Transport Mechanisms of Gases in Annealed Linear Low Density Polyethylene Films V. Compan ˜ , † M. Lo ´ pez-Lido ´ n, † A. Andrio, † and E. Riande* ,‡ Departamento de Fı ´sica Aplicada, Universidad Jaume I, Castello ´ n, Spain, and Instituto de Ciencia y Tecnologı ´a de Polı ´meros (CSIC), 28006 Madrid, Spain Received October 30, 1997; Revised Manuscript Received June 17, 1998 ABSTRACT: The effect of the pressure difference on the permeability of carbon dioxide, oxygen, and nitrogen through annealed LLDPE films is studied. The isotherms showing the dependence of the permeability coefficient on the pressure of the upstream chamber, p o, exhibit an anomalous decrease in the low-pressure region whose location is shifted to lower values of po as the temperature of the isotherms decreases. The diffusion coefficient increases with po, the increase being larger the higher is the temperature. The curves showing the dependence of the solubility coefficient on the pressure of the upstream chamber exhibits the same pattern as that of the permeability coefficient. The strong dependence of the diffusive characteristics of gases on p o in the low-pressure region was interpreted in terms of the dual mode theory. The values of the apparent diffusion coefficients for the molecules in the dissolved and the trapped modes are estimated. In the same way, the Henry’s law solubility coefficient, k D, and the Langmuir sorption capacity, C′H, are obtained. The values of kD and C′H for different gases are somewhat lower than those reported for glassy membranes. A detailed study on the effect of po on the activation energies associated with both the permeability and the diffusion coefficients is reported. Introduction Because of their good mechanical properties, espe- cially high tear strength, toughness, and good process- ability characteristics, coextruded films prepared from linear low-density polyethylene (LLDPE) present a wide variety of uses in the packaging industry. 1 The good mechanical properties of the films arise from the co- polymeric nature of LLDPE that makes possible the preparation of films that combine a relatively low crystallinity and a moderate orientation. Since these films are mainly used in food packaging, the study of the diffusive characteristics of gases through them, specially oxygen, nitrogen and carbon dioxide, is of a great importance. The permeation of gases in LLDPE semicrystalline films is a rather complex process. 2-6 Gas permeation through the amorphous phase of the films is a simple process, similar to that occurring through a liquid, and it is expected that the changes in the permeation characteristics with temperature obey Arrhenius be- havior. However, crystalline entities in the semicrys- talline films act as impermeable barriers to gases, forcing the penetrants to travel a longer path in the crystalline-amorphous interface than in the amorphous region, thus decreasing the diffusion coefficient. 3 More- over, as a consequence of the presumable changes in the crystalline-amorphous interface with temperature, the transport of gases in LLDPE films may not be a simple thermal-activated process. Earlier studies have shown that annealing causes a significant increase in the permeability coefficients of gases in coextruded LLDPE films without changing their overall degree of crystallinity. 7,8 On the other hand, the fact that the apparent diffusion coefficient only slightly decreases with annealing suggests that the most important consequence of this thermal process is a strong increase of the solubility of gases in the films. It would be expected, however, that annealing per- formed at relatively high temperatures (∼80 °C) would change the crystalline-amorphous interface in two opposing ways: favoring crystallites thickening, and increasing the transition order f disorder in the oriented molecules close to the crystalline-amorphous interface. In view of this, the increase of the apparent solubility coefficients by effect of annealing could be due either to a significant increase in the amorphous rub- bery region at the expense of both the oriented regions and the melting of the low size crystalline entities, or to the formation of molecular packing defects in the crystals and/or the crystalline-amorphous interface that could accommodate individual site molecules with- out disturbing the natural dissolution process of the gas in the amorphous region. 8 The first cause seems unlikely because a significant change in the overall crystallinity of the films is not detected. Therefore the increase in solubility could be attributed to adsorption processes taking place in defects in the crystals and/or in cavities formed in the crystalline-amorphous inter- faces. In this case, adsorption processes would play an important role in the gas transport. If this assumption is true, the dual mode model that gives a good account of gas transport in glassy membranes 9,10 would also describe the gas transport in annealed semicrystalline films. According to the model, diffusion would occur by jumps of dissolved molecules in the amorphous region (first mode) and partial mobility of trapped molecules in the cavities (second mode). To test the reliability of this assumption, attention was paid in this work to the study of the permeability of annealed coextruded LL- DPE films as a function of the pressure gradient. The results were further interpreted by assuming that Langmiur gas adsorption plays an important role in the solubility characteristics of the films. Attempts were also made to investigate how the combination of pres- † Universidad Jaume I. ‡ Instituto de Ciencia y Technologı ´a de Polı ´meros. 6984 Macromolecules 1998, 31, 6984-6990 S0024-9297(97)01600-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/15/1998