No description available.

The planar entry flow behavior of polystyrene, LDPE, and HDPE has been observed experimentally. The purpose of the work was to determine the cause of vortex growth and explain why this phenomenon occurs in some polymers but not in others. To accomplish this a die was constructed in which nearly any flow geometry could be formed by means of inserts. Flow visualization and flow birefringence experiments were performed using this die with 4:1 and 8:1 abrupt planar contraction geometries for τ_12w up to 7x10⁴ Pa and γ from 1 to 80 sec⁻¹. From these experiments it was determined that vortex growth in a polymer is caused by the resistance to flow under the influence of extensional strain. Although extensional viscosity measurements give an indication of whether or not vortices will form, the flow behavior is best correlated by the ratio of the centerline extensional stress to the downstream wall shear stress, τ₁₁ - τ₂₂ / τ_12w, measured in the entry region. The magnitude of this ratio was approximately 2 for LDPE, which exhibited vortex growth, but was approximately 1 for polystyrene, which did not exhibit vortex growth. Based on the experimental results, the numerical work was directed towards the use of a constitutive equation which could predict the extensional properties of the polymers being modelled. The Phan-Thien Tanner model was found to give adequate rheological property predictions and was used with the penalty finite element method to simulate the entry flow behavior of LDPE and polystyrene. Both qualitative (streamline patterns) and quantitative (extensional stress ratio) agreement was found between the experimental and numerical results. Vortex growth was predicted for LDPE, but incorporation of a zero relaxation time at the corner elements was necessary to increase the limit of convergence to the point where vortex growth was predicted. Support for the relation between vortex growth and extensional properties was given by the numerical results. lt was found that, holding all other rheological properties the same, an increase in the predicted extensional viscosity of a fluid results in the prediction of larger vortices. / Ph. D.

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Thesis (M.S.)--Rochester Institute of Technology, 1989. / Includes bibliographical references (leaves 131-133).

Thesis (M. Phil.)--University of Hong Kong, 1992.

Thesis (Ph. D.)--University of Texas at Austin, 2009. / Title from PDF title page (University of Texas Digital Repository, viewed on September 9, 2009). Vita. Includes bibliographical references.

The relationship between substituent chemistry and melt behavior for xylan derivatives was investigated by differential scanning calorimetry and parallel-plate dynamic viscometric measurements. Xylan esters exhibit characteristic Tg values which decrease with increasing size of substituent. However, these materials do not flow at temperatures well above Tg. The ether derivative hydroxypropyl xylan was found to flow at substitution levels higher than 0.5 degree of substitution, with melt viscosity decreasing as the degree of substitution increased. The influence of viscosity ratio and composition on the texture of melt-blends of hydroxypropyl cellulose and polystyrene was studied for viscosity ratios of 0.08 to 0.55. Blends were examined by SEM and TEM. It was found that composition was the determining factor in texture, and not viscosity ratio over the range tested. Extrusion problems limited the range of compositions tested to 40% biopolymer or less. Phase inversion was not observed, although inversion was expected for some samples based on predictive models in the literature. / Master of Science

