摘要:

A transient network model is presented which is based on the ideas of Lodge and Yamamoto. The network thus consists of different types of strands and the creation and loss rates are configuration dependent. As the model cannot be solved analytically one has to rely upon numerical tools. Using a continuous time algorithm this network can be simulated very efficiently. The material functions can be calculated as time‐dependent ensemble averages. Concentration is put onto a quantitative comparison between the rheological predictions of the model and experimental data for polyisobutylene subjected to simple and multiaxial elongational flows. The predictions of the model are found to be in very good agreement with the experimental data.

ISSN:0148-6055    

DOI:10.1122/1.550268

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

We consider the existence of solutions for some models of non‐Newtonian fluid. We have found that Poiseuille and Taylor–Couette problems may have no usual solutions in a large class of Oldroyd models of viscoelastic fluids. That is, axisymmetric Couette‐like solutions and unidirectional Poiseuille‐like solutions fail to exist at sufficiently large Deborah numbers. We have observed that some of the flow features become universal and independent of model parameters in the critical case, i.e., on the boundary between existence and nonexistence regions. We have described a duality between unidirectional flows of the 4‐constant Oldroyd fluid and plane potential flows of an ideal fictitious gas. Based on this duality we have interpreted the nonexistence of solutions as a ‘‘block‐up’’ phenomenon, in the language of gas dynamics.

ISSN:0148-6055    

DOI:10.1122/1.550270

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

One‐dimensional (1D) instabilities, some of them of new type, were found in simple shear and simple elongation for Giesekus, Leonov, and Larson viscoelastic constitutive equations (CEs) with single relaxation mode. Whereas 1D instabilities in shear flow were found for all three CEs, only the Larson model manifested some 1D instabilities (and even nonexistence of solution) in simple extensional flows.

ISSN:0148-6055    

DOI:10.1122/1.550271

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

Flexible polymeric chains (Kuhn chains) made up of nematogenic moieties are considered here. The simplest approach to the dynamics of such a system, in both the isotropic and the nematic phases, is that of modeling the chain as an elastic dumbbell where the ‘‘spring’’ law also accounts for the nematic interaction. The constitutive equation of this ‘‘nematic dumbbell model’’ is thereby derived. The rheological predictions so obtained are qualitatively compared with the most prominent features of LCP rheology as well as with previous theoretical results.

ISSN:0148-6055    

DOI:10.1122/1.550273

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

Flow studies of a high‐density polyethylene (HDPE) melt have been undertaken through planar slit dies with the purpose of establishing the birefringence pattern. Experimental results are compared with numerical solutions obtained by using an integral constitutive equation of the K‐BKZ type with a spectrum of relaxation times. The material parameters have been obtained by fitting viscosity and normal stress data as measured in shear, and elongational viscosity data available in the literature. The numerical simulations have been undertaken for a long (L/2H=2) and a short (L/2H=0.5) die at 180 °C. They show that as the flow rate increases, viscoelastic effects become important and manifest themselves in delayed relaxation of stresses along the slit and in stress overshoots at the die exit. This behavior is in close agreement with experimental birefringence patterns and in sharp contrast with purely viscous simulations which cannot predict such strong viscoelastic phenomena. Elastic recovery is also captured in an enhanced extrudate swell which is always higher for the short die.

ISSN:0148-6055    

DOI:10.1122/1.550366

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550274

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

The melt rheology of a new liquid crystalline copolyester Vectra E from Hoechst‐Celanese Corp. was investigated. A Rheometrics mechanical spectrometer was used to characterize the change in melt rheology produced by annealing below the melting temperature with dynamic, step shear rate, and step strain tests. The coupled relaxation model was employed in analyzing the step strain test data. Typically, thermotropic liquid crystalline copolyesters exhibit a reversible annealing effect which is erased by heating above the melting transition followed by rapid cooling. The unique feature of Vectra E is an irreversible annealing phenomenon. The unannealed copolyester melts near 330 °C, above which the viscosity decreases by at least two orders of magnitude. After annealing at 300 °C for 16 h, the copolyester exhibits a large increase in the melting transition temperature to 380 °C. Furthermore, upon melting the annealed sample the shear relaxation modulus and viscosity of the melt remain 2–3 orders of magnitude greater than those for the unannealed sample. These observations are consistent with a significant increase in the molecular weight of the Vectra E copolyester during annealing in the solid state at 300 °C.

ISSN:0148-6055    

DOI:10.1122/1.550275

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

Phase transition for blends of polystyrene (PS) (Mw=800,Mw/Mn=1.3)/polyisobutylene (PIB) (Mw=500) were studied in the quiescent state and under shear flow. The cloud points of blends were determined by turbidity measurements at a fixed cooling rate of 0.5 °C/min. The phase diagram of PS/PIB blends shows a UCST behavior with a bimodal miscibility gap at low temperature. The shear effects on the phase transition behavior of PS/PIB blend was investigated by a flow‐turbidity apparatus which was constructed by modifying a Rheometrics Mechanical Spectrometer (RMS). No shear‐induced phase separation was observed in the one‐phase region. For temperatures within the two‐phase region, the transmitted light intensity increased abruptly when the shear was applied. The intensities decreased and approached to the initial values when the shear was removed. Several possible explanations will be discussed to explain these results. The one that appears to be most valid is that of shear‐induced phase mixing.

ISSN:0148-6055    

DOI:10.1122/1.550276

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

The influence of phase separation on the linear viscoelastic response has been studied in miscible blends of polystyrene and poly(vinyl methyl ether) with a lower critical solution temperature near 110 °C. At temperatures between 25 and 155 °C, and for compositions in the range 20% to 60% polystyrene, the complex moduliG’andG‘were measured at frequencies in the range of 0.01 to 100 rad/s. Time–temperature superposition was applied in the single phase region to obtain the complex modulus over eight decades of frequency. Increasing the polystyrene content resulted in an increase in the zero‐shear viscosity and a shift of the terminal behavior to lower frequencies or longer times. The phase separation above the lower critical solution temperature was measured as a sudden increase in the fluorescence intensity of an anthracene‐labeled polystyrene (approximately 1 wt % in the blend), using an optical probe in the rheometer fixture. For the 20/80 and 40/60 PS/PVME samples, the terminal zone was in the accessible frequency window and phase separation was accompanied with a large increase inG’andG‘. In contrast, the complex modulus of the 60/40 PS/PVME blend could not be measured near the terminal zone and theG’andG‘did not exhibit any significant changes near the phase transition temperature.

ISSN:0148-6055    

DOI:10.1122/1.550277

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

The processing of high‐performance polyimides via their processible isoimide modifications is of great interest to aeronautical and aerospace applications. Rheological characterization during the isoimide‐to‐imide conversion, essential for identifying the processing conditions of such reactive polymers and their polymer blends, is reported. Using dynamic shear measurements, we obtained rheological information for a series of polyisoimide–imide resins as a function of temperature and cure history. Both homopolymers and copolymers have been investigated. We found that the processibility (e.g., melt viscosity of isoimide) and ultimate product properties (e.g.,Tgof polyimide) can be systematically varied by changing the chemical constituents and rigidity of the polymer backbone. Such correlations of chemical structures, rheological behavior, and processing characteristics provide the necessary database for tailor making new polymer systems with desirable processibility and mechanical properties. Practical considerations and difficulties in measuring the viscoelasticity of these materials are also addressed.

ISSN:0148-6055    

DOI:10.1122/1.550367

出版商:The Society of Rheology    

年代:1992

数据来源: AIP