Nonlinear Mechanical Behavior of Scarcely Crosslinked Poly(dimethyl siloxane) Gel: Effect of Strand Length Polydispersity

Abstract

Nonlinear mechanical behavior was examined for a scarcely crosslinked poly(dimethyl siloxane) gel (referred to as Gel-1/1) under constant-rate elongation and large step shear strains. The average molecular weight of the gel strands evaluated from the equilibrium modulus in the linear viscoelastic regime was Mc=190×103, and the strands had a significantly broad molecular weight distribution, Mw/Mn∼600 as estimated by fitting the linear viscoelastic moduli with a Rouse network model. In the elongational test at constant elongational rates dε/dt (=dλ/dt/λ; λ = elongational ratio), the Gel-1/1 sample exhibited dε/dt-insensitive strain hardening followed by rupture at λmax=4.5. This λmax was significantly smaller than the λmax° nominally expected for a gel composed of monodisperse strands having Mc=190×103; λmax°=53 and λmax/λmax°∼0.08 for those strands. In contrast, a reference experiment made for a Gel-U sample composed of monodisperse strands (Mc=15×103; including densely trapped entanglements) indicated that λmax of this gel was close to λmax°; λmax∼14, λmax°=16, and λmax/λmax°∼0.9 for Gel-U. These results suggested that the low-M fractions of the strands in the Gel-1/1 sample were highly stretched and broken at λ much smaller than the λmax° defined for the average Mc, thereby governing the nonlinear elongational behavior/rupture of Gel-1/1. Under large step shear strains γ (>2), Gel-1/1 exhibited nonlinear decay of the shear stress with time. Analysis of the linear viscoelastic moduli of Gel-1/1 after imposition of large strains indicated that the stress decay under large strains reflected scission of the low-M fractions of the gel strands as well as the motion of scission-formed long strands occurring with time. This behavior was qualitatively similar to the nonlinear elongational behavior, although a delicate difference related to time-dependent cessation/motion of the scission-formed long strands remained between the nonlinearities under the large shear and elongation.