Isothermal and Non-isothermal Crystallization in Liquid Crystals …
137
the Ozawa crystallization rate. For fixed temperatures, the plots of log(−ln(1 − D))
versus log(φ) are linear, with the slope being the parameter n O and the intercept
with the y-axis corresponding to log(Z(T )) (inset Fig. 11c). The values of n O were
found to range from 6.1 to 3.7 with increasing temperature, suggesting the presence
of considerable anisotropy in growth, resulting in the production of spiky crystals
(bundle-like or sheaflike crystallites) at slow heating rates and isotropic growth upon
fast heating [41]. Due to the broad temperature range of crystallization in 5P-EtFLEtP5, it was not possible to determine a reasonable number of points to construct the
Ozawa plot.
The activation energy of crystallization E c was calculated by means of an equation
proposed by Kissinger [45]:
ln
φ
T 2
p
= C −
E c
RT p
(15)
and by Augis and Bennett [46]:
ln
φ
T p − T o
= C AB −
E c
RT p
(16)
where C and C AB denote fitting parameters, φ refers to the cooling/heating rate,
T p is the maximum crystallization peak and T o indicates the onset temperature of
crystallization. The discrepancies between the crystallization activation barriers (E c )
estimated from two models are comparable within error limits. For 5P-EtFLEt-P5,
the crystallization energy values E c ≈ 90 kJ/mol at 293–306 K (points corresponding
to 1 K/min ≤ φ ≤ 5 K/min) and E c ≈ 30 kJ/mol at 317–344 K (points corresponding
to 5 K/min > φ ≥ 30 K/min) are consistent with findings obtained for isothermal
crystallization (Fig. 12). They also confirm that diffusive and thermodynamic factors
play dominant roles at low and high temperatures, respectively. Similar behavior was
identified for crystallization in nematic phase of other liquid crystal [41]. A different
scenario was observed for 5P-Am*FLAm*P5: Upon heating from the glassy state,
the metastable N* phase transformed to an isotropic state which then crystallized.
The activation energy of non-isothermal crystallization in such a condition (E a ≈ 54
kJ/mol) is less than half that observed for metastable N* in the isothermal experiment
(E a ≈ 114 kJ/mol). This difference can be attributed to the lower viscosity of the
material in the liquid state than in the N* phase, which facilitates molecular diffusion.
6 Crystallization in a Liquid Crystal with Smectic Phase
This part of the chapter presents the crystallization of 4-n-butyloxybenzylidene-4
-n
-
octylaniline (BBOA) in the smectic B phase (SmB) under various thermal conditions.
The SmB phase is an orthogonal hexagonal phase, occurring in two forms: the SmB Cr
phase, characterized by strong interlayer correlation, and the SmB hex phase, in which
137
the Ozawa crystallization rate. For fixed temperatures, the plots of log(−ln(1 − D))
versus log(φ) are linear, with the slope being the parameter n O and the intercept
with the y-axis corresponding to log(Z(T )) (inset Fig. 11c). The values of n O were
found to range from 6.1 to 3.7 with increasing temperature, suggesting the presence
of considerable anisotropy in growth, resulting in the production of spiky crystals
(bundle-like or sheaflike crystallites) at slow heating rates and isotropic growth upon
fast heating [41]. Due to the broad temperature range of crystallization in 5P-EtFLEtP5, it was not possible to determine a reasonable number of points to construct the
Ozawa plot.
The activation energy of crystallization E c was calculated by means of an equation
proposed by Kissinger [45]:
ln
φ
T 2
p
= C −
E c
RT p
(15)
and by Augis and Bennett [46]:
ln
φ
T p − T o
= C AB −
E c
RT p
(16)
where C and C AB denote fitting parameters, φ refers to the cooling/heating rate,
T p is the maximum crystallization peak and T o indicates the onset temperature of
crystallization. The discrepancies between the crystallization activation barriers (E c )
estimated from two models are comparable within error limits. For 5P-EtFLEt-P5,
the crystallization energy values E c ≈ 90 kJ/mol at 293–306 K (points corresponding
to 1 K/min ≤ φ ≤ 5 K/min) and E c ≈ 30 kJ/mol at 317–344 K (points corresponding
to 5 K/min > φ ≥ 30 K/min) are consistent with findings obtained for isothermal
crystallization (Fig. 12). They also confirm that diffusive and thermodynamic factors
play dominant roles at low and high temperatures, respectively. Similar behavior was
identified for crystallization in nematic phase of other liquid crystal [41]. A different
scenario was observed for 5P-Am*FLAm*P5: Upon heating from the glassy state,
the metastable N* phase transformed to an isotropic state which then crystallized.
The activation energy of non-isothermal crystallization in such a condition (E a ≈ 54
kJ/mol) is less than half that observed for metastable N* in the isothermal experiment
(E a ≈ 114 kJ/mol). This difference can be attributed to the lower viscosity of the
material in the liquid state than in the N* phase, which facilitates molecular diffusion.
6 Crystallization in a Liquid Crystal with Smectic Phase
This part of the chapter presents the crystallization of 4-n-butyloxybenzylidene-4
-n
-
octylaniline (BBOA) in the smectic B phase (SmB) under various thermal conditions.
The SmB phase is an orthogonal hexagonal phase, occurring in two forms: the SmB Cr
phase, characterized by strong interlayer correlation, and the SmB hex phase, in which