摘要:

Dynamic viscoelasticity and biaxial extensional flow behavior of polystyrene melts with various molecular weight distributions have been studied in wide ranges of temperature and strain rate ε̇B. Two patterns of time dependence of the biaxial stress growth coefficient η+Bare observed with increasing reduced strain rate ε̇Bτ0, where τ0is a characteristic relaxation time. At short times, η+Bcoincides with the linear viscoelastic curve; then it shows strain hardening after that, or strain softening followed by a quasisteady state. These behaviors are observed for polydisperse polystyrenes at much higher ε̇Bτ0than those for monodisperse polystyrenes. The quasisteady‐state viscosity ηB,qsdecreases with the strain rate. Addition of a very high molecular weight component causes considerable effects on τ0and on the strain rate dependences of shear ‖η*‖ and biaxial ηB,qsviscosities as well as on the stress growth coefficient η+B, where ‖η*‖ is the absolute value of the complex viscosity. The relaxation time τ0increases very much, and the strain rate dependences of ‖η*‖ and ηB,qsbecome much weaker. The upturn tendency of η+Bbecomes prominent when compared at the same reduced strain rate ε̇Bτ0. On the other hand, when compared at the same value of the reduced viscosity ηB,qs/6η0, a similar degree of strain softening in η+Bis observed for all polydisperse samples, where η0is the zero shear viscosity.

ISSN:0148-6055    

DOI:10.1122/1.550642

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Waxy maize starch in the solvent 90%DMSO–H2O exhibited the phenomena of shear‐thickening, quasihysteresis loops of shear stress versus strain rate that were anticlockwise and flow‐induced structure that was observed with phase contrast microscopy. These phenomena were not seen with normal maize starch and an enriched fraction of normal maize amylopectin. The ability of a waxy maize starch sample to have these properties could be destroyed by severe treatment. We consider the flow properties of shear‐thickening and anticlockwise shear stress/strain rate loops to be indicative of a flow‐induced structure that occurs with waxy maize starch amylopectin. Because these effects do not occur with normal maize starch we infer that there is a significant structural difference between the amylopectins of waxy maize starch versus normal maize starch.

ISSN:0148-6055    

DOI:10.1122/1.550643

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Often, blends of two immisible polymers have a morphology with one component building a matrix in which spherical inclusions of the other component are embedded. The rheological response of such blends contains an elastic contribution which can be attributed to the form relaxation of the inclusions. This process has a characteristic relaxation time which is proportional to the radius of the inclusions divided by the interfacial tension between the blends’ components. Thus a distribution of radii leads to a distribution of relaxation times. It is shown that rheological data together with an emulsion model can be used to determine the volume weighted sphere‐size distribution up to a scaling depending on the interfacial tension. The procedure is applied to data of four PMMA/PS blends and the results are compared with the corresponding distributions obtained from transmission electron microscopy (TEM). If the concentration of the spherical inclusions is small, both results are in excellent agreement. For larger concentrations, deviations between the results from rheology and TEM are observed.

ISSN:0148-6055    

DOI:10.1122/1.550720

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550644

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The kinetic progress of biopolymer gelation is measured in terms of the shear modulus–time curve available from mechanical spectroscopy. Attention is focused on the gel time, and on the limiting value of the modulus attained at long time. A theoretical treatment of these quantities is presented based on cascade theory. This model can take on a reversible, or irreversible, form depending on differential equations defining the growth of the degree of cross‐linking α(t), i.e., depending on whether a crosslink equilibrium is assumed or permanent branched crosslinking competes with irreversible intramolecular cyclization. Application of this model to limiting modulus–concentra‐ tion and modulus–temperature data is illustrated using results for a low DE pectin–calcium system. The model’s ability to explain modulus–molecular weight data is also demonstrated using literature data for kappa carrageenan gels. Where gel time–concentration data are concerned, success is much more limited, however, and this problem is discussed.

ISSN:0148-6055    

DOI:10.1122/1.550645

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The rheological behavior of protein gels with different network and strand structures has in a series of studies been investigated to elucidate how the structure of the gel influences its rheological properties. Nondestructive viscoelastic measurements and fracture techniques were used and the different types of structures studied were fine‐stranded gels and particulate gels. The influence of spatial inhomogeneities in the network structure was also investigated. The protein gel chosen as the model system was β‐lactoglobulin which forms gels on heating and develops different structures depending onpH: a particulate network at intermediatepH (4–6) and a fine‐stranded network below and above this interval. It can also form inhomogeneous networks depending onpH or heating rate. The fine‐stranded and the particulate gels had totally different mechanical properties. It was shown that the strand structure influenced both the modulus as well as the fracture properties of both fine‐stranded and particulate gels. Inhomogeneities induced by a slow heating rate also changed the mechanical properties the gels.

