Photoemulsion polymerization

The emulsion polymerization in the third step is carried out in the presence of a water soluble monomer, such as acrylic acid. The radicals formed by the photolysis of HMEM (Fig. 6.18b) on the surface start radical chain polymerization by a grafting-from technique (see Section 6.13.3) thus generating chains of poly(acrylic acid). The polymer chains remain bound to the surface by an ester bond which can be cleaved by hydrolysis to obtain the polymer for analysis. Thus the molecular weight of the bound polymer chains can be determined which gives their contour length Lc. The thiclcness L of the brush (Fig. 6.17) attached to the surface of the particles can be deduced from the hydrodynamic radius as measured by dynamic light scattering. 

It should be noted that since two radicals are generated for each decomposed initiator molecule (Fig. 6.18b), only half of the generated polymer will be attached to the surface, while the other half will grow in solution and must subsequently be removed from the latex, such as by extensive serum replacement against pure water (Guo et al., 1999 Guo and Ballauff, 2000). 

A combination of photopolymerization and conventional emulsion polymerization has been used recently (Guo et ah, 1999) for the synthesis of well de ned polyelectrolyte brushes (Fig. 6.17). 

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Lu Y, Wittemann A, Drechsler M, Ballauff M (2006) Preparation of poly(styrene)-poly()V-isopropylacrylamide) (PS-PNIPA) core-shell particles by photoemulsion polymerization. Macromol Rapid Commun 27 1137-1141... 

Figure 6.18 Schematic description of the method of preparation of spherical polyelectrolyte brushes by photoemulsion polymerization (a) general scheme (b) photodecomposition of initiator HMEM. (After Guo and Ballauff, 2000.)...

Figure 1.1. (a) Stmeture of the spherical polyelectrolyte brushes having cationic polyelectrolyte chains on their surface. The core consists of poly(styrene) and has diameters of approximately 100 nm. The chains are densely grafted to the surface of these cores by a grafting-from technique ( photoemulsion polymerization, cf. Ref. 24). (b) The core-shell microgel particles shown in a schematic fashion The core consists of poly(styrene) (PS) whereas the network consists of poly(iV-isopropylacrylamide) (PNIPA) crosslinked by JVdV -methylenebisacrylamide (BIS). 

Figure 18 Schematic representation of the preparation of Ag SPEB(PS-PAA) composite particles in situ. PS core particles are first prepared by a conventional emulsion polymerization. In the second step, the PS cores are covered with a thin layer of photoinitiator HMEM. In the third step, the shell of polyelectrolyte brushes is formed by photoemulsion polymerization Shining light on the aqueous suspension of these particles generates radicals at their surface, which initiate the radical polymerization of the functional monomer (silver acrylate) on the surface of the core particles. Concomitantly, the silver anions embedded in the PAA brushes are reduced to Ag-NP under UV irradiation. Reprinted with permission from Lu, Y. Mei, Y. Schrinner, M. etal.

Since chemically grafted SPB show greater stabifity and controllable grafting density, this method of preparation has become more popular than physical adsorption. Chemical methods include photoemulsion polymerization, thermocontrolled emulsion polymerization, atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer (PJVFT). 

ThermocontroUed emulsion polymerization has offered a new way to synthesize SPB. This method has some advantages over photoemulsion polymerization. For example, it is easy to scale up, and the polymerization... 

Figure 4.2 (A) Schematic representation of SPB preparation by photoemulsion polymerization (B) decomposition of HMEM.

Figure 4.4 PAA brushes on PB core by (A) direct thermoinitiated emulsion polymerization, and (B) photoemulsion polymerization (Li et al., 2012).

However, Li et al. (2012) prepared SPB on industrial poly(butadiene) (PB) latex particles, whose abundant residual C=C double bonds allow for grafting brushes through radical polymerization with a common initiator instead of the special initiators. In comparison, photoemulsion polymerization was also employed to prepare SPB on the PB core. The schematic diagram of these two processes is given in Fig. 4.4. 

Guo X, Weiss A, BaUaufFM Synthesis ofsphericalpolyelectrolyte brushes by photoemulsion polymerization. Macromolecules 32 6043-6046, 1999. 

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