Crystallization as Studied by Broadband Dielectric Spectroscopy by Unknown

Author:Unknown
Language: eng
Format: epub
ISBN: 9783030561864
Publisher: Springer International Publishing

Fig. 19Characteristic crystallization time (τcryt) as a function of reciprocal temperature for the isothermal melt crystallization from (SmBcryt) of BBOA. The inset presents the Avrami exponent nA versus temperature.

Adapted with permission from [15] (according to an open access Creative Commons CC BY license)

7 Conclusion

The study examines the isothermal and non-isothermal crystallization processes taking place in the isotropic and nematic phases of two mesogenic fluorene derivatives (5P-EtFLEt-P5 and 5P-Am*FLAm*P5) and in the highly ordered smectic B of 4-n-butyloxybenzylidene-4′-n′-octylaniline (BBOA) by means of BDS, DSC and POM methods.

5P-EtFLEt-P5 was found to have a higher tendency to vitrify than crystallize, even for cooling rates as low as 0.1 K/min: Crystallization of the melt nematic state can be achieved only by isothermal annealing for several hours. It was found that the crystallization process from the nematic melt state to Cr1 phase was faster than that observed for cold crystallization after softening of glass of nematic phase on heating. The phase diagram of 5P-Am*FLAm*P5 demonstrated a strong dependence on cooling rate: A sample in the chiral nematic (N*) phase forms a glassy state when cooled at ϕ ≥ 5 K/min but undergoes crystallization of Cr1 at ϕ < 5 K/min. Both materials crystallize upon heating when the glass softens. The crystals in 5P-EtFLEt-P5 display greater order than these found in 5P-Am*FLAm*P5, as evidenced by a higher fusion entropy value. The relaxation processes observed by BDS in the crystalline forms of 5P-Am*FLAm* indicate that the phases are conformationally disordered crystals (CONDIS). No such molecular motions were detected in the crystalline phases of 5P-EtFLEt-P5.

The energy barrier for isothermal cold crystallization of Cr1 in the metastable N phase of 5P-EtFLEt-P5 was found to increase from Ec ≈ 32 kJ/mol to Ec ≈ 100 kJ/mol for lower crystallization temperatures. This is consistent with studies performed under non-isothermal conditions, demonstrating Ec ≈ 32 kJ/mol on fast heating (5 K/min > ϕ ≥ 30 K/min, corresponding to crystallization at 317–344 K) and Ec ≈ 90 kJ/mol on slow heating (1 K/min ≤ ϕ ≤ 5 K/min, corresponding to crystallization at 317–344 K). The change in the crystallization energy was attributed to the fact that the crystallization process is dominated by thermodynamic and kinetic factors at high and low temperatures, respectively. A different crystallization behavior was observed for 5P-Am*FLAm*P5: On heating from the glassy state, the metastable N* phase transforms to an Is state which then undergoes crystallization to the Cr2 crystal. The activation energy of non-isothermal crystallization in the Is phase ( kJ/mol) is significantly lower than for isothermal crystallization in the N* phase ( kJ/mol). This can be attributed to the material having lower viscosity in the isotropic state than in the N* phase, thus better facilitating molecular diffusion. It was found that both compounds displayed a correlation between molecular dynamics and isothermal crystallization kinetics in the vicinity of the glass transition temperature.

Complex crystallization was also observed for BBOA. During cooling, part of the SmBcr phase crystallizes and the remainder vitrifies; the crystallization is completed upon subsequent heating, when the glass softens. POM investigations found that the degree

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