Melt elongation and recovery of polymer blends, morphology, and influence of interfacial tension

Blends of polystyrene (PS) and poly(methylmethacryl;ate) (PMMA) have been investigated in elongation at 170 degrees C. The melts of the pure PS and PMMA have only a small difference in their viscosities. Consequently, with 8-12-16-20 weight % PS in PMMA, the elongational viscosities of the melts show only small differences and are similar to that of PMMA. However, when the maximum elongation of 3.5 Hencky units (corresponding to a stretch lambda = 33 and obtained at a strain rate of 0.1 s(-1)) is followed by recovery, the recoverable strain strongly depends on the PS concentration and shows very large values with a maximum that corresponds to a recoverable stretch of lambda(R) = 14.5 (for the 20% PS blend). The morphology obtained by quenching the samples shows that the originally spherical PS droplets in the continuous PMMA are stretched into ellipsoids and finally into long needles. During recovery, the viscoelastic deformation (molecular orientation) recovers and the stretched needles change backwards into spheres. But these two procedures occur with different time scales. The driving force for the second procedure is the interfacial tension alpha. An analysis is given for the determination of alpha from the measured transient recovery. Conversely, the transient recovery can be predicted when alpha, the droplet size, and the equilibrium value lambda(R) are known. Resulting from the different blends the interfacial tension between the melts of PS and PMMA is between alpha = 1.5 and 2.2 mN/m. This range covers previously published results for the same blends measured by a very different method, viz. shear oscillations. (C) 1997 The Society of Rheology.