Date of Award
Doctor of Philosophy (PhD)
Zumbrunnen, David A
Grujicic , Mica
Li , Gang
Thompson , Lonny
A prototype In Situ Structuring Rheometer (ISSR) was designed and implemented to study changes in shear viscosity of polymer blends and composites while processing them in such a way as to control the evolution of microstructure. The ISSR is based on a regime of fluid mechanics known as chaotic advection, in which simple time-periodic flow fields can cause fluid particles to move chaotically. Chaotic advection is also the basis of Smart Blending, a technology employed to process polymer blends having a variety of morphologies at a fixed composition, and polymer composites in which the additives have been arranged into layered structures or networks. Smart Blending has been implemented as batch devices or continuous flow devices, with a device of the former type providing the basis for the ISSR.
Designed as a test cell to be fitted into a commercial instrument so as to leverage its measurement capability, the core of the ISSR is a cylindrical blending cavity the end surfaces of which are formed by rotatable disks which induce stirring. While the upper disk is an attachment for the commercial instrument, the lower disk has an independent drive system. The ISSR also includes a heating chamber, temperature control systems and a purge gas system. Alternate counter-rotation of the disks through an appropriate displacement leads to a chaotic flow. The design of the ISSR and experiments conducted using it were guided by modeling.
The result is that even as the microstructure in the sample is being controllably formed, the shear viscosity is measured each time the upper disk rotates. In contrast, conventional rheometry using a parallel-plate or cone-plate test cell involves mixing materials as melts beforehand, with a polymer blend usually having a droplet morphology and a composite usually having the additive randomly dispersed throughout the polymer matrix.
Three types of systems were processed and studied using the ISSR. At least three samples of each system were processed to different extents, cryogenically fractured and examined using scanning electron microscopy (SEM). By so doing, the trends in viscosity were related to progressive structure development, which is the controlled evolution and retention of particular blend and composite morphologies, as has been documented in previous chaotic advection blending studies.
The first system was a compatible blend of low density polyethylene (LDPE) and high density polyethylene (HDPE), for which the viscosity initially rose and eventually reached a plateau, which was consistent with a model that showed no change in viscosity with the number of layers. Blend samples at different stages of processing showed the initial formation of layers and the development of nanoscale features as these layers refined. The second system was a composite of linear low density polyethylene (LLDPE) and carbon black (CB), for which the shear viscosity slowly decreased with continued processing. Micrographs indicated that the carbon black initially formed coarse striations and may have subsequently formed networks, as was observed in previous studies using related chaotic advection blending devices. The third system, an immiscible blend of LDPE and polypropylene (PP), exhibited a nearly constant viscosity.
Repeatability of viscosity data was an issue for all three systems. Several problems with this prototype were identified as potential factors: misalignment of the cavity components, sample leakage, temperature cycling of the sample, and coordination of disk motions. To address these problems, it is recommended that the cavity seal be improved, the temperature control systems studied more thoroughly, and the disk motions coordinated automatically in a future ISSR.
Subrahmanian, Ramaswami, "Applying Chaotic Advection to Rheology: an In Situ Structuring Rheometer" (2012). All Dissertations. 791.