Macroscopic gelation

The process of macroscopic gelation that is sensed as a change in the rheological properties (a rapid increase in the steady-state shear viscosity) then involves the aggregation and crosslinking between these preformed gel particles to form an infinite network. 

Macroscopic gelation is characterised by the strong increase in viscosity, the beginning of elasticity and the formation of an insoluble gel fiuction. 

This system has been investigated in detail, and the tunability of this type of two-component gelation system has been exemplified, as explained below. The acid-base interaction between the two components plays a primary role in the formation of the gel—if the acid is protected as an ester, no gel formation takes place. It is proposed that this interaction (Fig. 20) generates the gelator complex, which is the species that hierarchically self-assembles to form fibrous gel-phase aggregates. Interestingly, it was reported that macroscopic gelation still occurred when the acid-base interaction was swapped for an interaction between a crown ether and a protonated amine (Fig. 21) [71]. This indicates that different supramolecular interactions between the components can easily be employed. 

In a key study, Smith, Hirst and co-workers reported the effect of varying the ratio of the two components [77]. Remarkably, increasing the amoimt of diamine relative to the dendritic branch changed the propensity of this system to induce macroscopic gelation, and ultimately gave rise to a completely new morphology in which micrometer-sized platelets were observed... 

Gelation is defined as the point during polymerization when the polymer transforms from a hquid to a rubbery state [97]. At the molecular level, this correlates to the moment at which the molecular weight approaches infinity upon incipient formation of a cross-linked network. Macroscopically, gelation is defined as an abrupt increase in viscosity after which the polymer loses its ability to flow and develops viscoelastic properties. The macroscopic definition of gelation does not necessarily correlate to gelation at the molecular level, since hnear polymers... 

Staudinger polymerized divinylbenzene under ultradilute conditions and predicted the formation of microparticle polymers [2]. The obtained solution had extremely low viscosity. Bobalek et al. [8] and Solomon and Hopwood [9] stopped the reaction immediately preceding the macroscopic gelation to synthesize microgels. When they synthesized an alkyd resin at... 

Fig. 7 Interparticle coupling in SI-ATRP on nanoparticles for bulk and miniemulsion systems. Tbe compartmentalization of particles in a miniemulsion prevents macroscopic gelation from occurring. Reprinted with permission from Bombalski et al. [87]...

In particle systems, as a result of the large surface-to-volume ratio, a sufficient number of surface initiators might be present to generate the required concentration of deactivator in the solution. However, fi"ee initiator and/or excess deactivator are still often added to this system. Free initiator is often added for SI-ATRP of particles for multiple reasons. One reason is to provide better control over the polymerization, another is to prevent macroscopic gelation resulting from interpaiticle coupling. 

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