The motivation for the determination of the viscosity of polymer solutions in dense fluids at the critical polymer concentration stems from the need to understand the factors that influence the time scale of phase separation in systems that undergo spinodal decomposition upon a pressure quench. In a recent investigation of PDMS + CO 2 and PE + n -pentane where molecular weights of the polymers and the critical polymer concentrations were comparable, significant differences were observed in the time evolution of new phase growth. Among the reasons that contribute to the difference in phase separation kinetics is the viscosity of the solutions. This thesis has been carried out to experimentally demonstrate the differences in viscosities of solutions at their critical polymer concentration. Specifically, the thesis focused on the high-pressure density and viscosity of solutions of poly(dimethylsiloxane) (M w = 93,700, M w /M n = 2.99) in supercritical carbon dioxide and of polyethylene (M w = 121,000, M w /M n = 4.3) in near-critical n -pentane. The measurements have been carried out a t the critical polymer concentrations, which is 5.5 wt % for solution of PDMS in CO 2 and 5.75 wt % for solution of PE in n -pentane. For PDMS + CO 2 system, the measurements were conducted at 55, 70, 85 and 100 o C and pressures up to 50 MPa. For iii PE + n-pentane system, the measurements were conducted at 140 and 150 o C and again up to 50 MPa. All measurements were conducted in the one-phase homogenous regions.At these temperatures and pressures, the viscosities were observed to be in the range from 0.14 mPa.s to 0.22 mPa.s for PDMS + CO 2 , and from 2.3 mPa.s to 4.6 mPa.s for PE + npentane systems. In both systems the viscosities increase with pressure and decrease with temperature. The temperature and pressure dependence could be described by Arrhenius type relationships in terms of flow activation energy (E # ) and flow activation volume (V # ) parameters. The flow activation energies in PDMS + CO 2 system were about 7 kJ/mol compared to about 18 kJ/mol for the PE + n -pentane system. The activation volumes were in the range 40-64 cm 3 /mol for PDMS + CO 2 system and 65-75 cm 3 /mol for the PE + n -pentane solution. The higher values of E # and V # represent the higher sensitivity of viscosity to temperature and pressure changes in the PE + n -pentane system. The viscosity d ata could also be correlated in terms of density using free-volume based Doolittle type equations. Density is shown to be an effective scaling parameter to describe T/P dependency of viscosity. The closed packed volumes suggested from density correlations were found to be around 0.33 cm 3 /g for the PDMS and 0.48 cm 3 /g for the PE systems. Comparison of the viscosity data in these systems with the data on the kinetics of pressure-induced phase separation confirms that the slower kinetics in the PE + n-pentane stems from the higher viscosity in this solution compared to the PDMS + CO 2 system, despite the similarity in the molecular weight of the polymer a...

We present a new sensor configuration and data reduction process to improve the accuracy and reliability of determining the terminal velocity of a falling sinker in falling body type viscometers. This procedure is based on the use of multiple linear variable differential transformer sensors and precise mapping of the sensor signal and position along with the time of fall which is then converted to distance versus fall time along the complete fall path. The method and its use in determination of high-pressure viscosity of n-pentane and carbon dioxide are described.

ABSTRACT:The phase behavior and volumetric properties of polyethylene (PE) in solutions of n-pentane and npentane/CO 2 were studied in a temperature (T) range of 370 -440 K at pressures up to 60 MPa. Measurements were conducted with a variable-volume view-cell system equipped with optical sensors to monitor the changes in the transmitted light intensity as the P or the T of the system was changed. Lower-critical-solution-temperature-type behavior was observed for all of the liquid-liquid (L-L) phase boundaries, which shifted to higher pressures in solutions containing CO 2 . The solid-fluid (S-F) phase boundaries were investigated over a P range of 8 -54 MPa and took place in a narrow T range, from 374 to 378 K in this P interval. The S-F phase boundary showed a unique feature in that the demixing temperatures showed both increasing and decreasing trends with P depending on the P range. This was observed in both the PE/n-pentane and PE/n-pentane/CO 2 mixtures. The density of these solutions were measured as a function of P at selected temperatures or as a function of T at selected pressures that corresponded to the paths followed in approaching the phase boundaries (S-F or L-L) starting from a homogeneous one-phase condition. The data showed a smooth variation of the overall mixture density along these paths.