Solid State Phase Transitions In Poly(Ethylene Oxide) Crystals Induced By Designed Chain Defects

We have used Cu(I)-catalyzed azide-alkyne cycloaddition to synthesize a new series of poly(ethylene oxide)s having in the center of their chains two 1,2,3-triazole (TR) rings separated by (CH2)(n) spacers with 2 <= n <= 4 (PEO11-TR-(CH2)(n)-TR-PEO11). The degree of crystallinity obtained by temperature-dependent WAXS measurements indicates that only one out of the two PEO11 chains of the three polymers forms a 7(2) helix upon cooling to -10 degrees C and crystallizes into a monoclinic unit cell known from PEO homopolymer. A solid-state phase transition occurs for all samples during heating below their melting temperature. Solid-state "C MAS cross-polarization and single-pulse NMR spectroscopy indicate the complete incorporation of the chain defects into the PEO crystals (PEO-TR phase) during this transition. The 2D WAXS pattern of an oriented PEO11-TR-(CH2)(2) -TR-PEO11 sample generates a structural model where the crystal lattice of the initial PEO phase becomes highly distorted during the solidstate phase transition due to C-H center dot center dot center dot pi interactions of the aromatic TR rings. Furthermore, an additional phase transition occurs for PEO11-TR-(CH2)(4)-TR-PEO11 after melting of the PEO-TR phase. This phase has complex characteristics; i.e., the typical 7(2) helix of PEO forms, but the two TR rings and the methylene groups of the alkyl spacer are in different chemical environments.