Basic characteristics of uniaxial extension rheology: Comparing monodisperse and bidisperse polymer melts

Abstract

We have carried out continuous and step uniaxial extension experiments on monodisperse and bidisperse styrene-butadiene random copolymers (SBR) to demonstrate that their nonlinear rheological behavior can be understood in terms of yielding through disintegration of the chain entanglement network and rubber-like rupture via non-Gaussian chain stretching leading to scission. In continuous extension, the sample with bidisperse molecular weight distribution showed greater resistance, due to double-networking, against the yielding-initiated failure. An introduction of 20 % high molecular weight (10 6 g/mol) SBR to a SBR matrix (2.4×10 5 g/mol) could postpone the onset of non-uniform extension by as much as two Hencky strain units. In step extension, the bidisperse blends were also found to be more resistant to elastic breakup than the monodisperse matrix SBR. Rupture in both monodisperse and bidisperse SBR samples occurs when the finite chain extensibility is reached at sufficiently high rates. It is important to point out here that these results along with the concept of yielding allow us to clarify the concept of strain hardening in extensional rheology of entangled polymers for the first time.

Catastrophic failure of polymer melts during extension

“…Actually, very little experimental evidence exist of the startup and particularly the steady extensional flow behavior of monodisperse polymer melts above Z = 34 entanglements [6]. For the sake of completeness figure 4 show the theoretically expected and measured extensional viscosities for the 240 kg/mole SBR melt from Wang et al (2011) [21], showing the expected agreement. Figure 5 contains the break of rectangular shaped 240 kg/mole SBR samples from Wang et al [21].…”

“…For the sake of completeness figure 4 show the theoretically expected and measured extensional viscosities for the 240 kg/mole SBR melt from Wang et al (2011) [21], showing the expected agreement. Figure 5 contains the break of rectangular shaped 240 kg/mole SBR samples from Wang et al [21]. These are shown together with the first published breaking study by Vinogradov et al [1] (data from their figure 1).…”

“…It depends solely on the number of entanglements. The 95 kg/mole SBR melt studied in Wang and Wang [20] was reported [6] to have Z = 24 entanglements, whereas a SBR melt with Z = 98 entanglements (corresponding to the 240 kg/mole SBR melt from Wang et al [21]) is expected to have a Rouse time of τ R = (24/98) · (1.34s/57.95s)λ max ≈ 0.48s. The increase in entanglement number has destabilizing effect at all extensional rates.…”

“…Basically, these were either the extension of a cylindrical shaped sample from the ends of the sample [1,11] with a fixture moving in the axial (e.g. extensional) direction, or the use of a dual-wind up system [2,3,20,21]. Both cylindrical [2,20] and rectangular strips [3] were applied in the dual-wind system.…”

“…Secondarily to discuss its consequences for the suggested cohesive type of failure mechanisms. As in most experimental studies [1,2,3,11,20,21] the focus will be on the extension and potential breakup of monodisperse polymer melts, applying narrow molecular weight distributed polystyrene (PS), polyisoprene (PI) and styrenebutadiene (SBR) melts.…”