Dual phase domain continuity

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. 

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. 

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]. 

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. 

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. 

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. 

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. 

In general, the use of the IPN concept yields better control over phase domain size and extent of molecular mixing. By various mechanisms, processibility is attained, yet the final products are all crosslinked. Because in many cases the phases interpenetrate to develop dual-phase continuity, a new type of bonding is achieved, to hold the two components in juxtaposition. 

Thus, the domain diameters of the second polymerized polymer may be predicted solely on the knowledge of the networks and their interaction. Equation (3) predicts domains of the order of 300-800 A for many IPN s of interest, in general agreement with experiment. Strictly speaking, equation (3) is valid only for small volume fractions of network II, since it is based on a spherical model. As will be illustrated further below, recent experiments suggest that the dual phase continuous structure of the IPN s arises from a modified type of spinodal decomposition. 

In [158,159] the structures of semi- and full polybutadiene-PS sequential IPNs were studied and the results of determining the equivalent sphere diameter were compared to TEM studies. It was found that the dimensional characteristics obtained by the two methods are in approximate agreement. Surface areas in the range of 150-200 m g indicated true colloidal sizes for the phase domains. Correlation lengths of 35-60 A were found for IPNs, 50-100 and 160-80 A for various semi-IPNs. It was also established that these IPNs are characterized by the dual-phase continuity. 

Dual-phase continuity, which is defined as the continuity of both polymer I and polymer II domains throughout the macroscopic sample, seems to be better described in terms of spinodal decomposition. In [170] an example of dual-phase continuity was given. PnBA/PS sequential IPNs of various compositions were prepared by UV photopolymerization. PnBA served as network I and PS as network II. Acrylic anhydride (AA) and DVB were used as labile and permanent cross-linkers. After IPN formation, the AA-containing network I was de-cross-linked and solvent extracted. Scanning electron microscopy (SEM) of network II revealed a porous but continuous structure formed by aggregates of fused spherical PS domains. It was shown that network I was continuous, since it could be quantitatively and easily extracted. The major conclusion from this paper relates to the dual-phase continuity of PnBA/PS in sequential IPNs. 

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