Surface End-Grafted Poly(acrylic acid) Monolayers. The synthesis and chemical characterization of surface end-grafted poly(acrylic acid) (PAA) monolayers as used in this study is described in detail in refs 9 and 10. In brief, the “grafting-from technique” was used to prepare the PAA (-CH2CHCOOH-)n monolayers. The polymerization initia-
tor was covalently bound via monochlorosilane headsgroups to the inorganic substrate (silicon wafer) and the PAA chains were synthesized under controlled/living radical po- lymerization conditions. The grafting density was afterward adjusted by partial cleavage of the PAA chains from the surface. An ester group that connects the initiator and the silane anchor group acts herein as a break-seal group.
Gel permeation chromatography (GPC) and X-ray photoelectron spectroscopy (XPS) we- re applied to the samples to determine the molecular weight of the PAA chains and to estimate their grafting density on the surface. This characterization opens up the possi- bility to directly compare GPC and AFM results obtained on the same samples.
In the following, it is referred to two PAA monolayer samples, which are for differentiation named “dense” (D) and “dilute” (d). The PAA chains from the “dense” monolayer have an average contour length ofhlciD ≈590 nm and a polydispersity of PDD = 1.3. The “dilute”
Langmuir 23 (2007), 6660-6666. 53
sample consists of PAA chains with an average contour lengthhlcid≈450 nm and PDd=
1.24. The grafting densities were estimated as follows:σD ≈0.005 molecule/nm2 ≈2.3σd. AFM Desorption Experiments. In AFM-based desorption measurements using end- grafted polymer monolayers, single polymer chains are adsorbed on the tip of an AFM cantilever and successively desorbed from the tip by separating the AFM tip and sam- ple surface (see Figure 1). With these experiments, we are able to probe a multitude of polymers grafted from a flat surface with the very same tip as follows: When approa- ching a bare AFM tip the chains will adsorb on the AFM tip as they gain adsorption enthalpy. Upon retraction of the AFM tip, the polymers successively desorb in an equi- librium process, which is due to a much faster dynamics of the surface-polymer contacts in comparison to the retraction velocity of the AFM tip (ca. 1 µm/s). The polymers are not pinned to the AFM tip but highly mobile; that is, the adsorbed polymer segment is able to slide on the tip surface.8 This means that the heterogeneities of the tip surface
are not reflected in the measured force as the polymer rearranges during the desorption in order to maximize its adsorption enthalpy. The corresponding force-distance curves thus show plateaus of constant force representing the desorption process. As the complete desorption of a polymer chain is indicated by a drop in force, the number of desorbed molecules equals the number of observed plateaus, and the absolute distance at which a force drop occurs states the bridging lengthlbr. At high adsorption strengths, the polymer
chain stays adsorbed until the end segment detaches, and the bridging length therefore represents the contour length of the polymer. Lateral displacements of the grafting point on the sample surface and the tip apex shorten the bridging length in comparison to the contour length of the polymer chain. However, in the case of contour lengths on the order of several hundred nanometers as used here (i.e., at length scales much larger than the end-to-end distance of the polymer chains), this effect becomes negligible.
The height of the force plateau represents the magnitude of the desorption force between the PEL chain and the surface, the mean desorption force is obtained by a statistical analysis. The desorption force histograms typically show values for the full width at half- maximum (fwhm) of ca. 6 pN, which enables a determination of the desorption forces with 1-2 pN precision. The analysis of the desorption forces is a bit more complicated here as the desorption forces are slightly dependent on the bridging length. From one measurement series, all plateaus are analyzed in terms of bridging length, but only the last plateau is considered for the desorption force as here no intermolecular interaction can adulterate the polymer-surface interaction. The desorption force is then taken from the bridging length that corresponds to the maximum position of the bridging length distribution.
All desorption experiments were conducted with a custom-made instrument using sili- con nitride (Si3N4) cantilevers (Microlever) purchased from Veeco Instruments. Nominal
54 Choose Sides: Differential Polymer Adhesion
calculated spring constants may have deviations of up to 10%; therefore, measurements series were performed with the same cantilever for being able to monitor minute force differences.
Figure 1. (a) Schematics of a single molecule desorption experiment with surface end-grafted polymers by means of an AFM. (b) Typical records of the de-adhesion force measured upon retracting the tip from the surface. A plateau represents a polymer sliding off the tip. The number of plateaus equals the number of desorbed polymer chains. (c) Statistical analysis of the bridging lengthlbr and the desorption force FA. The distribution of the bridging length is a function of the molecular length distribution of surface-grafted chains. The histogram of the desorption force can be fit by a Gaussian distribution with typical values for fwhm of ca. 6 pN. Histograms were taken from the “dilute” monolayer at pH 6.
Langmuir 23 (2007), 6660-6666. 55
Before the measurements, the Si3N4 tips were cleaned with UV light, and control measu-
rements on the interaction of the AFM tips with bare silica substrates were performed in order to exclude contaminations of the tip surface. In the desorption experiments, only these bare Si3N4 tips were used. The experiments were conducted in aqueous solution
containing 100 mM NaCl, and typically at least 1000 force-distance curves were measured on different spots on the surface to ensure that a representative ensemble of molecules is probed.