A Scanning Force Microscopy Study of Block Copolymers Containing a Conjugated Segment
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(PMMA) or a polystyrene (PS) block (Figure 1). The initial motivation for the synthesis of such copolymers was to exploit the presence of the nonconjugated segment to provide solubility in common organic solvents (PPP being notoriously insoluble in any medium). In this work, we aim at using the intrinsic immiscibility of the blocks in the solid state to generate nanostructures of a conjugated material with well-defined shape and size, depending on chain length and mass ratio of the components. Thin films of the block copolymers are prepared from solution, analyzed with
PDI-TMAFM, and the images are interpreted with the help of molecular dynamics calculations.
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Figure 1: Chemical structure of PMMA-PPP (left) and PS-PPP (right) block copolymers.
EXPERIMENTAL The PMMA-PPP and PS-PPP block copolymers were prepared from precursor copolymers, PMMA-Poly 1,3-cyclohexadiene (PMMA-PCHD) and PS-PCHD using previously described methods [7]. These precursors were synthetized by anionic polymerization with a lithium counter-ion in a non polar medium. This method allows a good control of the molecular weights and of the 1,4 binding of the resulting cyclohexene units. The PPP block is then obtained by dehydrogenation of the PCHD sequence. These soluble copolymers were characterized by size exclusion chromatography, light and neutron scattering. These techniques show a strong aggregation of copolymers in THF or CS2 solutions and the formation of rather polydispersed micelles with a PPP hard core surrounded by a shell of PS. This polydispersity makes it difficult to determine the aggregate shapes. Thin films, typically 20 ýim thick, were prepared on silicon or mica substrates by solvent casting from toluene solutions containing 1mg/ml of the compounds. After drying in air, the films were annealed at 150'C for 48 hours invacuum (10-7 T). All AFM images were recorded with a Nanoscope III microscope (Digital Instruments) operated at room temperature in air, using the microfabricated cantilevers provided by the manufacturer (spring constant of 30 Nmni). The system is equipped with the Extender electronics module to provide height and phase cartography. Images of each sample were taken at several locations, and the time for scanning was about 5 min. All images were obtained with the maximum number of pixels (512) in each direction. For image analysis, the Nanoscope image processing software was used. The images presented here were not filtered and are shown as captured. Repeated scans indicated that the observed structures were stable. RESULTS AND DISCUSSION As a first step into microscopic analysis, the surface smoothness of the area under study is checked using tapping-mode operation. For PDI measurements, it is best that the surface roughness be as small as possible, since topographic contrast usually makes the interpretation of 396
phase images more complicated. Figure 2 (top) shows a typical lxi ImW topographic image for a PMMA-PPP copolymer, with molecular masses of the blocks o
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