Some evidence for the existence of the liquid‐liquid transition: Melt rheology studies

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Abstract
In 1963 Boyer proposed the existence of a liquid‐liquid transition, Tll, in polystyrene above the glass transition Tg. Since that time there has been considerable discussion and disagreement on the existence of this transition. Detailed shear modulus data as a function of temperature in the range 165–215°C will be presented for a series of polystyrenes of various molecular weights. In each case, as the temperature is increased, the modulus gradually decreases until a specific molecular weight dependent temperature is reached and then suddenly the modulus decreases rapidly over a few °C and then returns to its previous gradual decrease with increasing temperature. The response is reminiscent of the modulus behavior going through Tg. It is suggested that the temperature at which this sharp decreased in modulus takes place is Tll. The molecular weight effects on Tll will be discussed in terms of several rheological properties that measure the temperature of this transition.