Glass transition of one-dimensional molecular chains of p-nitroaniline confined in AlPO4-5 nanopores revealed by dielectric spectroscopy

The dynamics of molecular chains of hydrogen-bonded p-nitroaniline (pNA), c onfined in the one-dimensional pore system of AlPO4-5, was studied by broad band dielectric spectroscopy (10(-2)-10(6) Hz). Two relaxation processes we re observed. A fast process (the alpha process) shows above 72 degrees C a Vogel-Fulcher-Tammann behavior typical of glass-forming materials, indicati ng cooperative dynamics of the "condensed" pNA chains. Below the glass temp erature the alpha-relaxation peak broadens significantly and obeys an Arrhe nius law. Both observations imply a change in length scale of cooperativity xi from a temperature-controlled to a chain-length-dependent one. The seco nd relaxation process is tentatively assigned to structural relaxations of the AlPO4-5 host lattice itself, since it is also present in calcined (empt y) crystals. (C) 1999 American Institute of Physics. [S0021- 9606(99)70437- X].