Cholesteryl hydrogen phthalate melts to an isotropic liquid at 431 K. The liquid readily supercools and undergoes an isotropic -" cholesteric transition at 367 K. On further cooling the turbid cholesteric phase becomes increasingly more viscous and undergoes a glass transition at 297 K, as indicated by differential thermal analysis. In the cholesteric phase3 the alignment of the molecular axes has a long-range order as in a nematic phase, but this order is restricted to within the plane and each subsequent plane has molecular orientations (or local director) twisted by an angle of<1. Thus the arrangement acquires a helical pattern with the optical axis coincident with the helical axis, which is normal to the planes. Measurements were made on unoriented samples contained in a parallel-plate, three-terminal dielectric cell whose temperature was controlled from 77 to 440 K. The distance between the highly polished surfaces of the electrodes was - 2 mm and the strength of the electric field was 20-25 V cm"1. The cholesteric phase between the electrodes had no preferred orientation; the symmetry of the sample was helical over distances of, say, a few micrometres and spherical over longer distances, because the order induced by the surface of electrodes extends to a distance of a few hundred micrometres, and thus the helical axes for the domains may assume all possible orientations. The capacitance and conductance, or dielectric loss factor, were measured from 0.1 to 100 Hz, using a low-frequency bridge4, and from 102 to 105 Hz, using a GR 1615 A capacitance bridge. No change in capacitance or conductance occurred with a change in the electric field strength from 2.5 to 25 V cm"1. The dielectric loss tangent, tan 8, at 1 kHz of the supercooled and glassy cholesteric phase is plotted against temperature in Fig. 1. The two peaks are due to molecular relaxations, in the supercooled liquid at 321 K and in the glass at 155 K whose rates correspond to the frequency of 1 kHz. These are referred to as main and secondary relaxations respectively. The presence of a secondary relaxation indicates that, even in the otherwise rigid matrix of a cholesteric glass, configurational states which involve changes in the electric polarization are available to the structure. The lower magnitude of its dielectric loss, or orientation polarization, also suggests that either the number of molecules contributing to the secondary relaxation and/or the dipole moment associated with the molecular reorientations is much lower than that in the main relaxation.Fig. 1 The dielectric loss factor, tan 6, at 1 kHz of the unoriented samples of cholesteric phase of cholesteryl hydrogen phthalate in the supercooled liquid and glassy states plotted against temperature. rg is the calorimetric glass transition temperature. The spectrum of the main relaxation in a frequency plane is asymmetric and is broader at the high-frequency than at the low-frequency side of the peak. The half-width of the peaks is 2 decades of frequency, which is significantly higher than 1.14 decades characteristic of a single Debye-type relaxation. The spectrum of the secondary relaxation given in Fig. 2 is also broad. Here the half-width of the peaks increases from 4 decades at 190 K to 5 decades at 139 K. Thus both the main and secondary relaxations have a distribution of relaxation times, but the distribution changes significantly with temperature only for the latter. The relaxation rates, which are given by the frequency, /m, of the peaks in the spectrum, are plotted-logarithmically against the reciprocal temperature in Fig. 3. The rate of the main relaxation follows the Vogel-Fulcher-Tamman equation of the type, /mAexp(-J3/(rr0)), where A, B and T0 are empirical contants. The extrapolated relaxation rate at the calorimetric Tg (= 297 K) is 104 s"1. Thus the molecular degrees of freedom causing the dielectric relaxation of the cholesteric phase are undoubtedly the ones whose freezing out causes the decrease in the heat capacity at Tg. The rate of the secondary relaxation in Fig. 3 follows the Arrhenius equation, /m = /0 exp (-E/RT). This gives an activation energy,, of 19.8 kJ mol"1 for the molecular processes in the cholesteric glass. The aforegiven features of both the main and secondary relaxations in the cholesteric liquid crystal are strikingly similar to those observed in amorphous polymers5'6, rigid molecular glasses7 and orientationally disordered crystals2. This similarity suggests that the potential energy barriers resisting the molecular motions, whose freezing out causes the glass transition, are independent of the internal degrees of freedom and the state of aggregation of molecules, and that the molecular mobility is intrinsic to the nature of the glassy state.In terms of the models for the structure of a glass, which involve dense random packing8'9, microcrystals10, dislocations11"14, mixed clusters of competing polymorphs15'16, discli-nations17 and dislocations18, the secondary relaxation may arise from hindered rotations of molecules in localized regions of loose molecular packing19 (referred to as 'islands of mobility') bounded by the relatively immobile close-packed structures. The localized regions may consist of the interstitial packing20 between the amorphous clusters, or polyhedra, if the structure of a glass consists of regions of varying molecular density. In such a case only a fraction of the total number of molecules in a glass may contribute to its secondary relaxation. Although such localized regions may exist due to the freezing-in of the short range structures corresponding to the density fluctuations in the liquid at Tg, it is also believed that the secondary relaxation indicates small angle movement of each molecule2 in the essentially equivalent molecular environments of a dense random packed structure of a glass. In this case all molecules in a glass contribute to the relaxation but each by a small amount.Owing to the nature of the packing of the rod- or lath-like molecules fewer regions of relatively loose structure would exist in a glass obtained from a cholesteric liquid than in one obtained from an isotropic liquid. Consequently, the magnitude of tan 8 in the secondary relaxation relative to the main would, in general, be lower in the former than in the latter type of glass. But, if the small angle movement of all molecules were responsible for the secondary relaxation, as is expected in the dense random packed structure of a glass, the aforementioned relative magnitude of tan 8 would be similar in the two types of glasses. The ratio of maximum tan 8 at 1 kHz of the secondary to the main relaxation in Fig. 1. is 1:58 in the cholesteric glass, which is much lower than the corresponding ratio of 1:10 or less, observed in most molecular2'7 and polymeric glasses5. Therefore, the secondary relaxation is likely to be associated with the regions of relatively loose interstitial packing in the strucure of a glass. These results are significant for our understanding of the molecular processes in disordered solids, for it seems that the features of molecular motions in them must have their origins in the non-equivalence of the molecular environment, rather than in the complexity of the molecules, their internal degrees of freedom, or the state of their orientational or positional disorder. The results also support a model for the structure of a glass as an assembly of polyhedra connected by loose interstitial packing, where molecular motion is possible even when the polyhedra are fixed in plage. Because supercpaoled thin films of cholesteric glass in our experiments are found to be anisotropic, we maintain that the polyhedra in the structure are not microcrystals or crystal nuclei arrested in various stages of their growth, for a random arrangement of such polyhedra would produce a macroscopically isotropic, not anisotropic, structure.Fig. 2 The dielectric loss factor, tan 6, plotted against frequency for the secondary relaxation in the glassy cholesteric phase at several temperature. Fig. 3 The frequency of maximum loss, /m, plotted logarithmically against the reciprocal temperature for main relaxation, and secondary relaxation, in the supercooled and glassy cholesteric phase. TK is the calorimetric glass transition temperature.The results have a further implication for our understanding of the glass transition in metal alloys and of the computer simulation of hard sphere glasses, for the configurational restrictions that lead to the formation of a glass in a mesomorphic phase involve mainly the positions (centres of mass) of the molecules and not the orientations or the local director, as no change in the order parameter occurs at Tg. In this respect the glass transition in them is similar to the glass transition in metallic alloys and in the computer simulated experiments on aggregates of hard spheres, where the choice of spheres precludes the possibility of an orientational disorder.