Using temperature-dependent Fourier transform infrared (FTIR) spectroscopy, we probe the molecular level, chain-structural dynamics associated with solid-solid transitions between 25 and 250 degrees C in a layered inorganic-organic silver dodecanethiolate, AgS(CH2)(11)CH3. Spectroscopic evidence presented here establishes two major transitions: the transition occurring at similar to 130 degrees C is characterized by an abrupt, but fully reversible, change in the chain conformational order from an initial all-trans state to the one characterized by mixed or partial chain disorder. The observation of this phase transition is consistent with the previous predictions of a rapid and drastic change in the structural motif from an initial bilayer to the final micellar state. The second transition at about 190 degrees C, which is consistent with the previous assignment of micellar amorphous transition, is furthermore irreversible and represents thermal degradation of the material. Implications of these results for the general family of chain molecular assemblies in constrained molecular environments are discussed.