Tailored Melting Temperatures and Crystallinity of Poly(ethylene oxide) Induced by Designed Chain Defects

The odd-even effect of a homologous series of two poly(ethylene oxide) chains separated by (CH)(n) spacers and joined by two 1,2,3-triazole rings with 2 <= n <= 8 (PEO11-TR-(CH2)(n)-TR-PEO11) is reported. Upon cooling to -10 degrees C, one PEO11 chain can crystallize and the other remains amorphous. However, temperature dependent WAXS measurements revealed for all samples solid-state phase transitions assigned to the complete or partial addition to the PEO crystal structure. This is based on small-angle X-ray data and C-13 NMR spectroscopic techniques. The odd-even effect in the melting temperatures of the model compounds TR-(CH2)(n)-TR is based on the match and mismatch of the pi-pi stacking of the aromatic TR rings. In contrast, the same effect in the polymers results from different chain tilt of PEO in crystalline layers necessary to compensate for the density mismatch between the crystalline and amorphous phase. An odd-numbered spacer length reduces the melting temperature more than an even-numbered one.