Thin crystal melting produces the low-temperature endotherm in ethylene/methacrylic acid ionomers

• Published in: Polymer (Guilford) Vol. 46; no. 14; pp. 5118 - 5124
• Main Authors: Loo, Yueh-Lin, Wakabayashi, Katsuyuki, Huang, Y. Evelyn, Register, Richard A., Hsiao, Benjamin S.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 27.06.2005; Elsevier
• Subjects: Applied sciences; Exact sciences and technology; Ionomer; Organic polymers; Physicochemistry of polymers; Properties and characterization; Secondary crystals; Small-angle X-ray scattering; Structure, morphology and analysis; Ionomer; Secondary crystals; Small-angle X-ray scattering; Methacrylic acid copolymer; Methacrylate salt copolymer; Melting; Crystal orientation; Supramolecular structure; Ageing; Temperature effect; Ethylene copolymer; Experimental study
• ISSN: 0032-3861; 1873-2291
• DOI: 10.1016/j.polymer.2005.04.043

Upon extended aging at room temperature, ethylene/methacrylic acid (E/MAA) ionomers develop an endotherm near 50 °C in their differential scanning calorimetry traces. The origin of this low-temperature endotherm has been the subject of considerable debate, in part because direct structural probes have been little applied to its study. Here, we use in situ small-angle X-ray scattering and wide-angle X-ray diffraction to show that this endotherm reflects the melting of thin polyethylene crystals, which gradually develop following primary crystallization. These secondary crystals form within the amorphous layers separating primary crystals. When the framework of primary crystals is oriented by crystallization during flow, the secondary crystals follow this orientation, indicating that the secondary crystals are not isometric fringed micelles but instead have a substantial lateral extent.