SURFACE-PROPERTIES OF OXIDIZED LDPE BY SCANNING FORCE MICROSCOPY WITHCHEMICALLY-MODIFIED PROBES

In this article, we present the results of a study on the surface prop erties of chromic acid-oxidized low-density polyethylene (LDPE) by sca nning force microscopy (SFM) and contact angle measurements. LDPE film s were surface modified by a chromic acid treatment with subsequent an nealing in argon and reconstruction in boiling water as described by R asmussen, Stedronsky, and Whitesides [J. Am. Chem. Sec., 99, 4736 (197 7)]. The LDPE oxidation in chromic acid was monitored in situ by conta ct mode SFM. Initially stacks of lamellae became exposed, and at later stages a granular morphology was observed. By tapping mode SFM, the s ample roughness was shown to increase during the first 10 min of oxida tion from initially ca. 20 nm to ca. 50 nm. Gold-coated SFM probes (ti ps) functionalized with self-assembled monolayers were used to determi ne the pull-off force characteristics in ethanol. Variations in the co ntact area between SFM tips and polymer surfaces that exposed sharp cr ystalline features were shown to obscure the results of pull-off force measurements. However, on annealed and subsequently reconstructed sam ples with lower roughness, the results of force measurements correlate d well with the measured contact angles. Over the range of surface ene rgies studied, the normalized pull-off force between carboxylic acid-m odified tips and these smooth samples was shown to depend approximatel y linearly on the cosine of the contact angle. (C) 1998 John Wiley & S ons, Inc.