Studying Polymer Self-Assembly by Combined Cryogenic and Liquid Phase Transmission Electron Microscopy

Abstract

The functionality of many materials depends on their hierarchical organization. Although in biology such materials can be formed through the self-organization of preformed building blocks, the bottom-up assembly of synthetic materials with predefined structures and properties is still out of reach. Our long term vision is to enable synthesis of hierarchical materials through multiscale assembly by combining the macromolecular self-assembly of predefined building blocks with their subsequent organization into mesostructured materials. The assembly of block copolymers into defined objects with well-defined structure, and the subsequent organization of these objects using colloidal assembly is one promising approach to this goal. However, to achieve this, we must first develop the capability to monitor structure formation in situ and on all relevant length and time scales, from the nanometer to the (multi)micron level. Liquid phase electron microscopy (LP-EM) is extremely suited to study materials formation processes on different length scales and to provide detailed information on the mechanisms involved [1][2][3].LP-EM is an emerging analytical technique for the study of materials formation, but has already begun to revolutionize our understanding of nanocrystal formation. Many experiments have been performed by the electron beam reduction of metal precursors, which initiates nanoparticle formation [4]. This convenient method of synthesis, along with the high convenient of the metal nanoparticles has greatly aided the success of LP-EM in this area. However, the synthesis of most materials is typically performed by controlled solution mixing, which is still a challenge in the liquid cells available for LP-EM experiments. Furthermore, to study the aqueous solution self-assembly of polymers it is necessary to ensure the liquid layer thickness is on the order of the particle thickness, as the electron density of the macromolecular materials is not vastly different from water. Therefore, we must create new experimental set ups for LP-EM which allow thin cells to be created and either create structures without controlled solution mixing, or allow controlled solution mixing to occur inside the cell.In this paper we will discuss our efforts to perform block copolymer self-assembly using a solvent switch inside the liquid cell. The system we have chosen comprises poly(caprolactone)-b-poly(ethylene oxide) PCL-b-PEO, and a solvent switch from acetone to water, which based on ex-situ cryo-TEM and self-consistent field (SCF) theory should form vesicles upon increasing water content (Figure 1). PCLb-PEO is a well-studied system, however little is currently known about the dynamics of assembly, or how the assembly route can be effected by changing experimental conditions. Adam et. al. studied the formation of PCL-b-PEO vesicles by scatting and cryo-TEM [5], showing evidence of the conventionally accepted micelle  cylinder  vesicle formation route. However they also suggested

Nanotubes, Plates, and Needles: Pathway-Dependent Self-Assembly of Computationally Designed Peptides

“…Cryogenic conditions preserve the hydrated state of the sample even under the electron microscope vacuum. − As shown in Figure A, the nanostructure at pH 4.5 is clearly hollow nanotubes as indicated by the image contrast changing from tube edge to center. From the average of measurements of the cryo-TEM images (histogram shown in Figure S6), the tube diameter was determined to be 13.3 ± 0.7 nm (measured from wall center to wall center across the tube diameter), and the wall thickness was 3.1 ± 0.5 nm.…”