The characteristics of extensional flow and the chain orientations of the isothermal film casting process utilizing a two-dimensional
(2-D) viscoelastic model with finite element methods (FEM) were studied. Steady state and transient solutions were obtained
for a relatively large aspect ratio regime by employing successive iterative schemes. In this work, higher aspect ratios of
the equipment caused highly oriented molecular structures because the aspect ratio increases as the flow changes from planar
to uniaxial extensional flow. Fluid viscoelasticity always alleviated the neckin phenomenon and led to the planar extension
regime even if dichotomous behavior was observed for draw resonance in extensional thickening and thinning fluids. Consequently,
the change in the characteristic of extensional deformation from uniaxial deformation to the planar extension deteriorated
the molecular orientation. 相似文献
The rheological behavior in extensional flow was investigated for silica dispersions in polymer solutions using the tubeless syphon technique. The flow field of tubeless syphon is unsteady in the Lagrangian sense; hence the data are taken under transient stress conditions. Due to the insensitive nature of the extensional rate to flow rate, the extensional viscosities at different flow rates lie very close to a single curve, once they are plotted against strain. Two important points are noted with regard to the behavior of the dispersions. First, the extensional viscosity decreases with increasing particle concentration. Second, the viscosity growth curves of the dispersions can be superimposed by a vertical shift, but the curve shape differs from that of the medium. This implies a change in the relaxation time. 相似文献
Summary
In this work, we explore the effect of ethanol addition on the extension thickening behavior of aqueous solutions of poly(ethylene
oxide) (PEO) in porous media and opposed-jets flow. The main aspect analyzed is the formation of transient entanglements of
polymer molecules, which are responsible for the sudden increase in pressure drops and apparent extensional viscosities with
strain rates observed in elongational flows of PEO solutions. The results show that changing the solvent by adding ethanol
modifies the solution behavior because of two effects. First, the increase in the solvent viscosity increases the disentanglement
time of the transient entanglements so that the onset of extension thickening occurs at lower strain rates in the presence
of ethanol. Second, ethanol solutions are poorer solvents than water for PEO. This promotes intermolecular interactions between
PEO coils when the solutions are semi-dilute. The net result is a lowering of the onset strain rate for extension thickening
to values that are much smaller than those expected for the same PEO chains dissolved in a higher viscosity and poorer solvent.
The results also show that coil-coil interactions in solution can be detected at lower concentrations in porous media flow
than in opposed jets flow.
Received: 24 August 2001/ Revised version: 29 October 2001/ Accepted: 6 November 2001 相似文献
Linear stability analysis has been applied to a coextrusion fiber spinning flow that consists of a Newtonian fluid as a core layer and a Phan-Thien/Tanner (PTT) fluid as a skin layer. These two chosen fluids show entirely different rheological behaviors. The stability of this coextrusion system was affected by the choice of three characteristic parameters: the skin layer fraction (f), the extensional parameter (ϵ), and the shear thinning parameter (ξ) in the Phan-Thien/Tanner model. The linear stability results indicate that the viscoelastic skin layer (PTT fluid) has a stabilizing effect that delays the onset of draw resonance. Under fixed compositions (f is fixed), the stability envelopes changed from upturned curves to flattened ones as extensional force dominated the system. The neutral stable curves closed to the horizontal line at a critical draw ratio of around 20, showing similar behavior to a Newtonian fluid where the system has a very high Deborah number or it is dominated by shear thinning effects. 相似文献
The controlled dispersion of Stöber silica nanoparticles (SiNPs) at the interface of a PS/PA6 (80/20 wt%) blend was achieved by means of surface modifications using 3-methacryloxypropyltrimethoxysilane (MPS). The final localization of SiNPs in the blend was predicted using wetting parameter calculation and confirmed by scanning electronic microscopy (SEM) observations. Stability of blends during annealing was evaluated qualitatively by laser diffraction particle size analyzer. Morphologies of the blends in the molten state were observed using optical microscopy. Flammability of blends was investigated using pyrolysis-combustion flow calorimeter (PCFC). Results showed that both microstructure stability during annealing and thermal degradation of the blend, were improved when MPS-modified SiNPs are located at the interface. SEM pictures revealed that the MPS-modified SiNPs form a solid barrier between PS and PA6 phases which inhibits coalescence process and modifies the thermal degradation mechanisms. 相似文献
Capillary thinning experiments of semi‐dilute solutions of schizophyllan in water and DMSO are performed to determine the relaxation behavior in extensional flows as experienced, for example, in the flow through porous media in enhanced oil recovery. The extensional relaxation time λE is found to scale with concentration following a dissimilar power‐law dependency for the two solvents, λE ∝ c1.52 in water and λE ∝ c0.90 in DMSO. It is shown that the extensional flow fields are strong enough to break, and prevent the rebuilding of, intermolecular structures, due to hydrogen bonding that was observed to alter the viscoelastic response in shearing flows of aqueous schizophyllan solutions.
A study of postextrusion relaxation (shrinkage) rates identified two separate processes, only one of which is influenced by carbon black structure and surface area. One of these processes is thought to be a solidlike fast elastic recovery and was found to be influenced by carbon black and polymer characteristics. The other process reflected slow randomization of oriented molecular chains: it was mainly influenced by polymer nature and carbon black had almost no effect. In the previous paper of this series, work with various carbon blacks in SBR–1500 indicated that both types of polymer deformation originate within the converging flow region at the die entrance, where extensional flow takes place. In the present work the extensional flow behavior of five polymers (filled with various carbon blacks) was examined at elevated temperatures. The magnitude of shrinkage caused by molecular alignment was related to extensional viscosity of the rubbers. This confirms the hypothesis stated above. Extensional viscosity was measured using the previously described instrument, which is inexpensive to construct, easy to operate, and can be attached to any standard stress–strain tester. The characteristics of extensional flow are defined and it is shown that for polymeric materials it differs substantially from shear flow, since a rapid orientation of the molecular chains takes place during the extensional flow. The effect of carbon black on the extensional viscosity at elevated temperatures is illustrated. 相似文献
Capillary flow studies on bulk molding compound (BMC) using an instrumented injection-molding machine are reported. The significance of extensional flow effects with fiber-reinforced materials is emphasized. The extensional flow behavior in converging dies is modeled, and a means of evaluating both extensional and shear viscosity from capillary flow data is proposed. Methods of correcting results for the effect of deformation heating are discussed. The shear and extensional flow behavior of BMC in the temperature region 18 to 58°C can be fitted to a simplified Arrhenius Law. 相似文献
The development of biaxial segmental orientation and stress in a flexible-chain polymer fluid subjected to steady biaxial extensional flow is analyzed. Closed-formula model based on the Padè approximation of the inverse Langevin function in the non-Gaussian distribution of the chain end-to-end vectors is considered. The approach is free from the limitations related to finite chain extensibility and slow convergence of the series expansion formulations at higher chain deformations.Segmental orientation is characterized by the average orientation tensor, related axial orientation factors and global orientation anisotropy. Orientational behavior and corresponding stresses in the biaxial elongational potential flow are discussed in a wide range of elongation rates. Orientation characteristics calculated for the biaxial flow deformation are much higher than those predicted for the affine biaxial stretch deformation in polymer solids. 相似文献