Mesophase composition

The synthesis parameters in determining the degree of mesostructure, mesophase composition, and morphologies of product have been emphasized and discussed on the... 

Some color properties of the various dye-Ca mesophase composites... 

Figure 2.6 Scheme of the mechanism for the formation of mesoporous silica. Silica polymers formed initially from silica monomers, and associated with surfactant monomers, which form composite self-organised primary particles which can either continue to grow via monomer addition (path 1) or themselves aggregate in a directional fashion (path 2) to form the final mesophase composite. Nondirectional aggregation would cause formation of disordered pore structures. Reprinted with permission from Nooney, R.I. Thirunavukkarasu, D. Chen, Y. Josephs, R. Ostafin, A.E., Synthesis of Nanoscale Mesoporous Silica Spheres with Controlled Particle Size, Chem. Mater., 14, 4721—4728. Copyright (2002) American Chemical Society... 

The polyamides are soluble in high strength sulfuric acid or in mixtures of hexamethylphosphoramide, /V, /V- dim ethyl acetam i de and LiCl. In the latter, compHcated relationships exist between solvent composition and the temperature at which the Hquid crystal phase forms. The polyamide solutions show an abmpt decrease in viscosity which is characteristic of mesophase formation when a critical volume fraction of polymer ( ) is exceeded. The viscosity may decrease, however, in the Hquid crystal phase if the molecular ordering allows the rod-shaped entities to gHde past one another more easily despite the higher concentration. The Hquid crystal phase is optically anisotropic and the texture is nematic. The nematic texture can be transformed to a chiral nematic texture by adding chiral species as a dopant or incorporating a chiral unit in the main chain as a copolymer (30). 

Because of their unique blend of properties, composites reinforced with high performance carbon fibers find use in many structural applications. However, it is possible to produce carbon fibers with very different properties, depending on the precursor used and processing conditions employed. Commercially, continuous high performance carbon fibers currently are formed from two precursor fibers, polyacrylonitrile (PAN) and mesophase pitch. The PAN-based carbon fiber dominates the ultra-high strength, high temperature fiber market (and represents about 90% of the total carbon fiber production), while the mesophase pitch fibers can achieve stiffnesses and thermal conductivities unsurpassed by any other continuous fiber. This chapter compares the processes, structures, and properties of these two classes of fibers. 

Rand, B., Carbon fibres from mesophase pitch. In Handbook of Composites, Vol. I Strong Fibres, ed. W. Watt and B. V. Perov. North-Holland, Amsterdam, 1985, pp. 495 575. 

Edie, D. D., Pitch and mesophase fibers. In Carbon Fibers Filaments and Composites, ed. J. L. Figueircdo et al. Kluwer Academic Publ., Dordrecht, The Netherlands, 1990, pp. 43 72. 

Low density, carbon fiber-carbon binder composites are fabricated from a variety of carbon fibers, including fibers derived from rayon, polyacrylonitrile (PAN), isotropic pitch, and mesophase pitch. The manufacture, structure, and properties of carbon fibers have been thoroughly reviewed elsewhere [3] and. therefore, are... 

As Carfagna et al. [61] suggested, the addition of a mesophasic polymer to an amorphous matrix can lead to different results depending on the properties of the liquid crystalline polymer and its amount. If a small amount of the filler compatible with the matrix is added, only plasticization effect can be expected and the dimensional stability of the blend would be reduced. Addition of PET-PHB60 to polycarbonate reduced the dimensionality of the composite, i.e., it increased the shrinkage [42]. This behavior was ascribed to the very low... 

Consequently, the composite may be considered as consisting of three phases, that is the matrix, the inclusions and a third phase, which is a layer of variable thickness, including all these changes and which surrounds each one of the inclusions. This hybrid phase is called the mesophase. 

A study of the effect of the mesophase layer on the thermomechanical behaviour and the transfer mechanism of loads between phases of composites will be presented in this study. Suitable theoretical models shall be presented, where the mesophase is taken into consideration as an additional intermediate phase. To a first approximation the mesophase material is considered as a homogeneous isotropic one, while, in further approximations, more sophisticated models have been developed, in which the mesophase material is considered as an inhomogeneous material with progressively varying properties between inclusions and matrix. Thus, improvements of the basic Hashin-Rosen models have been incorporated, making the new models more flexible and suitable to describe the real behaviour of composites. 

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 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. 

