AFM and X-ray Studies of Crystal and Ionic Domain Morphology in Poly(ethylene-co-methacrylic acid) Ionomers

• Published in: Macromolecules 2000-10, Vol.33 (21), p.7939-7949
• Main Authors: Sauer, Bryan B, McLean, R. Scott
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Structure, morphology and analysis
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma001110t

New AFM methods were applied to resolve the morphology of the ionic domains in poly(ethylene-co-methacrylic acid) copolymers. Using standard tapping AFM techniques with a precision in the lateral dimensions on the order of a nanometer, the crystalline lamellar structure and overall morphology consisting of stacks of ethylene-rich lamellae separated by sometimes broad noncrystalline regions were characterized. By operating the AFM under special low oscillation amplitude conditions where tip−ionic cluster interactions could be induced to dominate the phase signal, it was shown that this method could be used to uniquely resolve ionic-rich regions and individual ionic domains. The individual domains were found to be on the order of 2 nm in diameter. Small-angle X-ray scattering (SAXS) characterization was used to confirm some aspects of the morphology and to contrast the different levels of resolution of the two techniques for both lamellar crystals and ionic domains. By sequential images taken under different tapping AFM conditions, the “softer” amorphous regions were found to be the richest in ionic domains. Lamellar morphology and perfection were found to be controlled by mobility in the melt which depends on acid level, neutralization level, and counterion type in the different ionomers studied. The crystalline domains in the metal neutralized ionomers were compared to the unneutralized “acid” form of the ionomer. Data for the acid form verified that no ionic domains exist in this material.