Chain stretching and recoiling during startup and cessation of extensional flow of bidisperse polystyrene blends

Simultaneous measurements of stress and flow-induced chain alignment were collected on a series of bidisperse polystyrene blends during startup of uniaxial extension and after cessation of flow at 150 °C. Chain alignment was measured using a novel method that combines the use of a commercial Sentmanat extensional rheometer with time-resolved small-angle neutron scattering (SANS) measurements. The latter uses state of the art methods of neutron time stamping in the SANS detector and deconvolution protocols that yields scattering data with time resolutions of the order of seconds [M. A. Calabrese et al., Soft Matter 12, 2301–2308 (2016)]. Those measurements were used to confirm the direct correlation between strain hardening and chain stretching at strain rates corresponding to Rouse Weissenberg number $W i R >$ 0.5. Furthermore, a linear relation between the alignment factor, $A f$⁠, and the tensile stress, $σ E$⁠, was observed in the bidisperse blends for stress values below 65 kPa during flow startup. This result confirmed the validity of a simple stress-SANS rule (SSR), analogous to the stress-optic rule, which relates chain alignment to the extensional stress. For stresses below 65 kPa, the stress-SANS coefficient, $C E = A f / σ E$⁠, was found to have a value of 3.9 MPa⁻¹. Failure of the SSR is observed at stresses greater than 65 kPa. Further confirmation of the SSR was provided by the linear relation between $A f$ and $σ$ during the late regime of relaxation after flow cessation, which yield $C E$ values nearly identical to those obtained during flow startup.