摘要:

The chemorheological models used for the description of thermoset polyurethanes are too often specific to a given formulation. This paper discusses the application of a more generic modeling approach which describes the evolutive rheological behavior of a polyurethane reactive mixture from a set of fitted monomer characteristic parameters and its calculated molecular weight distribution (MWD) at a given conversion level. The MWD is evaluated numerically from stochastic simulations. Once the adjusted parameters are obtained, the model becomes applicable at any starting formulation using a liquid prepolymer built from the same monomer. The linear viscoelastic properties were obtained from MWDs using the double reptation mixing rule. The mixing rule was adapted to include a contribution of Rouse’s relaxation times to account for short chains species, as those encountered in the beginning of the polymerization reaction. This procedure was successfully applied to two different difunctional thermoset polymeric systems that included either hydroxy-terminated polybutadiene or polypropylene glycol.

ISSN:0148-6055    

DOI:10.1122/1.550938

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Based on the model of Bersted and Slee, and later Malkin and Teishev, both differential and integral methods were developed to determine the molecular-weight distribution (MWD) from viscosity data. The more sensitive differential method can detect small inflections in the viscosity data and convert these into MWD information. The integral method is, however, capable of handling moderately incomplete viscosity data. Combining self-consistent differential and integral approaches, we are able to resolve details of a MWD and quantify reasonably broad MWDs from many sets of limited viscosity data. These methods both have very short computation times. A reported overemphasis of the high-molecular-weight end of distribution is due to the excitation of Rouse modes during the rheological measurements at high frequencies, which masks the diffusive contributions of the low-molecular-weight components to the relaxation process.

ISSN:0148-6055    

DOI:10.1122/1.550951

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

A combined finite element and Brownian dynamics technique (CONNFFESSIT) is used to predict the steady-state flow field around an infinite array of square-arranged cylinders using kinetic theory models. A finitely extensible elastic (FENE) dumbbell model and a modified reptation model are considered. Since Brownian dynamics simulations are used to predict the stresses in the flow field, no closure approximations are necessary in the models, such as the Peterlin approximation, or independent alignment. Comparisons are made with analogous models that have closed-form constitutive equations, namely the FENE-P dumbbell, and Doi and Edwards reptation model with independent alignment using the same numerical technique. The modified reptation model contains information about the entire chain configuration instead of just single segment orientations. In this way, the problems involved with reversing flows for reptation with independent alignment can be avoided. Since the flow field presents alternately converging and diverging flows for fluid particles, it offers an important test for reptation models in reversing flows. We find significant quantitative difference between the predictions of the approximate and the more realistic models. For example, the FENE-P dumbbell underpredicts the magnitude of the normal stresses by as much as 25%, reptation with independent alignment underpredicts the magnitude of the normal stresses by as much as 22%. The calculations presented here are a starting point towards the realistic prediction of the onset of instability seen experimentally in these flows.

ISSN:0148-6055    

DOI:10.1122/1.550960

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

This paper discusses the effect of temperature on the dynamic rheological properties of both the model polystyrene gelatin and the industrial photographic coupler dispersions. The time temperature superposition (TTS) was used to bring experimental data at various temperatures together into single master curves. An Arrhenius-type TTS principle, rather than the Williams–Landel–Ferry equation, was used in this work to bring all dynamic moduli and dynamic viscosity curves at different temperatures into single master curves. The present investigation verified that the TTS principle, which was developed for polymeric materials, could also be used for model and industrial photographic coupler dispersions as well. Furthermore, not only was the TTS principle suitable for the dispersions in the sol state, but it could also be used for the data in the gel-like state as well. The TTS allowed the estimation of the rheological properties of the dispersions over the frequency range which is otherwise inaccessible to the range of experimental measurement. Therefore, the linear viscoelastic properties of these model and photographic coupler dispersions at very low frequency (which is useful in predicting the stability of the product), as well as properties at very high frequency (or large deformation, e.g., during coating or pumping processes), could be estimated.

ISSN:0148-6055    

DOI:10.1122/1.550962

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

The linear viscoelastic behavior of starches from various origins, blended with different amounts of a plasticizer using a twin screw extruder, has been investigated. Conditions of stability for reliable measurements and linearity domain have been determined. At low strain, the plasticized starch is found to behave as a viscoelastic gel-like material. This behavior is partially explained by its semicrystalline structure, as evidenced by x-ray diffraction patterns. Crystallites are assumed to participate in the formation of an elastic network, embedded in a viscoelastic amorphous phase. Additional thermomechanical treatment reduces crystallinity and leads to a decrease of the storage and loss moduli. The plasticizing role of water is found to be important. The higher molecular weight of the potato starch is responsible for its larger moduli. In the case of maize starch, the highly branched amorphous amylopectin softens the structured behavior, resulting in lower moduli compared to those of rich-content amylose.

