Microgel formation, dependence

Storey [32] observed some anomalies in the dependence of the gel point at higher concentrations of DVB which suggested some inhomogeneity and a tendency to microgel formation which explained the shift of the gel point towards higher conversions. 

The free-radical crosslinking polymerization can be regarded as a special example of specific diffusion control, in which the tendency to microgel formation and decrease of apparent reactivity of Internal double bonds depends on the size of the mlcrogel which in turn depends on the molecular weight of the primary chain. Polymerization of diallyl monomers exhibits much less of these features (W) because the degree of polymerization of their primary chains is extremely low due to degradative chain transfer. 

As mentioned above, the preparation of nanogels by addition reactions of functional macromolecular precursors is mainly used for biomedical applications. Thus, the choice of synthetic precursors for microgel formation is restricted to biocompatible materials. Moreover, as most applications are in drug delivery, the molecular weight of the gel precursors should be below the threshold for renal clearance, a value that depends on the molecular architecture and chemical nature of the polymer but that is usually smaller than 30kDa, which is set as the limit for linear PEG [97], Polymers that are mostly used and thus presented in more detail here are PEG, poly(glycidol) (PG), and polyethylene imine) (PEI). 

The data correlate well with turbidity data (Fig. 2.31, curves 2 ) and show that acceleration of the start of gel formation with 4-6 wt% OP-10 occurs because of accumulation of microgel formations under the influence of the surfactant at an earlier stage. Acceleration of the start of gel formation wih DC-10 is insignificant and remains directly dependent on DC-10 concentration, corresponding to the turbidity data that indicate that this surfactant does not influence the compatibility of the system components and speed of gel formation. 

Attempts of the authors [5-7] to linearize the dependence of (1-Q) on t for the system EPS-4/DDM by the Eqs. (87) and (88) of Chapter 1 were not successful. Therefore the following assumption was made [6], The Eq. (87) of Chapter 1 describes low-molecular substances reaction kinetics at large density fluctuations in EucUdean space with dimension d, which is equal to 3 in the considered case. If we assume that the fractal clusters (microgels) formation with dimension D defines reaction curing course in a fractal space with dimension D, the dimension d in the Eq. (87) of Chapter 1 should be replaced by D. The dependence of In (1-Q) on corresponding to the Eq. (87) of Chapter 1 with the indicated re-... 

The known Q and D, values allow to estimate parameter K,c t as t function according to the Eq. (61) of Chapter 2. Since the values Kj and c in paper [57] experiment conditions are constant, then the indicated parameter can be considered as reactive medium current viscosity i), expressed in relative units. In Fig. 32 the dependences ri(t) for the system EPS-4/DDM at T =383 and 393 K are shown. As one can see, up to microgels formation point (t = 1200 s) very weak q growth is observed and then sharp t increasing (on about two orders) occurs. Such q increasing can be explained theoretically within the frameworks of the model, proposed in paper [25], which uses percolation theory representations, q value is given as follows [25] ... 

Fig. 49. Calculated dependence of the polymer structure on the initial 1,4-DVB and n-BuLi concentrations in the anionic 1,4-DVB polymerization. The numbers I to IV represent the region for the formation of linear, branched, microgel and macrogel structures, respectively. The solid and dashed curves represent the transition regions between these structures. [Reproduced from Ref. 239 with permission, Hiithig Wepf Publ., Zug, Switzerland]. 

Ester formation by dimethylsulfate or diazomethane is not satisfactory because the microgels become insoluble when the reaction proceeds to higher conversions. With diazomethane part of the unsaturated groups is involved in a side reaction of a 1,3-dipolar cycloaddition [132]. A more efficient method for ester formation of microgels is the reaction with 0-alkyl-N,N -bisisopropyl isoureas of the alcohols. The alkyl ureas are easily separated from solutions in methanol [294-296]. The esterified microgels were isolated by precipitation and freeze-drying. Depending on the alcohol used for ester formation, the yields of... 

The formation and reactivity of microgels of a dense structure has been investigated by Funke and Seitz (46-53). By use of pure p-divinylbenzene and techn. DVB (55%), respectively, the polymers have a vinyl content corresponding to a 40 to 90% consumption of the second vinyl group of DVB. Depending on the reactants and the reaction conditions subsequent reactions may be localized at the surface of the particle. 

The process of polymer degradation is always accompanied by a parallel competitive process of crosslinking which results in the formation of a microgel. The crosslinking depends on numerous factors such as the structure of the macro-radicals, the cage effect, the presence of various free radicals and oxygen, and the concentration of the polymers in solution. 

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