Surlyn®

Some of the properties that make Surlyn® excellent for packaging applications are its sealing performance, formability, clarity, oil/grease resistance, and high hot draw strength. Good hot draw strength allows faster packaging line speeds and reduces packaging failures. Another well known application of Surlyn® includes it use in the outer covering of golf balls.

Surlyn® is the random copolymer poly(ethylene-co-methacrylic acid) (EMAA). The incorporation of methacrylic acid is typically low (< 15mol. %). Some or all of the methacrylic acid units can be neutralized with a suitable cation, commonly Na+ or Zn+2. Surlyn® is produced through the copolymerization of ethylene and methacrylic acid via a high pressure free radical reaction, similar to that for the production of low density polyethylene. 3. The methacrylic acid monomer is more reactive with itself than with ethylene. This leads to a higher reactivity ratio, around four, for methacrylic acid, and could give a blocky incorporation of methacrylic acid along the polymer chain. However, by polymerizing under elevated heat and pressure the reactivity ratios are driven toward one, thus promoting a random incorporation of the co-monomers. The neutralization of the methacrylic acid units can be done through the addition an appropriate base in solution, or in the melt mixing of base and copolymer.

The inclusion of a few mole % ionic groups along the backbone has a tremendous effect upon the morphology and properties of the polymer. Poly(ethylene-co-methacrylic acid) ionomers typically show an increased melt viscosity, toughness, clarity, and adhesion. The increased clarity, a desirable property in packaging applications, is due to the reduction of crystallinty in the copolymer. The presence of the methacrylic acid units and the neutralized carboxylate anion/cation pairs provides sites for ionic interaction. Hydrogen bonding between carboxylic acid moieties will form dimers. Interactions between ion pairs, and the non-polar nature of the backbone, will cause the ions to aggregate together. At low mole % incorporation of methacrylic acid, or low % neutralization, the ion pairs will exist as isolated polar groups in the bulk polymer. However, above a certain critical ionic concentration, the ion pairs will assemble into larger groups. The end result is the formation of an ion rich phase within the bulk of the polymer. The assembled ionic aggregates act as thermally reversible cross-links, greatly modifying the viscoelastic properties of the resulting polymer. These aggregations of ions have been termed "multiplets". 4. These multiplets, typically on the order of a few nanometers in size, serve as scattering centers for small angle x-ray scattering (SAXS). Figure 3 shows typical SAXS profiles for low density polyethylene, poly(ethylene-co-methacrylic acid), and for 90% Na+ neutralized poly(ethylene-co-methacrylic acid). In the Na+ neutralized ionomer profile we see a small angle peak, due to the presence of the ionic aggergates. In general, the size of a given multiplet is reasoned to be determined by a variety of factors, such as strength of ionic interaction, steric considerations of packing the ionic pairs, and conformational mobility of the polymer backbone. Since the mid 1960’s there have been several models of ionomer morphology. The key differences in the proposed models are generally in the modeling of polymer backbone mobility and of the packing/ordering of the ion pairs in the multiplet. A close examination of the details of these models is not practical in this paper. However, a few relative points will be discussed.

Before examining specific models, it is important to note that Surlyn® is a semicrystalline polymer in both the acid and neutralized forms. The presence of the methacrylic acid units limits the ability of the copolymer to pack into a crystalline lattice, and the ionic interactions slow the formation of spherulites. 5. It is for this reason that Surlyn has enhanced clarity over polyethylene. The existing crystalline domains in Surlyn are too small to effectively scatter light. As stated above, this is desirable for packaging materials, and is a property we will wish to retain in our composite materials. The semicrystalline nature of Surlyn® also serves to complicate an understanding of the morphology. The exact nature of interactions between the ionic domains, the amorphous component, and the crystalline regions is not clear. Their interactions will play an important role in determining Surlyn’s® bulk properties.

One of the first models for ionic polymers was proposed by Eisenberg in 1970. This model examined the driving force for ionic aggregation, the work required in the deformation of the polymer chains to facilitate ionic aggregation, and the destabilization of the cluster at some temperature, Tc. 7. The basics of this model set the foundation for many following models.

Another interesting, and elaborate, model was proposed by Mauritz and Hopfinger in 1980. Beginning with the concepts of Eisenberg’s model, this approach was concerned with the hydration of the ionic dipoles and incorporation of mobile ions. These concepts play an important role in certain applications, such as permselective membranes, and fuel cells.

The model of Tadano et al. (1989), introduced the concept of an order-disorder transition in the ionic multiplet. Dynamic scanning calorimetry of poly(ethylene-co-methacrylic acid) ionomers, shows a transition below the melting point of the polyethylene crystals. The authors of this model propose that below this transition the ionic pairs exist in an ordered state within the multiplets. At a temperature above this transition the ionic pairs lose their well ordered structure, yet remain in multiplets.

A relatively recent model has been proposed by Eisenberg, Hird, and Moore (1990). This model proposes the existence of a region of restricted chain mobility about the multiplets. The motions of the polymer chains within this region are sterically hindered due to their close packing. As the incorporation of ionic groups increases these regions can overlap one another forming continuous regions of restricted mobility, termed clusters. This large volume, characterized by restricted polymer motion, is proposed to be the origin of two phase behavior, i.e. a second, higher Tg.

Many of the models discussed above draw upon the large volume of experimental data available for ionomers. Of interest in these experimental studies are the effects on the ionomer properties of variation of the ionic content, degree of neutralization, type of counter-ion present, type of anion, nature of the polymer backbone (elastomeric vs. plastic), and the presence of small molecules, i.e. plasticizers.

In summary, Surlyn® is a commercially important polymer whose properties are dictated by its ionomeric nature and the interaction of the various regions with one another.

REFERENCES:

1. http://www.dupont.com/packaging/products/resins/E-48623-2/E48623-2.html
2.http://www.dupont.com/packaging/desingia/stuctures/granola.html
3.Jerome,R. and M. Mazurek in Ionomers: Synthesis, Structure, Properties and Applications, M. R. Tant, K. A. Mauritz and G. L. Wilkes, Eds., Blackie Academic and Professional: London, 1997, Ch. 1.
4.Eisenberg, A. Macromolecules. 1970, 3, 147.
5.Vol 8 Encyclopedia of materials…Ionic polymers
6.Wilson, F.C.;Longworth, R.; Vaughan, D.J. Polymer Preprints, American Chemical Society, Division of Polymer Chemistry. 1968, 9, 505.
7.Mauritz K.A in Ionomers: Synthesis, Structure, Properties and Applications, M. R. Tant, K. A. Mauritz and G. L. Wilkes, Eds., Blackie Academic and Professional: London, 1997, Ch. 3.
8.Mauritz K.A in Ionomers: Synthesis, Structure, Properties and Applications, M. R. Tant, K. A. Mauritz and G. L. Wilkes, Eds., Blackie Academic and Professional: London, 1997, Ch. 3.

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