ISSN:0148-6055    

DOI:10.1122/1.550900

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Two high molecular weight, narrow distribution polystyrenes dissolved in tricresyl phosphate were studied using a sliding plate rheometer. Shear stress and birefringence measurements in step strain allowed the determination ofN3for both solutions. Start-up of steady simple shear and large amplitude oscillatory shear measurements revealed that the critical shear stress for slip for both solutions is around 2500 Pa and that slip is dynamic in nature, allowing rheological measurements above the critical stress to be made at sufficiently high frequencies. The critical stress value was used in conjunction with data from the literature to develop a relationship between critical stress and concentration. Large-amplitude oscillatory shear stress measurements revealed that the solutions exhibit a limiting behavior at high strain rate amplitudes in the intermediate frequency range when the dependence on strain rate amplitude becomes small. Birefringence measurements showed that at high frequencies, where the time scale of the deformation is short, the oscillatory component ofN3vanished. This was found to occur at lower frequencies for the lower molecular weight solution. In contrast, the average value of the third normal stress difference, reflected in the zeroth harmonic, was found to vary little between the two solutions.

ISSN:0148-6055    

DOI:10.1122/1.550958

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

We demonstrate that viscoelasticity combined with nonlinear slip acts as a storage of elastic energy generating oscillations of the pressure drop similar to those observed experimentally in extrusion instabilities. We consider the time-dependent axisymmetric incompressible Poiseuille and extrudate-swell flows of an Oldroyd-B fluid. We assume that slip occurs along the wall of the die following a slip equation which relates the shear stress to the velocity at the wall and exhibits a maximum and a minimum. We first study the stability of the one-dimensional axisymmetric Poiseuille flow by means of a one-dimensional linear stability analysis and time-dependent calculations. The numerically predicted instability regimes agree well with the linear stability ones. The calculations reveal that periodic solutions are obtained when an unstable steady-state is perturbed and that the amplitude and the period of the oscillations are increasing functions of the Weissenberg number. We then continue to numerically solve the time-dependent two-dimensional axisymmetric Poiseuille and extrudate-swell flows using the elastic-viscous split stress method for the integration of the constitutive equation. Again, oscillations are observed in the unstable regime; consequently, the surface of the extrudate is wavy. However, the amplitude and the period of the pressure drop oscillations are considerably smaller than in the one-dimensional flow. The most important phenomenon revealed by our two-dimensional calculations is that the flow in the die is periodic in the axial direction.

ISSN:0148-6055    

DOI:10.1122/1.550959

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

The deformation and recovery of a poly(isobutylene) droplet with a lower viscosity embedded in a poly(dimethyl siloxane) matrix are directly observed from two directions after application of a large step shear strain. The droplet shape and recovery time strongly depended on the magnitude of the applied strain. Just after application of a large strain, a droplet deforms almost affinely to a flat ellipsoid. It then changes into a rodlike shape, a dumbbell, and to an ellipsoid of revolution, and finally to a sphere. Model calculations show that the primary driving force for the shape recovery is the interfacial energy in order to reduce the surface area of the deformed droplet.

ISSN:0148-6055    

DOI:10.1122/1.550932

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

A chemical-type theory for wall slip in polymer melts is developed by modeling the exchange of bridging sites between two opposing polymeric and solid surfaces. Kinetic equations, describing surface coverage by bridging monomers, are formulated and analyzed to evaluate the stability of adhesive contact and slip characteristics of the viscoelastic melt. Order of magnitude estimates of the kinetic coefficients suggest that the polymer–solid interface is always at equilibrium, even under slip. The model displays the following features. The polymer slips at all stresses; the slip velocity,vs,obeys time-free volume superposition and depends on both shear and normal stresses. At small stresses,vsis linear in shear stress and proportional to a function of the work of adhesion; the slip parameterb(the slip extrapolation length scale) takes on the same form as that proposed by de Gennes, but displays an additional dependence on adhesive energy. At constantvsthe shear stress is proportional to the adhesive free energy. A catastrophic loss of adhesion occurs at a critical stress that depends on the difference between the work of adhesion (polymer–solid) and the work of cohesion (polymer–polymer). Predictions compare favorably with literature data for slip of linear low-density polyethylene on metal.

ISSN:0148-6055    

DOI:10.1122/1.550901

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

The shear creep response of homeotropic monodomains of4,4′-n-octylcyanobiphenyl (8CB) and dilute solutions of a side-chain liquid-crystalline polysiloxane (LCP) inN-(4-methoxybenzylidene)- 4-butylaniline (MBBA) is studied in the absence and presence of electric fields applied along the director. In the absence of the field, oscillations in strain rate are observed for 8CB and the LCP/MBBA solutions, indicative of director-tumbling flow. In the presence of electric fields, the tumbling flow is suppressed and, with increasing field strength, a systematic evolution is observed toward flow-aligning, the apparent viscosity being determined by the balance in hydrodynamic and electric torques. Due to a larger dielectric anisotropy, the tumbling flow of 8CB is suppressed at a critical field strength much lower than that of the LCP/MBBA solution. Moreover, a field-induced asymmetry of the doublet peaks is observed in the creep deformation for 8CB at 34 °C. Based on Ericksen’s transversely isotropic fluid theory, simulation of the influence of electric fields on the flow-tumbling behavior is performed and qualitative agreement with the experimental results is obtained.

ISSN:0148-6055    

DOI:10.1122/1.550945

出版商:The Society of Rheology    

年代:1998

数据来源: AIP