Nematic-isotropic biphase

Fig. 1. The Nematic-Isotropic Biphase. Nematic fraction fjj measured by NMR on slow cooling from the isotropic phase (sample thermal history as in ref. 5). Comparison with jTxN IN be DSC peak width and Tc-T2 or biphase observed by microscopy (scanning rate 10 C/min. thermal history as in ref. 5). Sample DDA-9 Mn=4,000.

This chapter contains the following sections Brief overview of rigid-flexible (RF) polymers Supercooling at the isotropic-mesophase transition Memory of thermal history in the isotropic phase and Memory of thermal history in the nematic-isotropic biphase and aging of the mesophase. 

The temperature variation of the nematic order parameter S(T) was investigated in a number of systems. This is not represented by the standard monotonic curve observed in pure LMLC pronounced undulations are observed in the S T) curve. This effect is tentatively explained by the interplay between the natural increase of S with decreasing temperature and the sequential incorporation into the nematic phase of molecules with decreasing chain length and decreasing order parameter.However, detailed NMR investigation of the nematic-isotropic biphase and of sharp fractions will be required before this effect can be fully explained. 

The Nematic - Isotropic Phase Transition. For nematic solutions kept free from moisture, the phase transformations described in the preceding were not observed, but the nematic phase could be reversibly transformed to the isotropic phase over a temperature interval Tj - Tj lOK. For the sample with w = 0.041, this transition occurred over the range T = 92 C to Tj = 101 0. For temperatures between T and Tj, the sample was biphasic, with the isotropic and nematic phases coexisting. This behavior is similar to that observed in previous studies, in which Tj - Tj is observed to be independent of w over a range of w for which Tj increases with increasing w (3,4). 

The polymers were formed by condensation of 4,4dihydroxy-benzene and 2,2 -dimethyl-4,4 dihydroxyazoxybenzene with various diacid chlorides acting as flexible spacer groups Polydisperse homopolymers and copolymers, sharp fractions of homopolymers and mixtures of polydisperse polymers with a low mass mesogen were investigated. Supercooling at the mesophase-isotropic and solid-mesophase transitions, sharpness of the nematic-isotropic transition (range of N+I biphase), polymer crystallization from the mesophase melt, and enhancement of crystallinity upon addition of a low mass nematic, were studied. 

N+I biphase. The nematic-isotropic intervals as observed by microscopy provide a very inadequate representative of the extent of the N+I biphase. This is clearly illustrated on Fig. 1, where the [T2 Tc interval measured by microscopy is considerably smaller than the biphase observed by NMR. Microscopic observation under slow rates of scanning with measurement of the transmitted light intensity by means of a photomultiplier, to eliminate observer subjectivity, gives substantially the same results. Fig. 1 shows that the N+I biphase extends well into what is seemingly a "homogeneous nematic phase. In polymer DDA-9, for example, the shortest chains become isotropic approximately 90 before chains whose mass is Mn> 10,000 and act as isotropic "contaminants of the polydisperse mesophase. 

Extensive DR studies of rod-like polymers in solution covering the isotropic, biphasic and nematic states of solution have been carried out only for two different PAICs in toluene a 1/1 copolymer of n-butyl- and n-nonyl isocyanate (PBNIC) and homopoly(n-hexyl isocyanate) (PHIC). The relaxation process was studied as a function of both concentration and temperature. Only one relaxation process, with a broad distribution of relaxation times, was observed in the isotropic and nematic phases up at relatively low frequencies (10" --10 Hz) (see Fig. 4.15). The most important results are summarized in Fig. 4.16. All three characteristic dielectric parameters—the dielectric increment A8, the maximum of the loss factor 8, and the logarithm of the mean relaxation rate /c—undergo significant changes across the isotropic-biphasic-nematic concentration range. 

Fig. 12 Pn-4 (a, b) and Pn-8 (c, d) P-sheet variation measure by IR circles) and NMR triangles). 10 mg mL peptide solutions prepared in (a, c) D2O and (b, d) 130 mM NaCl in D2O. For Pii-4, I nematic gel, II flocculate, III nematic fluid, IV isotropic fluid. For Pn-8, I isotropic fluid, II biphasic solution. III nematic gel. Adapted from Carrick et al. [23]. Copyright 2007, with permission from Elsevier...

The coagulation process can now be considered in perspective of a ternary polymer-solvent-nonsolvent system, A schematic ternary phase diagram, at constant temperature, is shown in Figure 8. The boundaries of the isotropic and narrow biphasic (isotropic-nematic) regions are based on an extension of Flory s theory ( ) to a polymer-solvent-nonsolvent system, due to Russo and Miller (7). These boundaries are calculated for a polymer having an axial ratio of 100, and the following... 

It has been found for some systems, and may be true for all, that there is no transition directly from the isotropic to the nematic phase as the critical condition is attained. Instead, a narrow biphasic region is found in which isotropic and nematic phases co-exist. This behaviour was predicted by Flory 2), even although his initial calculations related to monodisperse polymers. It is accentuated by polydispersity (see Flory s review in Vol. 59 of Advances in Polymer Science), and indeed for a polydisperse polymer the nematic phase is found to contain polymer at a higher concentration and of a higher average molecular weight than the isotropic phase with which it is in equilibrium. 

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