Clays platelet encapsulation

In our work, we have tried to obtain exfoliated clay platelets encapsulated by polymer material. The challenges in clay encapsulation are shown schematically in Figure 3.1. 

In the past, many groups have tried to encapsulate clay platelets inside latex particles. This encapsulation poses some extra challenges because of the tendency of the clay platelets to form stacks and card-house structures. Most of the attempts resulted in the so-called armored latex particles, i.e. clay platelets in the surface of the latex. Recently, natural and synthetic clays were successfully encapsulated. The anisotropy of the clay resulted in non-spherical latex particles (Figs. 5 and 6), either peanut-shaped [63] or flat [64]. Clay platelets also turned out to be good stabilizing agents for inverse Pickering emulsion polymerizations [65]. 

Tactoid/agglomerate - polymer chains encapsulate stacks of clay platelets. 

Figure 3.1 Schematic representation of the challenges in encapsulation of clay platelets through emulsion polymerization and related techniques.

In contrast, for an inverse emulsion containing hydrophilic clay particles, there is a possibility that the hydrophilic clay particles may reside inside the monomer micelles (Figure 3.2b), and upon polymerization the clay platelets may, hypothetically, have a chance to be encapsulated effectively inside the polymer particles. 

Figure 3.2 Schematic representation of (a) oil-in-water (O/W) emulsion polymerization m the presence of hydrophilic clay platelets and (b) water-in-oil (W/O) inverse emulsion polymerization of hydrophilic monomer with hydrophilic clay which, hypothetically, may lead to clay encapsulated inside the latex particles.

Many of the papers described emulsion or miniemulsion polymerization in the presence of unmodified or modified clays (often MMT). The modifieation can be surface modification, edge modification or both. lanchis et al. claimed that they obtained both clay platelets (silylated MMT) inside the latex partieles and on the surface of latex particles. Although the elay was not easily visible in the TEM pictures presented, they indirectly inferred the presence of MMT platelets inside the latex particles by looking at the shape of the latex particles. Snowman morphologies were associated with the encapsulated clay. Recently, reversible addition-fragmentation chain transfer (RAFT)-mediated... 

The possible morphologies that can be created during the sonication process are depicted in Figure 10.1. There are two thermodynamically stable cases regarding the clay location, namely the preferential location at the droplet surface (Figure lO.le) and full encapsulation (Figure 10.1a). The first case is desirable when solid particles instead of surfactant molecules are used to stabilize a (mini)emulsion and one speaks of a Pickering (mini)emul-sion. The latter case of encapsulation of clay platelets into the latex particles is preferred when surfactant molecules are used to stabilize the (mini)emulsion. 

Figure 10.1 Schematic illustration of the miniemulsion process and the possible morphologies of polymer-clay droplets/particles during the miniemulsification step and miniemulsion polymerization (a) full encapsulated morphology (b) dumbbell-like or snowman-like morphology (c), (d) dispersed silicate platelets with miniemulsion droplets or polymer adsorbed to its surface (e) clay platelets acting at the surfaces of droplets/particles.

As shown in the simulation of the morphology of hybrid monomer-clay miniemulsion droplets (Figures 10.2 and 10.4), the encapsulation of clay platelets is possible provided that the clay/water interfacial tension is very high (superhydrophobic clay) and that the clay/monomer interfacial tension is low (high compatibility between clay and monomer). Another aspect that the simulations show is that the size of the clay platelets might also play a role in the encapsulation of the clay in the monomer droplets (polymer particles). 

A similar strategy involving Laponite or MMT platelets grafted with polymerizable organotitanate and organosilane molecules was recently reported by Voorn et al. [285, 286]. Here, starved-feed soap-free emulsion polymerization of MMA conducted in the presence of the organoclay led to clay encapsulation. However the solid content was quite low (typically around 7%). 

Figure 13.9 Example of clay encapsulation by starved-feed soap-free emulsion polymerization. The large MMT platelets are encapsulated inside dumbbell-like particles whereas small Laponite platelets are contained inside spherical latex particles. Adapted from reference 52 with permission of the American Chemical Society.

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