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The melt rheology of linear poly(lactic acid)s (PLA) characterized by a high content of the l-form of the monomer is comprehensively investigated. Measurements of dynamic, steady, and transient shear viscosities are presented. Extensional data on PLA are presented for the first time and show a strong strain hardening behavior. The Cox−Merz relationship is obeyed over a particularly wide range (roughly 3 decades of shear rate). Results for high molecular weight samples suggest that the plateau modulus is approximately 5 × 105 Pa. In addition, the zero shear viscosity, η0, for these materials is found to roughly scale with the expected 3.4 power vs molecular weight. The transient shear results are satisfactorily predicted using a truncated form of the K−BKZ constitutive equation and a set of Maxwell modes (G k , λ k ) derived from the dynamic spectra. However, to capture the observed extensional hardening, an additional long time relaxation mode must be added to the spectrum. Time sweep measurements demonstrate that the melt stability of the polymer precludes long time measurements. Chemical changes manifest themselves in a lack of adherence to the principle of time−temperature superposition. It is shown that stabilization of poly(lactic acid) using tris(nonylphenyl)phosphite is possible and leads to a material that is thermorheologically simple within the experimentally assessable rates of deformation.

A series of monodisperse polybutadienes has been used to perform mechanical relaxation measurements from the flow regime to the glassy state. Results on loss shear moduli have been analyzed within the framework of a coupling model. It has been found to represent well the frequency dependence in the glassy and terminal zones using a segmental motion coupling parameter Pa and an entanglement coupling parameter Pv. Moreover, our results on apparent shift factors resulting from time-temperature superposition are consistent with the lack of thermorheological simplicity predicted by the coupling theory due to the difference between pa and &. ,

Polymer gels have been shown to behave as viscoelastic materials but only a small amount of data is usually provided in the glass transition. In this paper, the dynamic moduli G ′ and G ′′ of polyacrylamide hydrogels are investigated using both an AFM in contact force modulation mode and a classical rheometer. The validity is shown by matching of the two techniques. Measurements are carried out on gels of increasing polymer concentration in a wide frequency range. A model based on fractional derivatives is successfully used, covering the whole frequency range. G 0 N , the plateau modulus, as well as several other parameters are obtained at low frequencies. The model also predicts the slope a of both moduli in the glass transition, and a transition frequency fT is introduced to separate the gel-like behavior with the glassy state. Its variation with polymer content c gives a dependence fT ∼ c 1.6 , in good agreement with previous theories. Therefore the AFM data provides new information on the physics of polymer gels.

The one-dimensional fractional derivative Maxwell model has been found very useful in modeling the linear viscoelastic response in the glass transition and α-relaxation regions. That motivated further work on generalizations to nonlinear viscoelastic fractional constitutive equations. In the work of Palade et al. [Int. J. Eng. Sci. 37, 315 (1999)], a fully objective constitutive equation for an incompressible fluid—reducible to the linear fractional derivative Maxwell model under the small deformations hypothesis—is given, together with a state of rest stability analysis. In the stability section of the aforementioned work, for some physically aceptable perturbation inputs, it has been shown that no continuously differentiable solutions may be found to the corresponding equations of motion. In this work, we extend the stability analysis framework to L2 Hilbert functional space and prove the existence of solutions to the corresponding equations of motions.