(361d) Crystallizable Ionic Polymer - Ionic Oligomer Blends: Coassembled Nanoscale Structure and Enhanced Properties

Ionomers are a type of ionic polymer in which a small amount (< 15 mol%) of ionic functional groups are covalently bonded to the backbone chains [1]. An important class of ionomers with considerable scientific and industrial interest are those prepared by neutralizing semicrystalline copolymers with a metal cation (e.g., Na or Zn), such as commercially available ethylene/(meth)acrylic acid (E/(M)AA) ionomers. These materials' nanostructural elements consist of stiff ethylene crystallites (5–10 nm thick), plus physical crosslinks formed by self-aggregated ionic groups (1–2 nm), coexisting in a soft amorphous matrix. This morphology in turn dictates many of the superior physical properties of such semicrystalline ionomers compared with either non-ionic or non-crystalline counterparts, including high cut resistance, adhesion and optical clarity [2]. One ideal way to control the material properties of ionomers is ion content variation, but there is a practical limit to the achievable properties in neat ionomers due to the high viscosity and consequent reduction in processability encountered at moderately high ion level [3]. We herein present novel ionic blends comprising semicrystalline ionomers and metal soaps [4-5]. Metal soaps, specifically the salts of C₁₆–C₂₂ saturated and unsaturated a-carboxylic acids (“fatty acids”), are the oligomer equivalent of E/(M)AA ionomers, with carboxylic ionic groups bound to long sequences of methylene units. Therefore, a compatible blend with intimate molecular interaction in each of ionic, amorphous organic and crystalline regions, as well as consequent synergistic material property enhancement, is expected. We investigated several different blends made from E/(M)AA copolymers of varying comonomer (and termonomer, when an acrylate termonomer is present) content and metal soaps with different chain lengths, saturation and counterions. The morphology was probed by small- and wide-angle X-ray scattering, while the thermomechanical properties were determined by uniaxial tensile testing, dynamic mechanical thermal analysis, and differential scanning calorimetry.

It was found that blending ionic oligomers into semicrystalline ionomers can indeed raise the total ion content without adversely affecting the processability; for example, in one 60-40 ionomer-metal soap blend, 100% neutralization was achieved with the resulting melt viscosity equivalent to that of ~30% neutralized neat ionomer. The structure and properties of ionomer-metal soap blends depend significantly on the characteristics of the metal soap employed. Saturated magnesium stearate (MgSt) and unsaturated magnesium oleate (MgOl) and erucate (MgEr) all subject the parent ionomers to potentially lower stiffness. They act as effective permanent plasticizers, lowering the ionomer T_g by 10–30^oC, while also inducing a strong suppression of primary crystallization upon cooling from the melt. However, only MgSt is capable of subsequently “co-crystallizing” into a rotator structure [6] with the ethylene sequence of the ionomer upon room-temperature aging. As a result, MgSt-modified ionomers are practical reinforced hybrids, with improved mechanical properties. The type of cation in the metal soap also has a substantial impact on the phase behavior of the blends. While MgSt remains intimately mixed (coassembled) with the ionomer, sodium stearate (NaSt) phase separates to form a composite consisting of domains of essentially pure ionomer and NaSt upon cooling. Lastly, the nature of the ionomer (acid type, acid content, and termonomer content) plays a role insofar as it affects the degree of crystallinity. In the case of MgSt-based blends, the crystallizability of the ionomer regulates the trade-off between the modulus reduction due to the suppression of primary crystallites and the modulus increase due to the formation of rotator-type cocrystals [7].

[1] Ionomers: Synthesis, Structure, Properties and Applications, ed. M.R. Tant, K.A. Mauritz, and G.l. Wilkes (New York: Chapman and Hall, 1997). [2] R. Longworth, in Ionic Polymers, ed. L. Holliday (New York: Wiley, 1975).

[3] R. A. Register and R. K. Prud'homme, in Ionomers: Synthesis, Structure, Properties and Applications, ed. M.R. Tant, K.A. Mauritz, and G.l. Wilkes (New York: Chapman and Hall, 1997).

[4] Metal Carboxylates, R. C. Mehrotra and R. Bohra (London: Academic Press, 1983).

[5] K. S. Markley, in Fatty Acids: Their Chemistry, Properties, Production, and Uses, vol 2, ed. K. S. Markley (New York: Interscience, 1960).

[6] E.B. Sirota, H.E. King, Jr., D.M. Singer, and H.H. Shao, J. Chem. Phys., 98, 5809 (1993).

[7] K. Wakabayashi and R.A. Register, Polymer, 47, 2874 (2006).