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Modeling the relaxation of polymer glasses under shear and elongational loads
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10.1063/1.4769253
/content/aip/journal/jcp/138/12/10.1063/1.4769253
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/12/10.1063/1.4769253
View: Figures

Figures

Image of FIG. 1.
FIG. 1.

Solid curves: local strain , 8 reduced relaxation time τ(t)/t w and tensile stresses of the polymer (p) and solvent (s) during loading of an infinite uniform cylinder. Here, t w = τ(0) is the age of the system when the experiment begins. Parameters are G s /G p = 8.5, μ = 12.5, t w 0 = 104, τ0 = 6 s; applied force/initial area f = 2.7G p. (The curve for T p , in red, initially lies below T s but crosses it during strain hardening.) The unload results for the basic model (θ = 1) is shown dashed; the solid curve after unload has θ = 0.1. The horizontal axis is marked both in dimensionless model units (top) and real time (converted using τ0), bottom. (As explained in Ref. 23 , the numerical solver introduces, in lieu of inertia, a small additional fluid viscosity η n = 0.05G p τ0 into Eq. (1) , whose magnitude has negligible influence on these plots.)

Image of FIG. 2.
FIG. 2.

Schematic evolution of the relaxation time τ(t) in a sample with τ(0) = t w —which is its age or “waiting time” in our model—subjected later to a step strain causing a sudden drop in τ. In the simple aging picture, τ(t) rebuilds from this point with the same slope as before (upper curve). However, if aging and rejuvenation are factorable, the slope of the curve drops by the same factor as τ does (lower curve).

Image of FIG. 3.
FIG. 3.

Solid curves: shear strain γ, reduced relaxation time τ(t)/t w and shear stresses of the polymer (p) and solvent (s) during shear loading. Parameters as in Fig. 1 , with a matched ratio of shear stress to solvent yield stress. (The curve for Σ p , in red, initially lies below Σ s but crosses it during strain hardening.) The unload results for the basic model (θ = 1) is shown dashed; the solid curve after unload has θ = 0.1.

Image of FIG. 4.
FIG. 4.

As in Fig. 3 but with shear stress increased by a factor 2.76. (This roughly matches the depth of the minimum in τ(t) to the elongational data of Fig. 1 .)

Image of FIG. 5.
FIG. 5.

Startup of steady shear at applied strain rate for a system with the parameters of Fig. 1 . (Top panel) Total shear stress (solid) and the polymer (dotted) and solvent (dashed) contributions when the flow is imposed to be spatially uniform. (Within the resolution of the plot, the total stress calculated allowing for inhomogeneity is indistinguishable.) (Second panel) Total first normal stress difference N 1 (solid) and polymer (dotted) and solvent (dashed) contributions under the same flow. (Third panel) The “degree of banding” (found by subtracting the smallest from the largest shear rate present at any time) for a 2D run with heterogeneity allowed. (A small diffusivity was added to the governing equations for all stress components and for τ, and the system was initialized with a small spatially varying noise; see Ref. 58 .) (Bottom panel) Snapshots of the strain rate as a function of position y in the flow gradient direction with symbols identifying strain values as in the middle panel.

Image of FIG. 6.
FIG. 6.

(Upper panel) Shear stress as a function of time in shear startup for the fluidity model without (lower curve) and with polymer. (G p /G s = 0, 1/8.5, respectively.) (Dashed regions) Transient instability as found by linear stability analysis. 58 Dotted curve is total stress allowing for inhomogeneous flow. (Middle panel) The resulting absolute value of shear rate variations with polymer (multiply bumped curve with added symbols) and without (singly cusped curve). 58 (Lower panel) Snapshots of strain rate profile with polymer present, at strain points identified by the symbols as in middle panel. Note the strong strain inhomogeneity (shear banding) at strains just beyond the stress overshoot. Parameter values G s , τ0 = 1, μ = 12.5, t w = 108, . Other parameters as in Fig. 1 and Ref. 58 .

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/content/aip/journal/jcp/138/12/10.1063/1.4769253
2013-01-02
2014-04-18
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Modeling the relaxation of polymer glasses under shear and elongational loads
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/12/10.1063/1.4769253
10.1063/1.4769253
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