Date of Award
Open Access Dissertation
Chemistry and Biochemistry
College of Arts and Sciences
The work contained herein, is focused on the design, synthesis, and characterization of polymer nanocomposite interfaces and the property enhancement afforded from said interface design. Through the use of reversible addition fragmentation chain transfer (RAFT) polymerization for the grafting of polymer chains to silica nanoparticles, the surface of silica nanoparticles can be manipulated to tune the properties of the nanocomposite as a whole.
In the first part of this work, heterogeneity is introduced onto the surface of silica nanoparticles via a sequential RAFT polymerization to afford a bimodal brush system. A densely grafted, short brush population is polymerized from the surface in order to provide screening for the enthalpic core-core attraction of the nanoparticles that can lead to agglomeration. Afterwards a second sparsely grafted, long brush population is polymerized to enable the nanoparticle to entangle with the polymer matrix overcoming the entropic preference of the grafted chains dewetting from the matrix chains. These two populations and all their respective molecular variables (graft density, chemistry, end-group chemistry, polydispersity, etc) can be controlled with this approach. With this control in place, the molecular variables were used to produce both bimodal and monomodal samples for comparison of their resulting properties when dispersed in a polymer matrix. It was found that not only do the bimodal samples improve dispersion when compared to monomodal brushes, but that the thermomechanical properties are enhanced as well. Tuning of the long chain graft density determined that very low graft densities were better for improving entanglement. The first bimodal kinetic study was performed to prove that control over the polymerization can still be obtained using RAFT even when a dense brush is already in place.
Secondly, following the information gained from our first bimodal samples, it was ascertained that with our bimodal system the enthalpic attraction of the particles and the entropic dewetting of the grafted chains were decoupled. This allowed us to pursue the synthesis of mixed bimodal samples. In a mixed bimodal sample the chemistries of the brush populations are distinct. If the long chains are the only population entangling with the matrix, then it can remain matrix compatible while the short brush can be varied to improve other desired properties of the nanocomposite. In order to test whether monomer/polymer incompatibility would allow for the diffusion of a monomer past a short but dense brush of polymer it phase separates with to the surface, a simple poly(methyl)methacrylate/polystyrene mixed bimodal brush was made. With both variations of either chemistries short or long, bimodal samples were possible with control of all previously mentioned molecular variables. In order to push that testing further, bimodal samples of poly(1H,1H-heptafluorobutyl methacrylate) short brushes and polystyrene long brushes were made. A film of these nanoparticles were drop cast onto various substrates showing increased water contact angle measurements when compared with untreated samples. The drop casting of this film onto a sheet of polystyrene followed by annealing shows that the long polystyrene chains of the mixed bimodal brush can still entangle with the polystyrene of the substrate.
For the third section, further work was performed to develop new approaches to the synthesis of bimodal brushes. Taking cues from our testing that showed lower graft densities improved entanglement, it was decided to pursue a one-pot bimodal brush synthesis using a grafting-to approach. While grafting-to is incapable of producing high graft density brushes, this was not needed for our improvement in dispersion and entanglement. Since RAFT polymerization allows for control of the polymer chain end chemistry, the efficiency of post-polymerization modification was compared to using a modified/activated RAFT agent. The activated RAFT agent showed higher graft densities while still allowing the use of a thermally initiated, bulk polymerization without decomposing at the higher temperatures required for it. This allows for decreased solvent use and therefore easier scale-up. Both long and short chains were attached in a one-pot approach. While not having the control of the sequential RAFT polymerization process, it is much simpler, more efficient, and more modular than the multi-stepped procedure. In addition to overcome issues with characterization of bimodal brushes produced via a one-pot procedure, a new anthracene-containing initiator was created and used to end-label one population of chains via a radical cross coupling mechanism. This allows for characterization of each chain population independently using a combination of UV-Vis and TGA.
Finally, new synthetic strategies towards the modification of the silica nanoparticle surface via different ligands while also focusing on improving efficiency. Previous approaches used a linear aminosilane for the coupling of the RAFT agent to the surface. While successful, the reactions take hours to complete. In a new approach, an amine-containing cyclic azosilane was used for the modification of the silica surface in under five minutes. This new ligand has the same ability as our previous method to be varied in loading in order to vary the graft density. RAFT polymerizations of poly(methyl)methacrylate and polystyrene were performed at various graft densities to show that the attached RAFT agent retained its viability after attachment.
Neely,, T. L.(2015). Modification of Nanoparticles for Designed Interfaces. (Doctoral dissertation). Retrieved from http://scholarcommons.sc.edu/etd/3717