Strain-hardening and breakdown in poly(vinylidene fluoride) gels in methyl-ethyl-ketone and heptanone

Abstract

Gelation kinetics and rheological properties of poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-co-HFP)] solutions in methyl-ethyl-ketone and in 2-heptanone are investigated. Small-angle X-ray scattering measurements indicate that the systems undergo phase separation by a nucleation process. For concentrations between 6 and 10 wt% of copolymer, strain-hardening appears when gels are sheared in the nonlinear regime, around 50% of deformation. At some critical shear amplitude, the rheological response changes abruptly, but reversibly, from hyperelastic towards viscous liquid. This indicates that the system undergo fracture or shear banding, in bulk or at walls. In other words the continuous network formed by the elastic, polymer-rich phase is locally broken under high amplitude oscillatory shear, thus breaking down the overall elastic response of the material. More interestingly, when the strain amplitude is progressively decreased back to zero, the initial nonlinear viscoelastic behavior is quantitatively recovered. In addition, when the strain is removed, the solution turns back to gel state very fast as compared with thermal gelation kinetics. These observations indicate that the initial structure can heal within a short time. It is proposed that strain-hardening depends on the intrinsic hyperelastic behavior of the polymer-rich phase, which should have a high density of effective crosslinks and/or entanglements.