Structural study of a poly(methylmethacrylate) Langmuir–Blodgett film on a graphite surface by scanning tunneling microscope

Abstract

In this study, molecular configurations of three different stereoregular poly(methylmethacrylate) (PMMA) (isotactic, atactic, and syndiotactic PMMA) monolayers, which were transferred onto graphite substrates by the Langmuir–Blodgett (LB) technique, were investigated using scanning tunneling microscope (STM). STM images of monolayer films taken in air in the constant height mode showed various types of film structures. First, polymer films in their different forms always showed long-range ordered chain structures. Second, these polymer chains were generally observed to arrange themselves in parallel chains with no backbone crossovers. However, the packing density of polymer chains on the surface varied with change of scanning area presumably due to the nonuniformity of the LB film and the effect of thermal fluctuation of polymer chains on the film. Third, a monolayer film of isotactic PMMA film showed the coexistence of helical and linear chains in most of the film areas suggesting that the structural transition from the linear chains to 51 single stranded helices occurs at the surface pressure of 8 mN/m.

Development of an Ultrafast Low-Energy Electron Diffraction Setup

“…Atactic and syndiotactic PMMA molecules both arrange in a folded-chain configuration with little to no backbone crossovers [110]. The chain-to-chain distances reported in the literature are 5.0(1.0)Å and 4.8(1.9)Å, respectively, with a repeating monomer unit length of about 2.5Å in direction of the polymer backbone.…”

“…STM as well as AFM studies have shown that thin PMMA Langmuir-Blodgett films on mica and graphite display long-range crystalline order independent of tacticity, in contrast to PMMA behavior in bulk [110,111]. This behavior is in principle well known for other types of polymers, too, when subjected to a strong potential template [112,113].…”

“…The chain-to-chain distances reported in the literature are 5.0(1.0)Å and 4.8(1.9)Å, respectively, with a repeating monomer unit length of about 2.5Å in direction of the polymer backbone. In the case of isotactic PMMA, linear as well as helical arrangements are found with a somewhat smaller interchain distance of the latter of 3.7(0.8)Å [110,115,116].…”

“…The superstructure is only visible on about half of the investigated area. [110]. [229] The first observation can be explained by the abundance of three superstructure domains with a periodicity in only one direction, instead of a hexagonally symmetric 2 × 2 superstructure (Sec.…”

“…Specifically, AFM and STM measurements have been performed for various PMMA types on mica and highly ordered pyrolytic graphite (HOPG) [110,111,230,231], finding tacticity-dependent configurations of so-called folded-chain crystals. Remarkably, this is also the case for atactic PMMA (Fig.…”