Synthesis and solid-state NMR characterization of long-chain aliphatic polyesters with regularly spaced “defects”
In order to gain a better understanding of structure-property relationships, particularly the influence of regular versus random branching on the crystallization and polymer morphology of polyethylene, and to obtain chemical control over chain folding and lamellar thickness, model polyesters were synthesized and thoroughly characterized by solid-state NMR. Polyesters with perfectly regular placement of defects along the backbone were obtained by melt polycondensation of long-chain aliphatic α,ω-diols and 13C-labeled short-chain branched and non-branched diacids, diluting the amount of ester functionalities along the backbone by steadily increasing the length of the employed diols from 22 to 32 and then 46 methylene units.
Wide-angle X-ray diffraction established that the minimum diol length necessary to produce polyethylene-like orthorhombic crystal structures is 32 methylene units. A non-branched system used as a benchmark for comparison of long-chain polyesters to polyethylene showed polyesters to possess a polyethylene-like crystal structure with diester layers. This shows that these long-chain polyesters are valid model systems for polyethylene. However, chain dynamics in the polyester crystallites were found to greatly differ from those in polyethylene. This must be attributed to the diester layering prohibiting chain diffusion.
Small-angle x-ray diffraction of regularly branched polyesters showed the desired control over lamellar thickness, which is invariable under varying crystallization conditions and thermal treatment. The regular placement of the non-crystallizable defects enforces quantization of the length of non-crystalline chain folds. The resulting marked length difference between tight and loose loops affects the crystallinity significantly. Crystallinities determined by solid-state direct polarization NMR were found to be well above 50%, proving tight chain folds. The location of ester moieties and their mobility was probed by 1H spin diffusion and wideline separation NMR, showing ester groups to be located either on the crystalline/amorphous interface or in the amorphous regions, depending on the length of the diacid segment. Finally, diacid segment conformations were probed by DOQSY NMR, which showed specific conformations of ester moieties compatible with tight chain folds. Future research on this topic should focus on the variation of side groups, incorporating functional groups or reactive side groups that allow for post-polymerization modification of the crystalline/amorphous interface.