Boundaries between mesophase

C. Note that the pure surfactant forms a liquid, which is miscible with water over a certain composition/temperature range. The clouding region arises from the partial miscibility of the Ci2EOg micellar solution with water above about 49 °C. This is caused by a net intermicellar attraction arising from EO-EO interactions between adjacent micelles. It is discussed further in [103] and the references therein. Whilst the clouding phenomenon is important for many micellar properties of these surfactants, its only influence on the mesophases is to determine the limit to which they can swell in water, that is it fixes the boundary between mesophases and the dilute aqueous solution (Fig. 14). 

A decisive factor for the physical behaviour of a composite is the adhesion efficiency at the boundaries between phases. In all theoretical models this adhesion is considered as perfect, assuming that the interfaces ensure continuity of stresses and displacements between phases, which should be different because of the proper nature of the constituents of composites. However, such conditions are hardly fulfilled in reality, leading to imperfect bonding between phases and variable adhesion between them. The introduction of the mesophase layer has as function to reconcile in a smooth way the differences on both sides of interfaces. 

Figure 13 depicts a coalescence event that took place in 7 sec. Like other coalescence events in which new disclinations are produced, a 2tt disclination appeared at the position of the former boundary between the mesophase spherules. Frenkel (27) has pointed out that the time constant t for such coalescence phenomena is given approximately by... 

The critical thicknesses are thus in the range of the dimensions of lamellar, cylindrical or spherical mesophases in block copolymers with ordered morphologies. The question is whether the phase boundary between the amorphous and the liquid-crystalline phase in a block copolymer will exert an ordering effect as assumed in the original theory or rather a disordering influence. The latter case and transitions between the two cases have also been treated recently by an extension of the theory (5). Therefore a theoretical framework exists, within which the transition behaviour of amorphous / liquid-crystalline block copolymers can be described. 

Lyotropic nematic phases were first reported by Lawson and Flautt (62) for mixtures of Cg and Cio alkyl sulfates, together with their corresponding alcohols in water. They are somewhat less common than the mesophases discussed so far. When they do form, they occur at the boundary between an isotropic micellar phase (LQ and the hexagonal phase (Li/HQ, or between Li and the lamellar phase (Li/L ). As their name implies, they have a similar micellar order to that of the molecules in a thermotropic nematic phase. This long-range micellar orientational and translational order is lower than in the other lyotropic phases described above. Like the thermotropic phases, they are of low viscosity and can be aligned in a magnetic field. It is possible to identify nematic phases optically because of their characteristic schlieren optical texture. 

Three-ring mesogens with two longer lateral alkyl chains (e.g. 12) do not show mesophases. However, cross-shaped compounds (30) have nematic phases (uniaxial) with clearing points above 100°C [46, 59]. That is surprising because 1,2,4-tris(4-n-alkyl-oxybenzoyloxy)benzenes (19f) are not liquid crystalline, but l,3,5-tris(4-n-alkyloxy-phenyl)benzoates exhibit nematic discotic phases [83]. Apparently, such substituted benzene derivatives are located at the boundary between calamitic and discotic compounds. However, there is no example of this type that shows both columnar and nematic or smectic phases, as occurs in poly-catenar and double-swallow tailed compounds (see Sec. 5 and Chap. XII of this volume). 

The present study is devoted to the examination of the structure of this boundary layer, which is called mesophase, and which is created between phases in the composite, mainly on the side of the softer phase. This new infinitesimal phase may be assumed as constituting an independent phase, lying between the two principal phases, with its own particular mechanical and physicochemical properties. 

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. 

A series of models were introduced in this study, which take care of the existence of this boundary layer. The first model, the so-called three-layer, or N-layer model, introduces the mesophase layer as an extra pseudophase, and calculates the thickness of this layer in particulates and fiber composites by applying the self-consistent technique and the boundary- and equilibrium-conditions between phases, when the respective representative volume element of the composite is submitted to a thermal potential, concretized by an increase AT of the temperature of the model. 

Liquid crystal polymers (LCPs) were introduced over the last three decades. In the liquid state, either as a solution (lyotropic) or a melt (thermotropic), they lie between the boundaries of solid crystals and isotropic liquids. This polymeric state is also referred to as a mesomorphic structure, or a mesophase, a combined term adopted from the Greek language (mesos = intermediate morphe = form). This state does not meet all the criteria of a true solid or a true liquid, but it has characteristics similar to both a solid and a liquid. For instance, the anisotropic optical properties of LC polymeric fluids are like those of crystalline solids, but their molecules are free to move as in liquids. 

Theocaris [85] proposed a model that incorporates an interphase which he named a mesophase, which constitutes a boundary layer between the main phases of the composite. From a physical basis, a continuous and smooth transition of the properties from one phase to the other is assumed. Because the mechanical properties of this region also contribute to the composite properties, the determination of the local mechanical modulus is important. Dynamic mechanical analysis is used to identify the mesophase properties, primarily the glass transition temperature (Tg), through changes in the loss modulus peak. 

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