Poly isotropic transition

Figure 2.9 Temperature-dependence of the axial ratio f = bx/d of Kuhn length bf to molecular diameter d for poly(hexylisocyanate) (PHIC) in toluene (O) and tetrahydrofuran ( ) and for hydroxypropylcellulose (HPC) in dimethylacetamide (A). Here is the transition temperature from the nematic or chiral nematic to the isotropic phase in the melt. The lines fit the expression

Thus on varying the length and parity of the flexible spacer in dimers the nematic-isotropic transition temperature exhibits a dramatic odd-even effect which attenuates as the length of the spacer is increased while the nematic-isotropic entropy also exhibits the same pronounced alternation but which appears not to attenuate as the spacer length is increased. Such behaviour is also observed for semi-flexible main chain liquid crystal polymers for example, Figs. 5 and 6 show the dependence of and ASni/R, respectively, on the length of the flexible spacer for the poly a,co-[4,4 -(2,2 -dimethylazoxy-phenyl)]alkandioates [9],... 

Fig. 5. The dependence of the nematic-isotropic transition temperature, T i, on the length of the flexible spacer, n, for the poly a,(u-[4,4 -(2,2 -dimethylazoxyphenyl)]alkandioates [9]...

The G-SmA-N-I transition temperatures of syndiotactic poly(6-[4 -(4"- -bu-toxyphenoxycarbonyl)phenoxyl)phenoxy]-hexyl methacrylate prepared by aluminum porphyrin initiated polymerizations also level off at approximately 25 repeat units [91]. Similarly, the glass and nematic-isotropic transition temperatures of poly[6-(4 -methoxy-4"- Z-methylstilbeneoxy)hexyl methacrylate] prepared by group transfer polymerization become independent of molecular weight at approximately 20 repeat units [48]. Both polymethacrylates reach the same transition temperatures as the corresponding polymers prepared by radical polymerizations, which have nearly identical tacticities. 

The thermotropic behavior of liquid crystalline polynorbornenes also reach their limiting values at 50 repeat units or less [22, 182, 188-190]. For example, Fig. 9 demonstrates that the glass and nematic-isotropic transitions of both terminally and laterally attached systems level off at 25-50 repeat units, and correspond to the transition temperatures of the infinite molecular weight polymers. The same is true of the crystalline melting and smectic-isotropic transition temperatures of poly ( )-endo, exo-5,6-di [ -[4 -(4"-methoxyphenyl)-phenoxy]hexyl]carbonyl bicyclo[2.2.1]-hept-2-ene) [190]. 

However, as shown in Table 12, the glass transition temperature of poly 6-[4 -(4"-methoxyphenoxycarbonyl)phenoxy]hexyl methacrylate] is essentially independent of tacticity. With the exception of the polymer prepared by anionic polymerization [45], the nematic-isotropic transition also appears to be independent of tacticity. However, as demonstrated by the data in Fig. 8, the extrapolated transition temperatures (G 44 N 105 I) of an infinite molecular weight polymer prepared by anionic polymerization are nearly identical to those of the rest of the polymers presented in Table 12. 

In contrast, the transitions of most well-defined SCLCPs prepared by controlled polymerizations are relatively narrow. The effect of polydispersity was therefore investigated by blending well-defined (pdi < 1.28) poly 5- [6 -[4"-(4 "-methoxy-phenyl)phenoxy]alkyl]carbonyl bicy-clo[2.2.1]hept-2-ene)s of varying molecular weights (DP =5, 10, 15, 20, 50, 100) to create poly disperse samples (pdi = 2.50-4.78) [22]. In this case, both monodisperse samples and multimodal blends underwent the nematic-isotropic transition over a narrow temperature range. Polydispersity also had no effect on the temperatures of transi-... 

Models for the Nematic-Isotropic Transition for Poly- and Monodomains... 

TABLE II. Nematic-Isotropic Transition Teit eratures for Some Poly(Terephthalic Hydrazide) Solutions... 

The vdues of A// for other polyphosphazenes are also reported in Table 4.4. Unfortunately, the data reported are difficult to compare, since the past history of the samples studied is not always precisely known. There are also no data on the heats of the isotropic transition of these polyphosphazenes. Dilato-metric studies showed [41, 53] that both transitions in polybistrifluoroethoxy-phosphazene are accompanied by a marked abrupt change in volume (Fig. 4.17). It was found that the relative changes in the volume on melting and at the isotropic transition are close, 6%. Similar results have been obtained for poly-... 

Fig. 1. The molecular structure of the thermotropic liquid crystal polymers, poly a,co-[4,4 -(2,2 -dimethylazoxyphenyl)]alkandioates and their nematic-isotropic transition temperatures NI as a function of the number of methylene groups n in the flexible spacer. 

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