Uniform Nonspherical Colloidal Particles with Tunable Shapes

Colloidal particles are ubiquitous for a broad spectrum of applications. Many current applications require nonspherical particles. They have been used for modifying optical properties and as a building block for self-assembled biomaterials. They are also beneficial for controlling suspension rheology and for engineering colloidal composites. In general, synthesizing uniform nonspherical particles is difficult because surface tension favors spherical shapes in overall length scales. This limitation may be overcome with advanced techniques such as microfluidics and clusterization separations; these precision processes generally produce small yields, severely limiting their utility for practical applications. Recently, several methods that can produce nonspherical particles such as dimers and higher-order clusters have produced larger yields. These methods all rely on random coagulation of single particles or on the formation of clusters by using emulsification techniques. However, a common limitation of these synthesis techniques is that a single batch contains many particle types, requiring a sorting process to produce homogeneous samples, thus limiting the yield for any given sample type. Thus, there exists a need for flexible and robust techniques to generate nonspherical particles with well-defined shapes and high yields. One method of controlling particle shapes is to use a controlled phase separation in the seeded polymerization technique. This technique can produce extremely high yields of uniform particles. In the typical technique, seed particles are first swollen with a polymerizable monomer-based solution, and subsequent polymerization induces phase separation. Depending on thermodynamic and kinetic factors, the phase separation results in a variety of particle shapes. Shapes of higher aspect ratio may be obtained by crosslinking the seed particles before the phase separation. The crosslinking makes the seed particles more elastic. The elastic forces can impart greater directionality to the formation of the newly polymerizing phase. This suggests that seed particle crosslinking might be used to control the shapes of nonspherical particles. In this Communication, we introduce a new seeded-polymerization technique that uses the crosslinking forces to synthesize a variety of uniform nonspherical particles in large synthesis scales. Our technique involves controlling the directionality of phase separations in the seeded-polymerization technique by manipulating the crosslinking density gradients of the seed particles. This enables us to generate a variety of particle shapes, including “rod”, “cone”, “triangle”, and “diamond” particles (Fig. 1). The uniform placement of the individual bulbs in nearly all of the particles in each batch is achieved solely by tuning the crosslinking properties of the seed particles. To illustrate how crosslinking controls particle morphology, we compare the synthesis of two different types of trimer particles: “rods” and “triangles” (Fig. 1a and b, respectively). Rods and triangles both result from the seeded polymerization performed on dimers. The difference is the degree of crosslinking in the two bulbs of the seed dimer. Dimers that contain two bulbs with slightly different degrees of crosslinking become rods when swollen and polymerized. By contrast, dimers that contain two bulbs with equal crosslinking become triangle particles (Fig. 2). This is demonstrated by a series of experiments in which we synthesized trimers from different types of dimers. First, we synthesized spherical crosslinked polystyrene (PS) particles from linear PS template particles by swelling them with styrene monomers containing a crosslinker, divinylbenzene (DVB). In this swelling step, step a, the concentration of DVB, [DVB]a was fixed at 1 vol % relative to the total monomers. After polymerizing the spherical particles, we performed another swelling step, step b, to produce dimers. In step b, [DVB]b was varied from 0.5 to 1.1 vol %. Polymerization after swelling step b resulted in symmetrical phase-separated PS dimers. The size and appearance of the dimers produced were identical for all samples. Finally, in step c, the dimers were again swollen with a similar mixture, and polymerized following the same procedure. This resulted in trimer particles of various shapes. The shape of the trimers depends only on [DVB]b. High values of [DVB]b produced triangle particles, intermediate values of [DVB]b produced triple rod particles, and the lowest value of [DVB]b gave rise to snowman particles. To understand this behavior, we examine how changing [DVB]b changes the internal network properties of the seed C O M M U N IC A IO N