摘要:

The effect of corona treatment and copolymer modification on linear viscoelastic properties of PS/PE blends in the molten state has been investigated. The results show that corona treatment has little effect on rheological properties of the blends. On the other hand, SEBS addition results in an appreciable decrease of the elastic modulus in the low‐frequency region and in a more evident secondary plateau. The data have been analyzed quantitatively by Palierne’s emulsion model, which takes into account the presence of an emulsifier agent. The model has been used to estimate the interfacial tension in PS/PE blends as modified by corona treatment and by SEBS copolymer modification. Addition of as little as 1 wt % copolymer resulted in a fivefold decrease of interfacial tension, whereas only a little change was observed in the case of corona modified blends. Further, corona treatment did not affect the size of the dispersed particles, whereas 1 wt % SEBS addition reduced the particle size of the dispersed phase by a factor of 5. Experiments as well as simulations also have been carried out to examine the individual effects of viscosity ratio, particle size distribution, and interfacial tension on dynamic moduli of the blends.

ISSN:0148-6055    

DOI:10.1122/1.550709

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Apparent slip within a few micrometers of a glass surface is visualized with a microscope using small glass spheres as tracers suspended in a highly entangled 20% solution of polystyrene of molecular weight 8.42 million, confined between glass plates in a plane Couette device. After step shears of 0.5–5 strain units, particles within a few micrometers of each surface move by 70–500 μm, enough to unload more than half of the imposed shear strain at intermediate strains. The slip displacement, when plotted logarithmically against time, show a peculiar ‘‘kink’’ in the vicinity of the ‘‘Rouse’’ time of 100 s, similar to the kink reported for the shear stress. These results show that the peculiar rheological properties reported for highly entangled solutions are produced by an effective slip phenomenon that occurs either at the wall or in a thin disentanglement layer near it.

ISSN:0148-6055    

DOI:10.1122/1.550710

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

A reevaluation of the previous rheo‐optical analysis of the system hydroxypropylcellulose/acetic acid (HPC/AA) (Edwards and McHugh, 1993) is made through the development of techniques to monitor the steady‐state and transient responses of the linear dichroism, linear birefringence, their orientation angles, and the circular birefringence. Shear and first normal stress measurements allow correlation of the rheological behavior with that of the optical properties. Results are reported for the quiescent circular birefringence of the HPC/AA solutions. Flow experiments indicate that the linear dichroism and birefringence, and the rheological response, are controlled by the overall orientation of the semirigid molecules; thus these optical properties have a common transient time scale. Circular birefringence is controlled by conformational rearrangements of the HPC which take place above a critical shear rate. The time scale of this intramolecular mechanism is much larger than that for the orientational ordering. Most likely, internal hydrogen bonds which enhance the optical activity of the HPC are broken under shear, causing molecular untwisting, followed by retwisting on flow cessation.

ISSN:0148-6055    

DOI:10.1122/1.550711

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Normal stresses in colloidal dispersions at low shear rates are determined theoretically for both dilute and concentrated suspensions of Brownian hard spheres. An evolution equation for the pair‐distribution function is developed and the perturbation to the microstructure in a general linear flow is shown to be regular toO(Pe2), where Pe=γ̇a2/D0. Here, γ̇ is the characteristic shear rate,ais the particle size, andD0is the bare‐particle diffusivity. The next term in the perturbation of the microstructure is shown to beO(Pe5/2). The bulk stress (nondimensionalized by ηγ̇, where η is the viscosity of the suspending fluid) for a dilute suspension in a general linear flow is determined toO(φ2Pe). For simple shear flow the theory predicts normal stress differences ofN1/ηγ̇=0.899φ2Pe andN2/ηγ̇=−0.788φ2Pe; there is no correction to the shear viscosity atO(Pe), however. A scaling theory is also presented for concentrated suspensions using the corrected time scalea2/Ds0(φ), whereDs0(φ) is the short‐time self‐diffusivity at the volume fraction φ.The appropriate Peclet number is now P̄e=γ̇a2/Ds0(φ). The scaling theory predicts that the dominant contribution to the stress comes from Brownian motion and scales as P̄eg(2;φ)/D̂s0(φ), whereg(2;φ) is the equilibrium radial‐distribution function at contact andD̂s0(φ)=Ds0(φ)/D0. As maximum packing is approached, φm, the normal stress differences are predicted to diverge as (1−φ/φm)−2P̄e, P̄e≪1. In the presence of interparticle forces there is an additional contribution to the stress analogous to the Brownian contribution. When the length scale of the interparticle force is comparable to the particle size, there is no qualitative change for the divergence of the normal stress differences near maximum packing. For a strongly repulsive interparticle force characterized by a length scaleb(≫a), the theory predicts that the appropriate Peclet number is now Peb=γ̇b2/D0and that near maximum packing based on the thermodynamic volume fraction φb=4πnb3/3, the normal stress differences diverge as (1−φb/φbm)−1Peb, Peb≪1.

