Polymer Vol. 37 No. 26, pp. M-5837, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-3861/96/$15.00+0.00 zyxwvutsr SOO32-3861(96)00443-O Effect of annealing on the permeation characteristics of gases of coextruded LLDPE films Vicente Compafi, Andreu Andrio and Maria L. L6pez Departamento de Fisica, Universitat Jaume I, 72080 Castelldn, Spain and Evaristo Riande” lnstituto de Ciencia y Tecnologia de Polimeros, CSIC, 28006 Madrid, Spain (Received 2 November 1995; revised 22 January 1996) The temperature dependence of both the permeability and diffusion coefficients of carbon dioxide, oxygen and nitrogen in annealed LLDPE films are studied. It is found that the values of the permeability coefficient through the annealed membranes are nearly four times larger than those through the non-annealed ones. The fact that annealing slightly diminishes the values of the diffusion coefficient leads to the conclusion that the rise in permeability detected in the films by effect of annealing should be attributed to an increase in solubility. The permeability characteristics of the films are interpreted in terms of the free volume theory. Copyright 0 1996, Elsevier Science Ltd. INTRODUCTION Extensive work carried out by Michaels and coworkers’>2 and others3 suggested that gas transport in polyethylene films occurs nearly exclusively through the noncrystalline phase, so that the crystalline regions only act as physical barriers to impede the flow. The effect of orientation on permeation was studied on ultrahigh modulus poly- ethylene films prepared by tensile drawing of poly- ethylene in the solid phase4. In this case the barriers impeding the flow and their effect on the packing density in the crystalline-amorphous interfaces depend on both the molecular orientation and the size of the permeant molecules. A thorough study dealing with the permeation and diffusive aspects of these films was carried out by Holden et aL5 showing that the solubility of the gas in the oriented film is proportional to the amorphous volume fraction as occurs in undrawn films. However, a significant decrease in the diffusion coefficient with increasing draw ratio was observed, which is particularly important in the case of larger size permeant molecules. Coextruded linear low density polyethylene (LLDPE) films, like highly oriented polyethylene, exhibit good ultimate properties which confers them important applications in the packaging of foodstuffs. The com- bination of a relatively low crystallinity and a moderate orientation, which are mainly responsible for the good ultimate mechanical properties of these film8, may affect the permeation of gases through them. The physical barriers to gas permeation provided by the crystalline entities and their influence on the packing density in the * To whom correspondence should be addressed relatively ordered crystalline-amorphous interfaces will depend on the molecular orientation of the films. The microstructure of the films is governed by the chemical composition of the linear low density raw materials and the processing conditions, mainly speed extruders, distance between die and chill roll, vacuum knife depression, distance between die and frost line and line speed. Earlier studies carried out on the permeation of coextruded LLDPE films did not show a noticeable dependence on the processing conditions. Thus, the analysis of the temperature dependence of the perme- ability coefficient, P, of carbon dioxide through these films showed in all cases a significant increase in the values of P between 27 and 52”C, followed by a small plateau in which the permeability coefficient remains nearly constant. The permeability coefficient of oxygen followed similar trends, although the changes of this parameter with temperature were less ostensible. The differential scanning calorimetry (d.s.c.) thermo- grams of the coextruded LLDPE films present a single peak associated with the melting of the crystalline phase whose departure from the baseline and maximum are located, respectively, at 60 and 120”C7. The peak’s breadth suggests a wide distribu- tion of crystalline size entities and for this reason it was felt interesting to investigate how annealing of the films would affect their permeation characteristics. Consequently, this work describes a comparative study of the temperature dependence of both the perme- ability and the diffusion coefficients of carbon dioxide, oxygen and nitrogen through two annealed films obtained using different processing conditions and the results are interpreted by means of the free volume theory. POLYMER Volume 37 Number 26 1996 5831