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Nanoscale Mapping of Chemical Heterogeneity in Polymeric Materials by the Use of Phase Imaging AFM and Humidity

T. Nguyen, X. Gu, L. Chen, M. Fasolka and J. Martin
National Institute of Standards and Technology, US

Keywords:
nanoscale, chemical imaging, nansocale chemnical characterization, AFM, polymer, heterogeneity

Abstract:
Nanoscale mapping chemical heterogeneity of polymer surfaces is the subject of great interest in chemical sciences and technologies. However, the development of techniques capable of providing chemical information of polymer surfaces at the nanoscale spatial resolution is still in the infancy state. By utilizing phase imaging in taping mode AFM and manipulating the relative humidity (RH) of the tip-sample environment in an AFM, we have successfully detected and imaged nano hydrophilic/hydrophobic regions in polymers. This talk will present data to demonstrate the capability of this technique. Samples from a variety of hydrophobic/hydrophilic block copolymers and polymer blends have been studied. A humidity generator connected to a NIST-patented humidity chamber fitted to an AFM is employed to provide tip-sample environmental RH between 0 and 95 %. The humidity in the environmental chamber could be controlled to within 3 % of the preset values. For all samples, the AFM phase image contrast between the hydrophilic region and hydrophobic region in a polymeric material is poor (undistinguishable) when the tip-sample environment was dry ( 0% RH) or below 50 % RH. However, the chemical image contrast increases markedly with RH > 50 %. At high RH levels, even chemical domains that have small difference in the polar surface free energy component could still be detected with this approach. No effect of RH on the height image contrast is observed. Further, 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. Adhesion force measurements show that the tip/sample interactions increase markedly with increasing RH for the hydrophilic domains but are essentially unaffected for the hydrophobic domains. This difference is believed to be responsible for the enhanced image contrast observed with high RHs. The increased tip-sample interactions in the hydrophilic with increasing RH may be due to increases in the number of hydrogen-bonding adsorption sites of both the tip and the sample, polymer plasticization, and capillary force.

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