Entropy of isotropization

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 extent of correlation between repeating units is dependent on molecular mass entropy of isotropization and nematic order parameter at the l/N transition both increase rapidly with molecular mass, before leveling off. The N/I transition temperature T l also follows a similar trend, leveling off at Mn 10,000. As a result of this molecular mass dependence of Tni> a N+I biphase is observed in polydisperse samples. 

Plastic crystals always show the expected AS on isotropization as deduced from Fig s. 5.118 and 5.119, while condis crystals have the highest entropy of isotropization. Figure 2.103 indicates a value of n AS, for polymers, and n AS,. + ASp + ASp for small molecules with n representing the number of remaining conformationally ordered bonds in the condis crystal (n < n) [43,46]. 

Figure 6,34 Spacer-length and molar-mass dependence of temperature, enthalpy and entropy of isotropization for a series of side-chain poly(vinyl ether)s. The abbreviation mru stands for mole repeating units. Data from Gedde et al. (1992). With permission from Butterworth-Heinemann Ltd, UK.

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