Poly isotropization temperature

TABLE 2.16 Melting Point, Tm (°C), and Isotropization Temperature, I) (°C), of Poly(l,4-phenylene terephthalate) LCPs Containing Various Substituents... 

Differences in tacticity were also reflected by thermal data. While the iso-tropization temperature of (-)-poly-(IV-ll) and ( )-poly-(IV-ll),synthesized using initiator 1, stayed approximately unchanged, the isotropization temperatures for the chiral liquid crystalline polymers shifted to higher values when initiators 2 or 3 were used. The difference was up to 7 °C. If the decreased glass transition temperatures (Tg) for the chiral analogues were taken into account, the temperature range of the liquid crystalline phase was broadened by up to 12 °C. This means that a certain diad must be responsible for this behavior. The authors assumed that the diad cmHT was most suitable one for the formation of stable liquid crystalline phases in poly(norbornene) main chains. 

Lor polymers in which the mesogen is separated by a spacer of six methylenes units from the polymer backbone, it is obvious that the more rigid poly(norbornene)s favor nematic liquid crystalline phases. Poly-(VI-6) with the rigid and bulky 2,5-dimethine oxacyclopentane-3,4-dicarboximide unit in the main chain does not show liquid crystalline behavior (Table 6, entry 12). The more flexible poly-(II-6) backbone allowed the formation of a nematic mesophase. If the mesogen density was increased, as realized in poly-(IV-6), the isotropization temperature was found to be 26 °C higher than that for poly-... 

II-6) (Table 6, entries 1 and 3). This indicates an enhanced stability, and consequently a higher state of order for the liquid crystalline phase. This is confirmed by the fact that polymers with a higher number of Z-double bonds (Table 6, entries 4 and 5) show a smectic mesophase. Smectic mesophases were also found in polymers with poly(acrylate) A, poly(methacrylate) B, poly(silox-ane) C, and poly(vinylcyclopropene) D backbones (Table 6 entries 13,15,17, 19). The isotropization temperatures of these polymers were approximately the same. 

C),than for block copolymers with NBCN and MTD, which showed glass transition temperatures above the isotropization temperature of I-n (poly-MTD Tg=214 °C, poly-NBECN Tg=116 °C). The amorphous glassy domains... 

Stabilization of the mesophase was observed as the degree of polymerization was increased. The Tg values of the poly(norbornene)-polymers were about 30 °C higher than those of the poly(butadiene) polymers. Both polymers showed similar isotropization temperatures, but they differed substantially in their liquid crystalline behaviors. Poly-(IX-n)s with a poly(norbornene) backbone exhibited textures typical of nematic mesophases, whereas the poly-(butadiene)-based polymers poly-(X-n) displayed textures representative of smectic A mesophases. The more flexible backbone of poly(butadiene) allowed a higher order of alignment of the mesogenic units, resulting in the more ordered liquid crystalline smectic A phase. 

Figure 22 (B) shows a nematic mesophase in poly-61, which was identified in the X-ray analysis as described later. On the other hand, in the case of 62 and poly-63, on cooling scan, 62 exhibits glass transition at —30 °C and isotropization temperature at 43 °C. And, on cooling from the isotropic state to the glass transition state, spherulite textures were ob-sered at 40 °C for 62 and at 32 °C for poly-63, respectively.

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 existence-range of the condis crystal of poly(tetrafluoroethylene), PTFE, can be seen from the phase diagram of Fig. 5.130. The calorimetric heat capacity analysis of PTFE is described as an example of the ATHAS applications in Fig. 2.63, and the entropies of transition, which lead to the high isotropization temperature, are discussed in Sect. 5.4.3. 

There are a number of other acyclic polysiloxanes. For example, symmetric polysiloxanes having m = 2-9 have been investigated, with some properties listed in several handbook articles. These specific polymers and some recent relevant studies on them are (i) poly(di-n-propyl-siloxane), (ii) poly(di-n-butyl-siloxane), (iii) poly(di-n-pentyl-siloxane, (iv) poly(di-n-hexyl-siloxane), (v)poly(di-n-heptyl-siloxane), (vi)poly(di-n-octyl-siloxane), (vii) poly(di-n-nonyl-siloxane), and (viii) poly(di-n-decyl-siloxane). The isotropization temperatures show an interesting increase with increasing m, the number of methylene groups in the side chains. The melting points of the same polymers also increase with m, but with a smaller... 

Fig. 4. Dependence of mdting point and isotropization temperature on number of CHj- xnips in the repeating unit of poly-otgano xane Si[(CHj) CHjyp. ...

Fig, 23. Plot of the isotropization temperature T, as a function of the degree of polymerization of atactic poly[oxy(decamethylcyclohexasiloxane-2,8-diyl)] and X-ray spectra in mesomorphic and isotropic states... 

Several symmetric polysiloxane elastomers having longer side chains also show liquid-crystalline behavior, and these range from poly(di-n-propylsiloxane) to poly(di-Ti-decylsiloxane). The temperatures at which the liquid-crystallinity disappears (the isotropization temperatures ) show a very interesting monotonic increase with the increase in the number of methylene groups in the side chains... 

Another notable observation regarding these random copolymers is that the thermal properties vary with the repeating unit ratio. As in the case of poly(bis-trifluoroethoxyphosphazene), (19) these copolymers exhibit a Tg, a transition to a hexagonal columnar mesophase (T(l)) and an isotropization temperature (Ti). Polymers la-d are completely amorphous and exibit only Tg. The DSC traces of the T(l) transitions for sevei random copolymers is display in Figure 6. 

Fig. 11. In a system of water and hydrocarbon a nonionic emulsifier with a poly(ethylene glycol) chain as the polar part dissolves in the aqueous phase at low temperatures (a) and in the oil phase at high temperatures (c). At an intermediate temperature (b) three isotropic Hquid phases may be found.

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