Synthesis and morphology of well-defined poly(ethylene-co-acrylic acid) copolymers

We report the detailed characterization of strictly linear ethylene-co-acrylic acid (EAA) copolymers synthesized via two modes of olefin metathesis polymerization, where control of the polymer microstructure leads to the generation of unique polymer morphologies based on distribution of carboxylic acid moieties along the polymer backbone. Acyclic diene metathesis (ADMET) was used to prepare three high molecular weight, high strength EAA materials bearing pendant carboxylic acid groups along the copolymer backbone precisely placed every 9th, 15th, and 21st carbon, and ring-opening metathesis polymerization (ROMP) was used to create EAA materials of equimolar acid concentrations with irregularly distributed pendant groups along the linear copolymer backbone. Primary structure characterization using FT-IR and NMR techniques are discussed as well as secondary structure analysis via DSC and X-ray scattering. Thermal analysis indicates significant effects of functional group placement on melting temperatures and enthalpies illustrating the importance of ethylene sequence lengths and distributions on polymer crystallization. X-ray scattering reveals broad scattering peaks with evidence of orthorhombic polyethylene-like crystallization in all irregular copolymers except the most highly functionalized, fully amorphous material. The precise carboxylic acid copolymers have X-ray scattering peaks indicative of acid-acid spacing along all-trans polyethylene segments of the polymer chain. Only the regularly functionalized EAA copolymer with the lowest acid content is semicrystalline with evidence of orthorhombic polyethylene-like crystallization, while higher concentrations of acids groups lead to amorphous materials. Drawing the semicrystalline ADMET acid copolymer produces an anisotropic morphology, indicating that the acid-acid spacing is parallel to the polyethylene chain axis.