Uniform Stretching of Polymer Melts

A cylindrical rod composed of a very viscous polymer can be stretched uniformly, meaning that the diameter changes with time but not with position on the rod. This may be done in several ways (1) at constant stretch rate, (2) at constant stress, (3) at constant velocity, or (4) at constant force. Although all four test conditions are encountered in the literature, two of the conditions have serious disadvantages. Thus, the stretch rate continually decreases in a constant-velocity experiment and the stress continually increases in a constant-force experiment. Consequently, a steady state cannot be obtained and the extensional viscosity cannot be determined if stretching is done either at constant force or at constant velocity. 

Sample Length Variable During the Test, Constant Stretch Rate 

FIGURE 2. Uniaxial extensiometer of Agrawal et ai (A) strip chart recorder, (B) camera, (C) strain transducer, (D) fixed clamp, (E) sample, (F) movable clamp, (G) steel cable, (H) falling weight, (I) moving cam, (J) motor, (K) speed controller. (From Reference 19.) 

It should be realized that the duration of the experiment is limited to the time taken for the moving end of the sample to traverse the length of the constant-temperature bath so that the entire experiment may be over in as few as four or five seconds. Errors arise if the sample temperature rises or if the sample does not deform uniformly. Also, it is necessary for the end of the sample to go instantly from rest to a finite predetermined velocity at the inception of stretching. In addition, especially for vertical instruments, the density of the supporting liquid has to match that of the polymer to prevent the force of buoyancy from influencing the results. Finally, the method is suitable only for polymers having a zero-shear viscosity in excess of about 1(H Pa sec at the temperature of interest. 

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