Overall research focus
The main focus and interest of the Zetterlund group is synthesis of polymers, polymeric (nano)particles and polymeric materials using radical polymerization in dispersed systems. Dispersed systems refer to heterogeneous systems where monomer droplets and/or polymer particles are dispersed in a continuous phase, which is typically water. We use a range of different such systems, e.g. emulsion polymerization, miniemulsion polymerization, microemulsion polymerization and dispersion polymerization. These approaches all have their advantages/disadvantages with regards to synthesis of particles of various size/shape/morphology, such as particles with specific internal composition gradients, hollow particles and non-spherical particles. We employ conventional radical polymerization as well as reversible deactivation radical polymerization (RDRP; also known as controlled/living radical polymerization (CLRP)) techniques, mainly reversible addition-fragmentation chain transfer (RAFT) polymerization. A significant part of the research program is synthesis of hybrid polymeric materials based on graphene and graphene oxide. Our research is conducted with a variety of applications in mind, ranging from coatings (films), advanced polymeric materials and biomedical applications. In the sections below, a number of more specific research areas are outlined.
Fig. 1. Controlled/Living Radical Polymerization in Dispersed Systems: An Update, P. B. Zetterlund, S. C. Thickett, S. Perrier, E. Bourgeat-Lami, M. Lansalot, Chem. Rev. 2015, 115, 9745.
Polymer/graphene hybrid materials
The material graphene was discovered in 2004 (Noble Prize awarded in 2010) – it is the strongest material ever measured, and this is accompanied by a range of other extraordinary physical properties such as high thermal conductivity and high electrical conductivity. Graphene is seen as the material of the future, with the potential to revolutionize a wide range of industries from electronics to healthcare, and there is currently immense worldwide research activity in this area. We are interested in preparing novel polymeric nanocomposite materials with superior physical properties using graphene and graphene oxide.
Fig. 2. Schematic illustration of modification of graphene with polymer brushes.
The addition of graphene as a component of polymer nanocomposites can result in superior material properties over the more common fillers such as carbon black. In order to fulfill the potential of such nanocomposites, a high degree of dispersion of graphene as individual two-dimensional sheets in the polymer matrix is essential. However, both pristine graphene and graphene oxide are incompatible with most hydrophobic polymers, and do not form homogeneous polymer composites. We specifically address these issues by employing various techniques involving (controlled/living) radical polymerization in aqueous dispersed systems for synthesis of polymer/graphene nanocomposite materials. The key in our approach is to exploit the amphiphilic properties of graphene oxide – it can behave as a surfactant in aqueous emulsions of oil (vinyl monomer) under suitable conditions. We thus conduct various types of dispersed phase polymerizations (emulsion, miniemulsion) using graphene oxide as surfactant, thereby accessing a novel route to graphene/polymer nanocomposite materials. It is particularly interesting to be able to prepare electrically conductive polymeric materials using these approaches - we are actively pursuing a range of applications for these novel materials, such as in the area of regenerative medicine as well as energy storage (batteries).
Fig. 3. Schematic illustration of the use of graphene oxide as surfactant in miniemulsion polymerization for synthesis of hybrid nanoparticles.
Advanced polymeric materials – multiblock copolymers
The nanoreactor concept
Polymerization reactions can be manipulated to one’s advantage by conducting the polymerizations within tiny confined spaces – “nanoreactors”. Within the context of polymerizations in dispersed systems (e.g. emulsion, miniemulsion etc), the polymer particles/monomer droplets can constitute such nanoreactors, making the polymerization proceed quite differently compared to the corresponding homogeneous system. For example, depending on the system, the livingness can be increased by exploitation of the segregation effect, whereas the control over the molecular weight distribution can be improved via the confined space effect. Such effects of “compartmentalization” are summarized in Fig. 4.
Fig. 4. Illustration of compartmentalization effects and multiblock copolymer.
We are mainly exploiting the effects of compartmentalization to synthesize high-order multiblock copolymers in aqueous dispersed systems such as RAFT emulsion polymerizations. Compartmentalization effects make it possible to access polymer structures (types of monomers, block orders etc) that are inaccessible in the corresponding homogeneous systems. Polymeric materials comprising block copolymers – polymer chains containing discrete segments of different monomer types – can undergo spontaneous self-assembly in the molten/solid state into complex nanostructured soft materials with a wide range of applications.
Synthesis of polymeric nanoparticles of specific morphology
Non-spherical polymer particles (e.g. worm-like and rod-like particles) as well as polymer particles of specific internal morphologies (hollow particles, vesicles etc) have potential applications in a wide range of fields such as nanomedicine and materials science. In our group, we exploit a number of approaches directed at preparation of such (nano)particles with a specific focus on mechanistic aspects and the level of control over the morphology. These approaches are often, but not always, based on reversible deactivation radical polymerization (RDRP).
Fig. 5. Use of CO2 pressure to tune particle morphology during polymerization-induced self-assembly.
Polymerization-induced self-assembly relies on formation of a diblock copolymer that self-assembles as it is formed. The technique can be conducted as an emulsion polymerization or dispersion polymerization using RDRP, or using a new approach developed recently in our team based on non-living addition-fragmentation chain transfer polymerization. Fig. 5 illustrates how one can employ the CO2 pressure to tune the particle morphology during polymerization-induced self-assembly.
We are also exploiting so called SPG membrane emulsification as a synthetic route to polymer particles, both via self-assembly based approaches and miniemulsion-type techniques. These techniques are based on the passage of one liquid through nano-sized pores of well-defined size into another liquid, thereby generating an emulsion in the case of two immiscible liquids.