ISSN:0148-6055    

DOI:10.1122/1.550712

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

A novel method of investigating the link between molecular features of polymer molecules and the rheological properties of dilute polymer solutions has been investigated. It applies the dissipative particle dynamics (DPD) computer simulation technique, which introduces a lattice‐gas automata time‐stepping procedure into a molecular‐dynamics scheme, to model bead‐and‐spring‐type representations of polymer chains. Investigations of static and dynamic scaling relationships show that the scaling of radius of gyration and relaxation time with the number of beads are consistent with the predictions of the Rouse–Zimm model. Both hydrodynamic interaction and excluded volume emerge naturally from the DPD polymer model, indicating that a realistic description of the dynamics of a dilute polymer solution can be obtained within this framework, and that very efficient computer simulations are possible.

ISSN:0148-6055    

DOI:10.1122/1.550713

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

The orientation field induced by the steady, radially diverging, Newtonian flow between flat, parallel disks is studied. Exact solution for the three‐dimensional rotation of a rigid, hydrodynamically isolated, neutrally buoyant, non‐Brownian, slender particle is developed throughout the flow domain from an Eulerian viewpoint. Hence, a closed‐form expression is obtained for the orientation distribution, which represents the accurate spatial characterization of the orientation field. In addition, the accuracy of the closure approximations used in complex suspension flow simulations is investigated. Both hybrid and quadratic closure approximations are found to yield considerably inaccurate results, including incorrect asymptotic behavior. The exact solution indicates a stable skin‐core orientation structure at large distances from the inlet, which is also confirmed by the available experimental data.

ISSN:0148-6055    

DOI:10.1122/1.550714

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

A method was developed for calculating the molecular weight distribution of a polymer melt from its rheology, specifically dynamic or relaxation moduli data. The molecular weight range covered by the solution is specified by the source data or the entanglement molecular weight, and the blending rule consistent with double reptation was used. The solutions for both the discrete relaxation spectrum and final molecular weight distribution were obtained by Tikhonov regularization with the molecular weight distribution solution being particularly sensitive to the value of the regularization parameter λRused in the calculations. Here, λRwas varied and patterns developed between the most appropriate values for an acceptable solution and the polydispersity of the corresponding samples. For relatively low λRvalues, the algorithm was precise enough to resolve individual components in nearly monodisperse, binary blend, and multimodal systems with as many as 13 components. Results for commercial materials of varying polydispersity generally agreed with the molecular weight distributions from chromatography with slightly higher λRvalues, particularly the location of peak molecular weights. Commercial polyolefins, which required still higher λRvalues, gave consistently poorer results due to restricted molecular weight ranges allowed by the source data.

ISSN:0148-6055    

DOI:10.1122/1.550723

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

摘要:

Using conoscopy, we have investigated the dynamic director response of nematics to oscillatory shear and step shear strain. In oscillation, the ratio of the amplitude of the director rotation to the strain amplitude and the phase difference between the director rotation and the applied strain are measured as functions of frequency for both 5CB and 8CB. Comparison with the Leslie–Ericksen theory allows extraction of several important material parameter ratios. In particular, data in the high frequency limit yield λ, the material parameter which indicatesflow‐aligning(λ≳1) ortumbling(λ<1) behavior. For 5CB at 32.5 °C, λ≊1.03, while 8CB at 34 °C gives λ≊0.42. An analysis is presented for director field relaxation following distortion induced by a step strain. Step strain experiments on 5CB and 8CB provide an independent measurement of viscoelastic properties. Aside from confirming the flow classification of these model nematics, these new experimental methods establish a framework for the study of director dynamics in polymeric nematic liquid crystals.

ISSN:0148-6055    

DOI:10.1122/1.550715

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550716

出版商:The Society of Rheology    

年代:1995

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550647

出版商:The Society of Rheology    

年代:1995

数据来源: AIP