Morphology of Polymer Blends

Several attempts have been made to predict this limit of cocontinuity. A simple relationship was identified experimentally by Avgeropoulos et ol. [54], who showed that the composition at which a phase inversion occurs and a cocontinuous 

This simple approach was found to be more accurate than models that used the shear viscosity ratio for blends with a viscosity ratio not far from unity [55], as the torque reflects to some extent also the effects of melt elasticity and elongational flow component. Lyngaae-Jorgensen and Utracki developed a model which predicts the range of cocontinuity, and not only the phase inversion point [56,57]. This model, which is based on the percolation theory, relates the degree of cocontinuity 0A. which can be determined experimentally from extraction experiments, to the volume fraction of the given blend component (pp (Eq. (3.27)). 

The (pcR is the critical concentration characterizing the onset of cocontinuity, and it depends on the shape of the dispersed particles (y5cR = 0.156 for mono-disperse spheres and v5cr 0.156 for anisotropic particles) k is an empirical parameter. 

An exhaustive overview of the Uterature covering cocontinuous morphology, its formation and properties, is available in the review of Potschke and Paul [60], 

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P. M. Subiamunian, Permeability Barriers by Controlled Morphology of Polymer Blends, SPE-RETEC, Mississauga, Ontaiio, Canada, Oct. 16, 1984, pp. 1-9. 

TEM is used to investigate the phase morphology of polymer blends and the dispersion of fillers. See Electron Microscope. 

Potente H, Bastian M, Bergemann K, Senge M, Scheel G, Winkelmann T (2001) Morphology of polymer blends in the melting section of co-rotating twin screw extruders. Polym Eng Sci 41 222-231... 

Immiscibility of polymers in the melt is a common phenomenon, typically leading to a two-phase random morphology. If the phase separation occurs by a spinodal decomposition process, it is possible to control the kinetics in a manner that leads to multiphase polymeric materials with a variety of co-continuous structures. Common morphologies of polymer blends include droplet, fiber, lamellar (layered) and co-continuous microstructures. The distinguishing feature of co-continuous morphologies is the mutual interpenetration of the two phases and an image analysis technique using TEM has been described for co-continuous evaluation.25... 

Applications. Optical microscopy finds several important applications in filled systems, including observation of crystallization and formation of spherulites and phase morphology of polymer blends. " In the first case, important information can be obtained on the effect of filler on matrix crystallization. In polymer blends, fillers may affect phase separation or may be preferentially located in one phase, affecting many physical properties such as conductivity (both thermal and electrical) and mechanical performance. 

Figure 1.31. The development of the characteristic morphology of polymer blends that have phase-separated by nucleation and growth compared with the case of spinodal decomposition. The local fluctuations in concentration leading to phase separation are also shown. Adapted from Olabisi (1979).

SAXS has been used to study the morphology of polymer blends in the solid state [Khambatta, 1976 Russel, 1979 Russel and Stein, 1982, 1983]. Eor example, in the interlameUar regions of PCL/ PVC blend the system is miscible on a molecular scale. Addition of PVC impeded crystallization of PCL. At high PVC concentration PCL remained in solution. The radius of gyration was larger than that under unperturbed conditions, in spite of the fact that at the same time the second virial coefficient, A, was virtually zero. SAXS was used... 

The structure and morphology of immiscible blends depends on many factors among which the flow history and the interfacial properties are the most important. At high dilution, and at low flow rates the morphology of polymer blends is controlled by three dimensionless microrheologi-cal parameters (i) the viscosity ratio, where r j is the viscosity of the dispersed liquid and r 2 that of the matrix (ii) the capillarity number, k = d / Vj2, where d... 

As shown in Table 4.7, several methods have been used to characterize the morphology of polymer blends containing a block copolymer. In the pioneering works, transparency of films cast from solutions was used as a measure of the emulsifying agent efficiency [Riess et al, 1967 Banderet et al, 1967]. Other studies have focused on the observation of phase size reduction in the scanning... 

Table 7.6 provides a partial reference to studies on the effects of flow on the morphology of polymer blends [Lohfink, 1990 Walling, 1995]. Dispersed phase morphology development has been mainly studied in a capillary flow. To explain the fibrillation processes, not only the viscosity ratio, but also the elasticity effects and the interfacial properties had to be considered. In agreement with the microrheology of Newtonian systems, an upper bound for the viscosity ratio, X, has also been reported for polymer blends — above certain value of X (which could be significantly larger than the... 

In the first part of the chapter several methods used to observe morphology of polymer blends are presented. Various optical microscopic methods are reviewed, including such modem techniques as photon tunneling microscopy (PTM), scanning near-field optical microscopy (SNOM), phase measurement interference microscopy (PMIM), surface plasmon microscopy (SPM) and optical waveguide microscopy (OWM). Many of these methods have been developed to study surfaces and thin films. However, they can also be applied to polymer blend morphology. 

First information on the morphology of polymer blends is simply obtained by visual inspection. Blending two transparent, colorless amorphous polymers that have different refractive indexes usually leads to an opaque material, in which the size of phases exceeds the wavelength of visible light (>500 nm). It is usually assumed that to have opacity the difference of the refractive indexes should be larger than 0.003 [Paul, 1978]. 

Approximate ranges of the experimental techniques to study different blend morphologies are summarized in Table 12.15. See also Chapter 8. Morphology of Polymer Blends in this Handbook. 

Covas, J. A., O. S. Cameiro, and J. M. Maia. 2001. Monitoring the evolution of morphology of polymer blends upon manufacturing of microfibrillar reinforced composites. International Journal of Polymeric Materials 50 445-A67. 

Blends of semiconducting polymers can also be used as active layers to achieve bipolar charge transport. Since controlling the morphology of polymer blends is challenging, there are only few reports based on polymer-polymer composite bipolar devices. 

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