Bulk Synthesis of Polymer-Inorganic Colloidal Clusters
Entry by Pichet Adstamongkonkul, AP 225, Fall 2011
Title: Bulk Synthesis of Polymer-Inorganic Colloidal Clusters
Authors: Adeline Perro and Vinothan N. Manoharan
Journal: Langmuir, 2010, Vol. 26, No. 24
Nonspherical colloidal particles can be used as model systems to understand phase behavior and as building blocks for three-dimensional photonic crystal structures if we can control their shape, size, composition, and yield, which in turn precisely define the morphologies and surface properties of those particles. An approach uses pre-formed spherical particles and make them aggregate in a controlled manner into clusters, which allows different morphologies to be produced. This paper is the generalization of a method previously developed, which makes polystyrene-silica colloidal clusters in bulk with high yield, to other materials that could have a wider applications. The general process can be divided into three steps: the grafting of coupling agents onto the surface of inorganic seed; the emulsion polymerization reaction on the inorganic nanoparticles; and silanization the clusters, which facilitates the functionalization, by introducing a silane derivative during the polymerization.
Preparation of Inorganic Nanoparticle Seeds
In this study, two types of nanoparticle seeds were made: titanium dioxide seeds and silica seeds. The synthesis of the two kinds of seeds is very similar. In case of titanium dioxide seeds, titanium (IV) ethoxide solution was added to the mixture of ethanol, KCl and nitrogen, stirring at ambient temperature. By varying the salt concentration, one can change the size and distribution of the particles; increasing the salt concentration decreases the size. On the contrary, silica seeds were synthesized via Stober sol-gel process; the mixture of ethanol and ammonia solution was stirred abd heated before the addition of tetraethoxysilane. To promote the growth of polymer on the seed surface, the authors adsorbed either a macromonomers of PEG or covalently grafted methacryloxymethyltriethoxysilane (MMS), a silane derivative onto the inorganic particles surface.
Synthesis of Colloidal Clusters
In the process of Polystyrene-Silica Cluster formation, the particles undergo the emulsion polymerization of monomers, in which the polymer beads saturate the inorganic surface and the emulsion was stabilized by the non-ionic surfactant (NP30). One can go even further to synthesize the Silanized Polystyrene-Silica Clusters by adding 3-methacryloxypropyltrimethoxysilane (MPS) on top of the polystyrene particles.
The MPS surface-modified clusters were subsequently modifiedto form the silica shell by slowly adding the tetraethoxysilane at the rate that keeps the mixture from silica nucleation. 3-aminopropyltriethoxysilane was added to the mixture, stirred overnight, dialyzed against water, then the aqueous suspension was mixed with citrate-stabilized gold particles, purified by centrifugation. Colloidal crystals were then made via slow sedimentation by resuspending the clusters and sealing the solution inside a sample cell.
In this study, the effect of varying the seed particle size was examined. The previous study suggested that the two-term potential energy function models the morphology of these clusters as they would follow the geometric rule. The results showed that the distribution of clusters is similar in width for beads with different sizes.
The precise control of the silica particle size is essential for controlling the number of polystyrene domains building up on these particles and for maximizing the yield. It was observed that the transition from one cluster morphology to another is discontinuous as the size of the seeds changes. This can be captured by enforcing the hard-sphere constraint, as the maximum number of hard spheres that can fit on a central hard sphere as a function of the ratio of the sphere size. The authors suggested that to optimize the cluster distribution, one would need to select the size of the seed between the sizes where the transition of morphology occurs.
The morphologies of all clusters with 2-8 polystyrene domains were found to be "Tammes packings".
- Clusters containing 6 domains - Octahedra
- Clusters containing 8 domains - Square antiprisms
- Clusters containing 9 domains - two slightly different variants of triaugmented triangular prism
Titanium Dioxide versus Silica seeds
- Titanium dioxide have a broader size distribution compared to the monodisperse silica particles, due to the rapid hydrolysis rates and water sensitivity of its precursor, titanium tetraethoxide, in ethanol in the presence of salt.
- Silica particles produce high yield of tetrahedral clusters while titanium dioxide produces a wide range of clusters and there are some in between the sizes leading to the tight packingof polymer spheres. Surface roughness or irregularity could potentially contribute to such distorted shapes.
- However, smaller titanium seeds produce high yields of regular morphologies and identical to that of the silica seeds, indicating that same mechanism is at play.
- Size distribution of titanium dioxide results in the distribution of clusters.
Silanization of Hybrid Polyhedral Particles
The authors observed that the addition of MPS in the late stage of polymerization did not affect the cluster distribution. The deposition of silane on the cluster surface leads to the rougher surface, as confirmed by the SEM images. The silanization allows further functionalization of the surface due to the silane chemistry. The authors also justified this approach by incorporate amino groups onto the surface and observe the attachment of these clusters to the gold nanoparticles. They concluded that silanization is a simple way to stabilize colloidal clusters and produce complex structures.
Fabrication of PMMA-silica Clusters
The polymerization conditions determine the degree fo nucleation of polymer onto a seed. Polymethylmethacrylate (PMMA) and PMMA-co-ethylene glycol dimethacrylate (EGDMA) particles showed similar polymerization speeds, which are faster than that of polystyrene. However, the addition of EGDMA double the diameter of PMMA particles, which is in the optimized range for optical applications. By varying the size of the silica seeds, regular polyhedral clusters with identical morphologies as before were observed.
In the case of PMMA-co-EGDMA particles on 100 nm silica seeds having 1 functional group per <math>nm^2</math>, most clusters are dimers. However, with 500 nm silica seeds having 10 functional groups per <math>nm^2</math>, core-shell particles with thick PMMA-co-EGDMA shells were observed, which may be due to the change in the silica wettability resulting from high coupling agent concentration.