Effect of Orientation on the Free Volume and Oxygen Transport of a Polypropylene Copolymer L. S. SOMLAI, 1, * R. Y. F. LIU, 1, y L. M. LANDOLL, 2, A. HILTNER, 1 E. BAER 1 1 Department of Macromolecular Science, Center for Applied Polymer Research, Case Western Reserve University, Cleveland, Ohio 44106-7202 2 Applied Extrusion Technologies, New Castle, Delaware 19720 Received 5 November 2004; revised 5 January 2005; accepted 7 January 2005 DOI: 10.1002/polb.20412 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The effect of uniaxial orientation on the free-volume and oxygen-trans- port properties of a propylene copolymer with 4.5 wt % ethylene was examined. The free-volume hole size and hole density were measured with positron annihilation life- time spectroscopy. Subsequently, the free-volume characteristics were correlated with the oxygen-transport properties. Orientation had only a small effect on the total amount of free volume: a small increase in the hole density was offset by a small decrease in the hole size. As a result, the oxygen solubility and amorphous-phase density were unchanged by orientation. However, a pronounced decrease in the oxy- gen diffusivity when the draw ratio exceeded 6 indicated a change in the dynamic free volume. This was attributed to an increasing number of taut tie chains, which retarded oxygen diffusion. The reduced amorphous chain mobility was also manifest in the increased glass-transition temperature, decreased bulk thermal expansivity, and decreased expansivity of free-volume holes. V V C 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1230–1243, 2005 Keywords: diffusion; free volume; orientation; oxygen transport; permeability; poly(propylene) (PP); solubility INTRODUCTION Atmosphere control is extremely important to the beverage and food packaging industries and is dependent on the gas-transport properties of poly- mer films. 1 Polypropylene is a candidate material for packaging applications because of its low cost and good thermal stability in comparison with polyethylene. Commercial polypropylene films are typically oriented by a tentering process in which an extruded film is quenched, reheated, and biax- ially drawn into a film. 2,3 Orientation improves both the mechanical and gas-barrier properties of the films. 4–6 To achieve appropriate atmosphere control, a fundamental understanding of gas transport, including the effect of orientation, is required. The orientation of amorphous polymers such as poly(ethylene terephthalate) can significantly enhance barrier properties. 7–9 The free volume can be noticeably reduced by orientation through cold drawing even though crystallization is pre- vented. As a result, the amorphous glass is densi- fied with a proportional decrease in the oxygen solubility. Drawing a semicrystalline polymer such as polypropylene, however, results in a struc- tural transformation of spherulites into microfi- brils. An improvement in properties such as the axial elastic modulus and sonic modulus upon stretching is ascribed to taut tie molecules, which *Present address: Baxter Healthcare, Cleveland, Mississippi, 38732. y Present address: 3M, St. Paul, Minnesota, 55144. Correspondence to: A. Hiltner (E-mail: pah6@po.cwru.edu) Journal of Polymer Science: Part B: Polymer Physics, Vol. 43, 1230–1243 (2005) V V C 2005 Wiley Periodicals, Inc. 1230