Smectics melting processes

It was first reported in the early 1970s that these melt processible polymers could best be described as thermotropic systems which usually display an nematic texture in the melt phase [5]. Subsequently, a number of additional phases have been reported ranging from discotic structures to highly ordered smectic E G systems with three dimensional order. In the last several years an IUPAC sponsored study on nomenclature on thermotropic LPCs has been underway. A more complete set of definitions will be available shortly as a result of Recommendation No. 199 IUPAC [6]. 

In the following, a description of the melting processes of liquid crystals will be given, and then the general structures of the nematic, smectic A and smectic C achiral and chiral phases that are involved in TGB phenomena will be discussed. When mesophases are formed by molecules (such as 1) that have asymmetric or dissymmetric structures, a reduction in the environmental space symmetry occurs, which in some cases can induce the creation of helical... 

Price and Wendorff31 > and Jabarin and Stein 32) analyzed the solidification of cholesteryl myristate. Under equilibrium conditions it changes at 357.2 K from the isotropic to the cholesteric mesophase and at 352.9 K to the smectic mesophase (see Sect. 5.1.1). At 346.8 K the smectic liquid crystal crystallized to the fully ordered crystal. Dilatometry resulted in Avrami exponents of 2, 2, and 4 for the respective transitions. The cholesteric liquid crystal has a second transition right after the relatively quick formation of a turbid homeotropic state from the isotropic melt. It aggregates without volume change to a spherulitic texture. This process was studied by microscopy32) between 343 and 355.2 K and revealed another nucleation controlled process with an Avrami exponent of 3. 

As in the case of low-molecular liquid crystals the majority of information about the structure of LC polymers is obtained from their optical textures and X-ray diffraction data. Because of high viscosity of polymer melts, which results in retardation of all structural and relaxation processes it is quite difficult to obtain characteristic textures for LC polymers. As is noted by the majority of investigators smectic LC polymers form strongly birefringent films as well from solutions, as from melts11 27-... 

The development of the -modification is controlled by the relative crystallization thermodynamics and kinetics of the stable a-modification and of the smectic phase towards the metastable / -phase. For PP homopolymers, it is generally accepted that under isothermal conditions, the a-phase grows more rapidly at temperatures below 105 and above 140 °C than its counterpart, which in turn is more prone to develop in between these two temperatures in the presence of selective -promoters [52,70,122]. An elegant way to get fully nucleated /3-PP specimens would consist of pressing /3-PP pellets above their melting temperature (ideally more than 250 °C to erase any a-nuclei in the system), cool the melt quickly up to a crystallization temperature in between 100 and 130 °C, let the sample crystallize, and then quench it to room temperature [70]. However, such a processing method is too time-consuming to be of industrial relevance. 

If isotactic polypropylene is quenched from the melt to room temperature a so-called smectic modification is formed which shows only two crystal reflexions. Cabarcos, Bbsecke, and Zachmann investigated the kinetics of the transition from this modification into the a-modification. Fig. 54 shows the change of wide angle scattering during isothermal annealing at 90 °C. One sees that the 040-reflexion of the a-modification appears after about 200 sec and continues to increase in intensity afterwards. The time until the constant temperature is reached is about 150 sec. Therefore we can say that most of the process occurs at constant temperature. If the same is done at 130 °C the process occurs so rapidly that it is almost finished as constant temperature is reached. 

Although the so-called a-phase of the fatty alcohols—a thermotropic type smectic B liquid crystal with hexagonal arrangement of molecules within the double layers—is initially formed from the melt during the manufacturing process, it normally transforms into a crystalline modification as it cools. However, the crystallization of the gel matrix can be avoided if the ot-phase can be kept stable as it cools to room temperature. This can be achieved by combining appropriate surfactants such as myristyl or lauryl alcohol and cholesterol, a mixture of which forms a lamellar liquid crystal at room temperature. Due to depression of the melting point, the phase transition temperature of crystalline to liquid crystalline as well as liquid crystalline to isotropic decreases. Therefore, a liquid crystalline microstructure is obtained at room temperature. 

Polymers without flexible spacer groups. The DSC curves of t he polymer 3 indicated the existence of a liquid crystalline glassy state at room temperature. The polymer was found to be smectic. Two melting peaks were observed in the temperature range between 300 and 310 deg. C. These peaks are not as easily observed as in the case of the polymers discussed above, since the decomposition takes place in the same temperature range. The occurrence of exothermic peaks on cooling nevertheless indicates that reversible melting and crystallization processes take place. 

We reconsidered the folded-chain fringed-micelle model, proposed nearly forty years ago, and found it to be appropriate to explain mesophase ordering and crystallisation in the polymer melt and amorphous state. Putting together the evidence provided by Strobl for crystallisation as a multi-stage process [13], the folded-chain fringed-micellar grain model [202-206], the smectic phase of iPP [6,152,153], density fluctuation before crystalliza-... 

By wide-angle X-ray diffraction, four different crystal forms have been identified (3,4). These polymorphs are referred to as the a, y and "smectic" forms and their unit cell structures have been determined to be monoclinic, hexagonal, triclinic and pseudohexagonal, respectively. The relative amounts of these phases are very sensitive to the conditions of crystallization. In the process of slow cooling and isothermal crystallization, the iPP melt crystallizes into the monoclinic, hexagonal and more rarely triclinic lattice. Quenching the melt into the liquid nitrogen produces a "smectic" phase. 

However, high pressure melt homogenization presents a distinct thermal stress for the formulation and thermally sensitive compounds and drugs may be degraded. If thermal stress is an issue for the respective formulation, the so-called solvent-evaporation method can be used as an alternative. The whole preparation process is carried out at room or slightly lower temperature. By this preparation method, smectic nanoparticles with a mean size distinctly below 100 nm can be obtained. A disadvantage of this method are potential residues of the organic solvent in the final formulations. 

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