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. An endothermic phase transition, which occurs between 130 and 140 °C in bulk, was studied using differential scanning calorimetry, optical microscopy, magic angle solid-state NMR, and X-ray crystallography. In annealed films of 1, two types of crystalline domains (I and II) can be distinguished with different optical, morphological, and friction properties. These domains were investigated at the molecular level by a combination of multimode scanning force microscopy (SFM), optical microscopy, and X-ray diffraction experiments. SFM force measurements as well as tapping mode phase images showed that different functional groups are exposed at the surface for the different domains. The friction 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 attributed to different orientations of the molecular crystal structure with respect to the scanning direction. Two lattices were observed by high-resolution SFM in type I domains, one of rectangular symmetry (x = 10.0 Å, y = 11.8 Å, α = 90°) and one of pseudohexagonal symmetry (d = 11.6 Å), which were in agreement with the parameters of the (010) and (011) facets of the X-ray single-crystal structure (α = 23.94 Å, b = 33.01 Å, c = 20.59 Å, and α,β,γ = 90°). In conclusion, the molecular packing and friction properties of the multidomain structure of thin films of 1 could be elucidated by SFM and complementary methods.