Syndiotactic polystyrene brittleness

Because of the chain-stiffening effect of the benzene ring the TgS of commercial materials are in the range 90-100°C and isotactic polymers have similar values (approx. 100°C). A consequence of this Tg value plus the amorphous nature of the polymer is that we have a material that is hard and transparent at room temperature. Isotactic polystyrenes have been known since 1955 but have not been of commercial importance. Syndiotactic polystyrene using metallocene catalysis has recently become of commercial interest. Both stereoregular polymers are crystalline with values of 230°C and 270°C for the isotactic and syndiotactic materials respectively. They are also somewhat brittle (see Section 16.3). 

Syndiotactic polystyrene has high heat resistance, excellent chemical resistance and dielectric properties[27], transparency, and good thermal processability. However, brittleness is major faults for many applications. It has been reported that mechanical properties such as tensile strength and modulus of the materials can be enhanced by inducing... 

Polystyrene (PS) in its atactic and syndiotactic forms is a brittle thermoplastic, even in an orientated state [4]. To improve the toughness of aPS, impact modification has been practised for a long time, either by polymerizing the styrene in the presence of a polybutadiene rubber leading to high-impact polystyrene, commonly called HIPS, or by blending the polystyrene with multi-block copolymers, mainly of the styrene-butadiene-styrene (S-B-S) type. 

Polystyrene is a linear polymer that, in principle, can be produced in isotactic, syndiotactic, and atactic forms. The commercial product or general-purpose polystyrene is atactic and as such amorphous isotactic polystyrene is more brittle and more difficult to process than atactic polystyrene. It is therefore not of commercial interest. 

Polystyrene produced by free radical polymerization techniques is largely syndiotactic but the polymer is non-crystalline. Isotactic polystyrene has been prepared by the use of Ziegler-Natta catalysts and n-butyllithium. Isotactic polystyrene has a high crystalline melting point of 230°C, which makes it a difficult material to process also it is less transparent and more brittle than the non-crystalline polymer. For these reasons isotactie polystyrene has not achieved commercial importance. 

Polystyrene is an important commercial thermoplastic that has been described by Priddy [121]. Its entanglement molecular weight is aroimd 18,000 and for both structure-rheology studies and commercial applications, molecular weights much higher than this are of primary interest. Nearly all commercial polystyrene is atactic and is thus a transparent glass at temperatures below its Tg, which is 100 °C. New catalyst systems are able to produce isotactic and syndiotactic versions, but these have not found practical applications to date due to their brittleness. 

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