Dual-phase morphology

Other papers reported the phase separation behavior for the composition showing dual phase morphology [7,20,21,43-45], Delides et al. [43] proposed that the viscosity at the point of phase separation is sufficiently large enough to inhibit diffusion of the epoxy through the rubber (CTBN) and result in the generation of the occluded phase, which is the inclusion of epoxy domains within the rubbery phase. 

An increase in curing temperature resulted in UPE gelation, which restricted the secondary phase separation and the coalescence of spinodal decomposition. To be specific the longer the demixing period, the larger or the more interconnected the dispersed phase became. Min et al. [20,21] observed the dual phase morphology at 15wt% polysulfone... 

The solvent-etched fracture surface of folly cured PEI modified epoxy with different composition is shown in Figure 3.8. For PEI content smaller than 10wt%, the PEI-rich phase is dispersed in a continuous epoxy-rich matrix [i.e., sea-island morphology is observed (a and b)]. Above 25 wt% PEI content, nodular structure was observed (e and f) where the epoxy-rich phase forms spherical nodules and the PEI rich phase forms the matrix. With PEI content between 15 wt% and 20 wt%, dual phase morphology, where sea-island morphology and epoxy nodular structure coexist, is present (c and d). Similar morphology was observed in PEI/BPACY blend [47],... 

Figure 3.16 Schematic illustration of mechanism of dual phase morphology formation, (a) bimodal UCST (b) two step temperature quenching in polymer blend...

In the composition range where the dual phase morphology was shown, the volume fraction of the PEI-rich phase increased with the increasing initial PEI content because the volume fraction of the PEI-rich phase at the onset point of secondary phase separation increases. 

The dual-phase morphology of the PEI/epoxy blend was observed in the composition range of 13—18wt%, 14—20wt%, and 15-20wt% PEI when curing temperature was 150,... 

The formation of a dual-phase morphology in PEI-epoxy IPNs is explained in the following way [90,171]. Two cases were discussed. First, when phase separation is induced by the curing reaction, PEI-modified epoxy shows bimodal UCST behavior (Fig. 30). At composition (p (volume fraction of component 1) phase separation starts at conversion x when the curing temperature is Ti. Because of the spinodal mechanism of decomposition and due to the lower viscosity of the medium, the system will have macroscale phase separation and domains with volume fraction cp[ and. As the reaction proceeds and the conversion X2 is reached, an abrupt change in the equilibrium composition of the PEI-rich phase occurs from cp l to (p " in a very short time. This abrupt change is similar to the effect of the two-... 

Step temperature quenching in polymer blends having UCST behavior, which results in the dual-phase morphology (Fig. 30b). This means that the first phase-separated PEI-rich composition of (p now has a higher viscosity and has experienced the jump on the UCST curve, which induces a secondary phase separation within both the domain and the matrix. The cited work is very important because it was clearly shown that the phase domains observed via electron microscopy have the variable composition and are formed by both components of the semi-IPN. 

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. 

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 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. 

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. 

The morphology of IPNs has been widely investigated via electron microscopy and dynamical mechanical spectroscopy. Many IPNs have dual-phase continuity, with phase domain sizes of the order of several hundred angstroms. For sound and vibration damping over broad temperature ranges, the two polymers are mixed in different extents in different parts of the material, usually in the submicron range. 

Scanning electron microscopy (SEM) is one of the very useful microscopic methods for the morphological and structural analysis of materials. Larena et al. classified nanopolymers into three groups (1) self-assembled nanostructures (lamellar, lamellar-within-spherical, lamellar-within-cylinder, lamellar-within-lamellar, cylinder within-lamellar, spherical-within-lamellar, and colloidal particles with block copolymers), (2) non-self-assembled nanostructures (dendrimers, hyperbranched polymers, polymer brushes, nanofibers, nanotubes, nanoparticles, nanospheres, nanocapsules, porous materials, and nano-objects), and (3) number of nanoscale dimensions [uD 1 nD (thin films), 2 nD (nanofibers, nanotubes, nanostructures on polymeric surfaces), and 3 nD (nanospheres, nanocapsules, dendrimers, hyperbranched polymers, self-assembled structures, porous materials, nano-objects)] [153]. Most of the polymer blends are immiscible, thermodynamically incompatible, and exhibit multiphase structures depending on the composition and viscosity ratio. They have two types of phase morphology sea-island structure (one phase are dispersed in the matrix in the form of isolated droplets, rods, or platelets) and co-continuous structure (usually formed in dual blends). 

Although the morphology found by Kim et aL, Figure 6.4, is intuitively expected for a SIN, even having simultaneous gelation of both polymers does not necessarily guarantee molecular mixing or dual-phase continuity. 

Piezodialysis is a novel desalination technique in which salt is preferentially transported across the membrane and removed from a feed using pressure as the driving force.The theory requires the membrane to consist of two continuous phases, one anionic and one cationic/ A suitable material should have dual-phase continuity, but with a minimum of molecular mixing between the phases. The morphological features attainable with IPN formation fit these theoretical requirements. " ... 

Phase Continuity in Sequentiai iPNs. The question has been repeatedly raised Do IPN morphologies really exhibit dual phase continuity Evidence for dual phase continuity in sequential IPNs was examined by Widmaier and Sperling (40,41). A series of sequential IPNs were prepared from poly(ra-butyl acrylate) and polystyrene. Two cross-linkers were used, divinyl benzene (DVB), which forms ordinary covalent cross-links, and aciyUc anhydride (AA), which forms labile crosslinks. The AA cross-links were cut by soaking the samples in a 10% aqueous ammonium hydroxide solution for about 12 hours. 

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