Nanoparticle Encapsulation by a Polymer Via In Situ Polymerization in Supercritical Conditions Babak Esmaeili, Jamal Chaouki, Charles Dubois De ´ partement de Ge ´ nie Chimique, E ´ cole Polytechnique de Montre ´ al, C.P. 6079, succ. Centre-ville, Montre ´ al, Que ´ bec H3C 3A7, Canada The encapsulation of aluminum nanoparticles by poly- vinylidene fluoride (PVDF) was carried out in supercriti- cal conditions via in situ polymerization. The aluminum particles possessed an average diameter of 43.7 nm. The presence of PVDF on the particles was validated by thermogravimetric analysis (TGA). This result was further approved by X-ray photoelectron spectroscopy (XPS), which showed high intensity peaks of fluorine and carbon on the particles after the encapsulation process, which are associated with the presence of hydrocarbon-based PVDF. As observed by transmission electron microscopy (TEM) images, the nanoparticles were uniformly coated by a polymer of a few nanometers in thickness. The results showed that there is a good consistency between the calculated thickness of the polymer coating and the results obtained by TEM. In addition, the effect of polymeriza- tion time on the kinetics of the reaction was investi- gated. Finally, it was found that the thickness of the polymer layer can be controlled by the duration of the encapsulation process. POLYM. ENG. SCI., 52:637–642, 2012. ª 2011 Society of Plastics Engineers INTRODUCTION Nanoparticle encapsulation by a polymer is a conven- tional method of protecting the surface of particles from oxidation and avoiding agglomeration during powder han- dling and processing. Accordingly, particle encapsulation encompasses a vast range of applications in the pharma- ceutical, electronic, food, cosmetic, and biomedical indus- tries. Conventional techniques for particle coating usually involve large amounts of organic solvents [1–5], which may raise serious environmental concerns. Besides, after the coating procedure, further processing such as filtering, drying, grinding, and sieving is required, thus causing a further increase in total cost and the overall length of the process. There has been a continuous challenge of eliminating organic solvents by performing coating processes in gas/ solid reactors [6–9]. By encapsulating nanoparticles in a gas/solid reactor such as a fluidized bed, all drawbacks relating to the presence of organic solvents are eliminated. However, the efficiency of the process in terms of coating particles individually decreases because of their tendency to agglomerate, especially when dealing with very fine particles [9, 10]. By applying some external forces such as vibration or ultrasound effects [11, 12], the nanopar- ticle agglomerates can be broken up into smaller ones, thus improving the quality of fluidization. However, the process becomes more complicated and more expensive. Therefore, particle encapsulation in a clean and effective way is of strong interest. Supercritical fluids have recently been of high interest, given that in comparison with organic solvents, they make the processes cleaner and more environmentally friendly. Among them, supercritical carbon dioxide (scCO 2 ) is known to be an appropriate candidate because of its excellent properties such as low cost, nontoxicity, nonflammability, and availability. More- over, scCO 2 provides both gas-like diffusivity and liquid- like densities, which are two important factors of an appropriate solvent or processing medium. Particularly, due to its relatively mild critical conditions (T c ¼ 31.28C, P c ¼ 7.38 MPa), carbon dioxide is often considered as an ideal processing medium for particle encapsulation by polymerization [13]. A number of research projects concerning the encapsu- lation of fine particles by polymers in scCO 2 have been reported [14–18]. Glebov et al. (2001) encapsulated me- tallic microsized powders by poly(vinylidene fluoride) and poly(4-vinylbiphenyl), using supercritical carbon dioxide as a solvent. The coated particles of the metal powders exhibited enhanced resistance to dissolution in different basic and acidic solutions. Polymeric films de- posited from scCO2 showed significant protective proper- ties even for polymers with very low solubility in scCO2 [14]. Yue et al. (2004) coated fine dechlorane particles, which are aliphatic chlorine-containing crystalline organic compounds, with polymethyl methacrylate and poly(1- vinyl-2-pyrrolidone) using in situ polymerization in super- Correspondence to: Charles Dubois; e-mail: charles.dubois@polymtl.ca Contract grant sponsor: Natural Sciences and Engineering Research Council of Canada (NSERC); Defence R&D Canada (DRDC). DOI 10.1002/pen.22126 Published online in Wiley Online Library (wileyonlinelibrary.com). V V C 2011 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—-2012