Study On The Forming Process And Machnism Of ? Phase Of Conjugated Polymer Polyfluorenes In Solution

As a kind of potential conjugated polymer material,Polyfluorenes(PFs)had attracted people's attention for the unique blue-light-emitting properties,high fluorescent efficiency and thermodynamic stability;Besides,Polyfluorenes had many complex phase structures in solid films,which making it different from that of the same kind of benzene polymer.One of the most special phases was the ? phase,which was mainly consisted of ? conformation.The ? conformation had good coplanar property and ordered structure,making it behaved low exited stated.There had been many researches proving that the introduction of ? phase was to the benefit of the ratio of singlet to triplet excisions,reducing light bleaching effect and improving the transmission of carrier in film devises.What's more,the ? phase had a good potential application in electrically pumped organic lasing.However,in most cases,thin film devises were spin coated from precursor solution.Once the film was formed,the molecular chains were piled up so tightly that it was difficult for them to be adjusted.A simple and feasible method is adjusting the content of ? phase in the precursor solution and the state of aggregation structure,so the study of the forming condition and mechanism of ? phase in solution is important.Based on those properties,in this work,we started our study from the precursor solution.By changing the solvent solubility,molecular weight and the alkyl side chain length we had studied two representative Polyfluorenes: poly(9,9-dioctylfluorene)PFO and Polydiarylfluerene(PnDPF).The changing rule of the ? phase with aggregation structures and the forming characteristics of ? phase of them were studied to clarify the forming mechanism of the ? phase.It would provide theoretical support to induce more ? phase in precursor solution and make devises with high content of ? phase,high carrier mobility and excellent photoelectric properties.The work was divided into the following three parts.In the first work,we studied the forming condition and mechanism of ? phase of PFO by changing solvent and PFO concentration.It was found that the formation of ? phase had direct relationship with the proportion of poor solvent ethanol in mixed solvent(toluene/ethanol).The ? phase can only be formed when the proportion of ethanol reached 40%;but when the proportion of ethanol was less than 40%,the ? phase cannot be seen even there were large aggregations existed in solution(high PFO concentration).Besides,the PFO concentration had great influence on the aggregation but little effect on the formation of ? phase,it was inferred that the ? phase was only associated with the solubility of solvent,and it was formed in aggregation.In addition,according to the result of light scattering,single chains became more and more collapsed and aggregations grow bigger and bigger when ethanol proportion was increased,which proved that the interchain interaction between different chains and intrachain interaction in single chain itself were all enhanced,we inferred that the ? phase was formed by the two kinds of interactions but not by only one kind of interaction,the ? to ? translation could be happened only when the solubility was decreased to a threshold value(such as ethanol 40%)and the two interactions were strong enough to overcome the energy barrier,this viewpoint was different with debates reported before.We also found that the ? phase was stable at room temperature,it could not disappear only if heated at high temperature,and this property is benefit for adjusting the content of ? phase in solution to make devises with excellent performance.In the second work,the effect of molecular weight on the formation of ? phase,the single chain conformation and aggregation state were studied in PFO solution,and the forming mechanism of ? phase and related dynamic process were deeply studied following the first part of work.By comparing the shape parameter Rg/Rh of single chain and persistence length of PFO,it was found that the single chain with higher molecular weight was longer and more flexible than that with lower molecular weight,which proved that single chains with different molecular weight were different even with the same polymer.Then according to the result of scaling law and TEM results,it was found that sample with high molecular weight could form small and loose aggregations but those with low molecular weight formed large and dense aggregations,this phenomenon was different from many classical polymers.Besides,samples with higher molecular weight were easier to form ? phase than those with lower molecular weight in the range of 47,000 to 145,000 g/mol.Finally,the forming mechanism of ? phase was deduced: ? phase was formed by both the interchian interaction and intrachin interaction,but the intrachin interaction was the main reason and the interchain interaction was the secondly.In other words,the ? phase was formed mainly by the fold of chain itself.In the third work,the effect of alkyl side chain length on the solubility of PnDPF and formation of ? phase were studied,and according to the research result we continue studying the dissolution process and the mechanism of ? to ? translation.The deduced conclusions are shown as following: 1?in chloroform solution,PnDPF could all be dissolved to single chains but no ? phase was formed;In toluene solution,P7 DPF had poor solubility and formed large aggregates,however,the ? phase could only be seen in P7 DPF.It was inferred that the alkyl side chain length 7 was a critical point at which the structure of chain could form ? phase.When the length was longer or smaller than the critical point,the chain structure was not benefit for the formation of ? phase and it was easier for them to dissolve in solution.2?The ? phase of P7 DPF in toluene solution was formed in aggregation,which would be disappeared once the aggregates were dissolved.So the forming reason of ? phase was both the interaction and intrachin interaction,which was similar with that of PFO.3?The mechanism of ? to ? translation with chloroform proportion increasing was shown as following: The solubility of solution was enhanced with the increase of chloroform proportion,meanwhile the interchain and intrachin interaction were both decreased so that the disorder degree of ? conformation was enhanced,and finally the ? conformation was translated to disordered ? conformation.This part of work broaden the research scope of ? phase,making it no longer limited to PFO,but extends to other polyfluorenes with different molecular structure.It will be helpful for further understanding of the ? phase of polyfluoene,and is of great guiding significance for the future research of other types of polyfluoenes have ? phase.