Enthalpy of isotropization

Figure 9 Enthalpies of isotropization for three sets of main-chain macromolecular liquid crystals. The ordinate at zero number of CH2-groups indicates the mesogen contribution.

This method is used to locate phase transitions via measurements of the endothennic enthalpy of phase transition. Details of the teclmique are provided elsewhere [25, 58]. Typically, the enthalpy change associated with transitions between liquid crystal phases or from a liquid crystal phase to the isotropic phase is much smaller than the melting enthalpy. Nevertheless, it is possible to locate such transitions with a commercial DSC, since typical enthalpies are... 

Data. Assume for Al-Zn alloys that a2Y is isotropic, the enthalpy of mixing of Al-Zn solutions is independent of temperature, and the entropy of mixing, s, is ideal that is,... 

Similarly with low-molecular nematics is manifested in that the nematic polymers may form equally well schlieren texture, typical for low-molecular nematics (Fig. 18a) (polymers B.3.3-B.3.4, Table 9). The enthalpy of transition from LC state to isotropic melt is also close to that for low-molecular nematics. At the same time, there also exist definite structural differences. X-ray patterns of the same polymers, even in unoriented state, display certain elements of structural ordering in the arrangement of side branches (a weak diffuse halo at small angles), which could indicate a sibotactic nematic type of ordering. These differences are most distinct for oriented polymer films. As an example Fig. 18b, c, present X-ray patterns of unoriented and oriented samples of one and the same nematic polymer 121 l24. In fact two sharp small angle... 

The low-temperature properties of H2Se were studied by Kruis and Clusius [37KRU/CLU] who found that the solid phase was characterised by two phase transitions at 80 to 87 K and at 172.5 K, respectively. The temperature span of the lower transition is indicative of a higher order transition, and the higher transition takes place between two isotropic phases. The heats of transformation for the two transitions are (1.574 + 0.002) and (1.116 + 0.002) kJ-mol, respectively. H2Se(cr) melts at (207.430 + 0.018) K with an enthalpy of fusion of ... 

When the clearing point enthalpies of the homologous series were examined, it was found that the enthalpy peak that occurs in the isotropic liquid was only observed near to transitions to TGB phases. For example. Fig. 21 shows the clearing point enthalpies taken on cooling for the related (S)-l-methyloctyl 4 -(4-n-alkoxyphenylpropioloyloxy)biphenyl-4-carboxylates... 

Figure 16 shows the change in enthalpy and entropy as a function of the length of the spacer for the three polymer series plotted in Fig. 14. It demonstrates that both the change in enthalpy and entropy of isotropization from the nematic and s mesophases increase linearly with increasing spacer length. Although such increases in AH, and AS, have been attributed to more efficient deeoupling of the mesogen from the polymer backbone and therefore to inereased order [231], it simply corresponds to a con-...

Figure 16. Change in enthalpy and entropy of isotropization from the nematic (A) and SmA ( ) mesophases of poly ( )-endo,exo-5,6-di [n-[4 -(4"-cyanophenyl)phenoxy]alkyl]carbonyl )bicyclo-[2.2.1 ]hept-2-ene) s (NBE2-CN, Z>P =41 -266, pdi= 1.21-1.60) [191] from the nematic (T) mesophase of poly 4- [n-[4 -(4"-cya-nophenyl)phenoxy]alkyl]carbonyl bicyclo[2.2.1]hept-2-ene s (NBEl-CN, DPn=43-290, pdi= 1.08-1.27) [189] and from the nematic (A) and SmA ( ) mesophases of poly n-[(4 -(4"-cyanophenyl)henoxy)alkyl]vinyl ethers (VE-CN, DP = 1 -32, pdi = 1.09-1.21) [122-127] as a function of the number of methylenic units in their n-alkyj spacers.

Table 8. Normalized [232] changes in enthalpy and entropy of isotropization per methylenic unit in the spacer of poly ( )-endo,exo-5,6-di [n-[4 -(4"-cyanopheny l)phenoxy ] alkyI]carbonyl) bicyclo[2.2.1 ]hept-2-ene s...

As expected. Figs. 16 and 17 and Table 8 also demonstrate that the enthalpy and entropy of isotropization from the more ordered smectic mesophase is higher than those from the nematic phase. This discontinuity and/or change in the slope with a change in the type of mesophase can therefore be used as additional confirmation that a phase change has occurred with the addi-... 

The data in Table 10 demonstrates that for a constant spacer length and mesophase, both the change in enthalpy and entropy of isotropization decrease as the flexibility of the polymer backbone increases from poly-norbornene to poly (vinyl ether). However, the change in entropy decreases more rapidly than the change in enthalpy, and the isotropization temperature (T = AH /AS ) therefore increases with increasing flexibility. Since lower entropies of fusion are associated with more rigid structures, the lower entropy of isotropization of poly(vinyl ether)s is obviously not due to a lack of inherent flexibility of its polymer backbone, but rather to the more flexible backbone being more ordered and therefore more... 

The relative decrease in enthalpy should represent the extent of disorder at the interface of the two blocks, and has actually been directly correlated to the thickness of the interphase. That is, the AH of isotropization of lamellar PS-PChEMA (Scheme 26) containing pchEMA=0.56 is 81.4% of that of the homopolymer, indicating that the interphase should be approximately 18.6% of the liquid crystalline lamellae (12.5 nm). The calculated value of 2.3 nm corresponds very well to that measured by TEM following preferential staining of the interphase [207]. Nevertheless, variations in the extent of disorder at the interface as measured by decreased enthalpies of transitions may be due to variations in sample preparation and thermal history. For example, Gronski et al. s H-NMR experiments on deuterated PS-PBAz (Scheme 21) indicate that the disordered interphase present in powder samples is eliminated when the samples are oriented by shear for extensive time in the nematic mesophase [203]. 

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