Hydrophobically modified ethoxylated urethanes (HEURs) belong to an important class of telechelic associative polymers for improving solution rheological properties. We designed and prepared a novel azobenzene end-functionalized HEUR polymer (AzoHEUR), which was used to investigate the effects of hydrophobicity change of end hydrophobes induced by photoisomerization of azobenzene on the solution aggregation and rheological properties. The concentrated AzoHEUR solutions show a reversible rheological property change upon alternative exposure to UV and visible light. We have demonstrated that a reversible change in hydrophilic–lipophilic balance of polymer followed by photoisomerization of azobenzene induces a reversible rearrangement of micellar junctions through loop–bridge or bridge–loop transitions, which reversibly changes not only the network connectivity but also the solution relaxation behavior. Moreover, a structural model is proposed to describe the rearrangement of micellar junctions induced by photoisomerization of azobenzene. The work will not only provide new insights into the effect of hydrophobicity change of stimuli-responsive end groups on the aggregation and rheological behavior of HEUR aqueous solutions but also open a new perspective for development of some special applications of HEURs in fabrication and transmission of soft materials, medicines, cosmetics, inks for inkjet printers, and flow rate controlling systems.
Liu Ya-Di,Zhang Qiang,Yu Xin-Hong,Liu Jian-Gang,Han Yan-Chun
For poly(9,9-dioctylfuorene) (PFO), β phase (coplanar conformation with the intra-chain torsion angle of 165°) has a greater conjugation length and higher degree of order compared to those of α phase, which favors charge carrier transport. However, the highest content of β phase obtained so far is 45%. We propose to increase the content of β phase by promoting the solution aggregation of PFO molecules and extending film-forming time. For this purpose, 1,8-diiodooctane (DIO) is added to PFO o -xylene solution, which enhances the interaction of PFO chains and improves the planarity of PFO backbone, resulting in the formation of ordered aggregation. The aggregates act as nucleation centers to promote the formation of β phase. The content of β phase increases with increasing DIO concentration and reaches a platform of 39% as DIO is more than 4 vol%. Furthermore, the film is kept in a sealed environment with o -xylene atmosphere for 3 h, thus the PFO molecules have enough time to diffuse to the crystallization front and achieve disorder-order transition. As a result, the crystallinity of PFO is improved significantly and the content of β phase increases to 52%, reaching the highest value reported so far.
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