Observation of a Single Adsorbed Polymer’s Conformation on Mica with Scanning Probe Microscopy: Effects of Retraction Rate and Dwell Time

Introduction The conformation of adsorbed polymers on surfaces is critical in the success of many industrial applications, such as flocculation and dispersion. For flocculation, adsorbed polymers with large loops and tails perform better due to an increased collision efficiency. Direct observation of the conformation of a polymer chain on a solid surface will aid in the design of new flocculants, dispersants, etc. Currently, polymer conformation on a surface is most commonly determined by indirect methods such as dynamic light scattering, ellipsometry, and surface force techniques. The results from these techniques allow the researcher to infer what the conformation is, using different assumptions. Less commonly used techniques, such as small angle neutron scattering (SANS) and scanning probe microscopy (SPM), require fewer assumptions by directly measuring the polymer’s adsorbed conformation. SANS does this by obtaining the density of a labeled polymer at different distances from a surface using neutron reflection. However, SANS testing is very dependent on the quantity of liquid the neutrons must travel through and the reflection that can occur from the solid interface. Also, this equipment is not readily available for most researchers. SPM, however, is a readily available technique which can also be used to directly measure the conformation of an adsorbed polymer. One recently published method to measure polymer conformation on a surface involved pulling a single polymer from a surface. To do this Haschke et al. attached a polymer with one terminal SCH2COOH group to a gold coated SPM cantilever tip using a covalent bond between the thiol group and the gold surface. The tip was then touched to the surface and retracted allowing detection of the deflection of the cantilever as hydrogen bonds or electrostatic interactions break. For Haschke’s system, and many commercial systems, physi-sorption of the polymer is the accepted method of attachment. Thus, the covalent bond of the polymer to the tip is stronger than any desorption forces and can be assumed to be unbreakable. Haschke’s method made significant advancements into the use of SPM to determine adsorbed polymer conformation. However, it did not offer proof that only a single polymer is adsorbing to the surface because the SPM tip provides a large surface area for polymer chains to adsorb on. This allows the possibility that multiple polymer chains will absorb regardless of solution concentration. Without the assurance of one polymer chain, the data can not be interpreted to find the length of polymer loops. Furthermore, because the entire tip is gold it is not known where the polymer chain is located. If the polymer is located far above the apex of the tip it is possible not to detect any polymer adsorption because it will never contact the surface. This work has resolved the problems of Haschke’s method, mentioned above, and given further guidelines towards successful use of this technique. The affect of the speed of retraction of the cantilever and the amount of time the tip is left in contact are also analyzed and reported. With the modifications made in this work it will be possible to easily and directly determine the loop length of a polymer in solution.