From Functional Group Ensembles to Single Molecules: Scanning Force Microscopy of Supramolecular and Polymeric Systems
Surfaces in supramolecular and polymeric systems were characterized by scanning force microscopy (SFM) using probe tips functionalized with self-assembled monolayers (SAMs). This approach allows one to control the forces between tip and surface by immobilizing suitable molecules, which expose selected functional groups, onto gold-coated probes (Scheme 1). The objective of this Thesis work was the extension of SFM with SAM-functionalized probes (so-called "chemical force microscopy", CFM) to technologically relevant surfacetreated polymers and elastomers with the ultimate aim of laterally resolved detection of functional group distributions on a sub-100 nm level. In measurements of interaction forces between a few or even individual molecules in supramolecular systems the transition from studying continuum to studying non-continuum properties was also attempted. In situ measurements of reaction kinetics using "inverted CFM" on a scale of 10 - 100 molecules were achieved. Furthermore, interactions between ensembles of functional groups and individual molecules were studied and provided fundamental insight into functional group distributions on polymer surfaces as well as rupture forces of individual host-guest complexes. Scheme 1 Schematic drawing of the contact zone between SFM tip and surface where interactions between functional groups on both monolayer modified tip and sample surfaces are measured in CFM. In order to understand the interaction of SAM-functionalized SFM tips with various surfaces in friction and pull-off force measurements, a large variety of model surfaces was studied. These model systems included SAMs of thiols, disulfides, and sulfides on gold, thin films of low molecular mass compounds, and structured, modified, or functionalized polymer surfaces. The merging of bottom-up and top-down approaches with its implications for technology on a nanometer scale forms the framework for this Thesis as discussed in Chapter11. Self-assembly as a strategy for a bottom-up approach on one hand and scanning probe microscopy as enabling technology in a top-down approach on the other hand were shown to be complementary in the development towards nanotechnology. Chapter 2 served as a brief introduction to the self-assembled monolayer model systems and the polymeric materials, as well as to the method of scanning force microscopy, which were used and investigated throughout this Thesis. The foundations for the experimental work described in the subsequent Chapters were provided and specific relevant topics, such as interaction forces and friction, were highlighted. For tip modifications, as well as for the formation model surfaces (SAMs on gold), a large variety of novel functional symmetric and unsymmetric disulfides was synthesized. The investigation of fundamental structure-property relationships of SAMs of these and other thiols, sulfides, and disulfides on Au(111) were discussed in Chapter 3. The lattice structure of SAMs of these compounds on Au(111) was unveiled by SFM with molecular (lattice) resolution. Based on the lattice structure, information on the tilt angle of the molecules could be obtained. For instance, Y-type thiols with two alkyl chains connected to one thiol group were shown to possess tilt angles of ca. 14°. The rigid headgroups of tetrasulfide derivatives of resorcinarene were concluded to force the alkane segments into a near-perpendicular orientation, in contrast to similar calixarene derivatives with tilted alkane chains. In addition, an information or "penetration" depth of the SFM tip of 6 Å could be estimated from results obtained on unsymmetric disulfides with different chain lengths. SAMs of the synthesized fluoroalkyldisulfides were shown to be useful for liquid crystal alignment. For instance, a transition from planar to homeotropic anchoring of nematic liquid crystals could be achieved by increasing the length of the alkane segments in partially fluorinated disulfides. In addition, in SAMs of mixed hydrocarbon-fluorocarbon disulfides evidence for small clusters and negligible diffusion constants at room temperature were found in a combined XPS and FT-IR study. In Chapter 4 the factors which influence friction and pull-off forces as measured by SFM with SAM-functionalized probe tips were discussed. The studies were carried out on thin films of tetranitrotetrapropoxycalixarene, lamellar polymer crystals, ultrathin polymer fibers obtained by electrospinning, lipid mono- and bilayers, as well as SAMs of various organosulfur derivatives on gold. These samples allowed us to vary surface energy, structure and phase state of mono- and bilayers, anisotropic crystal structure, and actual tip-sample contact area. Friction anisotropy was observed for films of tetranitrotetrapropoxycalixarene and lamellar polymer crystals due to a rectangular crystal structure and the ordered array of folds on the polymer crystals, respectively. In SAMs of unsymmetrically substituted disulfides on Au(111) the magnitude of friction forces was correlated with the conformational disorder as detected by FT-IR. Differences in surface mechanical properties also gave rise to significant contrast in friction forces measured on lipid mono- and bilayers. The contrast observed in friction forces was therefore concluded to be often not related to different chemical composition or different exposed functional groups, but rather to surface mechanical properties, energy dissipation, or friction anisotropy due to orientation of molecules. In contrast to this result, measurements of pull-off forces were found to correlate well with the nature of the exposed functional groups in absence of inelastic deformation. Thus, pull-off force measurements were used in the subsequently described studies on polymer surfaces in order to characterize the effects of surface treatments. Anisotropic friction was studied in detail on uniaxially oriented polymers which was discussed in Chapter 5. For these studies oriented polymer systems were prepared. Samples included HDPE and nylon-6 oriented by channel die compression, HDPE and PTFE oriented by sliding over heated glass substrates, and PE crystallized under high pressure. By means of SFM the local order was visualized on a molecular scale in order to prove the suitability of the samples for tribological studies. The friction data was collected in molecularly resolved SFM line scans which