Structure and Dynamics in a Polymorphic Nanophase-Separated Stiff Comblike Polymer

We have investigated the structure and molecular motions of the side and main chains in the comblike polymer poly(1,4-phenylene-2,5-n-didecyloxy terephthalate) with 10 carbons in the alkyl side chain (PPDOT) with respect to its two polymorphic states by means of 13C magic-angle spinning (MAS) NMR spectroscopy. Structure and polymorphism of such stiff-chain systems are relevant for many semiconducting polymers. Both modifications, called A and B, exhibit a layered morphology with well-ordered backbones forming p-p stacks separated by nanodomains of methylene sequences and are connected by a B-A solid-solid phase transition in the range 70-110 °C during heating. At ambient temperature, the polymorph A slowly converts back to the thermodynamically more stable modification B. We use 13C CP MAS spectra to observe the structural changes during the phase transition and the structural differences in both modifications. In 13C DIPSHIFT experiments, which inform about motionally averaged 1H-13C dipole-dipole couplings, we do not see any indication of a backbone-amorphous phase, and therefore we emphasize that the entire sample exhibits well-ordered p-p stacking for T < 180 °C. The polymorphs differ in the conformational statistics and dynamics of the side chains as well as in the structural conformation of the backbone. Above 180 °C we detect the formation of a liquid-crystalline phase, characterized by the loss of the p-p stacking and molecular motions of the side chains with correlation times tc < 0.01 ms.