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The adhesion or bonding of two contacting surfaces is essential for a variety of processes, both natural and artificial. For example, the ability of insects and lizards to move on vertical walls requires their legs to adhere to the surface of the wall. To name a few technical processes: medical prosthetics, soldering, and surface coatings all have adhesion at their core. Thus, such an abundant phenomenon requires a tool to access and accurately study adhesion at a variety of interfaces. Atomic force microscopy (AFM) enables the measurement of adhesion at the nano-/microscale due to its high accuracy in force detection and lateral position sensitivity. Polymers are a suitable substrate for studying adhesion peculiarities due to their stimuli-responsiveness, which allows for fine-tuning of material properties. Therefore, for the sake of a broader comparison, the adhesion of the polymer interfaces is studied here with different AFM-based techniques on the microscale to select the most accurate one. The AFM study of adhesion is based on the mechanical contact between the indenter on the cantilever and the sample. Thus, increasing the contact area between the indenter and the sample allows for an increase in the accuracy of the measurement because the measured force is larger. Therefore, the use of a soft deformable particle attached to the end of the cantilever, called a soft colloidal probe, increases the accuracy of the measurement. However, the use of a soft colloidal probe can introduce non-linear effects during the rupture of the contact between the probe and the substrate. The main goal of this work is to measure the thermodynamic work of adhesion, regardless of these effects. First, we aimed to demonstrate the applicability of soft colloidal probe AFM to complex substrates. To achieve this, we applied the soft colloidal probe AFM to an artificial polymer with a structure like a mussel foot protein (a-mfp) distributed on a glass surface. The polymer was synthesized having a functional group sensitive to the pH change from 5.5 to 6.8. The change in pH induced a change in the structure of the polymer. The soft colloidal probe AFM revealed that remarkably, the a-mfp coating was 5 times less adhesive at pH 5.5 than at pH 6.8 when the experiments were conducted separately using different substrates for each pH value. However, when we changed the pH in situ during the measurement, we detected only 2-2.5 higher adhesion values for the case of pH 6.8. We attribute this result to the deposition of a-mfp on the surface of the colloidal probe. The second step was to use the PNIPAM anchored to the substrate (PNIPAM brush) as a sample sensitive to the non-solvent effect. Previously, we relied only on the calculation of the force required to break the contact between the probe and the sample to access the adhesion properties. However, for the contact between elastic deformable bodies (i.e. soft colloidal probe, polymeric sample), the rupture of the contacts depends on the parameters of the rupture: the speed of the contact rupture and the pressure during the contact. Nevertheless, the use of the soft colloidal probe allowed an optical measurement of the contact area, which allowed us to access the work of adhesion without the rupture of the contact. In this way, we were able to compare the two approaches: The AFM-based measurement of the pull-off force (contact rupture) and the optical measurement of the contact radius (no rupture). It was found that the results of the two approaches were drastically different: the values of the work of adhesion from the pull-off force were around 10 times larger. The significant dependence of the pull-off-based work of adhesion on the contact history and the parameters of the pull-off event with the very short time scale of the pull-off event makes the values of the work of adhesion from the pull-off force very far from the equilibrium values. At the same time, the measurement of the contact area does not rely on fast processes. Thus, the adhesion work values from the contact area measurements were found to be much closer to the equilibrium values and therefore more accurate. We found that the work of adhesion of the PNIPAM brush was higher in the swollen state than in the collapsed state and attributed it to the adaptability of the polymer chains to change their conformation under contact. In other words, the freedom of movement of the chains significantly affects the work of adhesion of the PNIPAM brush. To further investigate this idea, we carried out a preliminary study on the PNIPAM hydrogel with redox stimuli-responsive and non-responsive cross-links, which allowed us to control the degree of cross-linking in situ. The decrease in the degree of crosslinking under the redox stimuli made the hydrogel more adhesive both in water and under co-nonsolvency conditions. Thus, we can conclude that the freedom of movement of the chain in the case of PNIPAM is crucial for its adhesive properties. Ultimately, this work demonstrates that the optical measurement of the contact area, rather than the theoretical prediction of the contact area, is critical to accessing the work of adhesion accurately and close to equilibrium. In addition, we have demonstrated that such measurements are applicable even to complex polymer systems. |
