Chain Movements in Amorphous State

The random chain arrangements in amorphous or molten polymer create extra empty space, called the free volume, which essentially consists of holes in the matrix. When sufficient thermal energy is available, the vibrations can cause a segment to jump into a hole by cooperative bond rotation and a series of such jumps can result in the polymer chain eventually changing its position. However, the complete movement of a chain cannot remain unaffected by the surrounding chains since considerable entanglement exists in the melt and any motion will be retarded by other chains. 

Problem 2.34 A plastics extrader is known to work best at a melt viscosity of about 20,000 poises. The vinyl polymer of DPw = 750 and Tg = 80°C, earlier used with the extruder, had this viscosity at 150°C. However, a batch of the same polymer received subsequently has DP , = 500. At what temperature should the extruder now be ran so that the optimum melt viscosity for processing is achieved (Sperling, 1986)  

Answer For a vinyl polymer, Z = 2(DPw). Since both samples have Z 700, Eq. (2.49) can be used for both. 

Since at high molecular weight Tg is essentially constant, both the polymer samples may be assumed to have the same Tg. Therefore, 

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