The Application Of Fluorescence Resonance Energy Transfer To The Conformation Transformation Of Polymer Chains In Shear Field

Polymer materials play an important role in the field of modern industry because of their excellent properties and wide application.In the process of processing,forming and use of polymer materials,phenomena such as shearing thinning,rod climbing effect and expansion from the mode are often generated due to the action of shear force.Therefore,the study of the conformation transformation of polymer chains in the shear field is of great basic research value and can also guide the practical application of polymer materials.Since the 1940s,researchers have been using simplified polymer chain model,combined with mathematical and physical theories,to construct a constitutive equation to describe the microstructure and motion of polymer,and to study the conformational transformation of polymer chain in the shear field.At the same time,many researchers have also studied it through experimental means.Common experimental methods,such as rheometer,can characterize macroscopic properties such as viscosity and elasticity of polymer fluids,but cannot deeply explore the conformational transformation of polymer chains.Light scattering method and fluorescence microscope method can obtain the conformational information of single polymer chains,but the research objects are usually high molecular weight polymers.Due to the limitation of characterization methods,most of the studies on the conformational transformation of low molecular weight polymer chains in shear field are theoretical derivation and computational simulation.Fluorescent resonance energy transfer（FRET）effects,due to their sensitivity to distance at the nanoscale,can be used to study conformational transformations of low molecular weight polymer chains in shear fields.Previous studies have shown that the distance between the fluorophore is far beyond the FRET sensitive range,and that FRET has not fully taken advantage of its advantages.By using appropriate FRET fluorescence pairs and appropriate labeling strategies,we can detect abundant conformational changes of polymer chains in the shear field,which provides new experimental evidence for theoretical research and numerical simulation.To this end,we have done the following:1.In this paper,the effect of solution concentration on chain conformation is studied.For polymers with larger molecular weights(M_n=9,700 g·mol^-1,M_n=12,300g·mol^-1),the FRET efficiency remained largely unchanged in the sparse solution due to the virtually nonexistent interchain interaction,whereas in the semidilulte solution,the chain was compressed due to the interchain interaction,and the FRET efficiency increased with increasing concentrations.For low molecular weights(M_n=6500g·mol^-1,M_n=5500g·mol^-1),the FRET efficiency remained essentially unchanged in the dilute solution,but decreased rather than increased in the semidilute solution.This phenomenon is due,on the one hand,to the inhibition of FRET efficiency by the self-diffusive motion of the low molecular weight polymer in the semidilute solution.On the other hand,because the low molecular weight polymer chain is semi-rigid,semi-rigid chain is more difficult to be compressed.2.The change of conformation of polymer chains in dilute solution with shear rate has been studied.For polymers with large molecular weight(M_n=1.26×10⁵g·mol^-1),the conformational change of stretching due to the increase of shear rate was detected by FRET method.For low molecular weight polymers(M_n=7900g·mol^-1,M_n=5500g·mol^-1),the FRET method detected different conformational transformations of the polymer chain at different shear rates.When the shear rate is lower than 150s^-1,FRET efficiency decreases with the increase of the shear rate,which is mainly affected by the elastic deformation of the polymer chain in the shear field.When the shear rate is greater than150s^-1,the end of the polymer chain is oriented in the direction of the velocity gradient,and the polymer chain is compressed in the direction of the velocity gradient,resulting in an increase in FRET efficiency with the increase of the shear rate.When the shear rate continues to increase to more than 1000s^-1,the low molecular weight polymer chain will form a U-shaped or S-shaped conformation,which is consistent with the results of previous computational simulation.3.The change of conformation of polymer chains with shear rate in solution of different concentrations has been studied.The experimental results show that FRET efficiency varies with the shear rate in dilute polymer solutions of different concentrations,which is consistent with the scaling theory.For semidilute solutions,however,FRET efficiency barely changes at low shear rates due to inter-chain interactions.However,when the shear rate exceeds a certain critical rate,FRET efficiency increases rapidly.We propose that this is due to the strong interaction between the polymer chains in the semidilute solution,and the fact that the lower shear rate does not result in conformational transformation of the polymer chains,and therefore FRET efficiency remains almost unchanged.However,when the shear rate exceeds a certain critical rate,the polymer chain is freed from the interchain interaction and has a higher degree of freedom,thus rapidly increasing FRET efficiency.