Direct One-Pot Synthesis of Chemically Anisotropic Particles with
Tunable Morphology, Dimensions, and Surface Roughness
Yanan Liu, Wang Liu, Yuhong Ma, Lianying Liu,* and Wantai Yang*
Beijing Engineering Research Center for the Synthesis and Applications of Waterborne Polymers, College of Materials Science and
Engineering, Beijing University of Chemical Technology, Beijing 100029, China *S Supporting Information
ABSTRACT: Previously, synthesis of anisotropic particles by seeded polymerizations has involved multiple process steps. In conventional one-pot dispersion polymerization (Dis.P) with a cross-linker added, only spherical particles are produced due to rapid and high cross-linking. In this Article, a straightforward one-pot preparation of monodisperse anisotropic particles with tunable morphology, dimensions, surface roughness, and asymmetrically distributed functional groups is described.
With a cross-linker of divinylbenzene (DVB, 8%), ethylene glycol dimethacrylate (EGDMA, 6%), or dimethacryloyloxybenzophenone (DMABP, 5%) added at 40 min, shortly after the end of nucleation stage in Dis.P of styrene (St) in methanol and water (6/4, vol), the swollen growing particles are inhomogeneously cross-linked at first. Then, at low gel contents of 59%, 49%, and 69%, corresponding to the cases using
DVB, EGDMA, and DMABP, respectively, the growing particle phase separates and snowman- or dumbbell-like particles are generated. Thermodynamic and kinetic analyses reveal that moderate cross-linking and sufficient swelling of growing particles determine the formation and growth of anisotropic particles during polymerization. Morphology, surface roughness, sizes, and cross-linking degrees of each domain of final particles are tuned continuously by varying start addition time and contents of crosslinkers. The snowman-like particles fabricated with DVB have a gradient cross-linking and asymmetrical distribution of pendant vinyl groups from their body to head. The dumbbell-like particles prepared using DMABP have only one domain cross-linked; i.e., only one domain contains photosensitive benzophenone (BP) groups. With addition of glycidyl methacrylate (GMA) or propargyl methacrylate (PMA) together with DVB or EGDMA, epoxy or alkynyl groups are asymmetrically incorporated. With the aid of these functional groups, carboxyl, amino, or thiol groups and PEG (200) are attached by thiol−ene (yne) click and photocoupling reactions. 1. INTRODUCTION
In the past decade, anisotropic particles with asymmetric geometry and specific surface reactive groups have attracted tremendous attention because of their fascinating properties including surface amphiphilicity, optical and magnetic properties, etc. Many strategies, typically including phase separation-,
Pickering emulsion-, and microfluid-based techniques, etc., have been developed for their fabrication, as addressed in recent review articles.1−6 Of much more interest in recent times is the exploration of relatively easy and reproducible processes7−11 that can provide high yields of uniform anisotropic particles suitable for self-assembly and new applications. Among such processes, controlled internal phase separation in seeded polymerizations is considered to be a prevalent technique.12−29 Spherical or nonspherical particles are usually taken as starting materials (seeds). When cross-linked seed particles are swollen with monomers12−15,22−29 and heated, phase separation (expelling of monomers as a liquid protrusion from seeds) is induced by relieving elastic stress built up on network. Subsequent polymerization of the expelled monomers enhances the phase separation and results in formation of anisotropic particles.
However, this technique still has some limitations. For instance, multiple process steps, including synthesis of seeds, additional modification to endow surface with hydrophilicity to increase interfacial tensions,23,26 cross-linking and swelling of seeds, and subsequent polymerization, are required.12−14,23−25
The swelling and repeated separating processes are timeconsuming. These could limit the achievable yields. Moreover, the seed domain of resultant anisotropic particle usually retains its smooth or coarse surface,22 while the newly formed domains are all smooth-bulb-shaped.22−24 In such cases, there are difficulties in control of surface roughness, shape, and sizes in the course of forming anisotropic particles. Besides, if different functionalities are desired on different compartments, a series of reactions, such as introduction of specific functional groups
Received: November 3, 2014
Revised: December 28, 2014
Published: December 30, 2014
Article pubs.acs.org/Langmuir © 2014 American Chemical Society 925 DOI: 10.1021/la504317m
Langmuir 2015, 31, 925−936 to seeds, conversion of the functional groups, and postfunctionalization of anisotropic particles, need to be carried out.29 The asymmetrically rough surface and distributed functional groups are important to self-assembly and new applications of anisotropic particles.30 So, the preparation of chemically anisotropic particles through a simple and efficient approach involving fewer steps is still strongly demanded.
It is known that dispersion polymerization (Dis.P) is a very simple method for synthesis of monodisperse polymer microspheres with functionalities. Its process can be divided into a nucleation stage and a particle growth stage. The growing particles can be considered as dynamically changing seeds.
They remain swollen by monomer and oligomers throughout the particle growth stage, until the very end of reaction.31−33
When the swollen growing particles are cross-linked via a delayed addition (two-stage Dis.P) technique developed by
Winnik’s group,31−33 the formed polymer network can be inhomogeneous. This tends to localize contraction forces.
However, to date, the swelling ability and inhomogeneity in cross-linking of growing particles have not been utilized for inducing phase separation and synthesizing anisotropic particle, except for preparing spherical core−shell (cross-linked) particles.34−36