A slit die with a rectangular slot placed transverse to the flow direction has been used for both flow visualization and direct pressure measurements of the hole pressure (Ph). The results from these measurements have been used to evaluate the Higashitani-Pritchard-Baird (HPB) equation which has been proposed for determining the magnitude of the primary normal stress difference (N1) from the values of the hole pressure. The slit die was run at higher shear rates than those used by Pike. Both the tracer method and flow birefringence technique were applied to visualize the streamlines and stress field, respectively, in the slot region . Effects of the slot on the flow pattern and on the stress field were examined by changing the slot width and by rounding the corners of the slot. The validity of the HPB equation, which is derived from the Higashitani-Pritchard theory (H-P theory), was tested by comparing values of N1 predicted by the HPB equation and slit die data with that obtained from the cone-and-plate rheometer. The validity of the HPB equation was also tested by changing the slot dimensions. Flow visualization experiments were performed for polystyrene (Styron-678) and polycarbonate (Lexan) melts. It was found that both the flow and stress fields are asymmetrically distributed about the slot centerline and that secondary flows exist in the upper part of the slot. However, no visible vortices was found for LOPE even though σ_w was increased to 70 Kpa. The shear rate at which the vortices became visible is lower for a wider slot. Rounding both corners of the slot seems to have an effect on smoothing the streamlines across the slot. Polycarbonate, which exhibits lower fluid elasticity than polystyrene does, the streamlines and stress field are more symmetrically distributed about the slot centerline. Five polymer melts were used in measuring Ph . It was found that the values of N1 predicted from the HPB equation correlate well with those obtained by the C&P apparatus at low shear rates. The predicted values of N1 also agree well with 2G' even though the shear rate was increased to 70 sec⁻¹ for most of the polymers investigated. Changing the width of the slot did not have a significant effect on the magnitude of Ph , whereas the magnitude of Ph depended largely on the polymers investigated. The measured Ph was nearly zero for polycarbonate at σ_w = 40KPa and was about 70 KPa for polystyrene at σ_w = 8O KPa. This was attributed to the significantly lower fluid elasticity of polycarbonate relative to polystyrene. / M.S.

Thesis (MSc)--University of Stellenbosch, 2006. / ENGLISH ABSTRACT: static density and magnetic structure functions of a melt of flexible polymers whose monomers possess bond-directed dipoles which interact with each other. In order to observe the effect of screening of the dipolar interaction on the structure functions we obtain results for cases with and without steric interactions and also for cases with and without Debye-H¨uckel screening of the dipole moments. / AFRIKAANSE OPSOMMING: Approximation” RPA) om die statiese digtheids- en magnetiese struktuurfunksies te bereken vir ’n smelt van hoogsbuigsame polimere, waarvan die monomere dipole langs die verbindings besit wat met mekaar in wisselwerking tree. Om die effek van afskerming op die dipolare wisselwerking en die struktuurfunksies te kan waarneem, bepaal ons resultate vir die gevalle met en sonder steriese wisselwerkings en ook vir gevalle met en sonder die Debye-H¨uckel afskerming van die dipoolmomente.

Polymer-layered silicate nanocomposites based on a variety of polymer matrices and several organoclays were prepared by melt processing. A detailed characterization of the thermal degradation of several commercial and experimental organoclays often used to form polymer nanocomposites was reported. The surfactant type, loading, and purification level of organoclay significantly affect their thermal stability; however, broadly speaking, the results suggest that these differences in thermal stability do not appear to have much effect on the morphology and properties of the nanocomposites formed from them. It seems that the thermal stability of organoclays is not the key factor in organoclay exfoliation in melt processed polymer nanocomposites, since the exfoliation/dispersion process may have been completed on a time scale before the degradation of surfactant progresses to a detrimental level. Polymer nanocomposites have been made from a variety of polymers; however, few matrices have demonstrated the ability to readily exfoliate the organoclay as well as nylon 6, especially for highly hydrophobic materials like polyolefins. Hence, a significant part of this research work was devoted to explore various routes to improve polyolefinorganoclay interactions, and thus, organoclay exfoliation in these systems. Amine grafted polypropylenes and a conventionally used maleic anhydride grafted polypropylene were used as compatibilizers for polypropylene based nanocomposites to improve the organoclay exfoliation. A series of ethylene vinyl acetate copolymers, the polarity of which can be adjusted by varying their vinyl acetate contents, based nanocomposites were prepared as the model system to address the relationship between the polarity of the polymers and their preferences over various organoclay structures. Attempts were made to explore the effect of degree of neutralization of acid groups in ionomers on the morphology and properties of nanocomposites, and it seems that the ionic units on the polymer chain provide a more favorable interaction between the polymer matrix and the organoclay compared to acid units and, thus, lead to better dispersion of the clay particles. It was determined that surfactants whose structure lead to more shielding of the silicate surface result in improved levels of exfoliation in all the above mentioned unmodified and modified polyolefin based nanocomposites. / text