摘要:

The relationship between molecular weight distribution (MWD) and linear viscoelastic response of polymer melts was investigated with three commercial polymers and two blends (11 and 13 components, respectively) of nearly mono‐ disperse materials. Dynamic modulus master curves in the plateau and terminal zones were compared with predictions based on theoretically motivated combining rules, various descriptions of response for monodisperse melts, and the molecular weight distribution of each of the samples. Agreement with the Tsenoglou combining rule was excellent for the model mixtures, in which MWD was known by construction. Departures at low frequencies were found for the commercial samples, in which MWD was determined by size exclusion chromatography and light scattering. The discrepancy is attributed to uncertainties in the high molecular tail of distributions obtained by the dilute solution methods.

ISSN:0148-6055    

DOI:10.1122/1.550363

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

Ultraviolet radiation cured polymeric systems, useful in photonic applications, are obtained through free radical polymerization of mixtures containing monomers and oligomers. We used aninsitucuring technique within a rheometer to study the effect of large amplitude oscillatory shear on sample gelation. Below the critical gel point (pre‐gel state), we found no effect of shearing and the samples were linearly viscoelastic for large strains. Materials in the post‐gel state, on the other hand, showed strain hardening with increasing strain. Measurements of moduli versus frequency following the shearing showed significant increases in the moduli indicative of shear induced gelation. This behavior can be explained by the presence of trapped free radicals existing both as small molecules (e.g., oligomers) and as part of the polymer network. Shear (i) causes the immobile radicals, which are part of the network, to encounter other reactive sites, and, (ii) enhances the diffusion of the small molecule radicals, both of which lead to increased cross‐linking. While rheological tests are consistent with this mechanism, direct confirmation of the presence of free radicals was obtained from electron spin resonance (ESR) measurements done on pre‐ and post‐gel samples.

ISSN:0148-6055    

DOI:10.1122/1.550364

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

Proper description of the resin flow through fibrous media is a very important input in the modeling of composite manufacturing processes. In this study, the complex phenomena associated with the flow of viscoelastic fluids through fibrous media are examined. In particular, the effects of fluid and fiber bed properties on the pressure drop observed during such flows were investigated, in conjunction with the specific conditions of typical composite manufacturing processes. To gain some insight into the problem, a theoretical and experimental investigation of Newtonian and non‐Newtonian flows through different geometric cylinder arrangements designed to simulate the actual fiber configurations was carried out. Particular care was taken to select the appropriate dimensionless flow parameters to demonstrate that the onset of the ‘‘excess’’ pressure drop due to fluid elasticity is independent of the flow geometry. Employment of this dimensionless group, which is related to the total strain on the macromolecules, yields superior predictions compared to previous results based on the Deborah number. In order to assess the applicability of the results in ideal fiber beds to actual manufacturing processes, flow of viscoelastic fluids through carbon fiber beds was studied under conditions that simulate the autoclave process.

ISSN:0148-6055    

DOI:10.1122/1.550365

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

We examine some theoretical and experimental aspects of the measurement of interfacial tension, stress relaxation in elongational flow, and yield stresses in organic liquids, blends of polymer melts, and liquid crystal polymers. This study is based on an instrument which is an improved version of the spinning drop apparatus that is commonly used to measure interfacial tension between melted polymers. Problems of vibrations at high speed, heating of the bearings, high temperatures required to melt the polymers, outgassing at the reduced pressures generated by rotation, and other problems have been eliminated in the improved apparatus. This same instrument can be used to generate curves of volume expansion versus temperature for blended systems and to detect and interpret yield stresses which occur in some polymers and strongly influence the properties of blends. The instrument has been enhanced for accurate measurements of drop diameter, length and shape as a function of time and initial conditions. A theory of upper and lower bounds for interfacial tension and a theoretically based method of exponential fitting has been developed to help to overcome the problems of slow approach to equilibrium between highly viscous melts. We have developed and propose to develop further a theory of relaxation in which transient measurements of drop diameter can be used to obtain rheological properties like elongational and yield stresses and interfacial tension and more generally to interpret the curves of relaxation generated in the experiments.

ISSN:0148-6055    

DOI:10.1122/1.550311

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

Above the critical stress for slip, the procedures normally used to analyze the results of capillary flow data give anomalous results. In particular, the Bagley plots are curved, even when a variation of viscosity with pressure is not anticipated, and the Mooney technique used to calculate the slip velocity gives results that indicate that the slip velocity depends on theL/Dratio. It is proposed that these phenomena arise from the dependence of the slip velocity on the wall normal stress, which implies a dependence on pressure. Based on this hypothesis, an approximate method is developed for interpreting the results of capillary flow experiments to determine the slip velocity as a function of both the wall shear stress and the pressure. The large available data set was used to incorporate into the model the effects of molecular weight parameters and temperature on the slip velocity. Finally, a detailed model for slip flow in a capillary was formulated that takes into account that the slip velocity and wall shear stress vary along the flow direction due to the pressure gradient. This model was used to evaluate the validity of the approximations used in the approximate data analysis technique for determining the slip velocity.

ISSN:0148-6055    

DOI:10.1122/1.550313

出版商:The Society of Rheology    

年代:1992

数据来源: AIP

摘要:

Shear‐induced particle structures of rheologically well‐characterized concentrated polymer dispersions were investigated by small angle neutron scattering (SANS) in a wide range of shear rates. The dispersions consist of electrostatically stabilized styrene–ethylacrylate–copolymer spheres in glycol or water. Their viscosity functions show pronounced shear thinning and strong shear thickening versus shear rate as measured by various rotational rheometers and by capillary rheometry. A quartz slit die, which could be tilted with regard to the neutron beam, enabled us to achieve wall shear rates as low as 10−5s−1and wall shear stresses up to 104Pa. Part of the measurements were repeated using a Couette shear cell. Spheres of 320 nm mean diameter at a volume concentration of 58.7% in glycol show an amorphous structure at rest. In the range of strong shear thinning the halo intensity becomes anisotropic, the intensity in flow direction being increased, but no long range particle structure shows up. This result shows that drastic viscosity transitions may occur, whereas the structural changes detectable by SANS may remain weak. Nearly monosized 165 nm particles at 52.3% solid in glycol exhibit in the shear thinning regime distinct hexagonal maxima superimposed on the halo and also in a second and third ring indicating the formation of a long range particle structure. With increasing shear rate these maxima disappear and an anisotropy of the halo intensity shows up. An intensity increase in flow direction at small angles is observed in the shear thickening regime. The particle structures observed after stop of flow depend on the preceding shear rate. Similar structural changes are found at 43.4% solid for the 165 nm particles. The results obtained in Couette flow are in qualitative agreement with the slit data but show much sharper intensity maxima. The underlying type of superstructure is compared with nonequilibrium molecular dynamics (NEMD) simulations for a soft sphere model fluid. The intensity patterns extracted from the simulation bear a remarkable similarity to the directly measured SANS data.

ISSN:0148-6055    

DOI:10.1122/1.550314

出版商:The Society of Rheology    

年代:1992

数据来源: AIP