Index of content:
Volume 60, Issue 1, January 2016
A nonlinear shear and elongation rheological study of interfacial failure in compatible bilayer systems60(2016); http://dx.doi.org/10.1122/1.4926492View Description Hide Description
This work aims to examine whether or not interfacial failure can occur in a compatible polymer bilayer system under large shear and elongation deformations, as well as to probe the sensitivity of nonlinear transient rheology to the presence of interface/interphase at neighboring layers. For this, stress relaxation after a step strain and fast startup in simple shear and uniaxial extension experiments have been performed on healed and coextruded poly(methyl methacrylate)/poly(vinylidene fluoride) bilayers with the presence of a robust diffuse interphase as evaluated by energy dispersive X ray. For unhealed bilayers, interfacial failure occurred in shear flows at intermediate deformations, while for healed bilayers the interphase greatly delayed the onset of interfacial failure to larger deformation steps and to a higher deformation rate in the startup shear. Extensional rheology demonstrated that the presence of an interphase in the bilayers greatly enhanced the transient extensional viscosityηE+(t) as well as the tensile relaxation modulus E(t) of the structure, even though the entanglement density was relatively low. Moreover, models are presented to predict the nonlinear relaxation behavior of bilayers and to estimate the relaxation behavior of the interphase. Fitting of the tube model to the shear relaxation indicates a dilated tube diameter in the interphase, confirming its weak entanglement intensity and its readiness to flow-induced disentanglement under large external deformations. Finally, the physics of the interfacial failure was assessed based on some recent molecular dynamic theories.
60(2016); http://dx.doi.org/10.1122/1.4935127View Description Hide Description
Polyphosphate is a highly water-soluble linear polyanion comprised of phosphate groups. A phase separation happens when divalent cations are added to polyphosphate solutions resulting in the formation of polyphosphate coacervates. Such coacervates could potentially be used as a glass precursor or in a variety of bioapplications including microencapsulation. In all of these applications, viscoelastic properties of polyphosphate coacervates directly affect their usage. Here, we show that these polyphosphate coacervates act as extremely viscous Newtonian liquids at low shear rates, with their viscosity highly dependent on the polyphosphate degree of polymerization (Dp) and type of divalent cations (Ca2+, Sr2+, or Ba2+) used for their preparation. For Ca2+-only polyphosphate coacervates, specific viscosity is directly related to Dp1.46 at 20 °C, similar to highly concentrated polymer solutions where chain entanglement is not important. For very long chain polyphosphate coacervates, however, elastic properties are dominant presumably caused by physical chain entanglement. Replacement of a fraction of Ca2+ with Sr2+ or Ba2+ results in coacervates having significantly more elastic characters and profoundly higher viscosity than their Ca2+-only counterparts. Overall, varying polyphosphate chain length and divalent cation type allows one to modify these materials for a desired application.