Dynamics of Star Polymers in Fast Extensional Flow and Stress Relaxation

Abstract

In the present work, we confirm the observation from Ianniruberto and Marrucci [1] that entangled melts of branched polystyrenes behave like linear polystyrenes in the steady state of fast extensional flow, by measuring a linear, an asymmetric star and a symmetric star polystyrenes with the same span molecular weight (180 kg/mol). We show that all three melts reach the same extensional steady-state viscosity in fast extensional flow (faster than the inverse Rouse time). We further measure stress relaxation following steady extensional flow for the three melts. We show that initially they relax in a similar way, most likely via arm retraction, at short time, but behave differently at long time due to both the length of the arm and the branch point. The terminal relaxation is described by a Doi and Edwards based model, i.e. considering pure orientational relaxation.

Stress growth and relaxation of dendritically branched macromolecules in shear and uniaxial extension

“…The (fewer) data with G4 are consistent with this picture. The important message is that these dendritically branched polymers strain harden significantly even when stretched at rates below their terminal rate, in sharp contrast to linear or star polymers [56]. Furthermore, in Figure 5 3b and 4b).…”

Structural Studies of Three-Arm Star Block Copolymers Exposed to Extreme Stretch Suggests a Persistent Polymer Tube

“…The relative large polydispersity may reflect a small amount long linear block copolymers, which may be the consequence of nonoptimal temperature during the polymer synthesis. The quality of the polymer was investigated further using rheology comparing the polymer of the present Letter with that of a well-defined low-PDI threearm fully protonated PS star (PDI < 1.1) [20]. Rheology measurements show that the relaxation of the partly deuterated material is only slightly slower at low frequency, indicating some side product of a high-molar mass linear PS.…”

“…The applied strain rate for the stretched sample was _ ϵ ¼ 0.06 s −1 , performed at T ¼ 125°C. The equilibrium Rouse time of a linear chain of the same span M W is 450 s at 125°C [20]. The nondimensional stretch rate based on this time constant is significantly greater than unity so we expect that the molecules are highly oriented and nonlinearly stretched.…”

“…It has been questioned whether the tube model can describe branched and other types of complex polymer systems [11]. Star polymers, for example, have been the subject of several studies in both melts and solutions [11][12][13][14][15][16][17][18][19][20]. Dondos et al discussed the elongation of starshaped polystyrenes in flowing solutions, showing how the molecular deformation depends on their number of branches [13].…”

“…Xu and Chen made a coarse grained molecular dynamic simulation, showing that star polymers align and deform for shear rates beyond a critical value [19]. Huang et al measured the nonlinear rheology of three-arm star polymer melts in fast extensional flows [20], showing that the extensional viscosity of star polymers was identical to that of linear polymer in agreement with theoretical predictions [21]. While the agreement between the extensional viscosity and the model prediction is significant, it provides at best an indirect confirmation that branched polymers in strong extensional flows become quasilinear by aligning the arms.…”