Impact of water and thermal induced crystallizations in a PC/MXD6 multilayer film on barrier properties
Graphical abstract
Introduction
Polymer films or membranes are present in our daily environment and used in several industrial fields like energy, transport, aircraft, building, biomedical, protective coatings or packaging for example. For technological and economic reasons, both lightening polymeric materials and enhancing their thermo-mechanical and barrier properties, are increasingly desired industrial goals [1], [2], [3].
An innovative process, multilayer coextrusion, has been developed for producing thin multilayer films, which consist of hundreds to thousands alternating layers of two polymers. This technique is based on the use of layer multiplying elements (LME) placed at the output of a conventional coextrusion process. The layer multiplying elements vertically separate the flow of polymer melts in two parts and subsequently recombine them by stacking. While keeping constant the whole thickness, after each passage through layer multiplying elements the number of layers is multiplied by 2, hence their individual thickness is divided by 2. Thus, depending on the number of multiplying elements, the final structure can be composed of a very large number of alternating layers. With the multiplication of nanolayers, new polymer morphologies in the film can be induced by crystallization under confinement arising from the forced assembly phenomenon [4], [5]. Such morphologies led to much improved barrier properties for many polymer pairs [6], [7], [8], compared to properties resulting from conventional polymer blends or micronic multilayer structures. Indeed, it has been shown that the best barrier properties are obtained when the crystallization of semi-crystalline polymers in a confined space leads to the formation of crystalline lamellae oriented in the in-plane direction [9], [10], [11], [12], that is to say perpendicular to the diffusing molecule pathway, thus creating a very high barrier layer. The development of polymer films made of ultra-thin alternating layers is then a new challenge both from the technological point of view and from the implementation of characterization tools suited to these new materials.
In a previous paper on the elaboration of a polycarbonate (PC)/poly(m-xylene adipamide) (MXD6) multilayer film [13], an improvement of the MXD6 barrier performances was obtained due to a confinement effect induced by the PC under multilayer form. Even if the degree of crystallinity of MXD6 was close to only 8%, the barrier properties of the MXD6 were improved by a factor of 60% in the case of nitrogen and oxygen and 13% in the case of water as permeant molecules. Knowing that the MXD6 can crystallize until ∼30% [14], it became interesting to reach this maximum amount by increasing the crystalline phase fraction and to see at which point the barrier performances of the multilayer film can be enhanced by post-processing treatment. Therefore, two types of crystallization treatments have been applied on the PC/MXD6 multilayer film. The first crystallization treatment consists in heating the multilayer film at the crystallization temperature of the MXD6 and the second one in immersing the sample into liquid water as MXD6 is able to crystallize in contact with water [13]. It has already been demonstrated that the water sorption into MXD6 produced a glass-to-rubber transition, which facilitated the crystallization of the polyamide thanks to the rearrangement of the polymer chains [15]. Nowadays, few works about the crystallization of MXD6 films by water or thermal treatments have been published and they lead to an improvement of the water and oxygen permeability [14], [16], [17] Gas (N2, O2, CO2) and water behaviors of these multilayer PC/MXD6 films were analyzed from flux permeation kinetics and the resulting barrier properties were correlated to the morphological and structural changes induced by these crystallized multilayer films. Because both the confinement effect induced in the multilayer and the crystallization may have concomitant effects on the barrier properties, the structure and transport properties of MXD6 film were also investigated.
Section snippets
Materials
PC, referenced as LEXAN 121R (Mw = 33,050 g/mol, Tg = 145 °C), was obtained from SABIC and MXD6, under the grade 6007 (Mn = 25,900 g/mol, Tg = 85 °C, Tm = 237 °C) was supplied by Mitsubishi Gas Chemical. The PC and MXD6 pellets were dried at 120 °C overnight and the residual moisture before processing was found to be less than 0.02% for PC and 0.1% for MXD6. In the typical range of shear rates occurring in the extruders (1–100 s−1) at a processing temperature of 240 °C, the viscosity ratio (η
Morphology
SEM experiments were carried out on the thermally and the water crystallized MXD6 films in order to verify if the structure of the film has been changed during the crystallization. As observed in Fig. 2, the cross section of the MXD6 water crystallized (Fig. 2.a) is not regular compared to the MXD6 film thermally crystallized (Fig. 2.b) which presents a flat and smooth surface. SEM images reveal that the water crystallized film has some empty spaces, similar to pinholes, which are located in
Conclusion
In this work, we report the crystallization of a PC/MXD6 film composed of a thousand micrometric layers, in order to further improve its barrier performances. Two crystallization treatments have been applied on this multilayer system, the first one by heating and the second one by immersing the sample in water because of the capacity of MXD6 to crystallize in contact with it. If both treatments led to the same degree of crystallization around 30%, the improvement in the gas and water barrier
Supporting information
Comparison of the DSC curves for the initial MXD6 film and the MXD6 films thermally and water crystallized.
Acknowledgment
The authors thank the GRR Crunch (supported by Upper Normandy region, France) for the financial support of the PhD fellowship of T. Messin.
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