Birefringence and stress growth in uniaxial extension of polymer solutions

Simultaneous measurements of extensional stresses and birefringence are rar e, especially for polymer solutions. This paper reports such measurements u sing the filament stretch rheometer and a phase modulated birefringence sys tem. Both the extensional viscosity and the birefringence increase monotoni cally with strain and reach a plateau. Estimates of this saturation value f or birefringence, using Peterlin's formula for birefringence of a fully ext ended polymer chain are in agreement with the experimental results. However , estimates of the saturation value of the extensional viscosity using Batc helor's formula for suspensions of elongated fibres are much higher than ob served. Reasons for the inability of the flow field to fully unravel the po lymer chain are examined using published Brownian dynamics simulations. It is tentatively concluded that the polymer chain forms a folded structure. S uch folded chains can exhibit saturation in birefringence even though the s tress is less than that expected for a fully extended molecule. Simultaneous measurements of stress and birefringence during relaxation ind icate that the birefringence decays much more slowly than the stress. The s tress-birefringence data show a pronounced hysteresis as predicted by bead- rod models. The failure of the stress optic coefficient in strong flows is noted. Experiments were also performed wherein the strain was increased linearly w ith time, then held constant for a short period before being increased agai n. The response of the stress and birefringence in such experiments is dram atically different and can be traced to the different configurations obtain ed during stretching and relaxation. The results cast doubt on the appropri ateness of pre-averaging the non-linear terms in constitutive equations. (C )2000 Elsevier Science B.V. All rights reserved.