Characterization of Long-Chain Aliphatic Polyesters: Crystalline and Supramolecular Structure of PE22,4 Elucidated by X-ray Scattering and Nuclear Magnetic Resonance M. G. Menges † and J. Penelle ‡ Department of Polymer Science and Engineering, UniVersity of Massachusetts, Amherst, Massachusetts 01003 C. Le Fevere de Ten Hove and A. M. Jonas UniVersite ´ de Physique et de Chimie des Hauts Polyme ` res, UniVersite ´ Catholique de LouVain, B-1348 LouVain-la-NeuVe, Belgium K. Schmidt-Rohr* Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011 ReceiVed July 17, 2007; ReVised Manuscript ReceiVed September 9, 2007 ABSTRACT: The crystalline packing and the phase structure of a long-chain aliphatic polyester, (O(CH 2 ) 22 - OOCCH 2 CH 2 CO) 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 13 C 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 13 COO-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 R 22 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-CdO 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 C 22 polymethylene section of one C 26 repeat unit. The C 22 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. Introduction Chain architecture of common commercial semicrystalline polymers such as polyethylene is of importance when consider- ing polymer properties such as crystallinity, strength, and viscoelasticity. For polyolefins, a family of materials with different properties can be tailored through appropriate choice of monomer, amount and nature of comonomer, and polymer- ization conditions. While high-density polyethylene (HDPE) consists predominantly of a polymethylene backbone with only a low branching content, linear low-density polyethylene (LLDPE) contains branches of a certain amount and given length dictated by comonomer content and nature that are randomly distributed throughout the backbone. These short chain branches act as “structural defects” and are usually rejected from the polyethylene crystal if longer than one carbon unit. Although in recent years more uniform incorporation of comonomer has been achieved through advances in metal catalysis, the distribu- tion of the distance between comonomer units is still statistical. This prevents those polyolefins from being useful as model polymers for studying relations between chemical architecture, crystalline microstructure, and chain dynamics in detail. While polyethylene copolymers allow only limited control over polymer crystallization and morphology through van der Waals and steric forces, polypeptides and polyamides have excellent control due to interchain hydrogen bonding. 1 Whether such a degree of control over polymer morphology can be attained through weaker van der Waals forces remains an open issue. To tackle this question, a valid model system will consist of an aliphatic backbone mimicking polyethylene and contain “structural defects” at controlled distances. The “structural defects” will then, like the short-chain branches in LLDPE, affect the lamellar thickness, crystallinity, chain dynamics, and ultimately strength. Out of the variety of defects imaginable, we have chosen ester groups. The PE-like polyesters can be synthesized through polycondensation of long-chain diols and * Corresponding author. E-mail: srohr@iastate.edu. † Present address: Institut fu ¨r Mikrotechnik Mainz GmbH, Carl-Zeiss- Strasse 18-20, 55129 Mainz. ‡ Present address: Institut de Chimie et des Mate ´riaux Paris-Est, 2-8, rue Henri Dunant, 94320 Thiais, France. 8714 Macromolecules 2007, 40, 8714-8725 10.1021/ma071590p CCC: $37.00 © 2007 American Chemical Society Published on Web 11/06/2007