Mesophase Glasses

There are again five pure phases possible (fully ordered crystal, mesophase glass, amorphous glass, mesophase, and isotropic melt). In addition, there is the possibility of seven two-phase materials (fully ordered crystal and mesophase glass, fully ordered crystal and amorphous glass, mesophase glass and amorphous glass, mesophase... 

Fig. 15. DSC-trace of a mesophase macromolecule with flexible spacers in the main chain. Heating rate 50 K/min. Cooling rate 50 K/min. The mesophase glass crystallizes on heating at about 345 K. and melts then at about 385 K. The reversible mesophase-isotropic transition is at about 320 K. Drawn after data from Ref. 27)...

In this section, the upper temperature limit of the crystalline state is explored on the basis of experimental data on the thermodynamics of melting, extrapolated to equilibrium. The more common nonequilibrium melting will see its final discussion in Sects. 6.2 and 7.2. The other condensed states of macromolecules, the mesophases, glasses, and melts are treated in Sects. 5.5 and 5.6. Much less is known about them than about the crystals. 

Kuhn length 48 letter abbreviation 17 melting, see polypropylene, stereoisomers, melting mesophase glass 561,562,567,... 

Focusing attention on PTEB, it has been found that, similar to the case of PDTMB, the mesophase experiences a very slow transformation into the crystal. Thus, only the isotropization is observed in a sample freshly cooled from the melt [27]. However, after a long time at room temperature, the transformation mesophase-crystal is produced, owing to a glass transition temperature of about 14°C. Moreover, several endotherms were obtained before the final isotropization for a sample of PTEB annealed at 85°C for 12 days, i.e., PTEB shows enantiotropic behavior. The different endotherms may arise from polymorphism or melting-recrystallization phenomena [30]. 

Measurements of heat capacity jumps at the glass-transition temperatures, Tg, in the matrix material and the composites, carried out from heat-capacity experiments, were intimately related to the extent of the mesophase thickness. Further accurate measurements of the overall longitudinal elastic modulus of the composites and the... 

The presence of a second viscoelastic phase, the mesophase, obviously affects the behaviour of the composite, which exhibits a glass-transition temperature, different than that of the matrix material. 

In order to simplify the procedure of evaluating the extent of mesophase and its mechanical and thermal properties, a simple but effective three-layer model may be used, which is based on measurements of the thermal expansions of the phases and the composite, below and above the transition zone of the composite, lying around its glass transition temperature Tgc. 

The experimental data show that the magnitude of the heat capacity (or similarly of the specific heat) under adiabatic conditions decreases regularly with the increase of filler content. This phenomenon was explained by the fact that the macromolecules, appertaining to the mesophase layers, are totally or partly excluded to participate in the cooperative process, taking place in the glass-transition zone, due to their interactions with the surfaces of the solid inclusions. 

In order to define the volume-fraction u of the mesophase for the particular composite studied, which was either a iron-epoxy particulate, or a E-glass-epoxy... 

Moreover, the mesophase-volume fractions Oj for the same inclusion-contents were determined from the experimental values of heat-capacity jumps ACp at the respective glass transition temperatures T f by applying Lipatov s theory. Fig. 7 presents the variation of the differences Ars oi the radii of the mesophase and the inclusion (rf), versus the inclusion volume content, uf, for three different diameters of inclusions varying between df = 150 pm and df = 400 pm. 

Indeed, the multi-layered model, applied to fiber reinforced composites, presented a basic inconsistency, as it appeared in previous publications17). This was its incompatibility with the assumption that the boundary layer, constituting the mesophase between inclusions and matrix, should extent to a thickness well defined by thermodynamic measurements, yielding jumps in the heat capacity values at the glass-transition temperature region of the composites. By leaving this layer in the first models to extent freely and tend, in an asymptotic manner, to its limiting value of Em, it was allowed to the mesophase layer to extend several times further, than the peel anticipated from thermodynamic measurements, fact which does not happen in its new versions. 

The definition of the extent of mesophase and the evaluation of its radius r, is again based on the thermodynamic principle, introduced by Lipatov 11), and on measurements of the heat-capacity jumps AC and ACf, of the matrix material (AC ) and the fiber-composites (ACP) with different fiber-volume contents. These jumps appear at the glass-transition temperatures Tgc of the composites and they are intimately related, as it has been explained with particulates, to the volume fraction of the mesophase. 

Fig. 14. The variation of the specific heat jumps at glass-transition temperatures of elacc-epoxy composites, versus the fiber volume content, uf. The values for the factor X and the mesophase, (uj and matrix, (nm) volume fractions, versus uf, as derived from the values of the respective AC, s are also plotted...

Fig. 17 presents the variation of the terms E((rf/r)n> and Em(rf/r), i in the mesophase layer for a 65 percent E-glass fiber-reinforced epoxy resin, as they have been derived from Eq. (48). It is wortwhile indicating the smooth transition of the Ermodulus to the Em-modulus at the region r == rf. Similar behaviour present all other compositions. 

Fig. 18. The variation of the elastic moduli of mesophases, versus the polar distance r from the fiber-matrix boundary, for a series of E-glass-epoxy fiber reinforced composites...

By using Lipatov s theory, interrelating the abrupt jumps in the specific heat of composites at their respective glass transition temperatures with the values of the extents of these boundary layers, the thickness of the mesophase was accurately calculated. 

The interference pattern depends both on the symmetry of the liquid crystal mesophase and on the arrangement of the molecules between the glass cover slips. Three examples are given in Fig. 8. 

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