Highly ordered Langmuir-Blodgett films of amphiphilic poly(propylene imine) dendrimers

Three amphiphilic poly(propylene imine) (PPI) dendrimers were synthesized by (1) attaching dodecanoyl chains to a PPI (DAB-dendr-(NH2)(8)) dendrimer core, (2) reducing the amide groups to secondary amines, and (3) methylating the secondary amines to tertiary amines. Monolayers of all three dendrimers on water were compressed to areas of 160 A(2)/molecule. The monolayers of the dendrimers were transferred to newly cleaved mica by the Langmuir-Blodgett method. Atomic force microscopy (AFM) measurements showed that the amphiphilic dendrimers formed stable homogeneous monolayers with surface roughness < 0.5 nm on mica. High-resolution AFM images displayed tetragonal order of a two-dimensional crystal with alkyl chain-to-chain spacing of 0.4-0.5 nm. Differential scanning calorimetry experiments detected glass transitions of the bulk dendrimers in the range of -12 to -60 degrees C and endothermic transitions from more ordered to less ordered molecular packing at higher temperatures.