Preparation And Damping Properties Of Self-healable Fluorinated Ionic Elastomers

For self-protection,the skins of human infant and many animals can not only ionically sense various environmental stimuli and autonomously repair wound,but also elastically absorb vibrational energy with excellent damping properties.However,it remains a great challenge to artificially synthesize skin-like stretchable ionic conductors that combine the self-healing and damping properties.This is because,damping always relies on strong polymer frictions,which generally contradicts the weak and dynamic crosslinking required for room-temperature self-healing.To reconcile these two properties,this thesis put forward a new solution based on the dynamic liquid crystalline self-assembly of fluorinated ionic elastomers.The ionic elastomers were prepared by one-step photo-induced copolymerization of both short and long fluorinated acrylates in the presence of lithium salt and ionic liquid,which were shown to be transparent,highly stretchable,damping,and self-healable.In a wide frequency range at room temperature,the ionic elastomer demonstrates very high values of loss factor and damping energy.By monitoring the changes of side-chain lateral distance and interlayer distance of smectic phase as well as morphology evolution,the elastomer is shown to contain a large number of dynamic liquid crystalline assemblies of sticky fluorinated long-chain units.As stretched,these assemblies are first strengthened and then fragmented.As revealed by IR and low-field NMR spectroscopy,the lithium ions form physical crosslinks with the carbonyl groups of fluorinated acrylates,which improved the elastomer’s mechanical strength and thermal stability.The high damping capacity and self-healing property of the present ionic elastomer are ascribed to the synergistic contribution from both dynamic liquid crystalline assemblies and lithium bond-induced physical crosslinks.The specific research content is as follows:（1）Optimized preparation:A series of ionic elastomers were prepared by copolymerizing short-chain 2,2,2-Trifluoroethyl acrylate（TFEA）and long-chain1H,1H,2H,2H-heptadecafluorodecyl acrylate（PFOEA）fluorinated monomers in the presence of lithium salt and ionic liquid.In consideration of the stretchability and elasticity,the 1:1 molar ratio of the two monomers was finally selected.（2）Structural and mechanical characterizations:In the present ionic elastomer,TFEA short side chains provide elasticity,PFOEA long side chains are able to self-assemble into the liquid crystalline form,while lithium salts and ionic liquids are the ionic conducting media.Cyclic tensile tests showed that the material is highly elastic with large dissipating energy.As elucidated by small-angle X-ray scattering（SAXS）and powder X-ray diffraction,the elastomer contains massive smectic assemblies from PFOEA segments with an interlayer distance of 2 nm,and side-chain lateral distance of 0.5 nm.As stretched,the liquid crystalline assemblies are reversibly strengthened and fragmented.Such a sticky assembly behavior leads to very high damping ability at room temperature.The loss factor（tanδ）of the ionic elastomer is larger than 0.2 in a very wide frequency range of 10^-9-10³ Hz,and the tanδpeak can be as large as 1.4,which would effectively shield traditional vibrations and noise.（3）Internal interactions:IR spectral analysis confirmed the formation of lithium bonds between lithium ions and carbonyl groups（i.e.ion-dipole interactions）,and the lithium ions preferentially adsorb on the PFOEA segments.With increasing PFOEA contents,the dipole-dipole interactions between fluorinated mesogens are gradually enhanced.Low-field NMR calculated the relaxation times of ionic elastomers with different copolymerization ratios.It is found that,the added lithium salts indeed act as physical crosslinkers which lead to the lower mobility of F atoms and lower relaxation times.Overall,this thesis proposed a new strategy for self-healable and highly damping ionic elastomers via the dynamic assembly of sticky fluorinated mesogens.The resulting ionic elastomer is easily prepared,highly stretchable,and optically transparent,and may serve as the optimal candidate for artificial ionic skins with energy-absorbing and vibration-damping features.On the other hand,in the last section,we also performed the analyses of two-dimensional correlation spectroscopy and low-field NMR spectroscopy to clarify the evolving mechanism of a heat-resistant perfluoroether elastomer.The sequential motions of different groups and polymer chains during heat were finally elucidated.