Natural copolymers, multiphase

Figure 7. Confirmation of multiphase nature of natural copolymers (a,b) and chemically homogeneous copolymer (c)...

Early morphological studies to determine the nature of multiphase polymers and blends were reviewed by Folkes and Keller [363]. Many studies were of extruded block copolymers of materials such as SBS where the dispersed phase, an unsaturated rubber stained with OSO4 (see Section 4.4.2), was observed in the form of spheres, cylinders, or lamellae [364]. An excellent example is shown in a TEM micrograph of a thin section of a poly(styrene-butadiene) diblock copolymer, stained with OSO4 [365], which depicts the (100) projection of a body centered cubic lattice (Fig. 5.79). 

Micromechanical studies have been carried out on thermoplastic elastomers. The latter are a special class of multiphase systems (block copolymers) exhibiting an unusual combination of properties they are elastic and at the same time tough and they show low-temperature flexibility and strength at relatively high temperatures (frequently ca 150 °C). In addition, they are easily processable. For this reason, they are nowadays of great commercial importance as engineering materials. In natural... 

D IR spectroscopy has been applied extensively to studies of polymeric materials. A recent review of 2D IR spectroscopy cites numerous applications in the study of polymers by this technique [6]. In this section, some representative examples of 2D IR analysis of polymers are presented. We will start our discussion with a simple homogeneous amorphous polymer then move to more complex multiphase systems, such as semicrystalline polymers. Alloys and blends consisting of more than one polymer components are of great scientific and technical importance. Both immiscible and miscible polymer blend systems may be studied by 2D IR spectroscopy. Analysis of microphase-separated block copolymers is also possible. Finally, the possible application of 2D IR spectroscopy to the studies of natural polymers of biological origin is explored. 

Many of the studies of multiphase polymers are conducted on unsaturated rubbers which are adequately stained by osmium tetroxide, which reveals the nature of the dispersed phase domains. Polymers with activated aromatic groups have been selectively stained by reaction with mercuric trifluoroacetate (Section 4.4.8). Hobbs [262] has successfully used this technique to provide contrast in blends of poIy(2,6-dimethyl-l,4-phenylene oxide) and Kraton G (SBS block copolymer). Although this stain is effective in enhancing contrast, a drawback of the method is that the material is not hardened or fixed by the stain. 

Processing plays a major role in the nature of the dispersed phase in multiphase polymers. Changes in the shear forces and the temperature provide different structures. In the case of PS modified with polyisoprene, TEM studies showed that smaller particles, broken down in size by melt shearing, resulted in lowered impact strength and increased tensile strength [183]. Particle dimensions have also been shown to be affected by the viscosity of the molten polymer and the concentration modifier. Heikens et al. [184] investigated copolymer modified PS and... 

Due to the nonmiscibility between blocks of different chemical nature (see Section 4.4), block copolymers are generally segregated in multiphase systems in which each phase preserves its properties. In that respect, they are definitely different from statistical copolymers having the same overall composition. 

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