Shear cessation

Rheological observations of the UHMWPE pseudo-gels of different concentrations under oscillatory shear conditions at different temperatures showed that these systems exhibit considerable drawability at temperatures above ambient. The deformation of the crystalline phase of the gel-like system is not reversible and, as shown in the sequence of photographs Figure 2, for a pseudo-gel of 4% concentration, it was greater when the sample was sheared under the same oscillatory conditions at higher temperatures. The displaced crystals of the UHMWPE pseudo-gel showed remarkable dimensional stability after shear cessation and removal of any compression load in the optical rotary stage. 

It is unfortunate that research in the area of polymer solutions has been deserted during the past ten years in favor of blend work. With the advent of shear cells, it is expected that research in polymer solutions will become fashionable again. For instance, the phenomenon of shear-induced apparent demixing of high molecular weight polystyrene in semidilute solutions (in DOP for example) is not understood. Kinetics measurements will hopefully permit a close monitoring of the remixing effect after shear cessation as well. 

Qi and Wang also observed a transient undulated HEX state during the melting of the HEX phase to the disordered state. This is consistent with the experimental observation in the shear cessation experiments of Bates and co-workers (Bates et al., 1994a Almdal et al., 1996). In these experiments, an initially disordered phase of the asymmetric poly(ethylenepropylene-b-ethylethylene) (PEP-PEE) diblock copolymer close to the order-disorder boundary is subjected to a steady shear which induces a transition to the HEX phase. The shear is then suddenly stopped and a transient modulated state is observed as the system relaxes back to the DIS. Insofar as a HEX... 

Fig. 8.16 (a) Optical micrograph of PA4,6/PPS (=80/20) blend aimealed in hot chamber for 10 min at 310 °C after shear cessation. Co-continuous morphology appeared, (b) Light scattering profile of this sample... 

The self-organizing nature of liquid crystal polymers is reflected in their complex flow behavior. The relaxation phenomenon of lyotropic polymer solution after shear cessation leads to band texture morphology that can be further induced to isotropic materials. Future research should focus on solvents influence on band size implicitly related to the induced pattern in polymers with different structures. Another aspect that could be explored is the imidization of patterned polyimide precursors and those conditions in which the texture is still maintained. 

Lin D, Qieng H, Zou F, Ning W, Han CC (2012) Morphology evolution of a bisphenol A polycarbonate/poly (styraie-co-acrylonitrile) blend under shear and afteu shear cessation. Polymer 53 1298-1305... 

System relaxation times have been determined from the relaxation of the stress after abrupt cessation of shear flow. Representative applications of the approach are found in Takahashi, etal. 9), who examined 355-3840 kDa polystyrenes in benzyl- -butylphthalate, at concentrations identified as showing dilute-solution behavior for the steady-state compliance Je and semidilute behavior for the zero shear viscosity. The stress relaxation after shear cessation, identified as the transient viscosity, decreased exponentially with time except at the shortest times studied, leading to an identification of an observed longest relaxation time Xm, whose c and M dependences were determined. 

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