Synthesis and properties of thermally cross-linkable main-chain azobenzene polymers containing diacetylene moieties

A series of main chain azobenzene polymers containing diacetylene moieties with different lengths of the spacer {-[CC-CH₂-O-C₆H₄-OCO-(CH₂)_m-O-C₆H₄-NN-C₆H₄-O-(CH₂)_m-OCO-C₆H₄-O-CH₂-CC]_n-, where m = 3, 6, 11} were synthesized by oxidative coupling polymerization. These polymers had molecular weights of 17,600-68,600 and polydispersity indices of 1.2-1.8 as determined by gel permeation chromatography using polystyrene as a standard. Their structures and properties were characterized and evaluated with NMR, FT-IR, X-ray diffraction (XRD), thermogravimetry (TG), differential scanning calorimetry (DSC) and nonlinear optical (NLO) analyses. All the polymers could be cross-linked at the elevated temperatures due to the polymerization reactions of the diacetylene groups in the polymer backbone, and the cross-linked polymers showed dramatically modified properties, such as thermal stability and solvent resistance. The third-order nonlinear susceptibilities of the cross-linked polymers were evaluated by means of the Z-scan technique and calculated to be 3.60 × 10⁻⁹, 2.73 × 10⁻⁹, 2.28 × 10⁻⁹ esu, respectively, whereas the un-cross-linked polymers showed no obvious NLO property.