Characterization of Long-Chain Aliphatic Polyesters: Crystalline and Supramolecular Structure of PE22,4 Elucidated by X-ray Scattering and Nuclear Magnetic Resonance

Abstract

The crystalline packing and the phase structure of a long-chain aliphatic polyester, (O(CH2)22OOCCH2CH2CO)n, PE22,4, have been studied in molecular detail by small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) experiments as well as various solid-state nuclear magnetic resonance (NMR) methods, in particular two-dimensional double-quantum spectroscopy (DOQSY) NMR. The volume crystallinity is 73 ± 3% according to quantitative 13C NMR and SAXS analyses. The DOQSY NMR spectra show signal characteristic of trans ester groups incorporated into straight poly(methylene) chains, as expected on the basis of WAXD. DOQSY spectra of singly 13COO-labeled diesters prove close proximity of ester groups in neighboring chains, confirming the ester layering deduced from SAXS, with three diester layers per crystallite. SAXS shows a 37° chain tilt with respect to the diester layers and crystallite surface, and the DOQSY NMR spectra confirm the resulting significant displacement of ester groups along neighboring chains. The data suggest a αβ22 tilting of the chain axes. NMR detects no significant disorder along the chain axes; this suggests that the disappearance of (h, k, l ≠ 0) reflections in WAXD is due to the small crystallite thickness, which is 5.6 ± 0.5 nm according to SAXS. The DOQSY NMR patterns show that the planes of the chains are far from the perpendicular relative orientation found in orthorhombic polyethylene, constraining the angle between the (normals to the) O−CO ester planes to 55° ± 20°. DOQSY NMR also indicates that ∼1/3 of the COO groups directly at the crystalline−amorphous interface are disordered. The chain loops in the amorphous phase contain only 6% of the esters and thus mostly consist of the C22 polymethylene section of one C26 repeat unit. The C22 loops connect ∼71% of the ends of crystalline stems, while 9% are terminated by chain ends and 20% are connected to a loose loop or tie molecule. NMR relaxation measurements confirm that, in spite of the relatively small fraction of ester groups among the poly(methylene) chains, they strongly suppress the fast 180° chain flips observed in polyethylene crystallites.