A New Approach to Studying Microcapsule Wall Growth Mechanisms Jian Li, † Adam P. Hitchcock, † Harald D. H. Sto ¨ver,* ,† and Ian Shirley ‡ BIMR and Deptartment of Chemistry, McMaster UniVersity, Hamilton, ON, Canada L8S 4M1, and Syngenta, Jealott’s Hill International Research Centre, Bracknell, Berkshire RG42 6EY, United Kingdom ReceiVed September 18, 2008; ReVised Manuscript ReceiVed January 16, 2009 ABSTRACT: Polyurea microcapsules were prepared by sequential interfacial reaction of different aromatic and aliphatic isocyanates. The resulting composite capsule walls were studied using scanning transmission X-ray microscopy (STXM) to quantitatively map the distributions of the resulting aromatic and aliphatic polyureas across the capsule wall. Application of this method to both cross-linked and non-cross-linked polyurea capsules resulted in observation of different capsule wall growth mechanisms, depending mainly on the choice of monomers. 1. Introduction Microcapsules are small, hollow devices designed to protect their contents from environmental degradation and to control the rate of release. Most microcapsules are prepared by interfacial polyaddition of oil-soluble polyisocyanates dispersed in water, with aqueous polyamines, though analogous ap- proaches are used to form polyurethane and polyamide capsules. The resulting spherical polymer capsules typically contain an organic active or fill, such as insect pesticides or pheromones, inks, and fragrances, and are mainly used in agriculture, carbonless papers applications, and personal care products. Recent applications involve capsules as catalyst supports 1,2 and in display devices. 3,4 Understanding the mechanisms of capsule wall formation and growth is key to controlling wall strength and permeability and hence to improving capsule performance, especially in terms of variable release 5 and release triggered by acid 6 or base. 7 The two major wall growth mechanisms are the moving boundary mechanism 8 and the stationary boundary mechanism, both illustrated in Figure 1. Pearson and Williams 5 determined that the reaction zone in interfacial encapsulations often moves during the reaction, with the water-soluble monomer diffusing through the initially formed polymer film to reach and react with the oil-soluble monomer, building up successive layers on the interior capsule wall. This model has been used by many researchers to interpret the encapsulation processes for poly- urethane, polyurea, and polyamide microcapsules. 9-14 In con- trast, the stationary boundary mechanism invokes the presence of a stable, stationary reaction zone during the encapsulation process (Figure 1b). Under this mechanism, capsule wall thickness does not increase with time once the initial polymer film has been formed, though its density may change. Encapsulation processes can be affected by many factors, including the polarity, diffusion coefficients and reactivities of monomers, the polarity and permeability of the forming polymers, solvent polarity, solution pH, and temperature. While a better understanding of the encapsulation process could lead to improved capsule design, experimental studies of the encapsulation process have been complicated by the high rate of interfacial polymerizations and the typically submicron thickness of the wall. Janssen and co-worker 12 used narrow-disperse capsules prepared from 2,4-toluene diisocyanate (TDC) and diethylen- etriamine (DETA) with diameters varying from 3 to 5 mm to study the wall formation process experimentally. They found that the capsule wall thickness increased with reaction time, in agreement with the moving boundary mechanism. In contrast, Jabbari, 15 studying smaller microcapsules prepared from hex- amethylene diisocyanate (HMDI) and hexamethylenediamine (HMDA), found that capsule wall thickness did not increase with time once the initial polymer film had been formed, implying that the boundary is stationary in this system. We aim to design an effective approach to studying polyurea capsule wall composition and wall formation mechanism. Our strategy involves using mixed isocyanate monomers of different reactivity in the organic or fill phase and studying their sequential interfacial polyaddition with aqueous amines to form composite polyurea capsules. The spatial distribution of the different isocyanate residues across the capsule wall should then reflect the capsule wall formation process. 16 We are using scanning transmission X-ray microscopy (STXM) to map the distribution of these different monomers across the capsule wall with submicron spatial resolution. We demonstrate here a strategy to study interfacial encap- sulation processes and determine the growth processes in two types of polyurea capsules as a function of cross-linking and polymerization temperature. Our approach involves using mixtures of aromatic and aliphatic isocyanates in an oil phase consisting mainly of p-xylene. Aromatic isocyanates are known to be significantly more reactive than their aliphatic counter- parts. 17,18 Sato reported the rates of phenyl isocyanate with methanol to be 50 times higher than the rate of reaction of ethyl isocyanate with methanol. 19 Kuck et al. reported the reaction of HMDI with secondary amines to be about an order of magnitude lower than the corresponding reaction of TDI. 20 Competitive interfacial polyaddition of a mixture of aromatic and aliphatic isocyanates should thus lead to sequential incor- poration of these two isocyanates. If the rate-limiting step is diffusion of the aqueous amine into the oil phase, then the polyurea formed early should be rich in aromatic residues, while polyurea formed later should be rich in aliphatic groups. Hence, observation of an aromatic/aliphatic composition gradient across the capsule walls would indicate that wall formation involves a moving reaction zone or boundary (Figure 1a), as the wall grows toward the interior of the capsule. Conversely, absence of such a compositional gradient would imply wall formation with a fixed though broad reaction zone, leading to an interpenetrating network (IPN) of sequentially formed aromatic and aliphatic polyureas, as illustrated in Figure 1b. We compared two types of polyurea microcapsules, both based on mixtures of aromatic and aliphatic isocyanates, but * To whom correspondence should be addressed. † McMaster University. ‡ Syngenta, Jealott’s Hill International Research Centre. 2428 Macromolecules 2009, 42, 2428-2432 10.1021/ma802130n CCC: $40.75  2009 American Chemical Society Published on Web 03/13/2009