Inhomogeneous chain relaxation of entangled polymer melts from stepwise planar extension in absence of free surface

In this work, we apply lubricated squeezing to perform planar extension of styrene-butadiene rubbers (SBRs) and illustrate how large step-strained SBR undergo chain relaxation. When the imposed step strain is moderate or the stepwise planar extension is imposed sufficiently slowly, the stress relaxation is spatially uniform. Upon a large stepwise extension imposed over a period much shorter than the overall chain relaxation time, the strained SBR undergo spatially inhomogeneous molecular relaxation, plausibly due to localized chain disentanglement that results from the molecular force imbalance. As a manifestation of localized elastic yielding, the SBR specimen appears wrinkled upon unloading in the middle of stress relaxation. In situ birefringence measurements confirm that uneven chain relaxation is ongoing during stress relaxation that involves keeping the sample height H fixed after squeezing from the initial height H0 to H. Thus, localized chain relaxation is shown to take place even in the presence of geometrical constraint, i.e., the absence of any free surface or meniscus. In comparison, homogenous stepwise deformation was found to result in nonquiescent relaxation for simple shear and filament breakup for uniaxial melt stretching.