| Exploring the relationship between polymer structure and deformation,fatigue,damage and performance at the molecular level is an important task to improve the properties,and monitor and extend the service life of polymer materials,which is also greatly beneficial to the needs of production and practical applications.Detecting micro-damage inside polymer materials before they fail can avoid economic losses and safety issues caused by material macro-damage.The currently reported non-destructive testing and evaluation methods require complex and expensive technology and equipment.Therefore,it is significant to develop a simple and easy detection method,and the covalent attachment of a few mechanophores into polymers to endow them with mechanochromic function provides great convenience for achieving this purpose.The easily detected optical signal generated by polymer mechanochromism can be used to realize direct visualization of damage and even dynamic stress in polymer material.The reported mechanophores have problems such as low mechano-sensitivity,complex synthesis,optical signal without reference,and single optical signal.Therefore,it is necessary to develop new mechanophores to overcome these drawbacks.This thesis mainly focuses on the exploration and development of novel mechanophores to obtain non-covalent mechanophores with simple synthesis,high mechano-sensitivity,and self-reference functions.Also,functional polymers with force-induced multistate or multicolor transition are prepared through rational molecular design and polymer structural optimization.From the perspective of basic research,the library of mechanophores has been expanded,and a new mechanism of non-covalent mechanophores has been explored.From the perspective of practical application,the quantitative relationships between optical signals and strain have been established,achieving the detection of multiple optical signals,and laying a good foundation for the visualization and failure analysis of polymer materials under load.The main contents of this paper are as follows:Firstly,the mechanophore bis-rhodamine RhRh was synthesized and covalently introduced to the center of liner polymethacrylates.The multistate ring-opening products of RhRh that cannot be obtained in photochemistry and piezochemistry were observed by monitoring the ultrasonic process of polymer solutions using absorption spectra,enabling mechanochromism based on polymer multistate mechanochemistry in solution.A mechanism of non-sequential ring-opening based on heat-force equilibrium for RhRh was proposed:RhRh directly generated a double ring-opening product RhRh OOunder force,which thermally cyclized to a single ring-opening product RhRh OC.On the one hand,RhRh OC further thermal cyclized to RhRh,and on the other hand,it underwent secondary ring-opening to generate RhRh OO.Then,a simple empirical model was established to describe the force-dependent relative distribution of the two ring-opening products.This polymer multistate mechanochemistry realized by the strategy of intramolecular coupling reveals a unique mechanism of mechanochemical reactions and is also helpful for the development of multifunctional mechanochromic polymers.Secondly,to enhance the mechano-sensitivity of the mechanophores in polymers,a non-covalent mechanophore ESIPT-1 based on the excited-state intramolecular proton transfer（ESIPT）process was designed and covalently incorporated into polyurethane for the first time.During the tensile process of polyurethane,the intensity ratio of enol emission and keto emission increases approximately linearly with strain,achieving real-time reversible polymer mechanochromism with a self-reference function.A new mechanism of mechanochromism was proposed:the force-induced torsion of the intramolecular dihedral angle disrupts the intramolecular hydrogen bond and ESIPT process,including the indirect change of dihedral angle by force-induced disaggregation of mechanophores and the direct action of force on the mechanophore to twist the dihedral angle.The development of ESIPT mechanophore enriches the non-covalent mechanophores,laying a foundation for real-time visualization of stress/strain distribution in polymers.Again,a new mechanophore Rh ESIPT-1 was obtained by intramolecular coupling of rhodamine structure and ESIPT structure through rational molecular design and covalently introduced into polyurethane.Under ultraviolet light,the enol emission and rhodol emission of polyurethane simultaneously increased with keto emission as a reference.The mechanism of cooperative photochromism was proposed:in the excited state,the proton transfer from the phenolic hydroxyl group on the xanthene to the nitrogen on the benzimidazole first occurred,followed by ring-opening of the spirolactam.The dual-mode force-induced multicolor transition of polyurethane under tensile and pressure was achieved using the difference in mechanochromic sensitivity of ESIPT structure and spirolactam structure:only the ESIPT structure twisted under tension,the enol emission rose and the fluorescence of polyurethane changed from light green to cyan;the torsion of ESIPT structure and the isomerization of spirolactam sequentially happened under pressure,and a color change of polyurethane from colorless to red was observed.This intramolecular coupling strategy not only can identify different modes of applied stress but also provide a reference for the design of novel multicolor mechanochromic mechanophores.Finally,to solve the problems of reduced fluorescence intensity of ESIPT and discontinuous activation process of spirolactam structure and ESIPT structure in the coupling strategy mentioned above,rhodamine mechanophore Rh-1 and ESIPT mechanophore ESIPT-1 were simultaneously covalently introduced into the first network of polyurethane-polymethacrylates double-network elastomer through polymer structural optimization.The pre-stretched characteristic of the first network significantly reduced the mechanochromic threshold of the rhodamine structure.The force-induced multicolor transition of fluorescence from cyan to blue to purple-red in the stretching process was achieved,and the dual-ratio strain sensing with the emission intensity ratios of enol/keto and rhodol/keto as indicators in low-and high-stress regions was realized with the mechano-activation threshold of rhodamine as the dividing point（275%/3.49 MPa）.This intermolecular cooperation strategy not only lays the foundation for gradient stress mapping of the polymer under load but also provides a multi-mode analysis of the whole loading experience. |
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