ISSN:0148-6055    

DOI:10.1122/1.550646

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

This paper serves as a review of work performed in the author’s and his collaborators’ laboratories over the last 15 years on the general topic of structure/property relationships for biopolymer (including food biopolymer) solutions and gels. In the first part, we describe how small deformation oscillatory measurements have enabled a distinction to be made between ‘‘entanglement networks,’’ ‘‘strong’’ and ‘‘weak’’ gels used, respectively, as food thickeners, gels, and stabilizers. At small enough strains both strong and weak gel systems give essentially the same mechanical spectrum, withG′≳G″, and with both moduli largely independent of frequency. However, the deformation dependence of these two classes of materials is very different. At large deformations strong gels will rupture and fail, while weak gels flow without fracture and but show recovery of solid (gel‐like) character. In this paper the various classes of food biopolymers are described, and their rheological properties related to differences in structure. The final part contrasts the behavior of a weak gel (xanthan gum) and entanglement solution (guar gum). This distinction is confirmed by their respective responses in start shear experiments. Guar solutions behave much like most other polymer solutions, whereas xanthan solutions show a very pronounced overshoot peak at low strains, and very long peak overshoot recovery times.

ISSN:0148-6055    

DOI:10.1122/1.550610

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The Wagner theory was used for gluten dough with 55% moisture content. Using the generalized Maxwell model with 24 parameters it was possible to simulate the linear memory function required in the Wagner formulation. The generalized Maxwell model also allowed prediction of the dynamic viscosity η′ and the out‐of‐phase component of the complex viscosity divided by frequency η′/ω in the frequency range studied. A damping function with two exponential terms was found to be suitable to simulate the nonlinear properties. With the incorporation of the linear memory function and the damping function, the Wagner model is able to predict the nonlinear relaxation modulus as well as the steady viscosity.

ISSN:0148-6055    

DOI:10.1122/1.550611

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

A waxy, a dent, and a high amylose starch were subjected to thermomechanical processing of varying degrees of severity. These treatments ranged from gentle dissolution in 90%DMSO to autoclaving, at temperatures up to 180 °C, to jet‐cooking. The treatments had major effects on viscosities of the starch dispersions and in particular on the intrinsic viscosities, which were determined to provide a relative measure of the molecular sizes of the processed starches. The intrinsic viscosities, from the least severe treatment to the most severe treatment, were 220 and 91; 181 and 104; 105–80 ml/g, respectively, for the waxy, dent, and high amylose starches. An unexpected characteristic of processed waxy maize was that the viscosity of 6% dispersions was extremely sensitive to intrinsic viscosity changes in the narrow range of 180–165 ml/g. In contrast, viscosities of waxy maize dispersions were relatively insensitive to changes in the lower intrinsic viscosity values (<160 ml/g) caused by more severe treatments. Dispersions of jet‐cooked waxy maize exhibited Newtonian flow; jet‐cooked dispersions of high amylose starch did not.

ISSN:0148-6055    

DOI:10.1122/1.550719

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Starch, especially the linear amylose fraction, is able to form helical inclusion compounds or complexes with molecules like flavor compounds, fatty acids, and emulsifiers. The effects of starch complexation on rheological properties were studied (a) in aqueous dispersions of 1%–4% starch content, (b) in high‐concentration gels of 40% starch content, and (c) in extrusion cooked low moisture starch blends. Complexation was followed by iodine binding capacity of starch and by differential scanning calorimetry. Rheological characterization was based on dynamic and steady shear measurements and on uniaxial force deformation tests. At low starch concentrations, the addition of complexing emulsifiers and flavor compounds induced gelation, provided that enough amylose was solubilized and swollen granules were present. Likewise, freshly prepared high‐concentration starch gels with the addition of complexing ligands exhibited higher moduli of deformability than control samples without additives. The sequence of moduli of elasticity became inverse during storage as complexation lowered the rate of amylose retrogradation. Melt viscosity during cooking extrusion increased with the addition of both complexing emulsifiers and noncomplexing triglycerides, while paste viscosity of ground extrudates increased with emulsifiers but not with triglycerides. Influence of starch complexation seems to be parallel for all moisture levels indicating that the main feature is the formation of insoluble amylose complexes which in turn lead to the formation of a firmer network between the dispersed starch granules.

ISSN:0148-6055    

DOI:10.1122/1.550612

出版商:The Society of Rheology    

年代:1995

数据来源: AIP