Imaging and Kinetic Study of Single Polymer Particle Growth

Abstract

The ability to directly observe chemical reactions at the single-molecule and single-particle level has enabled the discovery of behaviors otherwise obscured by ensemble averaging in bulk measurements. However powerful, a common restriction of these studies to date has been the absolute requirement to surface tether or otherwise immobilize the chemical reagent/reaction of interest. This constraint arose from a fundamental limitation of conventional microscopy techniques, which could not track molecules or particles rapidly diffusing in three dimensions, as occurs in solution. However, many chemical processes occur entirely in the solution phase, leaving single-particle/-molecule analysis of this critical area of science beyond the scope of available technology. Here we employed the 3D single-molecule active real-time tracking (3D-SMART) to untether the study of polymerization and monitor the growth of polymer particles in the solution phase. Another common feature of previous single-molecule and single-particle study is the use of external tag such as fluorescent labels. Doping of fluorescently labeled molecules have been playing an important role in the study of chemical reactions since the fluorescence microscopy offers high sensitivity and specificity. However, the labeling of molecules can be very complicated and the effect of the labels on the reactions are unknown or not fully understood. Here, we applied the stimulated Raman scattering (SRS) microscopy to study the growth of precipitated polymer particles. SRS can probe the intrinsic vibrational frequencies of chemical bonds or groups and image the material without adding external fluorescent labels. We also incorporated the Interferometric scattering (iSCAT) microscopy in the study of polymerization. Interferometric scattering (iSCAT) allows for sensitive non-fluorescent detection of nano-scale samples. It is label-free, fast and has high resolution, allowing for study of early stage of polymerization while the polymer particles are still small. Chapter 1 provides an overview of the single polymer particle study in the context of modern microscopic methods and introduces the research goals of this dissertation. Chapter 2 introduces the 3D single-molecule active feedback real-time tracking method that monitors the real-time growth of single polymer particles in solution phase. Radial growth rates of individual growing polynorbornene particles are measured and analyzed to reveal a spatial heterogeneity originated from accumulation of polymer chains and small aggregates at the bottom of solution. Chapter 3 demonstrate the hyperspectral Stimulated Raman scattering microscopy and the application of this method in the imaging of growing copolymer particles on surface. This method features the label-free process that relies on intrinsic signal of the polymer itself without external labeling technics. With a proper choice of functional groups (C=O and C=C), this method is also capable of resolving the ratio of two different monomers in the same copolymer particle. Chapter 4 introduces the Interferometric scattering microscopy and its application in the label-free and real-time monitoring of single polymer particles with smaller sizes (r=25-75nm). By introducing a Z scan of sample into the iSCAT experiment, unstable and uneven background signal as a common issue in previous iSCAT work is encompassed. Chapter 5 gives a comparison of three types of microscopic methods above and summarizes the key findings from the previous chapters.