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

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