Access Type

Open Access Dissertation

Date of Award

1-1-1998

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemical Engineering and Materials Science

First Advisor

Dr. Esin Gulari

Second Advisor

Dr. Charles W. Manke

Abstract

Dissolved gases in polymers behave as excellent plasticizers, reducing viscosity significantly through the chain dilution effect and the addition of free volume. Rheological measurements and theoretical modeling are presented for molten polystyrene with dissolved gases, including carbon dioxide, 1,1-difluoroethane (R152a), and 1,1,1,2- tetrafluoroethane (Rl34a), which are considered to be environmentally acceptable for replacing previous ozone-depleting hydrochlorocarbon (HFC) refrigerants. A modified pressurized capillary rheometer was employed for the viscosity measurement of molten polystyrene with dissolved gases under elevated pressures and at various temperatures. Experimental results show a significant reduction of viscosity due to dissolved gas, reaching as much as 3 orders of magnitude decrease in viscosity. The composition-dependent shift factor scaling method, which is similar to classical viscoelastic scaling techniques (time-temperature superposition), was adopted in order to evaluate the effect of gas concentration on viscosity. The method successfully consolidates the viscosity data of gas-loaded systems onto a single master curve, confirming that viscosity reduction by presence of dissolved gas can be represented quantitatively through the concentration shift factor. The extended free volume model was developed for the theoretical prediction of viscosity reduction by the dissolved gas. Free volumes were calculated from the specific volumes and the occupied volumes of pure components and mixtures. The specific volumes of polymer-gas mixtures were computed by the Sanchez-Lacombe equation of state model, which was parameterized using the solubility data of gases in molten polystyrene. The theoretical viscosity reduction was compared with the experimentally obtained shift factor. The effect of free volume on viscosity was found to be much greater than the effect of chain dilution. The viscosity reduction was maximized when the measurement temperature approaches the glass transition temperature of pure polymer, because of significant free volume increases due to the presence of dissolved gas. In addition, an extensive pressure correction procedure was developed in order to obtain representative viscosity data from capillary rheometry, and this method was successfully applied to both pure and gas loaded polystyrene melts.

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