FT-IR and 2D correlation spectroscopy were employed to study the microstructural changes ocurring during phase transitions of a liquid crystal poly( amidoamine) codendrimer ( PAMAM (L-1) 16(L-2)16) generation 3, functionalized on the terminal groups by one-chain promesogenic calamitic units (4-(4'-decyloxybenzoyloxy)salicylaldehyde (L-1)) and two-chain promesogenic calamitic units ( 4-(3', 4'-didecyloxybenzoyloxy) salicylaldehyde ( L2)). Spectral modifications associated with molecular conformation rearangements allowing for molecular shape change on going from a liquid-crystalline organization to another were found. The transition temperatures were calculated, and they are in good agreement with the DSC data. Spectral analysis gives evidence of the LC phase transitions and to an additional transition associated with the existence of conformers. Various types of hydrogen bonding have been established.