**8. Mechanism for microwave heating processes of liquid crystalline MBBA**

The microwave-induced local heating phenomena observed in liquid crystalline MBBA is shown schematically in **Figure 5A**. Microwave irradiation generates a small amount of the isotropic phase inside the liquid crystalline sample below the phase transition temperature (Tc) (**Figure 5A(b)**). The dielectric loss for the isotropic phase is expected to be higher than that for the liquid crystalline phase, as shown in the MD simulation discussed in a later section. As a result, the isotropic phase is heated more efficiently by microwave irradiation, which induces a relatively higher temperature in the isotropic phase region. This phenomenon can be considered to be due to a type of non-equilibrium localized heating state, as observed in liquid–solid system [24]. The isotropic phase forms small particles and the surface of these particles interact with the surrounding liquid crystalline molecules to generate different linewidths than those produced by the bulk isotropic phase. This non-equilibrium heating state can be maintained over long time spans because the rate at which heat is obtained by the small isotropic phase particles by the absorption of microwave energy is balanced with the rate at which heat is dissipated to the bulk liquid crystalline phase. At a higher microwave power level,

**177**

**Figure 6.** *A. Plots of 1*

*Microwave Heating of Liquid Crystals and Ethanol-Hexane Mixed Solution and Its Features…*

the bulk isotropic phase increases (**Figure 5A(c)**), and eventually the entire sample

**Figure 6A** shows the structure of MBBA and the chemical shift values for individual protons as a function of temperature for isotropic phase MBBA. It is noted

*H chemical shift against setting the temperature of NMR spectrometer. B. Temperature as a* 

*function of the microwave irradiation time at a microwave power of 65, 130, and 195 W [33].*

In conventional thermal heating, the surface of the liquid crystalline state begins to melt to the isotropic state as shown in **Figure 5B(f )** and subsequently undergoes a rapid change to the isotropic state over the entire region of the sample

*A: Proposed (a, b, c and d) microwave (MW) heating processes within the liquid crystalline state. A small fraction of the liquid crystalline domain (hot spots) changes to the isotropic phase during microwave irradiation. The rate of temperature increase in this isotropic phase domain is higher than that in the liquid crystalline phase because of the larger dielectric loss for the isotropic phase. B: Schematic diagram showing the thermal heating process (e, f, and g) starting from the liquid crystalline phase to the isotropic phase [33].*

transitions to the isotropic phase (**Figure 5A(d)**) [33].

**9. CSC-temperature of MBBA in the isotropic phase**

(**Figure 5B(g)**).

**Figure 5.**

*DOI: http://dx.doi.org/10.5772/intechopen.97356*

*Microwave Heating of Liquid Crystals and Ethanol-Hexane Mixed Solution and Its Features… DOI: http://dx.doi.org/10.5772/intechopen.97356*

**Figure 5.**

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

signals among the liquid crystalline phase signals (**Figure 4E**) even at a temperature lower than Tc. The temperature of the liquid crystalline phase was estimated to be 35°C from the assessment of the NMR linewidths with respect to the temperature. Such signals would not typically be expected until the temperature of the sample is close to its isotropic phase transition temperature of 40.5°C. This result indicates that microwave irradiation generated localized heating in the sample to form a small

*H NMR spectra of MBBA during the thermal heating processes. E. Series of 1*

*H NMR* 

*H NMR spectrum of MBBA in the liquid crystalline phase at 35°C. C. 1 H NMR spectrum of MBBA in the isotropic phase at 45°C, together with signal assignments for the individual* 

The temperature of the sample was successfully determined using *in situ* microwave irradiation NMR because the temperature of the MBBA liquid crystal was estimated with the NMR signal linewidth. It is noted that microwave irradiation generated a small fraction of the isotropic phase in the bulk liquid crystal at 35°C, even though this is 5.5°C lower than Tc (40.5°C). This result suggests non-

**8. Mechanism for microwave heating processes of liquid crystalline** 

The microwave-induced local heating phenomena observed in liquid crystalline MBBA is shown schematically in **Figure 5A**. Microwave irradiation generates a small amount of the isotropic phase inside the liquid crystalline sample below the phase transition temperature (Tc) (**Figure 5A(b)**). The dielectric loss for the isotropic phase is expected to be higher than that for the liquid crystalline phase, as shown in the MD simulation discussed in a later section. As a result, the isotropic phase is heated more efficiently by microwave irradiation, which induces a relatively higher temperature in the isotropic phase region. This phenomenon can be considered to be due to a type of non-equilibrium localized heating state, as observed in liquid–solid system [24]. The isotropic phase forms small particles and the surface of these particles interact with the surrounding liquid crystalline molecules to generate different linewidths than those produced by the bulk isotropic phase. This non-equilibrium heating state can be maintained over long time spans because the rate at which heat is obtained by the small isotropic phase particles by the absorption of microwave energy is balanced with the rate at which heat is dissipated to the bulk liquid crystalline phase. At a higher microwave power level,

equilibrium localized heating within the liquid crystalline sample [24].

region of the higher temperature isotropic phase.

**176**

**MBBA**

**Figure 4.**

*protons. D. Series of 1*

*A. Molecular structure of MBBA. B. <sup>1</sup>*

*spectra during the microwave heating processes [33].*

*A: Proposed (a, b, c and d) microwave (MW) heating processes within the liquid crystalline state. A small fraction of the liquid crystalline domain (hot spots) changes to the isotropic phase during microwave irradiation. The rate of temperature increase in this isotropic phase domain is higher than that in the liquid crystalline phase because of the larger dielectric loss for the isotropic phase. B: Schematic diagram showing the thermal heating process (e, f, and g) starting from the liquid crystalline phase to the isotropic phase [33].*

the bulk isotropic phase increases (**Figure 5A(c)**), and eventually the entire sample transitions to the isotropic phase (**Figure 5A(d)**) [33].

In conventional thermal heating, the surface of the liquid crystalline state begins to melt to the isotropic state as shown in **Figure 5B(f )** and subsequently undergoes a rapid change to the isotropic state over the entire region of the sample (**Figure 5B(g)**).
