Full tensor optical rheometry: Dynamics of flowing polymers in three -dimensional space
Control of material structure during processing is essential for the production of high performance parts. Polymeric materials are rheologically complex and flows during processing cause complex fluctuation in the molecular orientation, thus modifying the properties of the final product. The relationship between processing, rheology and the development of structure and phase separation, particularly for complex polymer systems such as blends, solutions and composites, is not fully understood.
Over the past few decades flow birefringence experiments have proven an increasingly valuable tool for determining the rheology and structure of flowing polymer systems. Birefringence measures the degree of extension and orientation of polymer molecules. By the use of the stress-optical rule, birefringence can also measure components of the stress tensor.
Initially, flow birefringence experiments were limited to steady-state measurements. This was due to the fact that birefringence experiments were incapable of measuring both the birefringence and angle of orientation of the polymers simultaneously. With the development of light-modulation techniques time-dependent measurements were possible. Conventionally, birefringence measurement techniques used a single probing laser beam, thus limiting measurements of polymer conformation to two-dimensional space.
The objective of this project is to determine the real-time, three-dimensional, full stress tensor by developing the Full Tensor Optical Rheometry (FTOR) technique. This technique utilizes three phase-modulated laser beams that impinge on the sample along three independent directions. Information about the molecular orientation in three-dimensions is measured and through appropriate geometric transformations the decoupling of this information yields the full conformation tensor.