Visualization of synthetic helical polymers by high-resolution atomic force microscopy

Abstract

Direct observations of the helical structures of artificial helical polymers, such as helical polyacetylenes and polyisocyanides, by atomic force microscopy (AFM) are described in this tutorial review. The two-dimensional helix bundle formation of specific helical polymers on substrates under solvent vapor exposure permits us to determine their helical structures, including their helical pitch and handedness, at a molecular level by AFM in the tapping mode. The direct observation of supramolecular helical structures based on stereoregular poly(methyl methacrylate)s is also described.

Synthesis and structure determination of helical polymers

“…Direct evidence for inversion of the macromolecular helicity of D-19 in different polar and nonpolar solvents has also been visualized for the first time by AFM with molecular resolution on exposure to each solvent vapor. 84 Regular 2D crystal formation on substrates after annealing in organic solvent vapors may be required for high-resolution AFM, 43 as the 2D crystals of polymer chains have the advantage of being sufficiently robust to resist being damaged by the tip during high-resolution AFM measurements; moreover, the flat surface of 2D crystals allows optimizing AFM settings for high-resolution imaging.…”

“…However, recent significant developments in microscopic instruments such as scanning probe microscopy, 41,42 coupled with precise polymerization techniques, have made it possible to directly observe the helical structures of certain helical polymers, including helical pitch and handedness, which provides more detailed structural information for understanding the principles underlying the generation of helical conformations. 43 In this review, the recent progress in the synthesis of helical polymers and their structural determination by means of CD and vibrational CD (VCD) spectroscopies, single-crystal X-ray analysis of uniform oligomers and the XRD of LC helical polymers, together with the direct observation of helical structures by high-resolution atomic force microscopy (AFM), which have been mainly performed in our laboratory, is described.…”

Chiral nanostructure in polymers under different deposition conditions observed using atomic force microscopy of monolayers: poly(phenylacetylene)s as a case study

“…In addition, the monolayer must be deposited on an appropriate substrate, because the interactions between the solid support and the polymer can affect, not only to the intermolecular assembly of the polymer chains, but also to its helical conformation. 47,48 During the last decade several procedures have been developed to create well-ordered monolayers of PPAs and good quality AFM images.…”

Amplification of macromolecular helicity of dynamic helical poly(phenylacetylene)s bearing non-racemic alanine pendants in dilute solution, liquid crystal and two-dimensional crystal

“…Figure 5 shows the typical AFM phase images of poly(1L m -co-1D n ) deposited on highly oriented pyrolytic graphite (HOPG) from a dilute benzene solution (0.005 mg ml À1 ), followed by benzene vapor exposure at B25 1C for 12 h. 21,22 This method is also very useful for constructing highly ordered 2D helix bundles with a controlled helicity for a dynamically racemic helical poly(phenylacetylene) 23 and helical polyisocyanides [46][47][48] on HOPG, and their helical structures were visualized by AFM. 49 Poly(1L m -co-1D n ) self-assembled into regular 2D helix bundles with a constant height (B1.6 nm), and the copolymer chains packed parallel to each other. These AFM images revealed the helical pitch, helical sense (right-or left-handedness) and molecular arrangements of the copolymers.…”