Novel polypropylene based microporous membranes via spherulitic deformation
Otto H. York Department of Chemical, Biological and Pharmaceutical Engineering
Doctor of Philosophy
Sirkar, Kamalesh K.
Gogos, Costas G.
Dreizin, Edward L.
Narh, Kwabena A.
A novel method for creating a microporous polypropylene membrane via spherulitic deformation is described. The microporous structure was generated by the combination of intra-spherulitic and inter-spherulitic deformations. Polypropylene was selected due to its unique cross-hatched lamellar morphology facilitating inter-spherulitic deformation. A precursor film with a spherulitic structure was made under low-stress melt processing condition. A tangential lamellae-rich spherulite was created and identified with a positive birefringence sign. A sequential annealing process improved the crystalline structure, and in particular the thickness of the tangential lamellae. The annealing process proved to be critical for initiating the inter-spherulitic deformation. The post-extrusion process conditions for initiating inter-spherulitic deformation to create microporous membranes by lamellar separation are delineated. The processing parameters are: annealing temperature, extension ratio, stretching rate, and stretching temperature. A fixed set of extrusion conditions was chosen for producing precursor films having similar spherulitic properties. A Wide Angle X-ray Scattering (WAXS) examination provides a quick characterization method for the inter-spherulitic deformation. Membrane porosity measurements showed a consistent correlation with the observed ø-form orientation index. A highly interconnected solvent-resistant porous polypropylene membrane having a pore size in the range of 50-400 nm and a porosity of about 0.18 was thereby developed in this study. This concept can be further expanded by using an cc-nucleating agent to reduce spherulite sizes and utilizing interfacial debonding between two different phases to enhance permeability. A highly methanol permeable membrane with an estimated porosity of 0.29 was produced with the nucleated polypropylene samples, and a reasonable permeability was also observed in the membrane made from an immiscible blend. However, the occurrence of debonding can also compensate for the energy to create inter-spherulitic deformation. Increasing extension ratio did not change the microstructure in the non-annealed sample; however, the lamellae can be further oriented in the annealed samples. Inter-spherulitic deformation became obvious at slow stretching rates; intra-spherulitic deformation was more favored at a fast stretching rate. The DSC thermal analysis of the precursor films showed two significant endothermic discontinuities (T 1 at 0°C and T 2 at 40°C ) in the non-annealed or annealed precursor films; T1 is believed to be the conventional T g of polypropylene; T 2 appears to be the result of the rigid amorphous fraction trapped within "lamellar wells" where the amorphous phase is surrounded by R-lamellae and T-lamellae. The lamellae could break down or slip from the lamellar knots as stretching temperatures are high enough to minimize the effect of the rigid amorphous fraction, and the annealed lamellae can still be oriented without a catastrophic cold-drawn deformation.
njit-etd2008-107 (149 pages ~ 18,220 KB pdf)
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Created November 12, 2008