Viscoelastic processes in vinyl alcohol–ethylene copolymers. Influence of composition and thermal treatment
Introduction
The relationship between composition and properties of vinyl alcohol–ethylene (VAE) copolymers is a topic of increasing interest, owing to the great potential of those copolymers as barrier materials used in foods and pharmaceuticals packaging. The gas barrier properties of VAE copolymers are enhanced in samples with composition rich in vinyl alcohol, around 60–80 molar fraction, as occurred in the samples studied by Séguelá et al. [1] and in the present work.
The composition peculiarities of VAE copolymers can be adequately studied by dynamic mechanical analysis. This technique allows to elucidate the different motions taking place in the macromolecular chain, which are affected by the possible presence of sorbed water tied by hydrogen bonds to the vinyl alcohol moieties. Taking into account the three mechanical relaxations exhibited in both “parent” homopolymers—poly(vinyl alcohol) and low-density polyethylene—the study of viscoelastic relaxations of VAE copolymers in comparison to those of homopolymers seems to be an adequate route to assess the influence of composition in the mechanical properties of VAE copolymers. This type of study has been only partially undertaken in a recent paper [1].
The present paper scans the dynamic mechanical behavior of three samples of VAE copolymers and two of the “parent” homopolymers, relating the results to those obtained either by differential scanning calorimetry (DSC) or X-ray diffraction (XRD). The latter technique is very useful to detect the polymorphism shown by these copolymers, reported extensively by us elsewhere [2].
Section snippets
Experimental
Three commercially available VAE copolymers, VAE1, VAE2 and VAE3 (from Solvay, Kuraray and Du Pont, respectively), were used. Table 1 shows the composition in vinyl alcohol determined by means of 1H and 13C NMR spectroscopies as well as the other characteristics of the samples supplied by the manufacturers. Sheet specimens were obtained as films by compression molding in a Collin press between hot plates (210°C) at a pressure of 2.5 MPa for 15 min.
Each one of the VAE samples was crystallized
Results and discussion
The DSC melting curves of the different VAE copolymers and the two homopolymers are displayed in Fig. 1 for the quenching treatment. The glass transition temperature in PVAL is observed around 72°C while those of copolymers are ranging within 50–60°C, depending on composition (Table 2). The melting temperatures, Tm, of the crystalline phase are clearly affected by vinyl alcohol content increasing as such a comonomer in the copolymers does. However, Tm is not influenced by the thermal treatment
Conclusions
Two relaxation mechanisms have been found in the three VAE copolymers investigated: β and α in increasing order of temperatures. The former relaxation is attributed to motions in the interfacial phase while the latter is associated to the glass transition process of such copolymers. An additional relaxation at very low temperature (around −125°C) is observed in VAE3, the copolymer with the highest ethylene content. This process is only occurring in VAE3 because sequences of, at least, three
Acknowledgements
The financial support of the CAM and the CICYT (Projects 07N/0051/1998 and MAT98-0961-C02-01) is gratefully acknowledged.
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