Microgels and Macrogels

Homogeneous Heterogeneous Low crosslink crosslinking crossinking density 

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In Fig. 48, the regions of the formation of linear or branched polymers, microgels and macrogels are shown as a function of the concentration of 1,4-DVB and of n-BuLi. Reactive microgels can be obtained at a monomer concentration below 50 g/1 and between 3 and 16 mol % of n-BuLi. The polymer structure approaches that of a macrogel when the concentration of 1,4-DVB or n-BuLi is increased. 

Fig. 48. Dependence of the polymer structure on the initial concentrations of n-BuLi and 1,4-DVB in the anionic 1,4-DVB polymerization in THF. The symbols represent linear ( ) branched (T) microgel ( ) and macrogel (M) structures. [Reproduced from Ref. 231 with permission, Hiithig Wepf Publ., Zug, Switzerland]. 

Anionic polymerization of 1,4-DVB by n-BuLi leading to the microgels was also reported by Eschwey et al. [236,237]. In their experiments, n-BuLi was used at very high concentrations of 17 and 200 mol % of the monomer. Contrary to the results of Hiller and Funke [231], they observed a transition from microgel to macrogel with decreasing n-BuLi concentration. Similar results were also reported by Lutz and Rempp [238]. They used potassium naphthalene as the initiator of the 1,4-DVB polymerization and THF as the solvent. Soluble polymers could only be obtained above 33 mol % initiator, whereas below this value macrogels were obtained as by-products. 

Thus, in order to interpret the above correlations between mechanical properties and cocrosslinkers, the function of glycol bis(allyl phthalate)s as cocrosslinkers on the polymerization process of DAP beyond the gel-point conversion should be considered in connection with the microgel and, moreover, macrogel formation. Here it should be recalled that PEGBAP showed a rather drastic effect ... 

Three distinct regions of microgelation, transition and macrogelation. 

Microgels have an advantage over linear polymers and macrogels, which is the speed at which the conformational change occurs. The time taken for the equilibration of microgels is reported to be approximately 1 s [67, 68] which is much faster than the time taken for polymers and macrogels in solution, which can be in the order of hours or days. The... 

With the ENB polymer the data points fall on one curve, although some deviation of points similar to that of the EP polymer occurs. It can be concluded that the straintime correspondence principle applies approximately to this sample. From the preceding argument, the gel particles in the ENB polymer must be microgels. On the other hand the EP polymer appears to contain long branching and macrogel. 

Figures 7.15 and 7.16 show the effect of the structure of carbon black on the strain amplification. The highest structure, N358, gave the highest amplification but the order is reversed between N330 and N326. This was shown with both samples ENB and EP. The reason for the reversal is not known at this time. Overall, sample ENB gave more pronounced amplification than EP did. This may come from the differences in gel-type and gel-content ENB containing 70% microgel and EP only 10% macrogel.

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