A satisfactory model for particulates is a modification of the well-known model proposed by Hashin 1 According to this model the composite consists of three phases the matrix, the inclusion, and a third phase, called the mesophase, which corresponds to the zone of imperfections, surrounding the inclusions2,3). 

The filled polymer is considered as a collection of repesentative volume elements (RVE) of many spherical or cylindrical composites of various sizes. Each of these contains a filler particle and two concentric spherical shells, a thin one corresponding to the mesophase, and another thicker, representing the matrix respectively. The volume fraction of the filler in each composite is the same, as the total volume fraction of the filler in the filled polymer. 

The above model has been successfully used to describe the thermomechanical behaviour of iron-particle reinforced resins. More precisely, the importance of this model is that it provides a quantitative means for assessing the adhesion efficiency between the phases and its effect on the thermomechanical properties of the composite. Moreover, by using this model the thermomechanical behaviour, as well as the extent of the mesophase developed in particulates could be described. The... 

Then, for a particulate composite, consisting of a polymeric matrix and an elastic filler, it is possible by the previously described method to evaluate the mechanical and thermal properties, as well as the volume fraction of the mesophase. The mesophase is also expected to exhibit a viscoelastic behaviour. The composite consists, therefore, of three phases, out of which one is elastic and two viscoelastic. 

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 solve the system of the above-described equations, and which are derived by applying the self-consistent model, applied for composites by Budiansky 7), it is necessary to evaluate experimentally the moduli of elasticity (tension, shear, bulk) and Poisson s ratios of the constituent phases and the composite. Thus, the only unknown are the radius r of the mesophase layer and its mechanical properties and thermal expansion coefficient, which are then derived. 

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. 

Fig. 2. A schematic variation of the thermal expansion, reduced to the gauge length 1, for the components of the composite (f corresponds to the elastic filler, m to the matrix, i to the mesophase and, c to the composite). The (Al/IJ of the composite is chaneing slope twice, at Tg, and Tgm. The Tgc is found approximately by the intersection of the two external linear branches of the (Al/lc) = f(T) curve...

Relation (18) correlates Tgc with the thermal properties of matrix and mesophase. Obviously, more accurate expressions for the thermal expansion curves, or the thermal expansion coefficient of the composite may provide a better approach to Tgc than the above formula. However, in many cases, it was found that this relation applies with satisfactory accuracy. 

Fig. 3. (a) Thermal expansion coefficients a for the inclusion (f), matrix (m), mesophase (i) and composite (c) of a typical iron-epoxy particulate composite, with 5 percent volume fraction for the inclusions, versus temperature, (b) the reduced longitudinal expansion of the same elements, normalized to the unit-length versus temperature (diameter of inclusions df = 150 pm)... 

Then, the three-layer model provides an easy method for evaluating the characteristics of the mesophase, by introducing a significant flexibility in the study of the physical behaviour of particulates. The drawback of the model is its instability to the values of the thermal expansions and the moduli of the composite, which must be evaluated with very high accuracy, fact which is a difficult task. Small deviations in measuring the a s and the E s may vary considerably the balance of characteristic values of the composite. However, the introduction of the influence of the mesophase to the physical behaviour of the composite, made in this model, is a certain advancement in the knowledge of the behaviour of these complicated substances. 

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. 

Thus, in the three-layer model, with the intermediate layer having variable physical properties (and perhaps also chemical), subscripts f, i, m and c denote quantities corresponding to the filler, mesophase, matrix and composite respectively. It is easy to establish for the representative volume element (RVE) of a particulate composite, consisting of a cluster of three concentric spheres, that the following relations hold ... 

Equation (20) yields the elastic modulus, Ec, of the composite in terms of the moduli and Poisson s ratios of the phases. If the Ec-modulus and vc-value are accurately measured and the Ef- and Em-moduli and the Poisson rations vf and vm are known, the average modulus of the mesophase, Ef, may be determined. Poisson s ratio of the mesophase may be found from the simple relation ... 

The law of mixtures for particulates, expressed by relation (20), yields the effective or average value of the elastic modulus of the mesophase, which may enter into any kind of law of mixtures, interconnecting the moduli of the phases and the composite. 

Lipatov 111 has indicated that the following relation holds between a weight constant X, defining the mesophase volume-fraction Uj, and the jumps of the heat capacity AC1 of the filled-composite and AC of the unfilled polymer for particulate composites ... 

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