摘要:

The rheology of a poly(vinyl acetate) (PVAc) ofM̄w=1.75×106,M̄w/M̄n=6.0,andB̄n(long branches/molecule)=3.2in diethyl phthalate and benzyl alcohol solutions has been investigated at concentration of 0.01–0.20 g/ml. The low shear‐rate viscosities, first normal stress differences, dynamic viscosities and moduli were measured by a mechanical spectrometer and the high shear values were obtained by a concentric‐cylinder viscometer. Consistent with the stress trend, it was observed that at lower concentration and higher temperature, non‐Newtonian effects occurred at higher shear rate and that the variation in viscosity with shear rate was greater at higher concentration. Solvent effects were found important even in concentrated solutions with the increase in viscosity with concentration more in diethyl phthalate than in a better solvent, benzyl alcohol. The critical concentration at which “entanglement” occurs was estimated to be ∼0.05 g/ml in both solvents. The viscosity data superimposed on a Graeasley viscosity‐shear‐rate master curve forM̄w/M̄n=1.65at lower shear rates for all concentrations, but not at higher shear rates. At low shear rates and frequencies, steady shear viscosity and dynamic viscosity coincided giving the so‐called zero‐shear viscosity. The applicability of several relationships which correlate steady‐flow and dynamic oscillatory behavior was examined. It was found that they are intercorrelated by a constant shift factor.

ISSN:0148-6055    

DOI:10.1122/1.549643

出版商:The Society of Rheology    

年代:1981

数据来源: AIP

摘要:

The unsteady viscoelastic flow problem of velocity profile development in a conduit owing to a step change in volumetric flow rate is considered. Using a three‐constant Oldroyd model, analytical solutions describing the time dependence of the velocity and shear stress distributions have been obtained. Particular analytical solutions are derived for conduits of different geometry, namely, annular, tubular, and planar. The analysis indicates that in contrast to Newtonian flow which exhibits a monotonic development of the velocity profile, viscoelastic liquids approach steady state behavior in an oscillatory fashion. The severity of the oscillatory behavior, however, is directly related to the elastic nature of the liquid.

ISSN:0148-6055    

DOI:10.1122/1.549614

出版商:The Society of Rheology    

年代:1981

数据来源: AIP

摘要:

Experimental data are reported for solutions of polystyrene in normal butyl‐benzene for concentrations,C,of450 kg/m3(with two exceptions). Polystyrene molecular weights,Mw,varied from 37,000 to2×106,and molecular weight distributions,Mw/Mn,varied from 1.06 to 22. The solutions were sheared in cone‐plate geometry in steady‐state, oscillatory, and stress growth and decay shearing modes. ForCMw>35,000,the viscosities depend on a molecular weight,M3,4.The first and second normal stress difference and related function dependences are similar. Both the first and second normal stress difference functions of the shear rate are similar and approach a constant “zero” shear rate asymptote. The ratios of the second to first normal stress difference coefficients for steady‐state shearing and the same ratios for the displacement and amplitude coefficients for oscillatory shearing have average values of −0.25, −0.225, and −0.22, respectively. The steady‐state shear compliances,Je0,computed from steady‐state “zero” shear rate first normal stress difference coefficients and the corresponding second normal stress difference properties are proportional toMw(Mz/Mw)3,5forCMw

ISSN:0148-6055    

DOI:10.1122/1.549617

出版商:The Society of Rheology    

年代:1981

数据来源: AIP

摘要:

The expansion of protein solutions extrudates was measured at the exit of horizontal capillaries. Zein solutions of 25.0, 29.4, 35.7, 40.0 and 45.4 wt % in ethanol were used. Two capillaries, with a length over diameter ratio(L/D)of 12 and 73 were used in order to study the viscoelastic relaxation of zein solutions in the high shear region,103–104 sec−1.At the lower zein concentrations (25.0 and 29.4 wt %) no significant effect of the(L/D)ratio of the capillary was observed. However, at higher zein concentrations (35.7, 40.0, and 45.4 wt %) significant extrudate swell ratios (χ), up to 1.50 were observed, and the extrudate swell ratio depended on the(L/D)ratio of the capillary. ForL/D≃12,shear rate affected the extrudate swell ratio (χ) at the higher concentrations. For the long capillary(L/D≃73),the extrudate swell ratio (χ) of all the solutions approached the Newtonian valueχ≃1.12forRe<16,as expected from the behavior of recoverable shear strain(SR)reported earlier at lower shear rates. The existence of the two concentration regimes, in terms of extrudate swell ratios (χ), is consistent with the transition in rheological behavior of zein solutions previously reported by Menjivar and Rha. The expansion behavior of the extrudates has been explained in terms of the viscoelastic characteristic times of these solutions and the counteracting effect of shear on protein‐protein interactions and particle orientation.

