Thermoplastic morphology

Figure 4.4 Transmission electron micrographs of two typical impact thermoplastic morphologies, (a) HIPS moulding material, with large rubber particles containing lighter PS inclusions, (b) ABS produced by a bulk process. (Adapted from Maul, J. etal. (2002) Ullmann s Encyclopedia of Industrial Chemistry. Wiley-VCH Verlag Gmbh Co., Germany and Priddy, D.B. (1998) Kirk-Othmer Encyclopedia of Chemical Technology, 4th edn. John Wiley Sons Inc., New York, Vol 42. pp. 1 0.)...

Pig. 1. Interpenetrating network morphology of thermoplastic elastomer where A = the crystalline domain, B = the junction of crystalline lamellae, and... 

Butadiene copolymers are mainly prepared to yield mbbers (see Styrene-butadiene rubber). Many commercially significant latex paints are based on styrene—butadiene copolymers (see Coatings Paint). In latex paint the weight ratio S B is usually 60 40 with high conversion. Most of the block copolymers prepared by anionic catalysts, eg, butyUithium, are also elastomers. However, some of these block copolymers are thermoplastic mbbers, which behave like cross-linked mbbers at room temperature but show regular thermoplastic flow at elevated temperatures (45,46). Diblock (styrene—butadiene (SB)) and triblock (styrene—butadiene—styrene (SBS)) copolymers are commercially available. Typically, they are blended with PS to achieve a desirable property, eg, improved clarity/flexibiHty (see Polymerblends) (46). These block copolymers represent a class of new and interesting polymeric materials (47,48). Of particular interest are their morphologies (49—52), solution properties (53,54), and mechanical behavior (55,56). 

The pseudocross-links, generated by the hard-segment interactions, are reversed by heating or dissolution. Without the domain crystallinity, thermoplastic polyurethanes would lack elastic character and be more gum-like in nature. In view of the outlined morphology, it is not surprising that many products develop their ultimate properties only on curing at elevated temperature, which allows the soft- and hard-phase segments to separate. 

Thermoplastic Elastomers. These represent a whole class of synthetic elastomers, developed siace the 1960s, that ate permanently and reversibly thermoplastic, but behave as cross-linked networks at ambient temperature. One of the first was the triblock copolymer of the polystyrene—polybutadiene—polystyrene type (SheU s Kraton) prepared by anionic polymerization with organoHthium initiator. The stmcture and morphology is shown schematically in Figure 3. The incompatibiHty of the polystyrene and polybutadiene blocks leads to a dispersion of the spherical polystyrene domains (ca 20—30 nm) in the mbbery matrix of polybutadiene. Since each polybutadiene chain is anchored at both ends to a polystyrene domain, a network results. However, at elevated temperatures where the polystyrene softens, the elastomer can be molded like any thermoplastic, yet behaves much like a vulcanized mbber on cooling (see Elastomers, synthetic-thermoplastic elastomers). 

Properties such as low permanent set, low creep and low hysteresis are really measures of the efficiency of the heat fugitive network system. This is a complex function of the morphology. As a very general statement, the problem would seem to be less important with the harder grades of thermoplastic elastomer. 

The blends of thermotropic LCPs and thermoplastics are generally two-phase systems where the dispersed LCP phase exists as small spheres or fibers within the thermoplastic matrix. Often a skin/core morphology is created with well-fibrillated and oriented LCP phases in the skin region and less-oriented or spherical LCP domains in the core. 

The effect of viscosity ratio on the morphology of immiscible polymer blends has been studied by several researchers. Studies with blends of LCPs and thermoplastics have shown indications that for good fibrillation to be achieved the viscosity of the dispersed LCP phase should be lower than that of the matrix [22,38-44]. 

Commercial thermoplastics are the engineering materials containing two or more compatibilized polymers that are chemically bounded in a way that creates a controlled and stable morphology with a unified thermodynamic profile. In view of multiplicity and contradictory requirements of various properties for most of the applications, almost all the commercial PBAs are made of two or more thermoplastics, elastomeric modifiers along with a series of compatibilizers with modifiers compounded together. A considerable number of blends have been appearing in the market regularly, some of which are listed in Table 9. 

Polyester thermoplastic elastomers, which are obtained by replacing a part of the R2 diol by dihydroxy polyether macromonomer, present biphasic morphology and rubberlike properties. 

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