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Entangled Linear Polymer Solutions at High Shear: From Strain Softening to Hardening
- Liu, Gengxin, Wang, Shi-Qing
- Macromolecules 2016 v.49 no.24 pp. 9647-9654
- glass transition temperature, polymethylmethacrylate, polystyrenes
- The present rheological study reveals for the first time that entangled polymer solutions made of linear polystyrene or poly(methyl methacrylate) can exhibit strain hardening due to non-Gaussian stretching during startup shear at sufficiently high rates and temperatures well above their overall glass transition temperatures: Tg,ₛₒₗᵤₜₑ > Tₑₓₚ > Tg,ₛₒₗᵤₜᵢₒₙ. The solutions made of high-Tg polymers only show partial yielding in the sense that both shear and normal stresses grow monotonically in time until a point of rupture, signified by an emergent cusp in the stress vs strain curve and macroscopic breakup along a shear plane. The shear softening-to-hardening transition, which occurs as a function of the applied shear rate, happens at lower equivalent rate with decreasing temperature, violating the time–temperature superposition principle.