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An investigation of phase transitions and molecular motions in polycrystalline n-heptadecylam- monium chloride (C17H35NH3Cl), employing difierential scanning calorimetry, x-ray powder difiraction and nuclear magnetic resonance techniques, is reported. This compound can occur in two virgin polymorphs at room temperature, one interdigitated and one noninterdigitated. The temperature at which crystallisation occurs determines the polymorph that forms. If these polymorphs are heated transitions to noninterdigitated ∞, fl and fi phases occur. Cooling to room temperature shows the same phase transitions, but the virgin phase is not formed. Instead, a noninterdigitated † phase is formed. Defect motions of chain-ends play a signiflcant role in the spin-lattice relaxation rates in all the phases. In the fi phase a degree of chain melting is present. The molecular dynamics of chains difiers during heating and cooling cycles in the † phase. If the temperature is kept constant during a heating cycle in this phase, fourfold motions of chains are frozen over a period of several hours, but the sample remains noninterdigitated. It seems that the interdigitation process is hampered by the population of defect orientations of chain-ends. The methyl group executes classical threefold reorientations and the NH3 group jumps in an asymmetric threefold potential well.

Phase Transitions and Molecular Reorientations in Bilayered n-Heptadecylammonium Chloride