Isothermal and Non-isothermal Crystallization in Liquid Crystals …
133
on the type of nucleation and the dimensionality of crystal growth. The Avrami plot
is shown in Fig. 8b.
Equation (10) can be presented as
ε N (t) = 1 − exp
−
t − t 0
τ cryst
n A
(11)
where τ crys = K
−1/n refers to the characteristic time of the process. To determine
the parameters according to the Avramov procedure, the first derivative of ε N (t)
was plotted as a function of ln(t) (Fig. 8c). The characteristic crystallization time
is obtained from the maximum of the derivative of ε N and the required condition
ensuring t 0 = 0 is fulfilled for the situation ε N = 1 − e
−1
= 0.63. The n A parameter
can also be estimated from the formula:
n A =
e
ln t 2 − ln t 1
(12)
where t 1 and t 2 are shown in Fig. 8c.
The comparison of data obtained for melt and cold crystallization revealed that,
contrary to the results reported previously for polymers, for 5P-EtFLEt-P5 the crystallization process from the nematic melt state was faster than that observed for
cold crystallization. As the molecules at the nematic/crystal interface are properly
oriented to join the crystal and they do not need to align during the crystallization process (Fig. 8a), these findings can be attributed to the fact that orientational
order of molecules is higher in the nematic phase achieved upon cooling than in the
metastable nematic phase obtained when the glass softens [38, 39]. Similar acceleration of the crystallization process associated with the liquid crystalline order has
also been observed for some other polymeric compounds [40].
5.2 Kinetics of Isothermal Cold Crystallization Process
in 5P-EtFLEt-P5 and 5P-Am*FLAm*P5: Molecular
Mobility, Morphology and Thermodynamic Properties
For 5P-Am*FLAm*-P5, it was only possibly to perform the isothermal experimnents
using BDS for cold crystallization, due to very short duration of the crystallization
process from the melt nematic N* state. Example of experimental data and the
results of Avrami analysis are given in Fig. 9. According to the classical theory of
crystallization, characteristic crystallization time is affected by kinetic and thermodynamic factors [41]. By studying the correlation between dynamic properties and
crystallization rates for a large number of materials, Ediger et al. [42] demonstrated
that coupling coefficient ζ, associated with decoupling of the crystal growth time
from viscosity due to dynamic heterogeneity, is related to fragility m f thus: ζ ≈ 1.1–
0.005m f . The extent of correlation between molecular mobility and the crystallization
133
on the type of nucleation and the dimensionality of crystal growth. The Avrami plot
is shown in Fig. 8b.
Equation (10) can be presented as
ε N (t) = 1 − exp
−
t − t 0
τ cryst
n A
(11)
where τ crys = K
−1/n refers to the characteristic time of the process. To determine
the parameters according to the Avramov procedure, the first derivative of ε N (t)
was plotted as a function of ln(t) (Fig. 8c). The characteristic crystallization time
is obtained from the maximum of the derivative of ε N and the required condition
ensuring t 0 = 0 is fulfilled for the situation ε N = 1 − e
−1
= 0.63. The n A parameter
can also be estimated from the formula:
n A =
e
ln t 2 − ln t 1
(12)
where t 1 and t 2 are shown in Fig. 8c.
The comparison of data obtained for melt and cold crystallization revealed that,
contrary to the results reported previously for polymers, for 5P-EtFLEt-P5 the crystallization process from the nematic melt state was faster than that observed for
cold crystallization. As the molecules at the nematic/crystal interface are properly
oriented to join the crystal and they do not need to align during the crystallization process (Fig. 8a), these findings can be attributed to the fact that orientational
order of molecules is higher in the nematic phase achieved upon cooling than in the
metastable nematic phase obtained when the glass softens [38, 39]. Similar acceleration of the crystallization process associated with the liquid crystalline order has
also been observed for some other polymeric compounds [40].
5.2 Kinetics of Isothermal Cold Crystallization Process
in 5P-EtFLEt-P5 and 5P-Am*FLAm*P5: Molecular
Mobility, Morphology and Thermodynamic Properties
For 5P-Am*FLAm*-P5, it was only possibly to perform the isothermal experimnents
using BDS for cold crystallization, due to very short duration of the crystallization
process from the melt nematic N* state. Example of experimental data and the
results of Avrami analysis are given in Fig. 9. According to the classical theory of
crystallization, characteristic crystallization time is affected by kinetic and thermodynamic factors [41]. By studying the correlation between dynamic properties and
crystallization rates for a large number of materials, Ediger et al. [42] demonstrated
that coupling coefficient ζ, associated with decoupling of the crystal growth time
from viscosity due to dynamic heterogeneity, is related to fragility m f thus: ζ ≈ 1.1–
0.005m f . The extent of correlation between molecular mobility and the crystallization