Dual-phase continuity

Dual Phase Continuity. Dual phase continuity has been shown to be important in numerous polymer blends and IPN s, to achieve special properties. Dual phase continuity is defined as a region of space where two phases maintain some degree of continuity. An example of dual phase continuity is an air filter and the air that flows through it. A Maxwell demon could traverse all space within the air filter phase, as well as within the air phase. 

Quantities useful for predicting phase continuity and inversion in a stirred, sheared, or mechanically blended two-phased system include the viscosities of phases 1 and 2, and and the volume fractions of phases 1 and 2, and ij. (Note These are phase characteristics, not necessarily polymer characteristics.) A theory was developed predicated on the assumption that the phase with the lower viscosity or higher volume fraction will tend to be the continuous phase and vice versa (23,27). An idealized line or region of dual phase continuity must be crossed if phase inversion occurs. Omitted from this theory are interfacial tension and shear rate. Actually, low shear rates are implicitly assumed. 

The reaction path is plotted in Figure 4, and compared with the theoretical line. For the 20/80 reaction system, polystyrene gelation takes place substantially simultaneously with the onset of dual phase continuity. The system was consequently not observed to undergo phase inversion. 

After polymerization was complete, transmission electron microscopy was carried out on thin sections of the 10/90 and 20/80 compositions. Confirming the optical micrographs, the polystyrene phase was continuous for the fully reacted product. As illustrated in Figure 5 for the 10/90 system, the oil phase (stained dark) contains a considerable amount of occluded polystyrene. For the 20/80 system, data not shown, dual phase continuity was found. The polystyrene phase was relatively pure, but the oil-rich phase had much occluded... 

This simple mixing rule demonstrates satisfactory agreement with experimental evidence from experiments with binary fluid blends [. l Furthermore, it is similar in form with the result from a continuum theory approach by Davis [ ], applicable for IPN with dual phase continuity but which are not mixed on a molecular level. This last model involves an exponent equal to 1/5 instead of 1/2 and is quite successful in predicting the experimental evidence [1 ] from permanent networks. 

Since the start of modern interpenetrating polymer network (IPN) research in the late sixties, the features of their two-phased morphologies, such as the size, shape, and dual phase continuity have been a central subject. Research in the 1970 s focused on the effect of chemical and physical properties on the morphology, as well as the development of new synthetic techniques. More recently, studies on the detailed processes of domain formation with the aid of new neutron scattering techniques and phase diagram concepts has attracted much attention. The best evidence points to the development first of domains via a nucleation and growth mechanism, followed by a modified spinodal decomposition mechanism. This paper will review recent morphological studies on IPN s and related materials. 

This review will examine the morphological features of sequential IPN s, starting with transmission electron microscopy (TEM) and modulus, and continuing on with SAXS, and SANS. Dual phase continuity in IPN s will be explored, with emphasis placed on splnodal decomposition in IPN systems. 

A major interest to sequential IPN s relates to dual phase continuity, defined as a region of space where each of two phases maintain some degree of connectivity. An example is an air filter with the air flowing through it. A Maxwell demon can transverse all space within both the filter phase and the air phase, both phases being continuous. 

While several experimental techniques provide Information relating to dual phase continuity, the two most important methods Involve scanning electron microscopy and dynamic mechanical spectroscopy [16,22-2A]. Donatelll, et al [1 ] performed the first mechanical study on PB/PS IPN s. Figure 5 [ 6] illustrates the fit provided by the Davies equation [22] and the Budlansky equation [25,26], both of these equations derived on the assumption of dual phase continuity. 

While the evidence for dual phase continuity provided by Figure 5 does not indicate directly any mechanism for phase separation, or the shape of the phases, dispersed, spherical polystyrene domains probably would not yield results of this type. By hind sight, the data are consistent with the notion of spinodal decomposition and cylindrical domains. 

While significant evidence supporting the notion of dual phase continuity was obtained, particularly from modulus-composition studies, no direct observation had been successful with TEM due to its two-dimensional limitation. 

The rapid Increase of transverse length and the presence of a maximum in the specific surface area suggests a macroscopic development of dual phase continuity. A similar trend was obtained in another study of IPN s with SANS [13]. 

Dual phase continuity offers many advantages, because rubber/plastic compositions yield tough, leathery materials. Many of the compositions described above, for example, contain two continuous phases, with cylinders of polystyrene meandering within the polybutadiene matrix. Since all IPN s are crosslinked, it may be that their greatest advantage will lie in products which are leathery or rubbery, but can not be permitted to flow. 

The previous section showed how IPNs and related materials can be synthesized. The several synthetic methods, such as sequential, simultaneous, latex, and thermoplastic IPN formation, will result in different morphologies. One of the main advantages of IPN synthesis relates to the ease of promoting dual phase continuity, i.e., for a... 

