Relaxation transitions liquid crystalline polymers

This paper presents some of our results on the synthesis and structure of thermotropic main-chain liquid crystalline polyethers based on bis(4-hydroxy-phenoxy)-p-xylene. It also deals with two areas in the field of liquid crystalline polymers that have received only little attention, namely the dielectric relaxation (5-10 and Gedde, U.W. Liu, F. Hult, A. Gustafsson, A. Jonsson, H. Boyd, R.H. Polymer submitted) and the kinetics of isotropic-mesomorphic state transitions (11-14. 32). They are both very important for the understanding of the nature of the mesomorphic state in polymers and for the understanding of similarities and differences of physical phenomena between liquid crystalline and semi-crystalline polymers. 

Small molecular mass liquid crystals do not respond to extension and shear stress. Liquid crystalline polymers may exhibit a high elastic state at some temperature due to the entanglements. However, the liquid crystalline network itself is an elastomer, showing rubber elasticity. In the presence of external stress, liquid crystalline networks deform remarkably and then relax back after the release of stress. The elasticity of liquid crystalline networks is more complicated than the conventional network, such as the stress induced phase transition, the discontinuous stress-strain relationship and the non-linear stress optical effect, etc. 

Elongational behavior is induced in the entrance of the spinning hole and in the transition region from backhole to actual capillary. In practice hardly any permanent orientation is built up in this way, however, because molecular relaxation is rapid. Spinning hole profiles are smoothened only to prevent the formation of vortices which would lead to extrudate distortion. Promoting orientation already in the spinning holes is not common for melt spinning. It could be beneficial for the orientation of melt-spun liquid-crystalline polymers, however, for example in the production of carbon fiber from pitch. 

Figure 10 shows the dielectric spectrum the ferroelectrk liquid crystalline polymer after Pfeiffer cl al. [35] in the temperature range of the smeetk A phase and also that in the smeetk C phsM. Just the imaginary component is shown for simplidly, as a function of the frequency. The rather intense relaxation transition observed around 100 Hz in the SmA region b attributed to the soft mode. It b thought to be assoebted with tilt angle movement, which becomes more and more intense as one approaches the SmA -SmC transition temperature (82 C). 

We will discuss some preliminary results, which have been performed recently l01). In Fig. 39a the results for polymer No. 2d of Table 10 are shown, which were obtained by torsional vibration experiments. At low temperatures the step in the G (T) curve and the maximum in the G"(T) curve indicate a p-relaxation process at about 120-130 K. Accordingly the glass transition is detected at about 260 K. At 277 K the nematic elastomer becomes isotropic. This phase transformation can be seen only by a very small step in G and G" in the tail of glass transition region, which is shown in more detail in Fig. 39 b. From these measurements we can conclude, that the visco-elastic properties are largely dominated by the properties of the polymer backbone the change of the mesogenic side chains from isotropic to liquid crystalline acts only as a small disturbance and in principle the visco-elastic behavior of the elastomer... 

Boyer also reported a transition at still higher temperature, T p Tll + 50 1.47 g, which he named the intramolecular relaxation transition, which separates structured polymer melts from true liquids [Boyer, 1977, 1980a,b, 1985, 1987]. This high-temperature transition may be related to melting. Formally, liquids remain supercooled below the melting point, 1.5Tg [van Krevelen, 1997]. Detection of this temperature is contingent on the polymer achieving sulHcient crystalline content. For example, poly(vinyl chloride), usually treated as an amorphous polymer, shows Tm = 444 to 452 K when the crystalline content of the syndiotactic isomer is about 2wt% [Marshall, 1994]. 

Typical examples of the temperature dependence of the relaxation rates of the a- and 6-relaxation (log and log 1 5) are plotted in Figures 8 and 9 for the polymers PM4 and PMg, respectively. The high temperature 6-relaxation process shows a behavior which is also found for liquid-crystalline polymethacrylates carrying phenylbenzoate as mesophase-forming unit (16,23). This means a step-like change of the temperature dependence of log 5 at the transition from the isotropic to the liquid-crystalline state. For the samples PMg and PM5 which have a smectic Wgh temperature phase no further step-like change at the phase transition fi om the smectic high temperature phase to the nematic low temperature phase is observed. This implies that... 

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