Morphology and glass transitions

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 

Because most IPNs are phase separated, usually they exhibit multiple glass transitions, one for each phase. However, since the phases are small with large volumes of interphase material, and if significant mixing of the components takes place, the glass transitions may be broadened or moved towards each other. 

Another advantage of using IPNs involves its thermosetting characteristics. By definition, IPNs will not flow when heated. A partial exception is the thermoplastic IPNs, which behave crosslinked at ambient temperatures, but flow at elevated temperatures. While some IPNs are tough, impact-resistant plastics, the crosslinking permits many other types of applications, such as sound and vibration damping, biomedical, adhesives and coatings uses, etc. (see Section 6.5). 

A powerful method of examining the morphology of many multicomponent polymer materials utilizes transmission electron microscopy [Woodward, 1989]. If the two phases are nearly equal in electron density, staining with osmium tetroxide or other agents can be used. For more detailed discussion on the methods of morphology characterization, see Chapter 8. 

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]. 

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Park et al. [20] reported on the synthesis of poly-(chloroprene-co-isobutyl methacrylate) and its compati-bilizing effect in immiscible polychloroprene-poly(iso-butyl methacrylate) blends. A copolymer of chloroprene rubber (CR) and isobutyl methacrylate (iBMA) poly[CP-Co-(BMA)] and a graft copolymer of iBMA and poly-chloroprene [poly(CR-g-iBMA)] were prepared for comparison. Blends of CR and PiBMA are prepared by the solution casting technique using THF as the solvent. The morphology and glass-transition temperature behavior indicated that the blend is an immiscible one. It was found that both the copolymers can improve the miscibility, but the efficiency is higher in poly(CR-Co-iBMA) than in poly(CR-g-iBMA),... 

Several fundamental studies of morphology and glass transition temperatures of poly(urethane-seq-diene) networks have been published 144,211 216). Phase separation was characterized by electron microscopy. 

By this procedure it is possible to synthesize [150] block copolymers, having thermoplastic elastomeric properties, with a micro-domain morphology and glass transition temperatures of -120 and 105 °C, corresponding to polysiloxane and poly(MMA) blocks, respectively. 

Table 35-3. Influence of the Morphology and Glass Transition Temperatures of the Phases on the Final Properties of Blends. Soft Phase is above the Glass Transition Temperature (If Amorphous) or Melting Temperature (If Crystalline) at the Temperature of Use Hard Phase is below the Transition Temperatures...

The miscibility of a polymer blend is usually ascertained by studying the optical, morphological, and glass transition behavior of the blend. When two amorphous polymers with different refractive indices mix intimately to form a miscible blend, the refractive index of the blend is uniform, and the blend appears transparent. On the other hand, when the two polymers do not mix intimately, the resulting blend is opaque. It must be cautioned that a two-phase immiscible blend may appear transparent if the refractive indices of the two polymers are closely matched or the domain size is smaller than the wavelength of the visible light. For a blend containing a crystallizable polymer, the blend may appear opaque even when the amorphous phases of the two polymers mix intimately. 

Because of their dual crosslinked nature, both networks exert a unique control over the size, shape, and composition of the phase domains in an IPN. The morphological detail strongly influences, in turn, the physical and mechanical behavior of the material. While Chapter 5 detailed several ways of synthesizing IPNs, little mention was made of how crosslink density, order of polymerization, overall composition, etc. affect the final product. The objective of this chapter will be to explore the interrelationships among synthesis, morphology, and glass transition behavior. Mechanical and engineering properties will be treated in Chapter 7. 

Morphology and Glass Transition Behavior This can be expressed as... 

N. Devia, J. A. Manson, and L. H. Sperling, Simultaneous Interpenetrating Networks Based on Castor Oil Elastomers and Polystyrene. III. Morphology and Glass Transition Behavior, Polym. Eng. Sci. 19(12), 869 (1979). Castor oil-polyester/styrene SINs. Electron microscopy and Tg. Studies in phase domain formation. 

K. C. Frisch, D. Klempner, S. Migdal, H. L. Frisch, and H. Ghiradella, Morphology of a Polyurethane-Polyacrylate Interpenetrating Polymer Network, Polym. Eng. Sci. 14(1), 76 (1974). Polyacrylate/polyurethane SINs. Morphology and glass transitions. 

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