ISSN:0148-6055    

DOI:10.1122/1.549642

出版商:The Society of Rheology    

年代:1981

数据来源: AIP

摘要:

Stress relaxation in uniaxial extension has been studied in a linear and a four‐arm star‐branched polybutadiene and mixtures containing 10 and 12% of the former. Their weight‐average molecular weights(Mw)were 344,000 and 655,000, respectively, andMw/Mnwas approximately 2. Relaxation in the pure star was much slower than in the pure linear, and the behavior of the mixtures was intermediate in character. Relaxation of the linear polymer in the terminal zone at different stretch ratios λ could be described by the equationf(t)=E(t)(A0/λ)ln λ,wheref(t)is the time‐dependent force andA0the unstrained cross‐section area, andE(t)can be identified with the small‐strain relaxation modulus of linear viscoelasticity. The time dependence ofE(t)agreed rather well with the Doi‐Edwards theory with molecular weight distribution taken into account. For the mixtures, the contribution of the linear speciesE1(t)was calculated by difference from the equationE1(t)=E(t)−V2E22(t),whereE22(t)is the relaxation modulus of the pure star andV2is an empirical weighting factor obtained from comparison ofE(t)andE22(t)at long times. It was found that(1−v22)−1E1(t)agreed withE11(t)within experimental error. It appears that in a star‐rich mixture the linear species relaxes at the same rate as in its homologous environment.

ISSN:0148-6055    

DOI:10.1122/1.549616

出版商:The Society of Rheology    

年代:1981

数据来源: AIP

摘要:

A molecular network theory is proposed to predict the nonlinear flow properties of entangled monodisperse polymers. The rate dependence of the viscosity and normal stress coefficients is attributed to the decrease in entanglement density with increasing shear rate. The probability for the existence of entanglements at a given shear rate is determined by equating the rates of two competing processes, i.e., entanglement creation and disengagement, which in steady flow are in a state of dynamic equilibrium. The entanglement creation process is driven by thermal diffusion and is assumed to be independent of shear rate. The entanglement loss process is caused primarily by the imposed shear and is assumed to be proportional to the shear rate to a powera,a parameter that accounts for the elasticity of the medium. The final equations are algebraically simple and can be easily adapted to most engineering calculations. The theory shows excellent agreement with experimental data on both melts and concentrated solutions.

ISSN:0148-6055    

DOI:10.1122/1.549644

出版商:The Society of Rheology    

年代:1981

数据来源: AIP

摘要:

Rheological properties of two copolyesters which exhibit liquid crystalline behavior in the melt state were determined using an Instron capillary rheometer. The two polymer melts with nematic liquid crystalline order consisted of copolymers of polyethylene terephthalate (PET) and 60 and 80 mole % ofp‐hydroxybenzoic acid (PHB). Data was also obtained on PET which was used as a control. Measurements included the temperature and shear rate dependence of viscosity, entrance pressure losses(ΔPent),and die swell(Dj/D).The viscosity of the liquid crystalline melts are as much as two orders of magnitude less than those of PET at temperatures at which they are ordinarily processed. The extrudate actually contracts at the lower end of processing temperatures but does increase to values greater than one with increasing temperature for the liquid crystalline polymers. Although the die swell data indicate that there is negligible elastic recovery at the capillary exit, values of the ratio ofΔPentto wall shear stress(τw)are considerably higher for liquid crystalline melts than for PET. Some explanation of these data is presented but more rheological measurements are needed before our understanding of these unique systems is complete.

ISSN:0148-6055    

DOI:10.1122/1.549645

出版商:The Society of Rheology    

年代:1981

数据来源: AIP