displayed a molecular stick-slip behavior. Imaging with lattice resolution was also achieved with chemically modified tips. This allowed us to control the adhesive forces between tip and polymer surfaces. For both polyethylene and polytetrafluoroethylene the systematic variation of the observed friction forces with the relative scan angle between SFM tip and polymer chain direction was measured. The dependence could be described semiquantitatively with the "Cobblestone model" of interfacial friction. The characterization of modified polymer surfaces using SFM with functionalized tips was described in Chapter 6. Pull-off forces and the degree of functionalization as measured by contact angle measurements of surface-treated polymers and rubbers could be successfully correlated. These results form the basis for differentiation and chemical imaging of polymer surfaces which expose different functional groups. Samples included isotactic polypropylene, polyethylene, polydimethylsiloxane, unvulcanized and vulcanized EPDM rubbers, and butyl rubber. Well-known surface treatments, such as oxyfluorination, chlorination, fluorination, chromic acid oxidation, and plasma treatments, were used to functionalize the polymers. In particular, the pull-off forces measured with carboxylic acid functionalized tips in ethanol were found to correlate with the hydrophilicity of the surfaces as determined by contact angle measurements. Possible limitations of pull-off force measurements on heavily corroded surfaces with high roughness values (chemically etched LDPE) were studied and additionally, the visualization of the dispersion of filler particles in EPDM rubbers and the differentiation of different filler particles was achieved. In Chapter 7 results on laterally resolved imaging of functional group distributions on a sub-50 nm level using scanning force microscopy with chemically modified tips were presented for the first time. Average pH-dependent pull-off force measurements carried out on oxyfluorinated films of isotactic polypropylene using OH-functionalized tips revealed a "force pKa" of 5.5 to 6.5. Laterally inhomogeneous pull-off forces were related to variations of local "pKa" values and different local hydrophobicity, and thus to inhomogeneous distribution of the polar functional groups introduced by the surface treatment on a sub-50 nm scale. In a second set of experiments the distribution of functional groups was investigated in thin films of plasma polymerized allylamine. For these samples a "force pKa" of 5.2 to 6.2 was detected which was independent of the content of amino groups in the films. The results constitute a significant step towards the ultimate aim of detecting functional group distributions in surface-treated or functionalized polymers with a lateral resolution in the sub-50 nm range. With further advances in fabrication of nanometer-sized pointprobes and optimized cantilever spring constants high resolution mapping of functional group distributions in (modified) polymers can also be anticipated in the future in systems that do not contain ionizable functional groups as presented here. By developing a novel approach, which we termed "inverted" chemical force microscopy, the difficulties of thermal drift and limited resolution due to the finite size of the contact area between AFM tip and sample surface in high resolution in situ AFM imaging of chemical reactions were circumvented. In this approach, as described in Chapter 8, the monolayer (i.e. sample) to be studied is immobilized at the surface of the tip, hence the name inverted. In situ force distance measurements on inert substrates using AFM tips coated with the reactants were shown to be capable of following reaction kinetics on a scale of 10 to 100 molecules. Reactivity differences related to different SAM structures were observed by inverted chemical force microscopy on the nanometer scale. These results agreed well with macroscopic behavior observed by FT-IR. These results together with additional force microscopy data supported the conclusion that for SAMs with closely packed ester groups, the reaction spreads from defect sites, causing separation of the homogeneous surface into domains of reacted and unreacted molecules. After describing the results of measurements between ensembles of molecules and the in situ studies of reaction kinetics at the surface of the modified AFM tips, the number of resolved interactions was further reduced. The detection of individual supramolecular hostguest interactions was discussed in Chapter 9. SAMs of b-cyclodextrin heptasulfide receptor molecules were shown to be highly ordered by high resolution SFM. Using these SAMs the successful study of supramolecular host-guest interactions of small molecules with fast unbinding kinetics by means of dynamic single molecule force spectroscopy became possible for the first time. The unbinding force between ferrocene moieties immobilized on AFM tips and b-cyclodextrin receptors in highly ordered self-assembled monolayers on Au(111) was found to be 56 ± 10 pN. Addition of an external guest (8-anilino-1-naphthalenesulfonic acid) was shown in situ to reversibly block most of the receptor sites. The observed single hostguest complex rupture force was found to be independent of the loading rate which is indicative for a thermodynamically controlled experiment. Chapter 10 described the investigation of the controlled positioning of metallodendrimers by means of directed assembly using SFM. Replacement of alkanethiol molecules and filling of defects in SAMs on Au(111) was used to incorporate different sulphide containing dendrimer and hemicarceplex molecules into monolayers. The resulting mixed monolayers were characterized by multimode AFM. Individual immobilized dendrimer molecules were visualized. In addition, the insertion mechanism was studied and the process was followed in situ by contact mode AFM. The initial SAM quality was found to have no significant influence on the surface concentration of dendrimer wedges. In contrast, an increase of the concentration of the dendrimer solution resulted in an increase of the number of observed molecules. The controlled positioning of sulfide derivatized metallodendrimers by means of tip-assisted directed assembly using AFM was described. Areas with high dendrimer concentration on a scale of 200 nm were created. This result proved that it is in principle possible to direct the assembly of the dendrimers, possibly on a scale of several tip-surface contact areas.