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Structure and Properties of Crystallizable Ionomers

 We are currently engaged in a systematic investigation of the structure-property-processing relationships in ethylene-methacrylic acid copolymers and ionomers.  These materials’ properties are strongly influenced by their crystallinity; in such materials, relatively thick “primary” crystals, which form on initial cooling from the melt, coexist with thinner “secondary” crystals which form over extended storage (days to months) at room temperature.  This leads to a dual melting behavior, where the secondary crystals melt first:

Dual-melting behavior in ethylene-methacrylic acid ionomers. Red circles: ionic aggregates; thin blue line segments: polyethylene crystal stems; bold blue curves: amorphous polyethylene chain segments. Small interlamellar crystallites melt first, followed by melting of the thicker primary crystals.

Such secondary crystals are not by themselves unusual:  they are a common feature in statistical ethylene copolymers, though the relatively high segmental friction in ionomers leads to an unusually large fraction of such secondary crystals.  But in nonionic ethylene copolymers, the influence of these secondary crystals is the same as that of the primary crystals:  they act as “hard” reinforcing filler.  Thus, as long as the amorphous phase is relaxed—that is, as long as its glass transition temperature is below the testing temperature—then the modulus should increase progressively as the crystallinity increases.  This is indeed observed for ethylene-(meth)acrylic acid copolymers, as shown in the figure below.  Moreover, the modulus for such semicrystalline copolymer is quantitatively well-described by the simple Davies Model, even vs. the more complex Halpin-Tsai model commonly used to represent fiber- or flake-reinforced composites. 

Modulus of ethylene-(meth)acrylic acid copolymers at 50oC.  The elevated temperature is necessary to ensure that the amorphous phase is fully relaxed (above Tg), especially for materials of higher acid content.  The simple Davies model (equation in inset, where  φ  is the volume fraction of phase i:  crystal (i=1) or amorphous (i=2)) gives an excellent quantitative representation of the data, using the independently-determined material parameters listed at right (E = Young's Modulus,  ν  = Poisson's ratio).  Note that the modulus varies by an order of magnitude over the range of comonomer contents examined.

The morphology of semicrystalline ionomers is considerably more complex than that of the copolymers, and this is reflected in their thermomechanical behavior.  Such materials commonly show multistep declines in their modulus-temperature curves.  We have recently identified the morphological origin of each of these steps, as shown below:

Neutralization causes the formation of ionic aggregates, which sequester essentially all of the acid comonomer units, leaving pockets of material which are essentially pure amorphous polyethylene.  These pockets soften first as the material is heated, near -20 ° C (Tgm).  But softening at Tgm is limited, because the secondary crystals and ionic aggregates form a percolated hard phase which spans the amorphous layer, producing the unusually high room-temperature modulus of ionomers.  This percolated hard phase is broken when the secondary crystals melt; the modulus of the material typically drops by a factor of five over a small temperature range covering Tm2.  Further heating loosens the constraints on chain segments near the ionic aggregates (at Tgi), and finally the primary crystals melt (at Tm1).  All four of these transitions are visible by dynamic mechanical thermal analysis of specimens with suitably-chosen thermal histories. 

Supported by  DuPont Packaging and Industrial Polymers

Recent/Current People and Projects:

Bob Scogna GS - Yield Behavior in Semicrystalline Ionomers
Katsuyuki Wakabayashi *06 - Structure-Property Relationships in Semicrystalline Copolymers and Ionomers 
Evelyn Huang *01 - Recrystallization and Crystal Thickness Distributions in E/MAA Ionomers

Selected Recent Publications:

K. Wakabayashi and R.A. Register, “Morphological Origin of the Multistep Relaxation Behavior in Semicrystalline Ethylene/Methacrylic Acid Ionomers”, Macromolecules, 39, 1079-1086 (2006).

K. Wakabayashi and R.A. Register, “Micromechanical Interpretation of the Modulus of Ethylene-(Meth)Acrylic Acid Copolymers”, Polymer46, 8838-8845 (2005). [John D. Hoffman Memorial Issue]

Y.-L. Loo, K. Wakabayashi, Y.E. Huang, R.A. Register, and B.S. Hsiao, “Thin Crystal Melting Produces the Low-Temperature Endotherm in Ethylene/Methacrylic Acid Ionomers”, Polymer46, 5118-5124 (2005).