Polystyrene recoverable strain

Fig. 21. Recoverable strain as a function of shear stress for polystyrene/toluene solutions with different molar masses at 5 wt%. (+) polyacrylamide/water solutions, ( ) 2 wt%, (x) 2 wt% (with surfactant), (O) 4 wt% and polyethylene melts...

Example 12.1 Recoverable Strain Graessley et al. (22) found that, with polystyrene extradate velocities of 1-3 mm/s in the temperature range of 160°-180°C, about 90% of the final D/Dq value was reached at an axial distance of 0.1 cm past the capillary exit. The rest of the swelling was completed in the next three centimeters. We wish to calculate the recoverable strain at 0.1 cm. 

Recently Maxwell and Cook presented data on die observation of Tn for six different molecular weight polystyrenes by five different theological measurements (1) an abrupt decrease in the shear modulus vs. temperature (2) an abrupt change in the recoverable strain characteristics as a function of temperature (3) a change in the recoverable strain as a fimction of time (4) an abrupt change of yield stress as a function of temperature and (5) a change in the stress relaxation characteristics. This data was obtained by several different researchers using two different rheometers. 

Figure 8.1 shows stress-strain curves of atactic polystyrene (PS) in compression at 295 K for two structures with different initial states well annealed, i.e., furnace cooled from Tg + 20 K to room temperature, and rapidly quenched into ice water (Hasan and Boyce 1993). In both cases there is a gradual transition to fully developed plasticity that is reached at the peak of a yield phenomenon which is more prominent in the annealed material. Both curves show several unloading histories, starting with one close to the upper yield peak. All unloading paths show prominent Bauschinger effects of plastic strain recovery that is independent of the pre-strain. These indicate the presence of strain-induced back stresses and some recoverable stored elastic strain energy. In both cases the flow stress moves toward a unique flow state attained at a strain of around 0.3. 

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