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 VCH2 in alkyl nanodomains for the overall packing state. We demonstrate that there is a large (≈30%) variation in the VCH2 values for different polymer series and packing states but no significant change in VCH2 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 VCH2 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.

Interrelations Between Side Chain and Main Chain Packing in Different Crystal Modifications of Alkoxylated Polyesters.

Interrelations between main chain and side chain packing are studied in a series of comb-like poly(1,4-phenylene-2,5-n-dialkyloxy terephthalate)s (PPAOTs) with C = 6-12 alkyl carbons per side chain by X-ray diffractometry. Two different polymorphic states, called modification A and modification B, are observed depending on thermal treatment and side chain length. For PPAOTs with short side chains (C ≤ 8), modification A is commonly observed. "As synthesized" (solution crystallized) PPAOTs with longer side chains (C ≥ 10) contain mostly modification B while modification A is growing during melt cooling. A solid-solid transition from modification B to modification A is observed above ≈70 °C for the decyl member (C = 10, PPDOT) while modification A converts under ambient conditions slowly to modification B. This indicates that modification B is thermodynamically stable at low temperature while modification A is stable at higher temperature. Crystallographic analysis shows that both modifications are characterized by an orthorhombic unit cell and a long-range ordered layered structure with alternating main chain and alkyl nanodomains. This is confirmed by 2D diffration data for shear oriented PPDOT fibers. The main difference between both modifications is the packing of the side chains which are in a crystalline state for modification B but disordered for modification A. This is concluded from values for the volume per CH2 unit VCH2 in alkyl nanodomains calculated without further assumptions from the obtained lattice parameters. Interestingly, the crystalline packing of the side chains in modification B leads to a significant increase (≈ 20%) in the application relevant π-π spacings within the main chain domains as compared to those for modification A. It is argued that the structure formation process and thermodynamic equilibrium in comb-like polymers might be strongly influenced by a competition of the individual packing tendencies of main and side chains.