Thermally Sensitive Core Shell Particles

Core-shell particles (bearing thermally sensitive polymer in the shell) were first reported by combining emulsion and precipitation polymerization processes [14,15]. The amount of NIPAM over styrene was reported to be the key parameter controlling the colloidal particle size and the layer thickness of the hydro-

Microgel

Core Shell

Composite

FIG. 3 Morphology illustration of thermally sensitive colloidal particles.

Microgel

Core Shell

Composite

FIG. 3 Morphology illustration of thermally sensitive colloidal particles.

philic shell. The styrene copolymerization with NIPAM monomer enhances both the conversion and the polymerization rate. This has been attributed to two things: the solubility of water of the NIPAM and the high propagation speed constant (kP) of NIPAM in comparison with that of styrene. The hydrodynamic particle size decreases as the NIPAM monomer increases in the polymerization recipe. The polymerization mechanism of such a system remains questionable even if various attempts have been made. The functionalization of such core-shell latexes was performed by investigating various processes such as batch polymerization and shot-grow process as recently reported by Duracher et al. [16]. In this domain, cationic amino-containing NIPAM-styrene copolymer latex particles were reported using aminoethyl methacrylate hydrochloride and the resulting particles were clearly characterized [17-19]. Two major results were interestingly pointed out from this functionalized core-shell latex: the increase of functional charged monomer in the polymerization recipe lead to (1) a thinner shell layer and (2) a high amount of water-soluble polymer. The behavioral result was mainly attributed to the radical transfer effect of such amino-contain-ing monomer [16]. It is interesting to note that when the functionalization was performed using a shot-grow process, the morphology of the final particles was found to be dependent on the conversion of the batch process (batch polymerization of styrene and NIPAM) at which the shot was performed. In fact, the surface roughness (raspberry-like structure) was observed when the shot was performed at low batch conversions (30-60%), whereas smooth morphologies were obtained when the shot was investigated at high batch conversions (above 70%). The morphologic behavior has been examined using SEM and atomic force microscopy. The thermal sensitivity character of the resulting colloidal particles was directly related to the amount of NIPAM monomer incorporated in the shell layer and the amount of functional monomer used in the elaboration [16].

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