Co-continuous structure

In polymeric materials, the morphology development upon spinodal decomposition proceeds through various stages [92,93]. In the early stage of decomposition a co-continuous structure develops. A dispersed two-phase structure results only in the late stage of phase separation and the shape of the domains is not uniform. The morphology development upon spinodal decomposition is presented in Fig. 6. 

In the unstable region, the concentration fluctuations are delocalized and there is no thermodynamic barrier to phase growth. Thus, separations that take place spontaneously lead to long range phase separation. This process is called spinodal decomposition (SD). In this mechanism, decomposition starts with a co-continuous structure and gradually shifts to a droplet morphology because of the breakdown of the continuous structure [41]. 

Potschke P, Paul DR (2003) Formation of co-continuous structures in melt-mixed immiscible polymer blends. J Macromol Sci Polym Rev C43 87-141... 

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... 

Miscibility of the blend components has an obvious effect on morphology (for more detailed discussions see Chapter 8). During processing, the hydrostatic and shear stresses can change the lower critical solubility temperature (LCST) by at least 60°C. This may result in formation (inside the processing unit) of a miscible blend. The blend emerging from the extruder may phase separates by the spinodal decomposition mechanism into a co-continuous structure, whose degree of dispersion can be controlled example PET/PC blends. 

Immiscible polymer blends normally have a sea-island stmcture, where one polymer is dispersed as (normally spherical) particles in the other polymer, which forms the matrix, or a co-continuous structure, where both polymers are equally distributed in the blend without one polymer forming a continuous phase. For the blends to have good mechanical properties, it is also important that there is good interaction between the different components in the blend. To ensure this, researchers have tried a variety of methods to compatibilize the polymers in blends. The most used method is to add a third polymer, which interacts well with the other two polymers, into the blend. Reactive blending is another well-used method, and recently, a lot of investigation went into the use of (especially clay) nanoparticles to improve the interaction between the polymer components by locating themselves on the interfaces between the polymers. 

With epoxy novolac and polyethersulfone (PES) modified TGAP (38), the elastic modulus was found to decrease monotonically with PES concentration, ultimate tensile strength was doubled by addition of 40% PES, and elongation to failure is also doubled at this PES concentration. Significantly, toughness decreases rapidly with diminishing volume of the phase-inverted co-continuous structure. 

Morphology of the bulk resin is determined by such factors as functionality and initial molecular weight, curing conditions, and thermoplastic concentration. Many investigators have found that the co-continuous structure has the best mechanical properties. 

As mentioned previously, thermoplastic modifiers can form co-continuous structures, also called interpenetrating networks, with epoxy. Epoxies (51) have also been used to form interpenetrating networks (IPNs) with polyurethane foams. As compared with the polyurethane rigid foams only, the IPNs have significantly higher compressive modulus and strength. Since only one is seen for the mixture, the domain size in the IPN is very small. 

Thermoplastic toughened epoxies have been found to have a variety of morphologies. These include discrete particles, a solid solution of the thermoplastic and the epoxy, a CO- continuous stmcture, as well as a phase inverted structure. Many investigators have found the co-continuous structure most effective for toughening. 

It should be noted that phase inversion prediction models focus on only a single composition, whereas in reality, co-continuous structures are observed over a composition range. Considering the definition of co-cmitinuous structure and equations based on the percolation theory, a model was proposed to correlate a continuity index (/) with the volume fraction at onset of co-continuity (0,- ) (see Table 7.3) (Lyngaae-Jorgensen et al. 1999). Numerical simulation predicted cr to be about 0.2 for classical percolation in three-dimensional systems (Dietrich and Airmon 1994 Potschke and Paul 2003). 

The co-continuous structure and the final rheological properties of an immiscible polymer blend are generally controlled by not only the viscoelastic and interfacial properties of the constituent polymers but also by the processing parameters. For example, the effect of plasticizer on co-continuity development in blends based on polypropylene and ethylene-propylene-diene-terpolymer (PP/EPDM), at various compositions, was studied using solvent extraction. The results showed more rapid percolation of the elastomeric component in the presence of plasticizer. However, the same fuUy co-continuous composition range was maintained, as for the non-plasticized counterparts (Shahbikian et al. 2011). It was also shown that the presence of nanoclay narrows the co-continuity composition range for non-plasticized thermoplastic elastomeric materials (TPEs) based on polypropylene and ethylene-propylene-diene-terpolymer and influences their symmetry. This effect was more pronounced in intercalated nanocomposites than in partially exfoliated nanocomposites with improved clay dispersion. It seems that the smaller, well-dispersed particles interfere less with thermoplastic phase continuity (Mirzadeh et al. 2010). A blend of polyamide 6 (PA6) and a co-polyester of... 

Mao et al. had tuned the morphology to improve the electrical properties of graphene filled immiscible polymer blends. PS and PMMA blends filled with octadecylamine-functionalized graphene (GE-ODA) were fabricated to obtain conductive composites with a lower electrical percolation threshold. The dependence of the electrical properties of the composites on the morphology was examined by changing the proportion of PS and PMMA. The electrical conductivity of the composites was optimal when PS and PMMA phases formed a co-continuous structure. For the PS/PMMA blend (50 wt/50 wt), the composites exhibited an extremely low electrical percolation threshold (0.5 wt%) because of the formation of a perfect double percolated structure (Mao et al. 2012). 

Co-continuous blend structures find industrial application in selective, reverse osmosis and ion exchange membranes requiring specific functional properties. For example, PA/polyester blends could be appropriate for hydrophilic microporous separation and filtratirMi membranes when the phases form a co-continuous structure with domain sizes ranging from 0.01 to 10.0 pm (Harrats and Makhilef 2006a). 

In tissue engineering, two biodegradable polymers can be blended to form a scaffold with a co-continuous structure, perhaps in the presence of an active drug to be dispensed by controlled release into the body. For example, hydroxyapatite can be combined with PLA and PMMA via melt extrusion processing to form a tissue scaffold or alternately be used in prosthetics. Blends of PP with SBR or SEBS can also be formulated to achieve the necessary body weight support and movement for the latter application (Harrats and Makhilef 2006a). 

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