AFM is a relatively recent technology development for imaging materials and has found wide applicability to the study of polyurethanes [38, 39]. Unlike electron microscopies, AFM depends on the interaction of a sample's surface with a scanning tip that can provide an electronic signal to recording electronics or be detected remotely by laser detection off of the tip about its coordinate position and its deflection in the z-position. This obviates the need for large vacuum chambers (although atomic-level detection may still employ high vacuum to prevent gas adsorption). These advantages mean that AFM can be well employed in ambient environments, in solvent-laden systems, or even on biological specimens. Numerous textbooks and research monographs [40, 41] are available with detailed information on instruments and techniques.
When an AFM tip is placed in direct contact with a surface (in essence a measure of the repulsive force), the technique is called "contact mode." Contact mode can be useful for investigating the surface of hard materials. It has the advantage of providing a direct interrogation of the surface but can provide misinformation of surface topology if the surface is deformable by the scanning tip and can also cause tip wear over time, which can affect measurement fidelity. In practice, the tip is scanned over the surface with a constant downward force and the tip position monitored by sensors.
In "noncontact" mode, the tip does not directly contact the surface . Instead, tip position is carefully controlled to hover just above the sample surface. In contrast to the repulsive forces monitored by contact mode, noncontact mode monitors attractive forces between the tip and the surface. The tip is not abraded by wearing on a surface, and the sample surface is not affected by contact with the high-modulus (silicon or silicon nitride) tip. The results of contact and noncontact modes applied to rigid samples will usually be very similar. However, soft materials, or materials with loosely bound surface layers, will be better understood using noncontact techniques. The scanning of the tip over the surface can be operated in a frequency modulation mode where intertip/surface forces modulate at a resonant frequency or can be operated with amplitude oscillation modulation mode where amplitude modulation or phase perturbation can be monitored. These modulations of a controlled frequency or amplitude can be correlated to characteristics of the probed material.
Tapping mode AFM  is in some ways a compromise between contact and non-contact methods of tip operation. The tip is oscillated above the surface of the sample, but it approaches the surface much more closely such that it intermittently samples the repulsive or contact regions of the tip/surface interactions. As the material taps the surface, it probes directly the surface topography but does not drag or distort the surface due to lateral surface interactions. Furthermore, the interactions incurred during the taps are readily related to material properties through the sinusoidal phase change induced in the oscillation. Figure 5.7 is an illustration showing a method of tip amplitude control and sample positional control used by one particular manufacturer of tapping mode AFMs.
Figure 5.8 illustrates the portion of the attraction/repulsion potentials sampled by each of these techniques. As the distance of the probe is brought into contact with the surface, the force measured by the probe is greater than zero. This allows a direct measurement of surface topography with position, but may lose information about
FIGURE 5.7 Simplified illustration showing sample and signal detection controls for a tapping mode atomic force microscope and the appearance of a working instrument. Images courtesy of Bruker Nano-Surface. (See insert for color representation of the Figure .)
Figure 5.8 Potential diagram of attractive and repulsive forces influencing an AFT tip as it approaches a surface in contact, noncontact, and tapping modes.
subtle material properties. It also may suffer some resolution insufficiency if the probe is too blunt to follow acute surface features. In tapping mode, the probe oscillates above but very close to the sample surface. As the probe approaches the surface near the minimum in the force, the attractive forces will bring the probe into physical contact with the surface and bring the measured force into the repulsive region of the potential. This allows the probe to not only sample surface topography but also directly probe material properties such as adhesion and modulus. Noncontact mode samples the region away from the sample surface and samples the positional change in the attractive forces of the material for the probe. As the probe samples the relative height of the sample, it will register as a change in attractive forces.
AFM has brought a new appreciation for the underlying microstructure responsible for the properties of polyurethanes [44—47]. It has furthermore elevated the theoretical basis for understanding the mechanism for polyurethane phase separation and the position polyurethanes occupy as a class of block copolymers . Figure 5.9 is an example of information available from, in this case, tapping mode AFM. The image shows the height image garnered from the potential forces experienced by the probe as it oscillates over the surface termed the "height." The phase image shows the information obtained by the probe in contact with the surface and as it is preferentially repulsed by the very-high-modulus hard segment (white in the image) and the relatively adhesive soft segment (dark). The image is consistent with proposed theories (see Chapter 4) related to mechanisms of phase separation and block copolymer equilibrium structures. Experimental details are provided in the Figure caption to impress upon the reader that even a relatively straightforward experimental technique must be performed with precision and attention to small details to obtain meaningful and useful results. Beyond that, the membrane/lacey structure of the hard segment is remarkable for its aesthetic value as well.
Figure 5.9 A piece of the foam was embedded in epoxy and then cured. The cured block was cryopolished at -110°C. The resulting face was exposed to RuO4 vapor, and then the block was repolished at room temperature. The face on which the TMAFM was done probably had regions that retained the Ru stain. TMAFM was obtained on a Digital Instruments MultiMode using a NanoScope IV Controller (Software v 5.12r3). Silicon cantilevers and tips were used (Nanosensors, LTESP, wafer number 1795L262, frequency = 158 kHz). Typical tapping conditions were Ao ~1.0 V, Asp ~0.6 V, and rsp ~0.60, with the tip/surface interaction repulsive in nature. Images courtesy of Greg Myers. Reprinted with permission from Ref. . © Elsevier Pub.