Preparation And Characterization Of Mechanochromic Hydrogels Based On Rhodamine

Stimuli-responsive hydrogels can change their physical or chemical properties,including shape or size,optical properties,electrical conductivity and mechanical properties,when they suffered from the stimuli of external environment.Therefore,they have versatile applications in many fields.Among them,mechanochromic hydrogels,which are novel stimuli-responsive soft materials,have attracted extensive attention.When they are subjected to an external force,their adsorption(chromogenic)or emission(fluorochromic)color will change;while the stress is removed or the gels are futher treated by other means,their optical properties can reverse back to the initial states.Based on this property,mechanochromic hydrogels have potential applications in many fields,such as stress/strain sensing,intelligent display and information storage,etc.Mechanophores are a class of force-sensitive small molecular groups,and they are often used to develop mechanoresponsive polymer materials.Among them,some exhibit mechanochromism.If these kinds of mechanophores were incorporated into hydrogels,the hydrogels might exhibit mechanochromic properties.Unfortunately,the mechanophores are often highly hydrophobic,and it is difficult to directly incorporate them into hydrogels.Therefore,designing and developing effective hydrogel preparation methods to covalently embed mechanochromic mechanophores into the hydrogel network,is of great significance for the development of novel mechanochromic hydrogels.In this thesis,utilizing the mechanochromic and mechanofluorescent rhodamine mechanophores as functional components,and in virtue of the development of new hydrogel preparation methods,we successfully fabricated mechanochromic micellar hydrogels and double-network hydrogels.In addition,a mechanochromic hydrogel that could respond to both force and temperature was also developed.The main contents of this thesis are as follows:(1)A mechanochromic and mechanofluorescent hydrogel based on a rhodamine mechanophore(Rh)was developed by micellar-copolymerization of Rh cross-linker in the presence of methyl acrylate and acrylamide within a uniform emulsion.As far as we kown,this is the first example that Rh are introduced into hydrogel system.The mechanical sensitivity of Rh in this hydrogel was significantly improved,and the enhancement mechanism of the mechanochromism of this hydrogel mainly includes three points.On the one hand,the micellar structure in the hydrogel leads to the stress concentration effect,facilitating the activation of Rh.On the other hand,the good segment mobility of the polymer chains in the hydrogel,is beneficial to the mechanical activation of Rh.In addition,water also can make a large contribution to the activation of Rh.The hydrogels could also respond to acid/alkali and exhibited outstanding properties of acidichromism and acidifluorochromism.This dual stimuli-responsive hydrogel system will help broaden the application fields of soft materials.(2)We prepared a novel mechanochromic hydrogel using a strategy that has been developed to prepare dual-network(DN)hydrogels.The mechanoactive rhodamine mechanophore(Rh)was covalently incorporated into a first network as a cross-linker.This first network embedded with Rh was highly pre-stretched.This guaranteed that the stress could be transferred extensively to the cross-linker Rh once the hydrogel was under an applied force.Thus the mechanochromic sensitivity of this hydrogel was high.Because of the improved sensitivity of the hydrogel to stress,the DN hydrogel exhibited reversible freezing-induced mechanochromism.Specifically,the DN hydrogel showed pH-regulated mechanochromic behavior that benefited from the sensitivity of Rh mechanophore to both pH and force.This hydrogel preparation strategy both introduces sensitive mechanochromism and preserves the advantageous mechanical properties of DN hydrogels.Our efforts will be beneficial to the design and preparation of mechanochromic hydrogels with high stress sensitivity.(3)We developed a mechanochromic dual-network hydrogel that could respond to both force and temperature.The first network of the hydrogel was prepared by the copolymerization of rhodamine mechanophore(Rh)cross-linker and two kinds of monomers:sodium 2-acrylamido-2-methylpropane sulfonate(NaAMPS)and Nisopropyl acrylamide(NIPAM).By adjusting the ratio of NaAMPS to NIPAM in the first network,a series of hydrogels with different structural morphology,mechanical strength,thermoresponsive behaviors and mechanoresponsive behaviors could be obtained.Hydrogels of different compositions all exhibited thermoresponsiveness.They also exhibited compression-induced mechanochromism/mechanofluorescence,in addition,upon cooling,they could also undergo freezing-induced mechanochromism.Since the hydrogels can respond to both force and temperature,they have potential applications in "smart windows" and rewritable materials.Our work has important guiding significance for the fabrication of multiple stimuli-responsive strong hydrogels.