Small Angle Neutron Scattering Observation of Chain Retraction after a Large Step Deformation

The process of retraction in entangled linear chains after a fast nonlinear stretch was detected from time-resolved but quenched small angle neutron scattering (SANS) experiments on long, well-entangled polyisoprene chains. The statically obtained SANS data cover the relevant time regime for retraction, and they provide a direct, microscopic verification of this nonlinear process as predicted by the tube model. Clear, quantitative agreement is found with recent theories of contour length fluctuations and convective constraint release, using parameters obtained mainly from linear rheology. The theory captures the full range of scattering vectors once the crossover to fluctuations on length scales below the tube diameter is accounted for.

Figure 1

Comparison of measured and predicted normalized structure factors after 0, $0.4{\tau }_{\mathrm{R}}$, $3.2{\tau }_{\mathrm{R}}$, and $15.7{\tau }_{\mathrm{R}}$ [(a)–(d), respectively]. The solid lines are predictions of the theory of Graham et al. [5], and the dotted lines correspond to a completely affine deformation on all length scales.

Figure 2

Data and theory for relaxation of the radius of gyration parallel and perpendicular to the stretch direction after deformation (a),(b) and experimental data highlighting the nonmonotonic region of $R_g^{\mathrm{perp}}$ (c).