To study the internal crosslinking of styrene-hydroxy ethyl methacrylate copolymers, it was necessary to prepare a range of well characterized materials containing only low concentrations of hydroxyethyl methacrylate. To ensure that the copolymers were of uniform composition, only low monomer conversions were used so that the composition of the monomer mixtures did not change appreciably during the reaction. Low polymerization temperatures were used to obtain high molecular weight copolymers. 

Preparation of a Superabsorbent Hydrogel Using Internal Crosslinkers... 

In the coatings industry, emulsion polymerization, as a synthetic tool for the preparation of acrylic and vinyl polymers, allows the manufticturer access to a wide array of polymeric systems, simply based on the types and ratios of the monomers used [7]. However, coatings derived fi"om these emulsion polymers often are deficient in performance because of lack of an internal crosslinking mechanism. Silanes can remedy this deficiency. 

Crosslinking through esterification is also possible without further additive, i.e. between the carboxyl and hydroxyl groups of the cellulose derivative chains. A striking example is the insoluble pharmaceutical tablet disintegrating agent croscarmellose sodium, which is produced by internal crosslinking mediated via pH and temperature control (see Sect 6). 

Internal crosslinking, by the liberation of Ca + from CaCOr particles, dispersed in the alginate solution, and changing the pH with acetic acid from the co-flowing soya bean oil. 

Matloka, P. P. Sworen, J. C. Zuluaga, F. Wagener, K. B., Chain-End and Chain-Internal Crosslinking in Latent Reactive Silicon Elastomers. Macro-mol. Chem. Phys. 2005, 206, 218-228. 

Pol miers with a very narrow molecular weight distribution can be made by known methods of polymerization, and these will lead to stabilized polymer particles of substantially uniform particle size, followed by internal crosslinking. In this way, polymer particles of very small size, with an average diameter in the range of 0.1-10 nm, can be obtained. 

In contrast, internally crosslinked polymers can be dispersed in a wide variety of solvents, but only scarcely affect the viscosity of the solution at all. This remarkable property makes these compositions potentially useful for imaging compositions, as well as in drug delivery applications (80). 

Of interest is the use of pol5miers that have a critical solution temperature. This is a temperature below which they are soluble in water, and above which they are insoluble in water. Such polymers can be dissolved in water to get a condensed, spheroidal conformation, and then internally crosslinked. 

The internally crosslinked materials can then be used for the delivery and the controlled release of other organic compounds such as drugs. The drug can be dispersed in a suspension of the crosslinked pol mer at a temperature above the critical solution temperature, at which the drug will be absorbed by the polymer in its collapsed-par-ticulate form. When the temperature is reduced below the critical solution temperature, the pol mer particle swells and slowly releases the drug. Pol mers that have critical solution temperatures are polymers of N-isopropylacrylamide (81). Here, the critical solution temperature can be adjusted using copol mers (80). Thus, the critical solution temperature can be lowered by copol meriza-tion with hydrophobic monomers such as acrylonitrile and raised by hydrophilic monomers such as acrylamide. The preparation of an internally crosslinked polymer is described below. 

Internal crosslinking can be carried out by other means besides y radiation. In some cases, irradiation of the aqueous dispersion with high intensity UV laser light will also cause internal crossUnking. 

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