Threading–Unthreading Transition of Linear-Ring Polymer Blends in Extensional Flow

Abstract

Adding small amounts of ring polymers to a matrix of their linear counterparts is known to increase the zeroshear-rate viscosity because of linear-ring threading. Uniaxial extensional rheology measurements show that, unlike its pure linear and ring constituents, the blend exhibits an overshoot in the stress growth coefficient. By combining these measurements with ex-situ small angle neutron scattering and nonequilibrium molecular dynamics simulations, this overshoot is shown to be driven by a transient threading-unthreading transition of rings embedded within the linear entanglement network. Prior to unthreading, embedded rings deform affinely with the linear entanglement network and produce a measurably stronger elongation of the linear chains in the blend compared to the pure linear melt. Thus, rings uniquely alter the mechanisms of transient elongation in linear polymers.

Nonlinear rheometry of entangled polymeric rings and ring-linear blends

“…8 for pure ring and linear components and their blends, as indicated in the caption (see also relevant discussion below). Over the examined regime of WiR < 1, the elongational deformation is larger than the shear one, especially for the R185 sample exhibiting the highest fractional deformation [8,9]. The peculiar extensional response of R185 is reported elsewhere [9].…”

“…The relaxation times have been extracted from the linear viscoelastic master curves, in terms of storage G' and loss modulus G", obtained through small amplitude oscillatory shear experiments at various temperatures. The inverse high-frequency crossover is assigned to the Rouse relaxation time of an entanglement strand, e [4,8]. For polystyrene linear polymer chains e is 0.002 s at 150 °C [4].…”

“…In particular, recent work with experimentally pure polystyrene rings having about Z = 5 and 12 entanglements indicates that they exhibit much weaker shear thinning compared to their linear counterparts [3,11]. Furthermore, the specific viscosity enhancement of entangled linear polymers upon addition of small fractions of rings was explained by invoking the constraint release process of the rings [4], whereas these blends were shown to exhibit significant strain hardening in uniaxial extension and an overshoot in the elongational stress growth coefficient which was associated to a transient threading-unthreading transition [8]. The above exciting results open new routes in molecular rheology and particularly in the direction of combining the features of rings and other macromolecular architectures to tune the viscoelastic properties of polymeric materials.…”

“…The reasons are the intriguing properties of these materials, which depart from their unlinked counterparts, and the potential applications of rings as elements for tailoring the flow properties of soft materials. Among the many manifestations of the uniqueness of non-concatenated entangled rings, we mention their power-law stress relaxation [1,2], their weaker shear thinning of viscosity compared to their linear counterparts [2,3], their role in enhancing the viscosity of linear polymers [4][5][6][7] and their strong resistance to stretching (strain hardening at low stretch rates) [8][9][10]. These unusual phenomena can be explained by the loopy conformations of the rings which yield self-similar relaxation, threading by linear chains and ring interlocking in an elongated melt, respectively.…”

“…It should be noted that progress in simulations has been remarkable as well. The weaker shear thinning of rings compared to their linear counterpart, their nonlinear response at low elongational rates accompanied by extension rate thickening and ring-ring interlocking, and the threading-unthreading transition represent the punchline of the rich outcome so far [8,9,[13][14][15][16][17]. Very recently, the N1 and N2 of moderately entangled poly(ethylene oxide) rings were investigated by means of atomistic molecular dynamics simulations and it was found that the ratio -N2/N1 is clearly larger compared to linear polymers [16].…”