SCANNING FORCE MICROSCOPY STUDIES ON MOLECULAR PACKING AND FRICTION ANISOTROPY IN THIN-FILMS OF TETRANITROTETRAPROPOXYCALIX[4]ARENE

Thin films of tetranitrotetrapropoxycalix[4]arene (1) show an unusual phase behavior and the formation of a complex multidomain structure. A n endothermic phase transition, which occurs between 130 and 140 degre es C in bulk, was studied using differential scanning calorimetry, opt ical microscopy, magic angle solid-state NMR, and X-ray crystallograph y. In annealed films of 1, two types of crystalline domains (I and II) can be distinguished with different optical, morphological, and frict ion properties. These domains were investigated at the molecular level by a combination of multimode scanning force microscopy (SFM), optica l microscopy, and X-ray diffraction experiments. SFM force measurement s as well as tapping mode phase images showed that different functiona l groups are exposed at the surface for the different domains. The fri ction forces observed in SFM depend on the type of domain, the applied load, and the orientation of the domain with respect to the scanning direction. The friction forces observed at normal forces below ca. 240 nN are lower for type I domains than for type II domains. For forces higher than 260 nN an inversion of the relative friction is observed. Moreover, type I domains exhibit a friction anisotropy that can be att ributed to different orientations of the molecular crystal structure w ith respect to the scanning direction. Two lattices were observed by h igh-resolution SFM in type I domains, one of rectangular symmetry (x = 10.0 Angstrom, = 11.8 Angstrom, alpha = 90 degrees) and one of pseudo hexagonal symmetry (d = 11.6 Angstrom), which were in agreement with t he parameters of the (010) and (011) facets of the X-ray single-crysta l structure (a = 23.94 Angstrom, b = 33.01 Angstrom, c = 20.59 Angstro m, and alpha, beta, gamma = 90 degrees). In conclusion, the molecular packing and friction properties of the multidomain structure of thin f ilms of 1 could be elucidated by SFM and complementary methods.