Despite the success of the Atomic Force Microscope operating in intermittent contact for microstructural characterization, important questions remain about the physical origin of the image contrast. A new technique (named Peak Force Tapping) based on the real time force distance curve analysis has been recently introduced to map the nanomechanical properties of materials (such as stiffness, adhesion, deformation, dissipation) simultaneously to the topography, with much higher resolution and less damage.

Beyond Imaging ...

The development of nano-sciences involves a large effort to study and understand nanometer-scale physical and chemical phenomena. Methods using local force probes (such as Atomic Force Microscopy - AFM) naturally provide important contributions to these studies. In particular, dynamic force techniques, i.e., using an oscillating probe, are well adapted to soft samples such as polymer materials or biological systems. The first demonstration of their potential was the elucidation of the phase-separated microstructure in thin films of block copolymers 15 years ago by phase detection imaging [1, 2]. Intermittent contact mode images can be of two different types: in the first one, the image corresponds to the changes in the piezo-actuator height that are necessary to maintain a fixed oscillation amplitude, through a feedback loop (the height image); in the second one, the image contains the changes in the oscillator phase delay relative to the excitation signal (the phase image). The phase measurement in many cases yields images reflecting tiny variations of the local properties of the sample surface. On that basis, it is possible to extract useful information from phase images of soft samples, especially for materials showing small-scale mechanical heterogeneities.

The height images are generally considered to display topographic information, but it must be kept in mind that the local mechanical properties of the sample (i.e., the possibility that the tip penetrates the surface slightly) may also contribute to the contrast in the height image. The major factors contributing to the phase contrast are still under debate, but are thought to be a result of viscoelastic response and adhesive forces rather than elastic surface behavior [3].

While it is well established that AFM can probe material properties through mechanical interactions, achieving semi quantitative or quantitative information on the mechanical properties is a real challenging task.

To address this important issue, a lot of methods have been developed in the last decade for local stiffness or adhesion measurements but they suffer from low operational speeds (long data acquisition times) and usually require large forces to be applied to analyze the sample surface, limiting high resolution and precluding measurements on soft materials such as polymers or biological samples. A new technique (named Peak Force Tapping [4]) based on the real-time force distance curve analysis during imaging has been recently introduced to map, simultaneously to the topography, with much higher resolution and less sample damage, the nanomechanical properties (such as stiffness, adhesion, deformation, dissipation, ...) of materials by applying very low forces to the sample (fig. 1). Here, we illustrate the major morphological properties observed (and sometimes unexpected) as well as the (semi) quantitatively measured nanomechanical properties by considering a series of complex multiphase polymer blends of water-borne polyurethanes mainly used as protective coatings.

Water-borne Polyurethanes: An Illustrative and Industrial Example!

Radiation curing technology has been used successfully in the industry for over 30 years. During the nineties, the introduction of water-based alternatives profoundly changed the landscape of the radiation curing market by combining the high performance and productivity of traditional radiation curing compositions with the low viscosity of water-based systems that makes them particularly suitable for application by spray, curtain or roller. They all largely respond to the environmental regulations due to their waterborne nature and the absence of additional coalescent, since the film formation (drying) and hardening (photo-curing) take place in two distinct steps. They usually target challenging performance areas within growing market segments for indoor and outdoor coating applications.

Radiation-curable polyurethane dispersions (UV-PUDs) are usually anionically-stabilized unsaturated polyurethane colloids in water showing a good colloidal stability and building on this strong environmentally-friendly asset. They encompass a very large range of compositions and molecular weights, from oligomers to very high molecular weight polymers with a varying amount of urethane, urea, allophanate and biuret functionalities and within a linear or branched polymeric architecture. As such, it has been possible to develop an intimate blend of hard and soft polymer dispersion delivering a superior balance of mechanical properties that has been valorized for the radiation curing of pigmented systems offering adhesion to various substrates on one hand, scratch and chemical resistance on the other hand.

In order to characterize the (micro)phase structure of these complex coatings, the thermal and thermo-mechanical properties were measured together with the tensile properties. A reinforcing effect is clearly observed in the multiple-phase structure obtained from hard and soft unsaturated polyurethanes. It can be explained by the presence of the hard domains embedded in the continuous soft matrix with the presence of an interphase [5]. These results have been correlated with the micro-phase characteristics determined by AFM and, more interestingly, the local nanomechanical properties such as stiffness, adhesion, dissipation and deformation can be simultaneously mapped by using Peak Force Tapping (fig. 2).

This methodology has been successfully extended to other polymeric systems with increasing complexity (block copolymers, hyperbranched polymers, supramolecular polymers) used for pressure-sensitive adhesives, biomimetic adhesives, hydrogels, packaging films, protective coatings, and thermoplastic elastomers. It demonstrates the wide range of applications of this novel technique in the field of materials science.

References
[1] Leclère Ph. et al.: Langmuir, 12, 4317-20 (1996)
[2] Kopp-Marsaudon S. et al.: Langmuir, 16, 8432-37 (2000)
[3] Leclère Ph. et al.: Scanning Probe Microscopy of Complex Polymer Systems: beyond imaging their morphology in "Scanning Probe Microscopies: beyond imaging", P. Samorì Ed., Edited by Wiley-VCH, 175-207 (2006)
[4] PeakForce QNM: Quantitative Nanomechanical Property Mapping, Bruker Applications Note (2011)
[5] Tielemans M. et al.: Proceedings of the 38th Annual International Waterborne Symposium, High-Solids and Powder Coatings Symposium, New Orleans, LA, February 28 - March 4, 449-463 (2011)
[6] Derjaguin B.V. et al.: J. Colloid. Interface Sci. 53, 314 (1975)