The full IPNs shown here (as in numerous other cases) have dual phase continuity. The domains, as cut in thin section for transmission electron microscopy, appear to be ellipsoidal. Actually, they are more probably thin sections of cylinders, cut at various angles. Other studies show that both phases may be continuous. Spinodal decomposition kinetics, thought to apply in many such cases, results in interconnected cylinders [Utracki, 1994]. 

X > 1, phase 1 is continuous X = 1, dual phase continuity or phase inversion is likely X < 1, phase 11 is continuous. 

The important points developed in this section are that sequential IPN synthesis tends to make dual phase continuous materials. Eor both sequential and simultaneous syntheses, a metastable phase diagram can be developed to study the kinetics of phase separation and gelation, so that better control of the morphology can be attained. The thermoplastic IPNs depend on equal volume and viscosity ratios to attain the dual phase continuity. 

The thermoplastic IPNs utihze physical cross-hnks, rather than chemical crosshnks. Usually, these materials wiU flow when heated to sufficiently high temperature (hence the terminology thermoplastic), but behave as thermosets at ambient temperature, with IPN properties, often possessing dual phase continuity. Most often, physical crosshnks are based on triblock copolymers (thermoplastic elastomers being the leading material), ionomers, or semi-crystalhne materials. 

Sometimes one of the components is chemically crosslinked. Usually, such materials undergo dynamic vulcanization, i.e., chemical crosslinking during melt shearing. The resulting action yields a dual-phase continuity product, where the chemically crosslinked component forms cyl-... 

Notes Dynamic vulcanizate usually, the EPDM is crosslinked via free radical methods during a shearing action of the blend, often accompanied by a partial phase inversion to dual phase continuity Based on G. Holden, Ch. 16, Table 16.10, in G. Holden, N. R. Legge, R. Quirk, and H. E. Schroeder, Eds., Thermoplastic Elastomers, 2nd Ed., Hanser, Munich, 1996 AES = Advanced Elastomer Systems... 

The field of IPNs is simultaneously one of the oldest in multicomponent polymer literature, and one of its newest and fastest growing fields. With IPNs, it is relatively easy to prepare very small domain sizes and/or materials with dual phase continuity. IPNs can be made via a multitude of ways sequential, simultaneous, latex, gradient, and thermoplastic, to name some of the more prominent materials. 

Dual phase continuity Ductile-brittle transition Durability... 

Any combination of the above two types of polymer can form thermoplastic IPNs in which some degree of dual-phase continuity is attained. Some of the thermoplastic IPNs have been claimed to be tough materials (44, 45). Though tribological properties of these new materials have not been investigated, they should be of... 

Recent research on IPN s has emphasized thermoplastic IPN s based on physical crosslinks, and the factors controlling the variation of domain sizes in sequential IPN s. Most recently, decross linking and extraction studies on sequential IPN s has led to an improved understanding of the dual phase continuity sometimes present in these materials. The sequential IPN system poly(n-butyl acrylate)/polystyrene is emphasized. 

In the case of thermoplastic IPN s, the crosslinks are of a physical, rather than a chemical, covalent nature. Important types of physical crosslinks include the hard blocks of a multiblock copolymer, ionomeric sites, or crystalline regions in semicrystalline polymers. Frequently, the polymers exhibit some degree of dual phase continuity. In all such cases, the thermoplastic IPN s behave as thermosets at use temperature, but as thermoplastics at some more elevated temperature. 

Dual phase continuity or phase inversion is controlled by two factorsthe volume fraction of each component, and its melt viscosity. Larger volume fractions or lower melt viscosities tend to make that phase continuous. Obviously, equal volume fractions and equal melt viscosities promote dual phase continuity. 

O SEBS Continuous SMAAI Continuous C Dual Phase Continuity 0 Phase Inversion... 

Fig. 2. Kraton G, poly (styrene-b-ethylene-co-butylene-b-styrene) (sees), and poly(styrene-co-methaerylie acid) (SMAAl) dual phase continuity map. ...

Fig. 2 illustrates the dual phase continuity aspects. Several compositions exhibited dual phase continuity, as shown by transmission electron microscopy. 

It should be emphasized that equation (l) assumes the formation of spherical domains. There is a growing body of evidence however, that some systems exhibit dual phase continuity. See below. 

One approach to the study of dual phase continuity in sequential IPN s is through the decross linking and subsequent dissolving out of one polymer or the other. For example, in a recent study of a polypropylene/EPM morphology, the ethylene-propylene rubber component was dissolved out, leaving a sponge-like structure of polypropylene, as observed via scanning electron microscopy. 

---