Chain Conformation In Polymer Solution And Confined Interface

Polymer solution theories occupies a very important position in polymer theory.The research of polymer solutions is of great value in both the theoretical basis and applied production.At present,the main methods for studying polymer solution are scattering methods,including: light scattering,neutron scattering,and X-ray scattering.The research of single chain conformation in solution,especially the low-molecular-weight polymer chain conformational transition,is limited by this detection method,which cannot perfectly present its experimental results,even may mislead our understanding of polymer solution.In this work,we used the fluorescence resonance energy transfer(FRET)method,which is based on the strategies of labeling fluorescent groups at different sites,firstly studying the conformational transition of polymer chains in solution,and finally studying the interaction between polymer chain and AAO nanotube interface.Firstly,we used fluorescent resonance energy transfer method to study the chain conformation transition of PS with molecular weight 200,000g/mol and PS with molecular weight 13,000g/mol.For PS with high molecular weight,randomly labeling strategy is utilized.For PS with low molecular weight,the strategy of double ends labelled is used by ATRP and click chemistry.According to the ratio I_A/I_C of the characteristic peak intensity(I_A)of the acceptor to the characteristic peak intensity(I_C)of the donor in the fluorescence spectrum,it is possible to determine the conformational transition of the polymer chain from the dilute to semi-dilute solution.The experimental results prove that both the PS with high molecular weight and PS with low molecular weight exhibit obvious conformational transitions.Interestingly,for high molecular weight PS,the critical overlap concentration obtained by the fluorescence method is consistent with the critical overlap concentration obtained from the viscosity experiment.And we think,when the critical overlap concentration is lower than the critical overlap concentration,polymer chain collapses slightly.However,for low molecular weight PS,the critical overlap concentration obtained by the fluorescence method is not consistent with the critical overlap concentration obtained from the viscosity experiment.And below the critical overlap concentration,the conformation of the polymer chain is basically unchanged.Secondly,most research on polymer solution have focused on high molecular weight or ultra-high molecular weight polymer,and there are few studies on the chain conformation of low molecular weight polymers,especially below the entanglement molecular weight.Therefore,we studied the conformational transition of a series of low molecular weight polymers at different concentrations in solution based on the method of fluorescence non-radiative energy transfer.Based on this special labeling fluorophores method,the efficiency of fluorescent energy transfer directly reflects the size of the polymer chains in solution.Experiment results have shown that for longer polymer chains,as the solution concentration gradually increases,I_A/I_C gradually increases,which means the polymer chains gradually collapse.But for shorter polymer chains,the trend of I_A/I_C changes with concentration is completely opposite.This similar phenomenon appears in PMMA.As a result of this,we believe that the energy transfer efficiency in solution is determined by its own structure and diffusion.In high molecular weight polymer solutions,the diffusion effect is negligible,but for low molecular weight,the effect of diffusion cannot be ignored.Finally,we applied the fluorescence resonance energy transfer method to study the interaction between the polymer chains and interface in AAO nanopores.In this system,we used the method of interchain labeling of fluorescent groups to graft the fluorescent donor onto the polymer chain and to graft the fluorescent acceptor anthracene on the AAO surface.The interaction between the polymer chain and interface is reflected by changes in the energy transfer efficiency.By changing the cooling rate,we found that under the rapid cooling rate,the energy transfer efficiency between the fluorescent donor and the acceptor is low,which proves that the polymer chain is completely detached from the AAO nanotube wall.But under the condition of slow cooling rate,the polymer chain is not completely detached,and there is a certain interaction between the wall and the surface.At the same time,we used DSC to test the glass transition temperatures of bulk polymer and the polymer filled into the AAO nanopores under the conditions of rapid cooling rate and slow cooling rate.We found that there is only one glass transition temperature under rapid cooling rate,but two glass transition temperatures under the condition of slow cooling rate.We believe that higher glass transition temperature that occurs under slow cooling rate is due to the presence of the nearby interface layer near the tube wall,which is consistent with our conclusions from fluorescence experiments.