160
K. Adrjanowicz and R. Richert
For a more detailed characterization of the field effect on VEC, protocol “B”
from Fig. 5 has been employed. Here, the sample is cooled to T = T g + 2 K =
173 K, and the heated to T c = 198 K, all without a dc field applied. As expected
based on the field free results of Fig. 6, the liquid crystallizes slowly under these
isothermal conditions, and according to the solid line in Fig. 9, all VEC has turned
into a crystalline state after about 14,000 s. The same temperature cycle has been
repeated, but after about 20% of the material had crystallized at T c , a bias field is
switched on until about 50% crystallinity is achieved, using fields of 51, 127, and
199 kV cm
−1 in three different experiments. Clearly, the rate of crystallization is
accelerated with increasing dc field. The situation in Fig. 9 is that nucleation and
some growth of the ordinary zero-field crystals had occurred prior to applying the
dc field, such that approximately 20% of the volume is crystalline. Therefore, the
faster crystallization could be accelerated crystal growth of already existing nuclei
or a high rate of nucleation, or both.
Interestingly, when the field is switched off at the point at which about half the
volume is crystalline, i.e., ε
(ν = 10 kHz) ≈ 30 in Fig. 9, the crystallization rate
does not return to the much slower level that prevailed before applying the field. This
could be seen as consistent with field-induced nucleation at T = T c , rather than field
controlled growth rates of existing crystallites.
Following the isothermal scans at T c for various fields shown in Fig. 9, a temperature scan from T c to T > T m was performed for each of the four field cases, and
these results are compiled in Fig. 10. As for the case of using protocol “A” of Fig. 5,
Fig. 9 Dielectric permittivity ε measured at a frequency ν = 10 kHz, measured isothermally at T c
= 198 K using the protocol “B” of Fig. 5. For each of the four separate experiments, the indicated
field was applied whenε reached 20 % of ε s , and removed again when ε reached 50% of ε s
K. Adrjanowicz and R. Richert
For a more detailed characterization of the field effect on VEC, protocol “B”
from Fig. 5 has been employed. Here, the sample is cooled to T = T g + 2 K =
173 K, and the heated to T c = 198 K, all without a dc field applied. As expected
based on the field free results of Fig. 6, the liquid crystallizes slowly under these
isothermal conditions, and according to the solid line in Fig. 9, all VEC has turned
into a crystalline state after about 14,000 s. The same temperature cycle has been
repeated, but after about 20% of the material had crystallized at T c , a bias field is
switched on until about 50% crystallinity is achieved, using fields of 51, 127, and
199 kV cm
−1 in three different experiments. Clearly, the rate of crystallization is
accelerated with increasing dc field. The situation in Fig. 9 is that nucleation and
some growth of the ordinary zero-field crystals had occurred prior to applying the
dc field, such that approximately 20% of the volume is crystalline. Therefore, the
faster crystallization could be accelerated crystal growth of already existing nuclei
or a high rate of nucleation, or both.
Interestingly, when the field is switched off at the point at which about half the
volume is crystalline, i.e., ε
(ν = 10 kHz) ≈ 30 in Fig. 9, the crystallization rate
does not return to the much slower level that prevailed before applying the field. This
could be seen as consistent with field-induced nucleation at T = T c , rather than field
controlled growth rates of existing crystallites.
Following the isothermal scans at T c for various fields shown in Fig. 9, a temperature scan from T c to T > T m was performed for each of the four field cases, and
these results are compiled in Fig. 10. As for the case of using protocol “A” of Fig. 5,
Fig. 9 Dielectric permittivity ε measured at a frequency ν = 10 kHz, measured isothermally at T c
= 198 K using the protocol “B” of Fig. 5. For each of the four separate experiments, the indicated
field was applied whenε reached 20 % of ε s , and removed again when ε reached 50% of ε s