Chain stiffening

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). 

In Flory s original treatment, llmtra accounts for the increase of the chain stiffness when the temperature is lowered. Flory93 described this chain stiffening by... 

Table 7.1 Chain-Stiffening Effects of Groups Present in Polymer Backbones...

Increased length of flexible side, pendant groups Inclusion of chain-stiffening units Addition of a and a,a substituents Addition of polar groups Increase in cross-link density... 

Since the sulfone group is chain-stiffening, these amorphous plastics are rigid. However, some flexibility is provided by the ether and methylene groups. The Tg of these materials is high (>190 °C), but polyalkyl sulfones, which... 

PPO has a high Tm because of the presence of the chain-stiffening 1,4-phen-ylene moiety, which makes up the major portion of the backbone of this polymer (the oxygen atom constitutes the remainder). Because of its very high T this polymer cannot be processed by traditional extrusion or molding techniques. 

A. Effects of intra-molecular interaction, which usually are the cause of chain stiffening. 

Impact strength It is a measure of toughness, that is, how well a sample will withstand the sudden onset of stress, like a hammer blow. Usually, chain stiffening lowers the impact strength and increases embrittlement. 

Metal-containing moieties typically act as chain stiffening agents increasing T and T but decreasing solubility. Flexibility is imparted by the hydrocarbon portions. 

Reversion and ring formation have been partially overcome through placement of chain-stiffening units in the silicone-backbone. Thus linear D2-m-carborane-siloxanes (11—14) with one to three trifluoropropyl moieties per repeating unit) exhibit better thermal and oxidative stability than silicones and fluorosilicones (J ). Initial degradation occurs in air about 300 to 350 C, almost 100 above that typically experienced for siloxanes and fluorosilicones. The carborane-siloxanes exhibit T s from -50 to 0 C ( 1 ). The carborane moiety also acts to inhibit formation of six-membered rings because of its size. 

Poly(styrene) 100 Chain stiffening due to pendant phenyl groups... 

Poly(p-phenylene terephthalamide) 345 In-chain stiffening from p-phenylene groups together with amide hydrogen bonding... 

---