Crystallization behavior of precision polymers containing azobenzene defects

Abstract

The crystallization of precision polymers containing (cis/trans)-azo-benzenes as defects within a polyalkylene-chain, prepared via ADMET and subsequent hydrogenation, is reported. The polymers are designed such that after exactly 18 –CH2− units an azobenzene-moiety (bearing either an o,o-dihydrogen (H-azo) or an o,o-difluorinated azo-moiety (F-azo) is placed, able to photochemically switch cis/trans configuration, thus changing the geometrical constraint exerted on the alkyl-moiety during crystallization and its crystal packing. The morphology of these polymers (Mn=3100 to 17,000g/mol) is investigated using differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS) and successive self-nucleation and annealing (SSA) experiments, putting special emphasis on the crystallization of the alkyl moieties. H/F-azo defects within the poly(alkyl)-chains significantly increase the melting point of the polymers compared to ADMET polymers without defect. Successive self-nucleation and annealing (SSA) and WAXS analysis show inclusion of the azo-moieties into the PE crystal. Changing the aromatic azo units from hydrogen (H-azo) to fluorine (F-azo) atoms also increases the enthalpy of melting and the melting point of the azo-polymers. Photochemical trans-to-cis isomerization of the (F-azo)-polymers increases lattice disorder throughout the entire sample to an extent sufficient to disrupt the long-range structuring of the sample, thus causing the reduction in their crystallinities.