Nanoscale spatial chemical information is essential to developing a molecular-level understanding of a variety of phenomena occurring at surfaces and interfaces, including adhesion, friction, and surface reactivity. Therefore, the ability to probe and image materials chemical heterogeneity with nanometer scale spatial resolution is sorely needed. Atomic force microscopy (AFM) is a powerful technique for characterizing surface morphology of materials with nanoscale resolution. However, the ability to identify and map the surface chemical heterogeneity has remained a challenge in the field of AFM. Chemical force microscopy (CFM), where the AFM probes are modified chemically, has been shown as a successful approach for enhancing the chemical sensitivity of model materials by AFM. However, this approach has not been experimented for real world and polymeric materials because the solvent medium used in the CFM technique to avoid the effect of the capillary forces can make irreversible changes on these chemically-complex materials. In this study, the application of a well-controlled humidity system is used to enhance the sensitivity of AFM in characterizing surface chemical heterogeneity of polymeric materials. The relative humidity (RH) of the tip-sample environment is controlled using a humidity generator and a novel small-volume environmental chamber. The relative humidity in the chamber can be controlled from nearly 0 % RH up to 95 % RH. The effects of RH on AFM image contrast are studied using well-defined hydrophobic and hydrophilic patterned self-assembled monolayer (SAM), hydrophilic/hydrophobic gradient SAM, and real-world polymer blend and copolymer samples. The effects of the tip geometry and chemical properties at the AFM tip surfaces are also investigated. Unmodified AFM tips, chemically-functionalized conventional AFM tips, and chemically-modified multiwall carbon nanotube (CNT) attached AFM tips are selected for this study.For all sample-tip combinations, the AFM image contrast between the hydrophilic region and hydrophobic region is very poor (undistinguishable) when the tip-sample environment is very dry ( l 0 % RH) or below 50 % RH. However, the contrast increases gradually with increasing RH from 50 % to 95 %. At high RH levels, even domains that are separated by a relatively low surface energy difference could still be detected with AFM imaging. For block copolymer samples, elevated RH levels not only increase the chemical heterogeneity contrast but also cause a substantial surface rearrangement, in which the areas occupied by the hydrophilic material increase with increasing RH and exposure time. The mechanism of enhanced AFM chemical contrast at elevated RH levels will be discussed.
Proceedings Title: Adhesion Society Meeting
Conference Dates: December 8-9, 2003
Conference Title: Adhesion Society
Pub Type: Conferences
AFM, atomic force microscopy, chemical heterogeneity, friction contrast, phase contrast, relative humidity