About different packing states of alkyl groups in comb-like polymers with rigid backbones

We report a comparative X-ray diffraction study on three series of comb-like polymers with rigid backbones and layered morphologies [regio-regular poly(3-alkyl thiophenes), alkoxylated polyesters, alkoxylated polyphenylenevinylenes] highlighting the importance of the volume per methylene unit V_CH in alkyl nanodomains for the overall packing state. We demonstrate that there is a large (≈30%) variation in the V_CH values for different polymer series and packing states but no significant change in V_CH depending on the length of the alkyl side groups. This calls into question commonly used structural models which are based only on tilting and interdigitation of ideally stretched alkyl side groups. We argue that a linear dependence of the layer spacings with side chain length can also be explained by a constant V_CH value and unchanged main chain packing. The potential importance of side chain packing for the occurrence of different (liquid-) crystalline modifications in various polymer series and possible interrelations between main and side chain packings are discussed.