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- EXTENSIONAL RHEOLOGY OF POLYISOBUTYLENE MELTS USING A COUNTER-ROTATING CYLINDERS RHEOMETER
- Sun, David
- 2011-12-05, 2011-12
Extensional rheology plays an immense role in many polymer processing operations, such as blow molding and fiber spinning. Extensional flow is...
Show moreExtensional rheology plays an immense role in many polymer processing operations, such as blow molding and fiber spinning. Extensional flow is a type of deformation that stretches a material and is sensitive to a polymer’s molecular structure. Elongational experiments are important in establishing flow models and verifying constitutive equations. Reliable extensional measurements and understanding of extensional rheology are vital for both academia and industry because they build upon the foundation for future theories, models, and applications. This study aims to understand the characteristics of a uniaxial elongation measuring technique and the validity of the data obtained. It focuses on the experimental properties of the Sentmanat Extensional Rheometer (SER), a specific uniaxial elongation rheometer, and tests the rheology for different molecular weights of polyisobutylene (PIB) melts. Through use of oscillatory shear, storage and loss modulus data are obtained and used to establish linear viscoelastic behavior. Using the SER, polyisobutylene was deformed to generate extensional viscosity data for different sample sizes, which was compared to the linear viscoelastic curve to check for consistency. Visual data analysis was used to examine deviations from ideal deformation. The results from this study were consistent with deviations seen by other researchers using the SER, and established experimental parameters that can improve performance of the SER. Based on elongational viscosity data, it is concluded that elongational viscosity does have a dependence based on sample size. In addition, this paper also quantifies the onset of surface instabilities, a phenomenon commonly seen, but not specifically reported. By utilizing the SER and different optical techniques, the development of the surface instability is examined. Analysis of the images demonstrates an appearance of surface striations that are consistent with different experimental parameters. By accurately capturing surface instabilities, they are found to be closely associated with sample deformations and onset of sample failure. By comparing visual and scattering images, the hencky strain (εinstability) at which instability occurs is consistently seen around 0.6 to 1.0. It is concluded that these striations are independent of strain rate, molecular weight, sample size, and technique. The validation of extensional viscosity’s dependence on sample size and onset of instability has great significance for uniaxial extension measuring tools, polymer processing, and extensional polymer modeling and simulation.
M.S. in Chemical Engineering, December 2011