Molecular origin of strain hardening in blend of ring and linear polystyrene

Ring polymers have attracted a great deal of scientific interest due to the lack of free ends which has dramatic consequences on their rheology. Previous studies show that the zero shear viscosity of pure rings is much lower than that of their linear counterparts with the same molecular weight.1 However, it has been shown that when linear polystyrene is mixed with rings in certain ratios, the zero shear viscosity of the blend is even higher than that of the linear. Uniaxial extensional rheology measurements of the blend show that the maximum stress was enhanced and followed by an unexpected stress overshoot at large Hencky strains. The strain hardening up until the maximum could be relevant e.g. for fiber spinning since strain hardening leads to increased molecular orientation and stretching that again leads to increased fiber strength. Ring linear blends may thus lead to stronger fibers in the future. In order to study the structural origin of the observed strain hardening, Very Small Angle Neutron Scattering (VSANS) is used in this study.In the present work, to explore how the rings affect the linear molecules, a blend of 30 % ring and 70 % linear polystyrene molecules (both of molecular weight 185 k and with 10 wt% deuterated linear chains) and a reference sample of pure linear polystyrene (also of molecular weight 185 k and 10 wt % deuterated chains) are used. The samples were prepared by using a filament stretching rheometer at 130oC with a constant Hencky strain rate of 0.003 s-1, and quenched at different times. One sample was quenched before the maximum stress where the segments start to be oriented. A second sample corresponds to maximum stress for the blend where the linear chains in the blend are stretched the most with respect to their counterparts in the pure linear. In this way, the level of molecular stretching of the linear component in the blend is quantified and compared with the linear material. Thus, we shine light on the origin of the strain hardening in the linear ring blend.